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Document:	The Ethernet A Local Area Network Data Link Layer and Physical Layer Specifications
Order Number:	AA-K759A-TK
Revision:	0
Pages:	94
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The

Ethernet

A Local Area Network
Data Link Layer
and
Physical Layer

Specifications

intgl

“XEROX

Digital Equipment Corporation

Intel Corporation

Xerox Corporation

Maynard, MA

Santa Clara, CA

Stamford, CT

Version 1.0

' September 30, 1980
AA-K759A-TK

The

Ethernet

A Local Area Network

Data Link Layer
and

Physical Layer
Specifications

lilgliltlal]

intg

XEROX

Digital Equipment Corporation

Intel Corporation

Xerox Corporation

Maynard, MA

Santa Clara, CA

Stamford, CT

Version 1.0

September 30, 1980

IMPORTANT INFORMATION AND DISCLAIMERS

1. This specification includes subject matter relating to a patent(s) of Xerox
Corporation. No license under such patent(s) is granted by implication, estoppel
or otherwise as a result of publication of this specification. Applicable licenses
may be obtained from Xerox Corporation.

2. This specification is furnished for informational purposes only. Digital, Intel, and
Xerox do not warrant or represent that this specification or any products made in
conformance with it will work in the intended manner or be compatible with
other products in a network system. Nor do they assume responsibility for any
errors that the specification may contain, or have any liabilities or obligations for
damages (including but not limited to special, indirect or consequential damages)
arising out of or in connection with the use of this specification in any way.
Digital, Intel and Xerox products may follow or deviate from the specification
without notice at any time.

3. No representations or warranties are made that this specification or anything made
from it is or will be free from infringements or patents of third persons.

ETHERNET SPECIFICATION: Preface

Preface

This document contains the specification of the Ethernet, a local area network
developed jointly by Digital Equipment Corporation, Intel Corporation, and Xerox
Corporation. The Ethernet specification arises from an extensive collaborative effort
of the three corporations, and several years of work at Xerox on an earlier prototype
Ethernet.

This specification is intended as a design reference document, rather than an
introduction or tutorial. Readers seeking introductory material are directed to the
reference list in Section 2, which cites several papers describing the intent, theory,
and history of the Ethernet.

This document contains 7 sections, falling into three main groups:

Sections 1, 2, and 3 provide an overall description of the Ethernet, including its
goals, and the scope of the specification.

Sections 4 and 5 describe the architectural structure of the Ethernet in terms of a
functional model consisting of two layers, the Data Link Layer and the Physical
Layer.

Sections 6 and 7 specify the two layers in detail, providing the primary technical
specification of the Ethernet.

Readers wishing to obtain an initial grasp of the organization and content of the
specification will be best served by reading Sections 1, 3, and 4. Readers involved in
actual implementation of the Ethernet will find Sections 5, 6, and 7 to contain the
Section 2 provides references, and the
central material of the specification.
appendices provide supplementary material.

The approach taken in the specification of the Data Link Layer in Section 6 is a
procedural one; in addition to describing the necessary algorithms in English and
control flow charts, the specification presents these algorithms in the language Pascal.
This approach makes clear the required behavior of Data Link Layer, while leaving
individual implementations free to exploit any appropriate technology.

Because the procedural approach is not suitable for specifying the details of the
Physical Layer, Section 7 uses carefully worded English prose and numerous figures
and tables to specify the necessary parameters of this layer.
Some aspects of the Ethernet are necessarily discussed in more than one place in this
specification. Whenever any doubt arises concerning the official definition in such a
case, the reader should utilize the Pascal procedural specification of the Data Link
Layer in Section 6.5, and the detailed prose specification of the Physical Layer in
Sections 7.2 through 7.9.

ETHERNET SPECIFICATION: Preface

One aspect of an overall network architecture which is not addressed by this
specification is network management.

The network management facility performs

operation, maintenence, and planning functions for the network:
- Operation functions include parameter setting, such as address selection.
- Maintenance functions provide for fault detection, isolation, and repair.
- Planning functions include collection of statisical and usage information, necessary

for planned network growth.
While network management itself is properly performed outside the Ethernet Data
Link and Physical Layers, it requires appropriate additional interfaces to those layers,

which will be defined in a subsequent version of this specification.

ETHERNET SPECIFICATION: Contents

Table of Contents

Preface
1. INTRODUCTION

2. REFERENCES
3. GOALS AND NON-GOALS
3.1 Goals
3.2 Non-Goals
4. FUNCTIONAL MODEL OF THE ETHERNET ARCHITECTURE
41 Lavering
4.2 Data Link Layer

4.3 Physical Layer
4.4 Ethernet Operation and the Functional Model
4.4.1 Transmission Without Contention
4.4.2 Reception Without Contention
4.4.3 Collisions: Handling of Contention
S. INTER-LAYER INTERFACES
5.1 Client Layer to Data Link Layer
5.2 Data Link Layer to Physical Layer

6. ETHERNET DATA LINK LAYER SPECIFICATION
6.1 Data link Layer Overview and Model

6.2 Frame format

6.2.1 Address Fields
6.2.1.1 Destination Address Field
6.2.1.2 Source Address Field
6.2.2 Type Field
6.2.3 Data Field
6.2.4 Frame Check Sequence Field
6.2.5 Frame Size Limitations

6.3 Frame Transmission

6.3.1 Transmit Data Encapsulation
6.3.1.1 Frame Assembly
6.3.1.2 Frame Check Sequence Generation
6.3.2 Transmit Link Management

iii

ETHERNET SPECIFICATION: Contents

6.3.2.1 Carrier Deference
6.3.2.2 Interframe Spacing
6.3.2.3 Collision Handling
6.3.2.3.1 Collision Detection and Enforcement
6.3.2.3.2 Collision Backoff and Retransmission

23
24
24
24
25

6.4 Frame Reception
6.4.1 Receive Data Decapsulation
6.4.1.1 Framing
6.4.1.1.1 Maximum Frame Size
6.4.1.1.2 Integral Number of Octets in Frame
6.4.1.2 Address Recognition
6.4.1.2.1 Physical Addresses
6.4.1.2.2 Multicast Addresses
Check Sequence Validation
Frame
6.4.1.3
6.4.1.4 Frame Disassembly
6.4.2 Receive Link Management
6.4.2.1 Collision Filtering

25
25
25
26
26
26
26
26
26
21
21
27

6.5 The Data Link Layer Procedural Model

6.5.1 Overview of the Procedural Model

6.5.1.1 Ground Rules for the Procedural Model

6.5.1.2 Use of Pascal in the Procedural Model

6.5.2 The Procedural Model
6.5.2.1 Global Declarations
6.5.2.1.1 Common Constants and Types
6.5.2.1.2 Transmit State Variables
6.5.2.1.3 Receive State Variables
6.5.2.1.4 Summary of Interlayer Interfaces
6.5.2.1.5 State Variable Initialization
6.5.2.2 Frame Transmission
6.5.2.3 Frame Reception
6.5.2.4 Common Procedures

31
34
34
35
35
36
37
38
42
4

7. ETHERNET PHYSICAL LAYER SPECIFICATION

7.1 Physical Channel Overview and Model

7.1.1 Channel Goals and Non-Goals
7.1.1.1 Goals
7.1.1.2 Non-Goals

45
45
45

7.1.2 Characteristics of the Channel

7.1.3 Functions Provided by the Channel

7.1.4 Implementation of the Channel
7.1.4.1 General Overview of Channel Hardware
7.1.4.2 Compatibility Interfaces

46
47
48

7.1.5 Channel Configuration Model

7.1.6 Channel Interfaces

ETHERNET SPECIFICATION: Contents

7.2 Transceiver Cable Compatibility Interface Specifications

7.2.1 Transceiver Cable Signals
7.2.1.1 Transmit Signal
7.2.1.2 Receive Signal
7.2.1.3 Collision Presence Signal
7.2.1.4 Power

53
53
54
54

7.2.2 Transceiver Cable Parameters
7.2.2.1 Mechanical Configuration
7.2.2.2 Characteristic Impedance
7.2.2.3 Attenuation

54
54
55
55
55

7.2.2.4 Velocity of Propagation
7.2.2.5 Pulse Distortion

7.2.2.7 Transfer Impedance

55
55

7.2.3 Transceiver Cable Connectors

7.2.4 Transceiver Cable Drive

7.2.5 Transceiver Cable Receive

7.2.2.6 Resistance

7.2.5.1 Load Impedance and Termination
7.2.5.2 Common Mode and CMRR

7.3 Coaxial Cable Compatibility Interface Specifications

7.3.1 Coaxial Cable Component Specifications
7.3.1.1 Coaxial Cable Parameters

7.3.1.1.] Characteristic Impedance

7.3.1.1.2 Attenuation
7.3.1.1.3 Velocity of Propagation
7.3.1.1.4 Mechanical Requirements
T 3.1.1.5 Pulse Distortion
7. 3.1.1.6 Jacket Marking
7. 3.1.1.7 Transfer Impedance
7.3.1.2 Coaxial Cable Connectors

7.3.1.3 Coaxial Cable Terminators
7.3.1.4 Transceiver-to-Coaxial Cable Connections
7.3.2 Coaxial Cable Signaling
7.4 Transceiver Specifications

7.4.1 Transceiver-to-Coaxial Cable Interface
7.4.1.1 Input Impedance
7.4.1.2 Bias Current
7.4.1.3 Transmit Output Levels

7.4.2 Transceiver-to-Transceiver Cable Interface
7.4.2.1 Transmit Pair
7.4.2.2 Receive Pair
7.4.2.3 Collision Presence Pair
7.4.2.4 Power Pair

7.4.3 Electrical Isolation

58
58

58
58
59
59
59

60
60
61
61

62
62
62
62

63
63

ETHERNET SPECIFICATION: Contents
7.4.4 Reliability
7.5 Channel Logic

7.5.1 Channel Encoding
7.5.1.1 Encoder
7.5.1.2 Decoder
7.5.1.3 Preamble Generation

65
65

7.5.2 Collision Detect Signal

7.5.3 Carrier Sense Signal

7.5.4 Channel Framing
7.5.4.1 Beginning-of-Frame Sequence

68
68

7.5.4.2 End-of-Frame Sequence

7.6 Channel Configuration Requirements

69
69

7.6.1 Cable Sectioning

7.6.2 Transceiver Placement

7.6.3 System Grounding

7.6.4 Repeaters

71
71
71
71
71
1

7.6.4.1 Carrier Detect and Transmit Repeat

7.6.4.2 Collision Detect and Collision Repeat
7.6.4.3 Repeater Signal Generation

7.6.4.3.1 Signal Amplification
7.6.4.3.2 Signal Timing

7.7 Environment Specifications

7.7.1 Electromagnetic Environment

7.7.2 Temperature and Humidity

vii

ETHERNET SPECIFICATION: Contents
Appendices

APPENDIX A: GLOSSARY

APPENDIX B: ASSIGNMENT OF ADDRESS AND TYPE VALUES

APPENDIX C: CRC IMPLEMENTATION

APPENDIX D: IMPLEMENTATION OF TRANSCEIVER CABLE
DRIVER AND RECEIVER

APPENDIX E: INTERFRAME RECOVERY

Figures and Tables

Figure 4-1: Ethernet Architecture and Typical Implementation
Figure 4-2: Architectural Layering

Figure 4-3: Data Link Layer Functions

Figure 4-4: Physical Layer Functions

Figure 6-1: Data Link Layer Frame Format

Figure 6-2: Structure of the Data Link Procedural Model

Figure 6-3: Control Flow Summary -- Client Layer Processes

Figure 6-4: Control Flow Summary -- Data Link Layer Processes

Figure 7-1: Physical Channel Configurations

Table 7-1: Physical Channel Propagation Delay Budget

Figure 7-2: Maximum Transceiver Cable Transfer Impedance

Figure 7-3: Typical Transceiver Cable Waveform

Figure 7-4: Maximum Coaxial Cable Transfer Impedance

Figure 7-5: Typical Coaxial Cable Waveform

Figure 7-6: Manchester Encoding

Figure 7-7: Preamble Encoding

Figure 7-8: Functional Logic ofcollisionDetect Signal

Figure 7-9: Functional Logic of carrierSense Signal

Figure C-1: CRC Implementation

Figure D-1: Typical Transceiver Cable Driver

Figure D-2: Typical Transceiver Cable Receiver

viii

ETHERNET SPECIFICATION: Contents

ETHERNET SPECIFICATION: Introduction

1. INTRODUCTION

The Ethernet local area network provides a communication facility for high speed
data exchange among computers and other digital devices located within a moderatesized geographic area.

Its primary characteristics include:

Physical Layer:
Data rate: 10 Million bits/sec

Maximum station separation: 2.5 Kilometers
Maximum number of stations: 1024

Medium: Shieldea coaxial cable, base-band signalling
Topology: Branching non-rooted tree
Data Link Layer:

Link control procedure: Fully distributed peer protocol, with statistical
contention resolution (CSMA/CD)
Message protocol: Variable size frames, "best-effort” delivery

The Ethernet, like other local area networks, falls in a middle ground between long
distance, low speed networks which carry data for hundreds or thousands of
kilometers, and specialized, very high speed interconnections which are generally

limited to tens of meters. The Ethernet is intended primarily for use in such areas as
office automation, distributed data processing, terminal access, and other situations
requiring economical connection to a local communication medium carrying bursty

traffic at high peak data rates.

Use in situations demanding resistance to hostile

environments, real-time response guarantees, and so on, while not specifically

excluded, do not constitute the primary environment for which the Ethernet is
designed.

The precursor to the Ethernet specified in this document was the "Experimental

Ethernet", designed and implemented by Xerox in 1975, and used continually since
that time by thousands of stations.

The Ethernet defined here builds on that

experience, and on the larger base of the combined experience of Digital, Intel, and
Xerox in many forms of networking and computer interconnection.
In specifying the Ethernet, this document provides precise detailed definitions of the
lowest two layers of an overall network architecture. It thus defines what is generally

ETHERNET SPECIFICATION: Introduction

referred to as a link-level facility.

It does not specify the higher level protocols

needed to provide a complete network architecture.
would

generally

include such

functions

Such higher level protocols

internetwork communication,

error

recovery, flow control, security measures (e.g. encryption), and other higher level

functions that increase the power of the communication facility and/or tailor it to
specific applications.

In particular, it should be noted that all error recovery

functions have been relegated to higher level protocols, in keeping with the low error
rates that characterize local networks.

One of the main objectives of this specification is compatibility. As stated in Section

3, it is intended that every implementation of the Ethernet be able to exchange data
with every other implementation. It should be noted that higher level protocols raise
their own issues of compatibility over and above those addressed by the Ethernet and

other link-level facilities.
compatibility, however.

This does not eliminate the importance of link-level

While the compatibility provided by the Ethernet does not

guarantee solutions to higher level compatibility problems, it does provide a context
within which such problems can be addressed, by avoiding low level incompatibilities
that would make direct communication impossible.

ETHERNET SPECIFICATION: References

2. REFERENCES

The following three papers describe the Experimental Ethernet, and are reprinted in:

"The Ethernet Local Network: Three Reports,”
Technical Report CSL-80-2.

Xerox Palo Alto Research Center

(February, 1980.)

[

Metcalfe, R. M. and Boggs, D. R., "Ethernet: Distributed Packet Switching for

[

Crane, R. C. and Taft, E. A.

Local Computer Networks,"
Network Design,"

Communications of the ACM 19 7 (July 1976).
"Practical Considerations in Ethernet Local

Presented at Hawaii International Conference on System

Sciences (January, 1980).

Shoch, J. F. and Hupp, J. A.

Network,"

"Measured Performance of an Ethernet Local

Presented at Local Area Communications Network Symposium

Boston (May 1979).
The following references describe the ISO Open Systems Model:

Zimmermann, H.,

"OS1 Reference Model -- The ISO Model of Architecture

for Open Systems Interconnection,”

IEEE Transactions on Communication

COM-28 4 (April 1980).

International Organization for Standardization (ISO),
Open Systems Interconnection,”

"Reference Model of

Document no. ISO/TC97/SC16 N227 (June

1979).

The following references describe the Pascal language (used in the Data Link Layer
procedural model) and its derivative Concurrent Pascal:

Jensen, K. and Wirth, N.,

Brinch Hansen, P.,

Pascal User Manual and Report, 2nd Edition.

Springer-Verlag (1974).
Concurrent Pascal Report.

Technical Report CIT-IS-TR

17, California Institute of Technology (1975).

The following references discuss the CRC code used for the frame check sequence:

18]

Hammond, J. L., Brown, J. E. and Liu, S. S., "Development of a Transmission
Error Model and an Error Control Model,"

Technical Report RADC-TR-75-

138, Rome Air Development Center (1975).

Bittel, R,

"On Frame Check Sequence (FCS) Generation and Checking.,”

ANSI working paper X3-S34-77-43, (1977).

ETHERNET SPECIFICATION: Goals and Non-Goals

3. GOALS AND NON-GOALS

This section states the assumptions underlying the design of the Ethernet.
3.1 Goals
The goals of the Ethernet design are:

Simplicity:

Features which would complicate the design without substantially

contributing to the meeting of the other goals have been excluded.
Low cost: Since technological improvements will continue to reduce the overall

cost of stations wishing to connect to the Ethernet, the cost of the connection
itself should be minimized.

Compatibility: ~ All implementations of the Ethernet should be capable of
exchanging data at the data link level.

For this reason, the specification

avoids optional features, to eliminate the possibility of incompatible variants
of the Ethernet.

Addressing flexibility: ~ The

addressing mechanisms

should

provide the

capability to target frames to a single node, a group of nodes, or to all nodes
on the network.

Fairness:

All nodes should have equal access to the network when averaged

over time.

Progress:

No single node operating in accordance with the protocol should be

able to prevent the progress of other nodes.

High speed:

The network should operate efficiently at a data rate of 10

Megabits per second.

Low delay:

At any given level of offered traffic, the network should introduce

as little delay as possible in the transfer of a frame.
Stability: The network should be stable under all load conditions, in the sense

that the delivered traffic should be a monotonically non-decreasing function
of the total offered traffic.

Maintainability:

The Ethernet design should allow for network maintenance,

operation, and planning.

Layered Architecture: The Ethernet design should be specified in layered terms

to separate the logical aspects of the data link protocol from the physical
details of the communication medium.

ETHERNET SPECIFICATION: Goals and Non-Goals

3.2 Non-Goals

The following are nor goals of the Ethernet design:

Full duplex: At any given instant, the Ethernet can transfer data from one
source station to one or more destination stations.
Bi-directional
communication is provided by rapid exhange of frames, rather than full
duplex operation.

Error control: Error handling at the data link level is limited to detection of bit
errors in the physical channel, and the detection and recovery from collisions.
Provision of a complete error control facility to handle detected errors is
relegated to higher layers of the network architecture.
Security:
The data link protocol does not employ encryption or other
mechanisms to provide security. Higher layers of the network architecture
may provide such facilities as appropriate.
Speed flexibility:

This specification defines a physical channel operating at a

single fixed data rate of 10 Megabits per second.

Priority.

The data link protocol provides no support of priority station

operation.

Hostile user: There is no attempt to protect the network from a malicious user
at the data link level.

ETHERNET SPECIFICATION: Functional Model of the Ethernet Architecture

4. FUNCTIONAL MODEL OF THE ETHERNET ARCHITECTURE

There are two important ways to view the Ethernet design, corresponding to:
Architecture, emphasizing the logical divisions of the system, and how they fit
together.
Implementation, emphasizing the actual components, and their packaging and
interconnection.

Figure 4-1 illustrates these two views as they apply to a typical implementation,
showing how each view groups the various functions.
This document is organized along architectural lines, emphasizing the large-scale
separation of the Ethernet system into two parts: the Data Link Layer and the
Physical Layer. These layers are intended to correspond closely to the lowest layers

of the ISO Model for Open Systems Interconnection [4,5]. Architectural organization
of the specification has two main advantages:
Clarity: A clean overall division of the design along architectural lines makes the
specification clearer.
Flexibility: Segregation of medium-dependent aspects in the Physical Layer allows

the Data Link Layer to apply to transmission media other than the specified
coaxial cable.

As is evident in Figure 4-1, the architectural model is based on a set of interfaces

different from those emphasized in the implementations. One crucial aspect of the
design,

however,

must be

interfaces: compatibility.

addressed

largely

terms of the

implementation

Two important compatibility interfaces are defined within

what is architecturally the Physical Layer:
Coaxial cable interface: To communicate via the Ethernet, all stations must adhere
rigidly to the exact specification of coaxial cable signals defined in this document,

and to the procedures which define correct behavior of a station. The mediumindependent aspects of the Data Link Layer should not be taken as detracting
from this point: communication via the Ethernet requires complete compatibility at
the coaxial cable interface.

ETHERNET SPECIFICATION: Functional Model of the Ethernet Architecture

Client Layer
ARCHITECTURE
Data Link Layer

Physical Layer

Data Link Controller

FUNCTIONS
0

Station

Data

station | Interface |

|Encapsulation 7

Physical Channel

Link

Encode &

Transmit &

Mgmt

Decode

Receive

Ethernet Controller Board

Transceiver

Coax

aple

Cable

TYPICAL
IMPLEMENTATION

0 1/0 bus, etc.

Compatibility

Interfaces

Figure 4-1: Ethernet Architecture and Typical Implementation

ETHERNET SPECIFICATION: Functional Model of the Ethernet Architecture
Transceiver cable interface: It is anticipated that most stations will be located some
distance away from their connection to the coaxial cable. While it is necessary to
place a small amount of circuitry (the ransceiver) directly adjacent to the coaxial
cable, the majority of the electronics (the controller) can and should be placed

with the station. Since it is desirable for the same transceiver to be usable with a
wide variety of stations, a second compatibility interface, the transceiver cable
interface, is defined.

While conformance with this interface is not strictly

necessary to insure communication, it is highly recommended, since it allows
maximum

flexibility in intermixing transceivers and stations.

4.1 Layering

The major division in the Ethernet Architecture is between the Physical Layer and
the Data Link Layer, corresponding to the lowest two levels in the ISO model. The
higher levels of the overall network architecture, which use the Data Link Layer,
will be collectively referred to in this document as the "Client Layer" since, strictly
speaking, the identity and function of higher level facilities are outside the scope of

this specification. The intent, however, is that the Ethernet Physical and Data Link
Layers support the higher layers of the ISO model (Network Layer, Transport
Layer, etc.).

The overall structure of the layered architecture is shown in Figure 4-2.

Client Layer

Interface

Data Link Layer

Interface

Physical Layer

Figure 4-2: Architectural Layering

ETHERNET SPECIFICATION: Functional Model of the Ethernet Architecture

In the architectural model used here, the layers interact via well defined interfaces.
The interface between the Client Layer and the Data Link Layer includes

facilities for transmitting and receiving frames, and provides per-operation
status information for use by higher-level error recovery procedures.
The interface between the Data Link Layer and the Physical Layer includes

signals for framing (carrier sense, transmit initiation) and contention resolution
(collision detect), facilities for passing a pair of serial bit streams (transmit,
receive) between the two layers, and a wait function for timing.
These interfaces are described more precisely in Section 5.
As mentioned in the preface, additional interfaces are necessary to allow a higher

level network management facility to interact with the Data Link Layer and Physical
Layer to perform operation, maintenance and planning functions.
4.2 Data Link Layer

The Data Link Layer defines a medium-independent link level communication
facility, built on the medium-dependent physical channel provided by the Physical

Layer. It is applicable to a general class of local area broadcast media suitable for
use with the channel access discipline known as carrier-sense multiple-access with
collision-detection (CSMA-CD).

Compatibility with non-contention media (e.g.,

switched lines, token-passing rings, etc.), while a worthwhile topic for further
research, is not addressed in this specification.
The Data Link Layer specified here is intended to be as similar as possible to that

described in the 1ISO model.

In a broadcast network like the Ethernet, the notion

of a data link between two network entities does not correspond directly to a
distinct physical connection.

Nevertheless, the two main

functions generally

associated with a data link control procedure are present:
Data encapsulation

- framing (frame boundary delimitation)
- addressing (handling ofsource and destination addresses)
- error detection (detection of physical channel transmission errors)
Link management
- channel allocation (collision avoidance)

- contention resolution (collision handling)
This split is reflected in the division of the Data Link Layer into the Data
Encapsulation sub-layer and the Link Management sub-layer, as shown in Figure 43.

NET SPECIFICATION: Functional Model of the Ethernet Architecture

Client Layer

Jy
t—

Client-to-Data Link Interface =——

Transmit

Receive

Data Encapsulation

Data Decapsulation

A
Data Link Layer

Y
Transmit

Receive

Link Management

Data Link-to-Physical Interface et

Physical Layer

Figure 4-3: Data Link Layer Functions

In terms of the ISO model, the Ethernet Data Link Layer provides a multiendpoint connection between higher-layer entities wishing to communicate. The
connection provided is called a data link, and is implemented between two or more
Data Link Layer entities called data link controllers via a Physical Layer connection
called the physical channel.
4.3 Physical Layer

The Physical Layer specified in this document provides a 10 MBit/sec physical
channel through a coaxial cable medium.

Because one purpose of the layered

architecture is to insulate the Data Link Layer from the medium-specific aspects of
the channel, the Physical Layer completely specifies the essential physical
characteristics of the Fthernet, such as data encoding, timing, voltage levels, etc.

Implementation details are left unspecified, to retain maximum flexibility for the
implementor. In all cases, the criterion applied in distinguishing between essential
characteristics and implementation details is guaranteed compatibility: any two
correct implementations of the Physical Layer specified here will be capable of
exchanging data over the coaxial cable, enabling communication between their

ETHERNET SPECIFICATION: Functional Model of the Ethernet Architecture

respective stations at the Data Link Layer.

The Physical Layer defined in this specification performs two main functions
generally

associated with physical channel control:

Data encoding

- preamble generation/removal (for synchronization)

- bit encoding/decoding (between binary and phase-encoded form)
Channel access

- bit transmission/reception (of encoded data)
- carrier sense (indicating traffic on the channel)
- collision detection (indicating contention on the channel)

This split is reflected in the division of the Physical Layer into the Data Encoding
sub-layer and the Channel Access sub-layer, as shown in Figure 4-4.

Data Link Layer

Data Link-to-Physical Interface =

Transmit

Receive

Data Encoding

Data Decoding
Iy
Physical Layer

Y
Transmit

Receive

Channel Access

#
Ethernet Coaxial Cable
Figure 4-4: Physical Layer Functions

ETHERNET SPECIFICATION: Functional Model ofthe Ethernet Architecture

4.4 Ethernet Operation and the Functional Model

This section provides an overview of frame transmission and reception in terms of
the functional model of the architecture.

This overview is descriptive, rather than

definitional; the formal specifications of the operations described here are given in

Sections 6 and 7.
4.4.1 Transmission Without Contention
When the Client Layer requests the transmission of a frame, the Transmit Data

Encapsulation component of the Data Link Layer constructs the frame from the
client-supplied data and appends a frame check sequence to provide for error

detection. The frame is then handed to the Transmit Link Management component
for transmission.
Transmit Link Management attempts to avoid contention with other traffic on the
channel by monitoring the carrier sense signal and deferring to passing traffic.
When the channel is clear, frame transmission is initiated (after a brief interframe

delay to provide recovery time for other data link controllers and for the physical
channel). The Data Link Layer then provides a serial stream of bits to the Physical

Layer for transmission.
The Data Encoding component of the Physical Layer, before sending the actual bits

of the frame, sends an encoded preamble to allow the receivers and repeaters along
the channel

to synchronize

their clocks

and

other circuitry.

It then

begins

translating the bits of the frame into encoded form and passes them to the Channel
Access component for actual transmission over the medium.
The Channel Access component performs the task of actually generating the
electrical

signals

the

medium

which

represent

the

bits

of the

frame.

Simultaneously, it monitors the medium and generates the collison detect signal,
which, in the contention-free case under discussion, remains off for the duration of
the frame.

When transmission has completed without contention, the Data Link Layer so
informs the Client Layer and awaits the next request for frame transmission.
4.4.2 Reception Without Contention
At the receiving station, the arrival of a frame is first detected by the Receive

Channel Access component of the Physical Layer, which responds by synchronizing
with the incoming preamble, and by turning on the carrier sense signal.
encoded

bits arrive

from

Decoding component.

the medium, they

are passed to the

As the

Receive

Data

ETHERNET SPECIFICATION: Functional Model ofthe Ethernet Architecture

Receive Data Decoding translates the encoded signal back into binary data and
discards the leading bits, up to and including the end of the preamble.

It then

passes subsequent bits up to the Data Link Layer.

Meanwhile, the Receive Link Management component of the Data Link Layer,
having seen carrier sense go on, has been waiting for the incoming bits to be
delivered.

Receive Link Management collects bits from the Physical Layer as long

as the carrier sense signal remains on. When the carrier sense signal goes off, the
frame is passed to Receive Data Decapsulation for processing.

Receive Data Decapsulation checks the frame’s destination address field to decide
whether the frame should be received by this station. If so, it passes the contents of
the frame to the Client Layer along with an appropriate status code.

The status

code is generated by inspecting the frame check sequence to detect any damage to
the frame enroute, and by checking for proper octet-boundary alignment of the end
of the frame.

4.4.3 Collisions: Handling of Contention

If multiple stations attempt to transmit at the same time, it is possible for their
transmitting data link controllers to interfere with each others’ transmissions, in
spite of their attempts to avoid this by deferring. When two stations” transmissions

overlap, the resulting contention is called a collision. A given station can experience
a collision during the initial part of its transmission (the "collision window"), before
its transmitted signal has had time to propagate to all parts of the Ethernet channel.

Once the collision window has passed, the station is said to have acquired the
channel: subsequent collisions are avoided, since all other (properly functioning)
stations can be assumed to have noticed the signal (via carrier sense) and to be
deferring to it. The time to acquire the channel is thus based on the round-trip
propagation time of the physical channel.

In the event of a collision, the Transmit Channel Access component of a
transmitting station’s Physical Layer first notices the interference on the channel
and turns on the collision detect signal. This is noticed in turn by the Transmit
Link Management component of the Data Link Layer, and collision handling
begins.

First, Transmit Link Management enforces the collision by transmitting a

bit sequence called the jam. This insures that the duration of the collision is
sufficient to be noticed by the other transmitting station(s) involved in the collision.
After the jam is sent, Transmit Link Management terminates the transmission and
schedules a retransmission attempt for a randomly selected time in the near future.
Retransmission is attempted repeatedly in the face of repeated collisions. Since
repeated collisions indicate a busy channel, however, Transmit Link Management

attempts 10 adjust to the channel load by backing off (voluntarily delaying its own
retransmissions to reduce its load on the channel). This is accomplished by

ETHERNET SPECIFICATION: Functional Model of the Ethernet Architecture

expanding the interval from which the random retransmission time is selected on
each retransmission attempt.

Eventually, either the transmission succeeds, or the

attempt is abandoned on the assumption that the channel has failed or has become
overloaded.

At the receiving end, the bits resulting from a collision are received and decoded by
the Physical Layer just as are the bits of a valid frame.

In particular, collisions do

not turn on the receiving station’s collision detect signal, which is generated only
during transmission.

Instead, the fragmentary frames received during collisions are

distinguished from valid frames by the Data Link’s Receive Link Management

component, by noting that a collision fragment is always smaller than the shortest
valid frame.

Such fragments are discarded by Receive Link Management,

ETHERNET SPECIFICATION: Inter-Layer Interfaces

5. INTER-LAYER INTERFACES

The purpose of this section is to provide precise definitions of the interfaces
between the architectural layers defined in Section 4. In order to provide such a
definition, some precise notation must be adopted. The notation used here is the
Pascal language, in keeping with the procedural nature of the formal Data Link
Layer specification (see 6.5).

Each interface is thus described as a set of procedures

and/or shared variables which collectively provide the only valid interactions
between layers. The accompanying text describes the meaning of each procedure
or variable and points out any implicit interactions among them.

Note that the description of the interfaces in Pascal is a notational technique, and in
no way implies that they can or should be implemented in software. This point is
discussed more fully in 6.5, which provides complete Pascal declarations for the
data types used in the remainder of this section. Note also that the "synchronousTM

(one frame at a time) nature of the frame transmission and reception operations is a
property of the architectural interface between the Client Layer and the Data Link
Layer, and need not be reflected in the implementation interface between a station
and its controller.
5.1 Client Layer to Data Link Layer

The two primary services provided to the Client Layer by the Data Link Layer are
transmission and reception of frames. The interface through which the Client
Layer uses the facilities of the Data Link Layer therefore consists of a pair of
functions.
Functions:
TransmitFrame

ReceiveFrame

Each of these functions has the components of a frame as its parameters (input or
output), and returns a status code as its result.
The Client Layer transmits a frame by invoking TransmitFrame:
function TransmitFrame (
destinationParam:

AddressValue;

sourceParam: AddressValue;
typeParam: TypeValue;

dataParam: DataValue): TransmitStatus;

The TransmitFrame operation is synchronous, in the sense that its duration is the
entire attempt to transmit the frame, so that when the operation completes,
transmission has either succeeded or failed, as indicated by the resulting status
code:

type TransmitStatus

(transmitOK, excessiveCollisionError);

ETHERNET SPECIFICATION: Inter-Layer Interfaces

Successful transmission is indicated by the status code transmitOK: the code
excessiveCollisionError indicates that the transmission attempt was aborted due to

excessive collisions, because of heavy traffic or a network failure.

Implementations

may define additional implementation-dependent status codes if necessary.

The Client Layer accepts incoming frames by invoking ReceiveFrame:
function

ReceiveFrame (

var destinationParam: AddressValue;

var sourceParam: AddressValue;
var typeParam: TypeValue;
var dataParam: DataValue): ReceiveStatus;
The ReceiveFrame operation is synchronous, in the sense that the operation does
not complete until a frame has been received. The fields of the frame are delivered
via the output parameters, along with a status code:
type ReceiveStatus = (receiveOK, frameCheckError, alignmentError);

Successful

reception

is indicated by the

status code receiveOK.

The code

frameCheckError indicates that the frame received was damaged by a transmission

error in the physical channel.

The code alignmentError indicates that the frame

received was damaged, and that in addition, its length was not an integral number
of octets. Implementations may define additional implementation-dependent status
codes if necessary.

5.2 Data Link Layer to Physical Layer
‘The interface through which the Data Link Layer uses the facilities of the Physical
Layer consists of a function, a pair of procedures and three Boolean variables.

Function:

Variables:

ReceiveBit

collisionDetect

TransmitBit

transmitting

Procedures:

carrierSense

Wait

During transmission, the contents of an outgoing frame are passed from the Data
Link Layer to the Physical Layer via repeated use of the TransmitBit operation:
procedure TransmitBit (bitParam: Bit);

Each invocation of TransmitBit passes one new bit of the outgoing frame to the
Physical Layer.

The TransmitBit operation is synchronous, in the sense that the

duration of the operation is the entire transmission of the bit, so that when the
operation completes, the Physical Layer is ready to accept the next bit immediately.
(Note: this does not imply that all invocations of TransmitBit are of exactly equal

duration; for example, if the Physical Layer must perform some initial processing -e.g., preamble generation -- before transmitting the first bit of a frame, the first

ETHERNET SPECIFICATION: Inter-Layer Interfaces
invocation of TransmitBit may

take significantly longer.)

The overall event of data being transmitted is signaled to the Physical Layer via the

variable transmitting:
var transmitting: Boolean;

Before sending the first bit of a frame, the Data Link Layer sets transmitting to
true, to inform the Physical Link that a stream of bits will be presented via the
TransmitBit operation. After the last bit of the frame has been presented, the Data

Link Layer sets transmitting to false to indicate the end of the frame.

The presence of a collision in the physical channel is signaled to the Data Link
Layer via the variable collisionDetect:
varcollisionDetect: Boolean;

The collisionDetect signal remains true during the duration of the collision.
(Note:
Since an entire collision may occur during the first invocation of
TransmitBit -- e.g., during preamble removal -- the Data Link Layer must handle

this possibility by monitoring collisionDetect concurrently with its transmission of
outgoing bits.

See 6.5

for details.)

The collisionDetect signal is generated only during transmission and is never true
at any other time; in particular, it cannot be used during frame reception to detect

collisions between overlapping transmissions from two or more other stations.

During reception, the contents of an incoming frame are retrieved from the
Physical Layer by the Data Link Layer via repeated use of the ReceiveBit
operation:
function ReceiveBit: Bit;

Each invocation of ReceiveBit retrieves one new bit of the incoming frame (i.e.,

not including any preamble bits) from

the Physical

Layer.

The ReceiveBit

operation is synchronous, in the sense that its duration is the entire reception of a
single bit.

(As with TransmitBit, the first invocation of ReceiveBit make take

significantly longer -- e.g., due to preamble removal).

Upon receiving a bit, the

Data Link Layer must immediately request the next bit until all bits of the frame
have have been received.

(See 6.5

for details.)

The overall event of data being received is signaled to the Data Link Layer via the
variable carrierSense:
var

carrierSense: Boolean;

When the Physical Layer sets carrierSense to true, the Data Link Layer must
immediately begin retrieving the incoming bits via the ReceiveBit operation.

When carrierSense subsequently becomes false, the Data Link Layer can begin
processing the received bits as a completed frame.

Note that the true/false

ETHERNET SPECIFICATION: Inter-Layer Interfaces

transitions of carrierSense are not defined to be precisely synchronized with the
beginning and end of the frame, but may precede the beginning and lag the end,
respectively.

If an invocation of ReceiveBit is pending when carrierSense

becomes false, ReceiveBit returns an undefined value, which should be discarded
by the Data Link Layer.
(See 6.5 for details.)
The Data Link Layer must also monitor the value of carrierSense to defer its own
transmissions when the channel is busy.
The Physical Layer also provides the procedure Wait:
procedure Wait (bitTimes: integer);

This procedure waits for the specified number of bit times. This allows the Data
Link Layer to measure time intervals in units of the (physical-channel-dependent)
bit time.

Another important property of the Physical Layer which is an implicit part of the
interface presented to the Data Link Layer is the round-trip propagation time of the
physical channel. This figure represents the maximum time required for a signal to
propagate from one end of the network to the other, and for a collision to
propagate back. The round-trip propagation time is primarily (but not entirely) a
function of the physical size of the network. The round-trip propagation time of
the Physical Layer is defined to be at most 450 bit times (see 7.1.2).

ETHERNET SPECIFICATION: Data Link Layer

6. ETHERNET DATA LINK LAYER SPECIFICATION
6.1 Data Link Layer Overview and Model

As defined in Section 4, the Ethernet Architecture consists of the Data Link Layer,

and below it, the Physical Layer. Furthermore, the Data Link Layer is divided into
wo sub-layers (see Figure 4-3).
Data encapsulation
-

framing

addressing

- error detection

Link management
-

channel allocation

contention resolution

This model is used throughout this section to structure the detailed specification of
the Data Link Layer.

6.2, 6.3, and 6.4.

An English description of the Data Link Layer is given in

A more precise algorithmic definition is given in 6.5, which

provides a procedural model for the Data Link Layer in the form of a program in
the language Pascal.
Note that whenever there is any apparent ambiguity
concerning the definition of some aspect of the Data Link Layer, it is the Pascal
procedural specification

in 6.5 which should be consulted for the definitive

statement.

6.2 Frame Format

The data encapsulation function of the Data Link Layer comprises the construction
and processing of frames.

The subfunctions of framing, addressing, and error

detection are reflected in the frame format as follows:
Framing: No explicit framing information is needed, since the necessary framing
cues (carrierSense and transmitting) are present in the interface to the Physical
Layer.

Addressing: Two address fields are provided to identify the source and destination
stations for the frame.

Error detection: A Frame Check Sequence field is provided for detection of
transmission errors.

Figure 6-1 shows the five fields of a frame: the addresses of the frame’s source and
destination, a type field for use by higher layers (see 6.2.2), a data field containing

the transmitted data, and the frame check sequence field containing a cyclic
redundancy check value to detect transmission errors. Of these five fields, all are of
fixed size except the data field, which may contain any integral number of octets

between the minimum and maximum values specified below (see 6.2.5).

ETHERNET SPECIFICATION: Data Link Layer

[€— 1 octet ——»
physical/multicast bit

—»T
F

E
E

6octets

Destination

6octers

Source

]

Joctets

Tie

frame transmitted

Data

Top-to-bottom

46-1500 octets

Octets wn.h.m

]

Frame Check Sequence
4 octets

LSB

MSB
Bits within

L- octet transmitted left-to-right
Figure 6-1: Data Link Layer Frame Format

Relative to Figure 6-1, the octets of a frame are transmitted from top to bottom, and
the bits of each octet are transmitted from left to right.
NOTE

This document does nor define an order of transmission for the octets of standard

multi-octet data types (strings, integers, etc), since no values of such data types
appear in the data link frame format. The order in which implementations of the

Ethernet store the octets of a frame in computer memory, and the manner in
which higher level protocols interpret the contents of the data field as values of

various multi-octet data types, are beyond the scope of this specification.
The Ethernet itself is also totally insensitive to the interpretation of bits within a
octet as constituting the digits of an 8-digit binary numeric value.

Since some

uniform convention is helpful, however, in avoiding needless incompatibility

among different station types, the interpretation is arbitrarily defined to be that

the left-most bit (first transmitted) is the low-order (20) digit and the right-most
bit (last transmitted) is the high-order (27) digit.

ETHERNET SPECIFICATION: Data Link Layer

6.2.1 Address Fields

Data link addresses are 6 octets (48 bits) in length.
wo

A data link address is of one of

types:

Physical address: The unique address associated with a particular station on the
Ethernet. A station’s physical address should be distinct from the physical
address ofany other station on any Ethernet.
Multicast address: A multi-destination address, associated with one or more
stations on a given Ethernet. There are two kinds of multicast address:

- Multicast-group address: An address associated by higher-level convention
with a group of logically related stations.
- Broadcast address: A distinguished, predefined multicast address which
always denotes the set of all stations on a given Ethernet.
The first bit of a data link address distinguishes physical from multicast addresses:
0 = physical address
1 = multicast address

In either case, the remainder of the first octet and all of the subsequent octets form a
47-bit pattern.
bits.

In the case of the broadcast address, this pattern consists of 47 one-

There is no standard "null" address value.

The procedures for assigning suitably unique values for physical and multicast

addresses are discussed in Appendix B.
6.2.1.1 Destination Address Field

The destination address field specifies the station(s) for which the frame is intended.
It may be a physical or multicast (including broadcast) address.

For details of

address recognition by the receiving station(s), see 6.4.1.2.
6.2.1.2 Source Address Field
The source address field specifies the station sending the frame.
field is not interpreted at the Data Link Layer.

The source address

It is specified at the data link level

because a uniform convention for the placement of this field is crucial for most
higher level protocols.

6.2.2 Type Field
The type field consists of a two-octet value reserved for use by higher levels (in
particular, to identify the Client Layer protocol associated with the frame). The type
field is uninterpreted at the Data Link Layer.

It is specified at this level because a

uniform convention for the placement and value assignment of this field is crucial if
multiple higher level protocols are to be able to share the same Ethernet network
without conflict.

Appendix

B discusses the assignment of type field values.

ETHERNET SPECIFICATION: Data Link Layer

6.2.3 Data Field

The data field contains a sequence of n octets, where 46 < n < 1500 Within this
range, full data transparency is provided, in the sense that any arbitrary sequence of
octet values may appear in the data field.
6.2.4 Frame Check Sequence Field
The frame check sequence (FCS) field contains a 4-octet (32-bit) cyclic redundancy

check (CRC) value.

This value is computed as a function of the contents of the

source, destination, type and data fields (i.e., all fields except the frame check
sequence field itself).
The encoding is defined by the generating polynomial:

Gx) = x32 + x2 + xB 4 x2 4 x16 4 x12 4 4l 4
X0+ %8 4+ %7 + x5+ x* + 22+ x + 1
(This polynomial is also

used in

the Autodin-II network;

its properties are

investigated in [8].)

Mathematically, the CRC value corresponding to a given frame is defined by the
following procedure:

1. The first 32 bits of the frame are complemented.

2. The n bits of the frame are then considered to be the coefficients of a
polynomial M(x) of degree n-1.

(The first bit of the destination address field

corresponds to the TM1 term and the last bit of the data field corresponds to
the x0 term.)
3. M(x) is multiplied by x32 and divided by G(x), producing a remainder R(x) of
degree < 31

4. The coefficients of R(x) are considered to be a 32-bit sequence.
5. The bit sequence is complemented and the result is the CRC.
The 32 bits of the CRC value are placed in the frame check sequence field so that the

x3! term is the leftmost bit of the first octet, and the x° term is the rightmost bit of
the last octet. (The bits of the CRC are thus transmitted in the order x3, x30,..
x1,
x0)
Appendix C discusses CRC implementation issues.
6.2.5 Frame Size Limitations

Given the limitations on the size of the data field specified in 6.2.3 and the 18 octet
total size for the other four fields, the smallest valid frame contains 64 octets and the
largest valid frame contains 1518 octets.

ETHERNET SPECIFICATION: Data Link Layer

6.3 Frame Transmission

The Data Link frame transmission and reception are as follows:
Frame transmission includes data encapsulation and link management aspects:

Transmit Data Encapsulation includes the assembly of the outgoing frame (from
the values provided by the Client Layer) and frame check sequence generation.
Transmit Link Management includes carrier deference, interframe spacing,
collision detection and enforcement, and collision backoff and retransmission.
The performance of these functions by a transmitting data link controller interacts
with corresponding actions by other data link controllers to jointly implement the
Ethernet data link protocol.
6.3.1 Transmit Data Encapsulation
6.3.1.1 Frame Assembly

The fields of the data link frame are set to the values provided by the Client Layer as
arguments to the TransmitFrame operation (see 5.1), with the exception of the
frame check sequence, which is set to the CRC value generated by the data link
controller.

6.3.1.2 Frame Check Sequence Generation

The CRC value defined in 6.2.4 is generated and inserted in the frame check
sequence field, following the fields supplied by the Client Layer. Appendix C
discusses CRC implementation.

6.3.2 Transmit Link Management
6.3.2.1 Carrier Deference

Even when it has nothing to transmit, the data link controller monitors the physical
channel for traffic by watching the carrierSense signal provided by the Physical
Layer. Whenever the channel is busy, the data link controller defers to the passing
frame by delaying any pending transmission of its own. After the last bit of the
passing frame (i.e., when carrierSense changes from true to false), the data link
controller continues to defer for 9.6 psec to provide proper interframe spacing (see
6.32.2). At the end of that time, if it has a frame waiting to be transmitted,
transmission is initiated independent of the value of carrierSense. ~ When
transmission has completed (or immediately, if there was nothing to transmit) the
data link controller resumes its original monitoring of carrierSense.

When a frame is submitted by the Client Layer for transmission, the transmission is
initiated as soon as possible, but in conformance with the rules of deference stated
above.

ETHERNET SPECIFICATION: Data Link Layer

6.3.2.2 Interframe Spacing
As defined in 6.3.2.1, the rules for deferring to passing frames insure a minimum

interframe spacing of 9.6 usec. This is intended to provide interframe recovery time
for other data link controllers and for the physical channel.

Note that 9.6 usec is the minimum value of the interframe spacing.

If necessary for

implementation reasons, a transmitting controller may use a larger value with a
resulting decrease in its throughput. The value should not exceed 10.6 psec.
6.3.2.3 Collision Handling

Once a data link controller has finished deferring and has started transmission, it is
still possible for it to experience contention for the channel. As discussed in 4.4.3,
collisions can occur until acquisition of the network has been accomplished through
the deference of all other stations’ data link controllers.
The dynamics of collision handling are largely determined by a single parameter

called the slor time.

This single parameter describes three important aspects of

collision handling:

- Itis an upper bound on the acquisition time ofthe network.

- Itis an upper bound on the length ofa frame fragment generated by a collision.
(See 6.4.2.1)

- It is the scheduling quantum for retransmission. (See 6.3.2.3.2)
In order to fulfill all three functions, the slot time must be larger than the sum of the
Physical Layer round-trip propagation time (450 bit times; see 7.1.2) and the Data
Link Layer maximum jam time (48 bit times, see 6.3.2.3.1). The slot time is defined
to be 512 bit times.

6.3.2.3.1 Collision Detection and Enforcement

Collisions are detected by monitoring the collisionDetect signal provided by the
Physical Layer.

When a collision is detected during a frame transmission, the

transmission is not terminated immediately. Instead, the transmission continues until

at least 32 (but not more than 48) additional bits have been transmitted (counting
from the time collisionDetect went on).

This collision enforcement or "jam"

guarantees that the duration of the collision is sufficient to insure its detection by all
transmitting stations on the network. The content of the jam is unspecified; it may
be any fixed or variable pattern convenient to the data link controller
implementation, but should not be the 32-bit CRC value corresponding to the
(partial) frame transmitted prior to the jam.

ETHERNET SPECIFICATION: Data Link Layer

6.3.2.3.2 Collision Backoff and Retransmission
When a transmission attempt has terminated due to a collision, it is retried by the
transmitting data link controller until either it is successful, or 16 attempts (the
original attempt plus 15 retries) have been made and all have terminated due to
collisions. Note that all attempts to transmit a given frame are completed before any
subsequent outgoing frames are transmitted.
is

determined by

The scheduling of the retransmissions

a controlled randomization process called

exponential backoff".

"truncated binary

At the end of enforcing a collision (jamming), the data link

controller delays before attempting to retransmit the frame. The delay is an integral

multiple of the slot time. (See 6.3.2.3). The number of slot times to delay before the

nTM retransmission attempt is chosen as a uniformly distributed random integer r in
the range 0 < r < 2X where k = min(n, 10). If all 16 attempts fail, this event is
reported as an error.

Note that the values given above define the most aggressive behavior that a station
may

exhibit in

attempting to

retransmit after a collision.

the course of

implem.enting the retransmission scheduling procedure, a station may introduce extra
delays which will degrade its own throughput, but in no case may a station’s
retransmission scheduling result in a lower average delay between retransmission
attempts than the procedure defined above.

6.4 Frame Reception

Frame reception includes both data decapsulation and link management aspects:
Receive Data Decapsulation comprises framing, address recognition, frame
check sequence validation, and frame disassembly to pass the fields of the
received frame to the Client Layer.
Receive Link Management’s main function is the filtering of collision fragments
from complete incoming frames.

The performance of these functions by a receiving data link controller interacts with
corresponding

actions

other

data link

controllers

to jointly

implement the

Ethernet data link protocol.
6.4.1 Receive Data Decapsulation
6.4.1.1 Framing
The data link controller recognizes the boundaries of an incoming frame by
monitoring the carrierSense signal provided by the Physical Layer.

There are two

possible length errors that can occur, which indicate ill-framed data: the frame may

be too long, or its length may not be an integral number of octets.

ETHERNET SPECIFICATION: Data Link Layer

6.4.1.1.1 Maximum Frame Size

The receiving data link controller is not required to enforce the frame size limit
specified in 6.2.5, but it is allowed to truncate frames longer than 1518 octets and
report this event as an (implementation-dependent) error.
6.4.1.1.2 Integral Number of Octets in Frame

Since the format of a valid frame specifies an integral number of octets, only a
collision or an error can produce a frame with a length that is not an integral
multiple of 8. Complete frames (i.e., not rejected as collision fragments; see 6.4.2.1)
that do not contain an integral number of octets are truncated to the nearest octet
boundary. If frame check sequence validation (see 6.4.1.3) detects an error in such a
frame, the status code alignmentError is reported.
6.4.1.2 Address Recognition
The Ethernet data link controller is capable of recognizing physical and multicast
addresses, as defined in 6.2.1.
6.4.1.2.1 Physical Addresses

The data link controller recognizes and accepts any frame whose destination field
contains the physical address of the station.
The physical address of each station is set by network management to a unique value
associated with the station, and distinct from the address of any other station on any
Ethernet.

The setting of the station’s physical address by network management

allows multiple data link controllers connected to single station all to respond to the

same physical address. The procedures for allocating unique addresses are discussed
in Appendix B.
6.4.1.2.2 Multicast Addresses

The data link controller recognizes and accepts any frame whose destination field
contains the broadcast address.
The data link controller is capable of activating some number of multicast-group

addresses as specified by higher layers.

The data link controller recognizes and

accepts any frame whose destination field contains an active multicast-group address.
An active multicast-group address may be deactivated.
6.4.1.3 Frame Check Sequence Validation
FCS validation is essentially identical to FCS generation.

If the bits of the incoming

frame (exclusive of the FCS field itself) do not generate a CRC value identical to the
one received, an error has occurred and is reported as such.
are discussed in Appendix C.

Implementation issues

ETHERNET SPECIFICATION: Data Link Layer

6.4.1.4 Frame Disassembly

The frame is disassembled and the fields are passed to the Client Layer via the
output parameters of the ReceiveFrame operation (see 5.1).
6.4.2 Receive Link Management
6.4.2.1 Collision Filtering
As specified in 6.2.5, the smallest valid frame must contain at least 64 octets.

Any

frame containing less than 64 octets is presumed to be a fragment resulting from a
collision and is discarded by the receiving data link controller. Since occasional
collisions are a normal part of the link management procedure, the discarding of
such a fragment is not reported as an error to the Client Layer.
6.5 The Data Link Layer Procedural Model
6.5.1 Overview of the Procedural Model
The functions of the Ethernet Data Link Layer are presented below, modeled as a
program written in the language Pascal [6]. This procedural model is intended as the
primary specification of the functions to be provided in any Ethernet Data Link Layer

implementation. It is important to distinguish, however, between the model and a real
implementation. The model is optimized for simplicity and clarity of presentation,

while any realistic implementation must place heavier emphasis on such constraints as
efficiency and suitability to a particular implementation technology or computer
architecture. In this context, several important properties of the procedural model
must be considered.

6.5.1.1 Ground Rules for the Procedural Model
a) First, it must be emphasized that the description of the Data Link Layer in a
programming language is in no way intended to imply that a data link controller

must

implemented

program

executed

computer.

The

implementation may consist of any appropriate technology including hardware,
firmware, software, or any combination.

b) Similarly, it must be emphasized that it is the behavior of Data Link Layer
implementations that must match the specification, not their internal structure.
The internal details of the procedural model are useful only to the extent that
they help specify that behavior clearly and precisely.

ETHERNET SPECIFICATION: Data Link Layer

V] The handling of incoming and outgoing frames is rather stylized in the

procedural model, in the sense that frames are handled as single entities by most
of the Data Link Layer and are only serialized for presentation to the Physical
Layer.

In reality, many data link controller implementations will instead handle

frames serially on a bit, octet or word basis.
typically

perform

the

A serial implementation would

required functions (address recognition,

frame

check

sequence generation/validation, etc.) in an overlapped, pipelined fashion.

This

approach has not been reflected in the procedural model, since this would only
complicate the description of the functions without changing them in any way.

d) The model consists of algorithms designed to be executed by a number of
concurrent processes; these algorithms collectively implement the Ethernet data

link control procedure.

The timing dependencies introduced by the need for

concurrent activity are resolved in two ways:
- Processes vs. External events: It is assumed that the algorithms are executed

"very fast" relative to external events, in the sense that a process never falls
behind in its work and fails to respond to an external event in a timely
manner.

For example, when a frame is to be received, it is assumed that the

data link procedure ReceiveFrame is always called well before the frame in
question has started to arrive.
- Processes vs. Processes: Among processes, no assumptions are made about
relative speeds of execution.
processes

must

respective speeds.

This means that each interaction between two

structured

work

correctly

independent

of their

Note, however, that the timing of interactions among

processes is often, in part, an indirect reflection of the timing of external
events, in which case appropriate timing assumptions may still be made.
It is intended that the concurrency in the model reflect the parallelism intrinsic to
the task of implementing the Ethernet data link, although the actual parallel
structure of the implementations is likely to vary.

ETHERNET SPECIFICATION: Data Link Layer

6.5.1.2 Use of Pascal in the Procedural Model
Pascal was chosen for the procedural model because of its relative simplicity and
clarity, and its general acceptance.
Several observations need to be made about the way in which Pascal is used for the
model, including:

a) Some limitations of the language have been circumvented in order to simplify
the specification:
1) The elements of the program (variables, procedures, etc) are presented in
logical groupings, in top-down order.

Certain Pascal ordering restrictions

have thus been circumvented to improve readability.

2) The process

and cycle constructs of the Pascal derivative Concurrent

Pascal

been

[7]

have

concurrent activity.

introduced

indicate

the

sites

of autonomous

As used here, a process is simply a parameterless

procedure that begins execution at "the beginning of time" rather than being
invoked by a procedure call. A cycle siatement represents the main vody of
a process and is executed repeatedly forever.
3) The lack of variable array bounds in the language has been circumvented
by treating frames as if they are always of a single fixed size (which is never
actually specified).

In fact, of course, the size ofa frame depends on the size

of its data field, hence the value of the "pseudo-constant" frameSize should
be thought of as varying in the long-term, even though it is fixed for any
given frame.

4) The use of a variant record to represent a frame (both as fields and as bits)
follows the letter but not the spirit of the Pascal Report, since it allows the
underlying representation to be viewed as two different data types.

(It also

assumes that this representation is as shown in Figure 6-1.)
b) The model makes no use of any explicit interprocess synchronization primitives .

Instead, all interprocess interaction is done via carefully stylized manipulation of
shared variables. For example, some variables are set by only one process and

inspected by another process in such a manner that the net result is independent of
their execution speeds. While such techniques are not generally suitable for the
construction of large concurrent programs, they simplify the model and more nearly

resemble the methods appropriate to the most likely implementation technologies (e.g.
microcode, hardware state-machines, etc.)

ETHERNET SPECIFICATION: Data Link Layer

6.5.2 Procedural Model
The procedural model used here is based on five cooperating concurrent processes.

Of these, three are actually defined in the Data Link Layer.

The remaining two

processes are provided by the Client Layer and utilize the interface operations

provided by the Data Link Layer.

The five processes are thus:

Client Layer:
Frame Transmitter Process Frame Receiver Process
Data Link Layer:

Bit Transmitter Process

Bit Receiver Process

Deference Process
This organization of the model is illustrated in Figure 6-2, and reflects the fact that
the communication of entire frames is initiated by the Client Layer, while the timing

of collision backoff and of individual bit transfers is based on interactions between
the Data Link Layer and the Physical-Layer-dependent bit-time.
Figure 6-2 depicts the static structure of the procedural model, showing how the
various processes and procedures interact by invoking each other. Figures 6-3 and 64 summarize the dynamic behavior of the model during transmission and reception,
focusing on the steps that must be performed, rather than the procedural structure
which performs them.

The usage of the shared state variables is not depicted in the

figures, but is described in the comments in 6.5.2.1.

ETHERNET SPECIFICATION: Data Link Layer

: FrameTransmiter :

Client

—_

TransmitFrame

ReceiveFrame

itDataE:

ransmitDat

ReceiveDataD

nca

ecelvebal

CRC32

DATA

ENCAPSULATION

RecognizeAddress

DATA LINK LAYER

Y
TransmitLinkMgmt

WatchForCollision

ReceiveLinkMgmt

BackOff

StartReceive

StartTransmit

LINK

Rand

MANAGEMENT

nsmitter

StartJam

NextBit

TransmitBit

RealTimeDelay

Wait

ReceiveBit
Physical Layer

TRANSMIT

RECEIVE

Figure 6-2: Structure of the Data Link Procedural Model

ETHERNET SPECIFICATION: Data Link Layer

TransmitFrame

ReceiveFrame

assemble frame

start receiving

deferring on?

done receiving?

[€———

start transmission

frame too small?

(collision)

collisionDetect ?

send jam
transmission done?

recognize address?

¥
increment attempts

yes

disassemble

frame

yes

100 many

attempts

good

validate frame

check sequence

compute

backoff
+

extra bits

?
wait backoff

truncated from frame?

ves

time

Done:

transmitOK

Done:

excessiveCollisionError

Done:

receiveOK | | frameCheckError

Done:

alignmentError

FrameTransmitter process

FrameReceiver process

(Invoking Data Link TransmitFrame operation)

(Invoking Data Link ReceiveFrame operation)

Figure 6-3: Control Flow Summary -- Client Layer Processes

ETHERNET SPECIFICATION: Data Link Layer

I
transmission started?

channel busy?

—>

transmita bit

end of frame?

deferring on

transraission done

Bit Transmitter process
yes

wait

interframe spacing

¥
deferring off

receiving started?

—]
>

receive a bit

frameWaiting ?

Deference process

carrier sense on?

receiving done

Bit Receiver process

Figure 6-4: Control Flow Summary -- Data Link Layer Processes

ETHERNET SPECIFICATION: Data Link Layer

6.5.2.1 Global Declarations
6.5.2.1.1 Common Constants and Types
The following declarations of constants and types are used by the frame transmission
and reception sections of each data link controller:
const

addressSize = 48;
typeSize = 16;

dataSize =
crcSize = 32;

(48 bit address = 6 octets}

{16 bit protocol type = 2 octets}

. .; {see6.5.1.2, note
3}
(32 bit CRC = 4 octets}

frameSize = ...;

{ = 2*addressSize
+ typeSize + dataSize + crcSize...see 6.5.1.2,

note 3}

slotTime = 512; {unit of time for collision handling}
type

Bit = 0..1;

AddressValue = array [1..addressSize] of Bit;
TypeValue = array [1..typeSize] of Bit;
DataValue = array [1..dataSize] of Bit;

CRCValue = array [1..crcSize] of Bit;
ViewPoint = (fields, bits);
Frame = record

{Two ways to view the contents of a frame}

{Format
of data link frame}

case view: ViewPoint of
fields: (
destinationField: AddressValue;
sourceField: AddressValue;

typeField: TypeValue;
dataField: DataValue;
fcsField: CRCValue);

bits: (

contents: array [1..frameSize] of Bit)
end; {Frame}

ETHERNET SPECIFICATION: Data Link Layer

6.5.2.1.2 Transmit State Variables
The following items are specific to frame transmission.

(See also 6.5.2.1.4 on

interfaces)
const

interFrameSpacing = 9.6; {minimum time between frames, in microseconds}
attemptLimit = 16; {Max number of times attempt transmission}

backOffLimit = 10; {Limit on number of times to back off}

jamSize = 32;

{jam may be 32 to 48 bits long}

var

outgoingFrame: Frame; {The frame to be transmitted}
currentTransmitBit, lastTransmitBit: 1..frameSize; {Positions of current and
last outgoing bits in outgoingFrame}

deferring: Boolean; {True implies any pending transmission must wait for the
channel to clear}

frameWeaiting: Boolean; {indicates that outgoingFrame is defzrring}
attempts: O..attemptLimit; {Number of transmission attempts on outgoingFrame}
newCollision: Boolean; {indicates that a collision has occurred but has not yet
been jammed}

transmitSucceeding: Boolean; {Running indicator of whether transmission is
succeeding}

6.5.2.1.3 Receive State Variables
The following items are specific to frame reception. (See also 6.5.2.1.4 on interfaces)
var

incomingFrame: Frame; {The frame being received}
currentReceiveBit: 1..frameSize; (Position of current bit in incomingFrame}
receiving: Boolean; {indicates that a frame reception is in progress}
excessBits: 0..7; {Count of excess trailing bits beyond octet boundary}
receiveSucceeding: Boolean; {Running indicator of whether reception is
succeeding}

ETHERNET SPECIFICATION: Data Link Layer

6.5.2.1.4 Summary of Interlayer Interfaces
The interface to the Client Layer, defined in 5.1, is summarized below:
type

TransmitStatus = (transmitOK, excessiveCollisionError); (Result of
TransmitFrame operation}

ReceiveStatus = (receiveOK, frameCheckError, alignmentError); {Resuit
of ReceiveFrame operation}

function TransmitFrame (
destinationParam: AddressValue;

sourceParam: AddressValue;
typeParam: typeValue;

dataParam: DataValue): TransmitStatus; {Transmits one frame}
function ReceiveFrame (
var destinationParam: AddressValue;
var sourceParam: AddressValue;
var

typeParam: TypeValue;

var

dataParam: DataValue): ReceiveStatus; {Receives one frame}

The interface to the Physical Layer, defined in 5.2, is summarized below:
var

carrierSense: Boolean; {indicates incoming bits}
transmitting: Boolean; {indicates outgoing bits}
collisionDetect: Boolean; (indicates channel contention}

procedure TransmitBit (bitParam: Bit); {Transmits one bit}
function ReceiveBit: Bit; (Receives one bit}

procedure Wait (bitTimes: integer); {Waits for indicated number of bit-times}

ETHERNET SPECIFICATION: Data Link Layer

6.5.2.1.5 State Variable Initialization

The procedure /nitialize must be run when the Data Link Layer begins operation,
before any of the processes begin execution.
state variables to their initial values.

reinitialized before each use.)

Initialize sets certain crucial shared

(All other global variables are appropriately

Initialize then waits for the channel to be idle, and

starts operation of the various processes.
procedure Initialize;
begin

frameWaiting : = false;
deferring : = false;
newCollision : = false;
transmitting

: =

false; {In interface to Physical Layer; see below}

receiving : = false;
while carrierSense do nothing;
{Start execution of all processes}
end; {Initialize}

ETHERNET SPECIFICATION: Data Link Layer

6.5.2.2 Frame Transmission
The algorithms in this section define data link frame transmission.
The function TransmitFrame implements the frame transmission operation provided

to the Client Layer:
function TransmitFrame (
destinationParam: AddressValue;
sourceParam: AddressValue;
typeParam: typeValue;
dataParam: DataValue): TransmitStatus;
procedure TransmitDataEncap; ... {nested procedure; see body below}
begin

TransmitDataEncap;
TransmitFrame : = TransmitLinkMgmt
end; {TransmitFrame}

First, TransmitFrame calls the internal procedure TransmitDataEncap to construct
the frame.

It then calls TransmitLinkMgmt to perform the actual transmission. The

TransmitStatus returned indicates the success or failure of the transmission attempt.
TransmitDataEncap builds the frame and places the 32-bit CRC in the frame check

sequence field:
procedure TransmitDataEncap;
begin
with outgoingFrame do

begin {assemble frame}
view : = fields;

destinationField : = destinationParam;
sourceField : = sourceParam;
typeField : = typeParam;
dataField : = dataParam;
fcsField : = CRC32(outgoingFrame);

view
: = bits

end {assemble frame}
end; {TransmitDataEncap}

ETHERNET SPECIFICATION: Data Link Layer

TransmitLinkMgmt attempts to transmit the frame, deferring first to any passing

traffic. If a collision occurs, transmission is terminated properly and retransmission is
scheduled following a suitable backoff interval:
function TransmitLinkMgmt: TransmitStatus;
begin

attempts : = O; transmitSucceeding : = false;
while attempts < attemptLimit and not transmitSucceeding do
begin {loop}

if attempts > 0 then BackOff;
frameWaiting : = true;
while deferring do nothing;

{defer to passing frame, if any}

frameWaiting : = false;

StartTransmit;
while transmitting do WatchForCollision;
attempts : = attempts + 1

end; {loop}

if transmitSucceeding then TransmitLinkMgmt : = transmitOK
else TransmitLinkMgmt : = excessiveCollisionError
end; {TransmitLinkMgmt}

Each time a frame transmission attempt is initiated, StartTransmit is called to alert
the BitTransmitter process that bit transmission should begin:
procedure StartTransmit;
begin
currentTransmitBit : = 1;
lastTransmitBit : = frameSize;
transmitSucceeding : = true;

transmitting : = true
end; {StartTransmit}

Once

frame transmission

has been

initiated,

TransmitLinkMgmt monitors the

channel for contention by repeatedly calling WatchForCollision:
procedure WatchForCollision;
begin

if transmitSucceeding and collisionDetect then
begin

newCollision : = true;
transmitSucceeding : = false
end

end; {WatchForCollision}
WatchForCollision,

upon

detecting a collision, updates newCollision

proper jamming by the BitTransmitter process.

to insure

ETHERNET SPECIFICATION: Data Link Layer

After transmission of the jam has completed, if TransmitLinkMgmt determines that
another attempt should be made, BackOff is called to schedule the next attempt to

retransmit the frame.
var maxBackOff: 2..1024; {Working variable of BackOff}
procedure BackOff;

begin
if attempts = 1 then maxBackOff : = 2 else if attempts< backOffLimit

then maxBackOff : = maxBackOff*2;
Wait(slotTime*Random(0, maxBackOff))
end; {BackOff}
function Random (low, high: integer): integer;
begin
Random : = ...{uniformly distributed random integer r such that low < r <
high}
end; {Random}

BackOff performs the truncated binary exponential backoff computation and then
waits for the selected multiple of the slot time.

The Deference process runs asynchronously to continuously compute the proper

value for the variable deferring.
process Deference;

begin
cycle {main loop}

while not carrierSense do nothing;
deferring : = true;

{watch for carrier to appear}

{delay start of new transmissions}

while carrierSense do nothing;

{wait for carrier to disappear}

RealTimeDelay(interFrameSpacing);

deferring : = false;

{allow new transmissions to proceed}

while frameWaiting do nothing {allow waiting transmission (if any)}
end {main loop}

end; {Deference}

procedure RealTimeDelay (usec: real);
begin

{Wait for the specified number of microseconds}

end; {RealTimeDelay}

ETHERNET SPECIFICATION: Data Link Layer

The BitTransmitter process runs asynchronously, transmitting bits at a rate
determined by the Physical Layer's TransmitBit operation:
process BitTransmitter;
begin

cycle {outer loop}

while transmitting do
begin {inner loop}

TransmitBit(outgoingFrame[currentTransmitBit]); (send next bit to
Physical Layer}

if newCollision then StartJam else NextBit
end {inner loop}
end {outerloop}
end; (BitTransmitter}

procedure NextBit;
begin

curreniTransmit3it : = currentTransmitBit + 1;
transmitting : = (currentTransmitBit < lastTransmitBit)

end; {NextBit}

procedure StartJam;
begin

currentTransmitBit : = 1;

lastTransmitBit : = jamSize;
newCollision : = false
end; (StartJam}

BitTransmitter, upon detecting a new collision, immediately enforces it by calling
StartJam to initiate the transmission of the jam. The jam may contain 32 to 48 bits
of arbitrary data. (StartJam uses the first 32 bits of the frame, merely to simplify
this program).

ETHERNET SPECIFICATION: Data Link Layer

6.5.2.3 Frame Reception
The algorithms in this section define data link frame reception:

The procedure ReceiveFrame implements the frame reception operation provided

to the Client Layer:
function ReceiveFrame (

var destinationParam: AddressValue;
var sourceParam: AddressValue;
var typeParam: TypeValue;

var dataParam: DataValue): ReceiveStatus;
function ReceiveDataDecap: ReceiveStatus; ... {nested function; see body
below}

begin
repeat

ReceivelLinkMgmt;
ReceiveFrame : = ReceiveDataDecap;
until receiveSucceeding

end; {ReceiveFrame}
Receiveframe calls ReceiveLinkMgmt to receive the next valid frame, and then

calls the internal procedure ReceiveDataDecap to return the frame’s fields to the
Client Layer if the frame’s address indicates that it should do so. The returned
ReceiveStatus indicates the presence or absence of detected transmission errors in

the frame.
function ReceiveDataDecap: ReceiveStatus;
begin

receiveSucceeding : = RecognizeAddress

(incomingFrame.destinationField);
if receiveSucceeding then with incomingFrame do
begin {disassemble frame}
view : = fields;

destinationParam : = destinationField;
sourceParam : = sourceField;

typeParam

ypeField;

dataParam : = dataField;

if fcsField = CRC32(incomingFrame) then ReceiveDataDecap : = receiveOK
else if excessBits = 0 then ReceiveDataDecap
: = frameCheckError
else ReceiveDataDecap : = alignmentError;

view
: = bits
end {disassemble frame}

end; {ReceiveDataDecap}

ETHERNET SPECIFICATION: Data Link Layer

function RecognizeAddress (address: AddressValue): Boolean;

begin
RecognizeAddress : = ... {Returns true for the set of physical, broadcast, and
multicast-group addresses corresponding to this station}

end; {RecognizeAddress}
ReceiveLinkMgmt attempts repeatedly to receive the bits of a frame, discarding any

fragments from collisions by comparing them to the minimum valid frame size:
procedure ReceiveLinkMgmt;

begin
repeat

StartReceive;

while receiving do nothing;

{wait for frame to finish arriving}

excessBits : = frameSize mod 8;
frameSize : = frameSize - excessBits; {truncate to octet boundary}
receiveSucceeding : = (frameSize > slotTime); {reject collision fragments}
until receiveSucceeding
end; {ReceiveLinkMgmt}
procedure StartReceive;
begin

currentReceiveBit : = 1;

receiving : = true
end; {StartReceive}

The BitReceiver process run asynchronously, receiving bits from the channel at the
rate determined by the Physical Layer's ReceiveBit operation:
process BitReceiver;

var b: Bit;

begin
cycle fouter loop}
while receiving do

begin finner loop}
b := ReceiveBit;

(Get next bit from physical link}

if carrierSense then
begin{append bit to packet}
incomingFrame[currentReceiveBit] : = b;
currentReceiveBit : = currentReceiveBit + 1

end; {append bit to packet}
receiving : = carrierSense

end {inner loop}
end (outer loop}

end; (BitReceiver}

ETHERNET SPECIFICATION: Data Link Layer

6.5.2.4 Common procedures
The function CRC32 is used by both the transmit and receive algorithms to generate
a 32 bit CRC value:
function CRC32 (f: Frame): CRCValue;
begin

CRC32:= {The 32-bit CRC as defined in 6.2.4}
end; {CRC32}

Purely to enhance readability, the following procedure is also defined:
procedure nothing; begin end;

The idle state of a process (i.e., while waiting for some event) is cast as repeated calls
on this procedure.

ETHERNET SPECIFICATION: Physical Layer
7.

ETHERNET PHYSICAL LAYER SPECIFICATION: Baseband Coaxial System

7.1

Physical Channel Overview and Model

The Ethernet physical channel (henceforth referred to as the channel) provides the
lowest layer in the Ethernet architecture.

It performs all the functions needed to

transmit and receive data at the physical level, while supporting the Data Link to
Physical Layer Interface described in 5.2.
This section describes the requirements for interface and compatibility with a
baseband coaxial implementation of the channel.
7.1.1

Channel Goals and Non-goals

This section states the objectives underlying the design of the channel.
7.1.1.1 Goals

The following are the goals of the channel:

1. Provide a means for communication between Ethernet Data Link Entities.
2. Define physical interfaces which can be implemented compatibly among
different manufacturers of hardware.
3. Provide all clocks, synchronization, and timing required for both itself and
the Ethernet Data Link.
Provide high bandwidth and low bit error rates.
Provide for ease of installability and serviceability.
Provide for high network availability.
Support the Ethernet Data Link to Physical Link interface.
8. Low cost.
7.1.1.2 Non-Goals
The following are not goals of the baseband coaxial channel design:
1. Operation at data rates other than 10 megabits per second.
2. Operation with media other than the specified coaxial cable.
3. Simultaneous use of the channel by transmitters using signals not specified in
this document.

4. Protection against a malicious user or a malfunctioning Data Link Entity is
not provided by the channel as specified. However, higher layers (above the

Data Link) and/or physical security means may be employed to acheive this.

ETHERNET SPECIFICATION: Physical Layer

712

Characteristics of the Channel

The channel provides (and the data link assumes) the following characteristics:
L The ability to send and receive information (non-simultaneously) between
any two or more data link entities on the same network.
The ability to detect the presence of another station’s transmission while not
transmitting (carrier sense).
. The ability to detect the presence of another station’s transmission while
transmitting (collision detect).
A total worst-case round trip signal propagation delay (including actual
propagation time, synchronization time for all intervening electronics, and
signal rise time degradation) of 450 bit times (equal to 45 ps for this 10 Mbit
channel).

713

Functions Provided by the Channel

The channel hardware provides the following functions in the performance of ‘ts

role:
1L Means for transmitting and receiving serial bit streams between the data link
layer and the media.

Generation of clock for synchronization and timing.
3. Means for detecting carrier (non-idle channel).
Means

for detecting

collisions

(simultaneous

transmission

attempts

multiple stations).
Coding and decoding of the data link bit stream into a self-synchronizable
sequence of electrical signals suitable for transmission on the media provided
by the channel.
Generation

and

removal

coding-specific

preamble

information

synchronizing header sequence inserted before the first bit of the frame) to

ensure that all channel electronics are brought to a known steady-state before
the data link frame is transmitted.

7.14

Implementation of the Channel

The physical

channel

specification

is implementation dependent;

most of the

channel hardware is fully specified, and little leeway is given to the individual
designer.

This is done in the interest of compatibility; any system which allows

different implementors to use different channel cables, connectors, clock speeds and
the like will not be compatible across manufacturer boundaries. Only the design of
channel components which are not critical to system compatibility is left to the

implementor.

ETHERNET SPECIFICATION: Physical Layer

7.1.4.1 General Overview of Channel Hardware

The channel minimally consists of the following functional blocks:
1. The passive broadcast medium (coaxial cable),

2. The transceiver (transmitter-receiver for the coaxial cable),
3. The means for connecting transceivers to a coaxial cable segment and for
connecting coaxial cable segments together,
4. The channel clock,

5. The channel data encoder and decoder,
6. The preamble generator and remover,
T The carrier and collision detect circuits.

The coaxial medium is the only element common to the entire network.
transceiver is required for each station connected to the medium.

must be located adjacent to the coaxial ~able.

The transceiver

The latter four components are

generally located within, and tightly coupled to, the station hardware implementing
the data link function.

It may be useful to be able to physically separate the transceiver from the rest of
the channel hardware. This allows topological flexibility, packaging advantages,
and improved system availability, as well as allowing for independent manufacture
of station hardware and transceivers. To ensure that compatibility is maintained, a
physical interface (known as the transceiver cable) is identified and specified to
connect the transceiver to the station.

Finally, it may be necessary to add repeaters to the system, to reach the maximum
allowable distance between stations, and to provide additional topological
flexibility. Repeaters are implemented using standard transceivers, plus a simple,
non-buffered finite state machine.

ETHERNET SPECIFICATION: Physical Layer

7.1.4.2 Compatibility Interfaces
There are a number of possibilities for implementing systems or subsystems
compatible in whole or in part with this specification.

It is important that all

implementations be compatible at some point, so that heterogenous systems from
different manufacturers’
medium.

implementations can be interconnected on the same

It is not necessary in every case to implement all of the components

described herein; e.g., it is possible to design an integrated station/transceiver
(without requiring the transceiver cable). The implementor must make the required
trade-offs between topological flexibility, system availability, configurability, user
needs, and cost when designing the system.

For a device to be considered compatible, it must meet the applicable requirements
at either the transceiver cable or the coaxial cable interface, as appropriate, in
addition to the Data Link compatibility required for all stations connected to the
network.

All Ethernets must be compatible at the coaxial cable.
If a transceiver cable is used, it should be the one specified in this document. This
allows device manufacturers to build hardware compatible with the Ethernet at the

transceiver cable level, without concerning themselves with the details of transceiver
implementation.

Devices implementing transceiver cable compatibility should be

capable of using transceivers designed and built by another manufacturer, on the

specified coaxial cable.
Equipment designed for connection to the specified coaxial cable either without a
physically separate transceiver or with a non-standard transceiver cable interface

will be capable of communication.

However, a sacrifice may have been made with

respect to interchangeability with other stations.
This scheme of multiple compatibility interfaces allows individual designers some
flexibility in making system tradeoffs, yet allows cable manufacturers, transceiver
manufacturers and systems manufacturers to use standard commodity parts to
produce a compatible communications system.

ETHERNET SPECIFICATION: Physical Layer
7.1.5

Channel Configuration Model

Certain physical limits have been placed on the physical channel.

These revolve

mostly around maximum cable lengths (or maximum propagation times), as these

affect the slot time as defined in the data link. While the precise specification (in
later sections) specify these maxima in terms of propagation times, they were
derived from the physical configuration model described here.
The maximum configuration is as follows:

1. A coaxial cable, terminated in its characteristic impedance at each end,
constitutes a cable segment. A segment may contain a maximum of 500
meters of coaxial cable.

2. A maximum of 2 repeaters in the path between any two stations. Repeaters

do not have to be located at the ends of segments, nor is the user limited to
one repeater per segment.

In fact, repeaters can be used not only to extend

the length of the channel, but to extend the topology from one to threedimensional. Repeaters occupy transceiver positions on each cable segment
and count towards the maximum number of transceivers on a segment just as
do the logically distinguishable stations.
3. A maximum total coaxial cable length along the longest path between any
two transceivers of 1500 meters. The propagation velocity of the coaxial cable

is assumed to be 0.77 ¢ worst-case.

(c is the velocity of light in vacuo;

300,000 kilometers per second.) The total round-trip delay for all the coaxial
cable in the system is therefore 13 us worst-case.

4. A maximum of 50 meters of transceiver cable between any station and its
associated transceiver.

Note that in the worst case the signal must pass

through six 50 meter transceiver cables, one at the transmitting station, one

at the receiving station, and 2 at each repeater (two repeaters possible). The
propagation velocity of the transceiver cable is assumed to be .65 ¢ worst-

case. The total round-trip delay for all the transceiver cables is therefore 3.08
us

worst-case.

5. A maximum of 1000 meters of point-to-point link anywhere in the system.
This will typically be used as a way of linking cable segments in different

buildings. Note that a repeater with this internal point-to-point link can be
used to repeat signals between segments many hundreds of meters apart.
The worst-case propagation velocity of the link cable is assumed to be

.65 c; the round-trip propagation delay for 1000 meters is 10.26 ps.
Table 7-1 summarizes the allocation of the round-trip propagation delay to the
individual components in the channel. Figure 7-1 shows a minimum, typical, and
large-scale channel configuration.

ETHERNET SPECIFICATION: Physical Layer

Coaxial Cable Segment

(500 M max)

Transceiver Cable

50 M max

Coaxial Cable

i
Station ——pf

Transceiver & Connection
to Coaxial Cable
(100 max per segment)

Figure 7-1a: Minimal Configuration

Segment 1
1

Repeater

Segment 2

Figure 7-1b: A Typical Medium-scale Configuration

]

ETHERNET SPECIFICATION: Physical Layer

il
Segment 1

Station

[]—

|
Segment 2

Repeater

Segment3

Remote repeater —DCS

Coaxial Cable

Point-to-point link
(1000M max)

Segment 4

[ }——
Segment 5

—f
}_._..

Figure 7-1c: A Typical Large-scale Configuration

ETHERNET SPECIFICATION: Physical Layer

Table 7-1: Physical Channel Propagation Delay Budget (Note 1)

Element

Unit
Steady-State
Delay

Unit
Startup
Delay

Encoder

0.1pS

5.13 nS/M

Transceiver.Cable

Transceiver

0.50 pS

02pS

Transceiver

0.50 uS

05pS

Transceiver

0.5pS

(transmit path)
(receive path)
(collision path)

# Units
Forward
Path (Note 2)

# Units
Return
Path

Total
Delay

Coaxial Cable

4.33n8/M

1500 M

13.00 uS

Point-to-Point

5.13nS/M

1000 M

10.26 S

Repeater

0.8 1S

1.60 pS

Repeater
(collision path)

02 S

040 S

Decoder

0.1pS

08 puS

1.80 uS

Carrier Sense

02pS

0.60 uS

Collision Detect

0.2pS

0.60 uS

Signal Rise Time

0.1pS

030 S

2.7uS

8.10 S

Link Cable

(repeat path)

(t0 70% in 500 M)
(Note 3)

Signal Rise Time
(50% to 94% in 500 M)
(Note 4)

Total Worst-Case Round-Trip Delay

44.99 uS

Note 1: All quantities given are worst-case (both number of units and unit delays per unit).
Note 2: The propagation delay has been separated into “"forward-path" and “return path" delay.

This is because in one direction it is carrier sense which is being propagated through the channel,
and in the return direction it is collision detect which is being propagated. The two signals have
different propagation

delays.

Note 3: In the worst-case, the propagated signal must reach 70% of its final value to be detected as
valid carrier at the end of 500 meters of coaxial cable. This rise time must be included in the
propagation

delay budget.

Note 4: In the worst-case the propagated collision on the return path must reach 94% of its final
value 1o be detected as a collision at the end of 500 meters of coaxial cable.

ETHERNET SPECIFICATION: Physical Layer
7.1.6

Channel Interfaces

The channel specification hinges around three well-defined entities; the transceiver
and coaxial cables (shown as compatibility interfaces in Figure 4-1), and the logical
interface between the physical channel and the data link controller (shown in
Figure 4-4). Note that the former two are physical interfaces specific to the
channel, and are specified in the interest of compatibility. The latter is provided as
a means by which the data link controller can interact with the physical channel.
The channel access component of the logical interface (discussed in 4.4.1) comprises
the collision and carrier detect functions described in 7.5.2 and 7.5.3, as well as the
actual transmission of signals on the media. The data encoding and decoding
functions described in 4.4.1 comprise the generation and decomposition of encoded
signals suitable for transmission (described in 7.5.1), the generation and removal of
code-specific preamble (described in 7.5.1.3 and 7.5.4.1), and the serial bit stream
interface between the layers.

Section 5 describes the interface between the data link and physical layers as a
series of Pascal procedures, functions, and shared variables.
The data link
specification in section 6 shows how the data link uses this interface to
communicate between client layers. However, this specification will not attempt to

model the operation of the physical channel in Pascal. The interface between layers
is supported by the physical hardware which provides the ability to send and
receive bit streams, provide timing, and signal carrier sense and collision detect to
the data link.

The remainder of this section specifies the requirements for compatibility at both
the transceiver cable and the coaxial cable. In addition, the specifications for the
transceiver, which interfaces the transceiver cable to the coaxial cable is given, as
well as the specification for the logic required between the transceiver cable and the
interface to the data link.

7.2

Transceiver Cable Compatibility Interface Specifications

The transceiver cable is the means by which a physically separate transceiver is
connected to a station. It provides one of the compatibility interfaces described in
7.1.4.2.

7.2.1

Transceiver Cable Signals

The transceiver cable carries four signals: Transmit, Receive, Collision Presence,
and Power. Each signal is carried on a twisted pair of conductors in the cable.
7.2.1.1 Transmit Signal

The transmit pair carries encoded data for which the data link is requesting
transmission on the channel. This signal is generated by the data encoder, with the
transceiver cable drive characteristics specified in 7.2.4.

ETHERNET SPECIFICATION: Physical Layer

7.2.1.2 Receive Signal

The receive pair carries encoded data from the transceiver to the station.
typically goes to the data decoder and the carrier sense circuitry.

In the steady-

state, all transitions and lack of transitions on the coaxial cable become transitions
and lack of transitions on the receive pair, with the transceiver cable drive
characteristics specified in 7.24.

(During start-up, the first few bits may be

absorbed by the transceiver to attain steady-state.)
In the case of a station transmitting without collision interference, the station’s own
transmit transitions on the coaxial cable will also appear on the receive pair, after a

delay due to propagation through the transceiver. During collisions (whether or not
that transceiver is involved in the collision) transitions on the receive lead are

undefined; they may or may not meet decoder phase requirements, or they may not
be present at all for extended periods.

Thus the receive signal on the transceiver

cable cannot be used alone to deterministically generate the carrier sense signal.
This is described in more detail in 7.5.3.
7.2.1.3 Collision Presence Signal
The collision presence pair is used by the transceiver to indicate the presence of
multiple transmission attempts on the coaxial cable. This is done by transmitting a
square

wave

with

10MHz

fundamental

transceiver cable driver (described in 7.2.4).

frequency

through

the

standard

An oscillator is used instead of a

simple level shift to allow AC coupling at the transceiver. Transceivers use the

collision presence signal to indicate one of two conditions: the transceiver is
transmitting and there is an attempt by another station to transmit at the same time,
or there is a simultaneous transmission attempt by three or more stations regardless

of whether the transceiver in question is transmitting.
7.2.1.4 Power
A pair of wires is designated for providing power to the transceiver.

When the

transceiver cable is implemented, the station end of the cable must supply a voltage

between +12 and +15 Vdc + 5% with at least 0.5 Amperes available to the cable
for remotely powering the transceiver.

The power source must meet applicable

requirements for UL Class 2 wiring devices.
7.2.2

Transceiver Cable Parameters

7.2.2.1 Mechanical Configuration
The transceiver cable consists of four stranded, twisted pair conductors, plus an
overall shield and insulating jacket. The conductor and jacket insulating material
may be polyethylene or other suitable material.

The flammability characteristics of

the insulating material must be suitable for the installed environment.

ETHERNET SPECIFICATION: Physical Layer

7.2.2.2 Characteristic Impedance

The differential mode characteristic impedance of all pairs shall be 78 @, + 5 @, in
the configuration.

7.2.2.3 Attenuation

The signal attenuation of any pair shall not exceed 3 dB (measured at 10 MHz) for
the total length between the transceiver and the station.
7.2.2.4 Velocity of Propagation

The minimum velocity of propagation of the transceiver cable shall be 0.65 c.
7.2.2.5Pulse Distortion
Pulse distortion shall not exceed + 1 nS at the end of 50 meters of cable when
driven with random 10 Mbit data encoded in accordance with 7.5.1.
7.2.2.6 Resistance
The reristance of the conductors used for the power pair shall not exceed 40
milliohms per meter.
7.2.2.7 Transfer Impedance
The common mode transfer impedance of the transceiver cable shall not exceed the

values shown in Figure 7-2 as a function of frequency. The differential mode
transfer impedance of the cable with respect to any pair shall be 20 dB lower than
the specified common mode transfer impedance.
100

10
7

m§ /meter

10KHz

100K

TMM

10M

Frequency
Figure 7-2:

Transceiver Cable Transfer

Impedance

100M

56
7.2.3

ETHERNET SPECIFICATION: Physical Layer
Transceiver Cable Connectors

The connectors used at the ends of the transceiver cable shall be 15 conductor ‘D’
subminiature types (Cinch type DASM-15 or equivalent).

The end of the cable

that mates with the transceiver must use a female connector with a slide lock
assembly (Cinch type DA 51220-1 or equivalent).
mating male connector with locking posts.

The transceiver must provide a

The other end of the transceiver cable

(which mates with a female connector at the station) must use a male connector
with locking posts (Cinch type D 53018 or equivalent). The station must provide a

female connector with the slide lock assembly.
Because of the end-to-end matching of the connectors, transceiver cables may be
extended by concatenating transceiver cable sections.
sections function as 'extension cords’.)

(The transceiver cable

A cable with multiple sections must still

meet the cable loss characteristics of 7.3.1.1.2.
The pin assignment is given in the following table:
Transceiver Cable Connector Pin Assignment
1.

Shield (See note)

Collision Presence +

Collision Presence -

Transmit +

10.

Transmit -

Reserved

11.

Reserved

Receive +

12.

Receive-

Power return

13.

Power

Reserved

14,

Reserved

15.

Reserved

Note: Shield must be terminated to connector shell as well as pin 1.
Metal, metallized plastic, or otherwise shielded connector backshells must be used
to ensure shield integrity.
7.2.4

Transceiver Cable Drive

This section describes the requirements for driving any of the signal pairs in the
transceiver cable: transmit, receive, and collision presence.
The AC signal levels presented to the transceiver cable shall be + 700 mV nominal,

balanced differential drive into 78 + 5 ©. The common mode voltage presented to
the transceiver cable shall not exceed that allowed at the receiver, as specified in
7.2.5.2.

Signal waveform shall be as shown in Figure 7-3.

ETHERNET SPECIFICATION: Physical Layer

Preamble

Data

1‘0]1---0‘1‘1‘1---0

H0.7V =

,’

o—

>i‘7 Idle
o

Idle —><¢

-0.7V——

A o ¥ e
1. Voltages are measured differentially at output of transceiver cable driver.
2. Rise and fall times meet 10,000 series ECL requirements.
Figure 7-3: Typical Transceiver Cable Waveform

The transceiver cable driver must be capable of maintaining the specified minimum
differential signal into the worst case low cable impedance (73 @ differential, 18.5 @

common mode) in the environment specified in section 7-7.
The idle state of the output shall be high (+ 700 mV nominal); the first transition
presented is negative-going, the last transition must be positive-going.

Note that

the presence of AC coupling may cause the voltage as specified at the output of the
transceiver cable drive circuit not to appear on the transceiver cable in the idle
state.

A typical transceiver cable drive circuit is given in Appendix D.
7.2.5

Transceiver Cable Receive

The following sections specify the requirements for receiving signals from any
signal pair in the transceiver cable: transmit, receive, and collision presence. The
circuit must be capable of receiving the signals from the transceiver cable driver
specified in 7.2.4 through the cable specified in 7.2.2 in the worst case.
receive circuit is given

in Appendix

A typical

7.2.5.1Load Impedance and Termination
The termination impedance shall be 78 @ + 1% differential mode, and 18.5 @
minimum common-mode, over the frequency range of 3-20 MHz.
7.2.5.2Common Mode and CMRR
The common mode range and the common mode rejection ratio shall be sufficient
to maintain a 5:1 signal to noise ratio in the environment specified in 7.7, measured
at the input to the transceiver cable receiver. The common mode DC voltage at the
input of the receiver shall be in the range of zero to

+5 Vdc..

ETHERNET SPECIFICATION: Physical Layer

7.3

Coaxial Cable Compatibility Interface Specifications

The coaxial cable is the common, shared broadcast medium through which stations
communicate.

It provides one of the compatibility interface points described in

7.14.2.
7.3.1

Coaxial Cable Component Specifications

The cable is of constant impedance, coaxial construction.

It is terminated at each

end by a terminator (specified in 7.3.1.3), and connection provided for each
transceiver. Coaxial cable connectors are used to make the connection from the
cable to the terminators, and between cable sections (if needed).

The cable has

various electrical and mechanical requirements which must be met to ensure proper
operation.
7.3.1.1Coaxial Cable Parameters
7.3.1.1.1

Characteristic Impedance

The average characteristic impedance of the cable shall be 50 + 2 @, measured
according to Mil. Std. C17-E. Periodic variations in impedance along a single piece
of cable may be up to + 3 @ sinusoidal, centered around the average value, with a
period < 2 meters.

Note that the proper operation of the network is dependent

upon the cable characteristic impedance; its value and tolerance are critical.
7.3.1.1.2

Attenuation

The attenuation of a cable segment shall not exceed 8.5 dB measured at 10 MHz,
nor 6.0 dB measured at 5 MHz.
7.3.1.1.3

Velocity of Propagation

The minimum acceptable velocity of propagation is 0.77 c..

ETHERNET SPECIFICATION: Physical Layer
7.3.1.14

Mechanical Requirements

The cable used should be suitable for routing in various environments, including
but not limited to, dropped ceilings, raised floors, and cable troughs.

The jacket

must provide insulation between the cable sheath and any building structural metal.
Also, the cable must be capable of accepting coaxial cable connectors, described in

7.3.12.

The cable must in addition conform to the following requirements:

1. The center conductor must be 0.0855" + .0005" diameter solid, tinned

oos

copper,

The core dielectric material must be foamed,
The inside diameter of the innermost shield must be .242" minimum,

The outside diameter of the outermost shield must be .326" + .007",
The outermost shield must be greater than 90% coverage tinned copper
braid,

6. The jacket O.D. must be 0.405" nominal,
7. The cable concentricity must be 90% minimum.
The cable must also meet applicable flammability criteria and local codes for the
installed environment.

Different (e.g., polyethylene and Teflon dielectric) types of

cable sections may be interconnected, while meeting the sectioning requirements of
7.6.1.

7.3.1.1.5

Pulse Distortion

Pulse distortion shall not exceed + 7 nS at the end of 500 meters of cable when
driven with random 10 Mbit data encoded in accordance with 7.5.1.
7.3.1.1.6

Jacket Marking

The cable jacket must be marked with annular rings in a color contrasting with the
background color of the jacket.

The rings must be spaced at 2.5 meter + 5 cm

regularly along the entire length of the cable.

It is permissible for the 2.5 meter

spacing to be interrupted at discontinuities between cable sections joined by
connectors.

(See 7.6.2

for transceiver placement rules which mandate cable

markings.)
7.3.1.1.7 Transfer Impedance
The transfer impedance of the cable shall not exceed the values shown in Figure 74 as a function of frequency.

ETHERNET SPECIFICATION: Physical Layer

100
He

m® /meter

X
%,

/f
01

10KHz

100K

10M

100M

Frequency

Figure 7-4: Maximum Coaxial Cable Transfer Impedance

7.3.1.2 Coaxial Cable Connectors

Coaxial cable connectors are used to join cable sections and attach terminators.
Three types of connectors may be necessary; male plugs, female jacks, and femaleto-female barrels. Plugs are used exclusively at the ends of all cable sections. Jacks
are used to house cable terminators. Barrels are used to join cable sections.
All connectors are N series, 50 € constant impedance types. Since the frequencies
present in the transmitted data are well below UHF range (being band-limited to
approximately 20 MHz), military versions of the connectors are not required (but
are acceptable).

Means must be provided to ensure that the connector shell (which connects to the
cable sheath) does not make contact with any building metal, or other unintended
conductor. A sleeve or boot to be slid over the connector at installation time is
suitable.

7.3.1.3Coaxial Cable Terminators

Coaxial cable terminators are used to provide a termination impedance for the
cable equal in value to its characteristic impedance, thereby eliminating any
reflection from the ends of the cables. Terminators shall be packaged within an
inline female jack connector. The termination impedance shall be 50 @ + 1%
measured from 0-50 MHz, with the magnitude of the phase angle ofthe impedance
not to exceed 5 degrees. The terminator power rating shall be 1 watt or greater.

ETHERNET SPECIFICATION: Physical Layer

7.3.1.4 Transceiver-to-Coaxial Cable Connections

A means must be provided to allow for attaching a transceiver to the coaxial cable.
The connection must disturb the transmission line characteristics of the cable as

little as possible; it must present a predictably low shunt capacitance, and therefore
a negligibly short stub length.

For this reason, the transceiver must be located as

close to its cable connection as possible; they are normally considered to be one
assembly.

Long (greater than 3 cm) connections between the coaxial cable and the

input of the transceiver are not acceptable.

The transceiver-to-coaxial cable connection shall present less than 2 picofarads
shunt capacitance to the coaxial cable, not including any transceiver electronics. If
the design of the connection is such that the coaxial cable must be severed to install
the

transceiver,

the

coaxial

cable segment

must still

meet

the

sectioning

requirements of 7.6.1. Any coaxial connectors used on a severed cable must be type
N, as specified in 7.3.1.2.
732

Coaxial Cable Signaling

The AC component of the signal on the coaxial cable due to a single transceiver as
measured on the coaxial cable immediately adjacent to the transceiver connection
shall be + 16 mA nominal (14 mA min, 19 mA max). The DC component shall be
one-half the AC component, plus 45 mA (4 mA min, 5 mA max). The actual
current measured at a given point on the cable is a function of the transmitted
current and the cable loss to the point of measurement. Positive current is defined

as current out of the center conductor of the cable (into the transceiver). Cable loss
is specified in 7.3.1.1.2.

The 10%-90% rise and fall times shall be 25 + 5 nsec.

Figure 7-5 shows typical

waveforms present on the cable. Harmonic content generated from a 10 MHz
fundamental periodic input shall meet the following requirements:
Second and Third Harmonics: -20 dB min
Fourth and Fifth Harmonics:

-30 dB min

Sixth and Seventh Harmonics: -40 dB min
All Higher Harmonics:

-50 dB min

The signals as generated from the encoder (described in 7.5.1.1) shall appear on the
coaxial cable without any inversions.

ETHERNET SPECIFICATION: Physical Layer

Idle —+

Preamble

Data

-1.825V

1. Voltages given are nominal; worst case is given in text.

2. Rise time is 25 nS nominal.
3. Voltages are measured on coaxial cable adjacent to transceiver.

Figure 7-5: Typical Coaxial Cable Waveform

Transceiver Specifications

The following sections specify the requirements for a transceiver.
7.4.1

Transceiver-to-Coaxial Cable Interface

The following sections describe the interface between the transceiver and the

coaxial cable. Positive current is defined as current into the transceiver (out of the
center conductor of the cable).
7.4.1.1Input Impedance

The shunt capacitance presented to the coaxial cable by the transceiver circuitry
(not including the means of attachment to the coaxial cable) shall not exceed 2
picofarads. The shunt resistance presented to the coaxial cable shall be greater than
50 Ke.
These conditions must be met in both the power off and the power on, not
transmitting states.
7.4.1.2Bias Current

The transceiver must draw between -2 and +50 uA in the power-off and the
power-on, not transmitting states.

+— Idle

ETHERNET SPECIFICATION: Physical Layer

7.4.1.3 Transmit Output Levels

Signals received from the transceiver cable transmit pair must be transmitted onto
the coaxial cable with the characteristics specified in 7.3.2. Note that 7.3.2 specifies
the current level on the coaxial cable. Since the coaxial cable proceeds in two
directions away from the transceiver, the current into the transceiver is actually
twice the current measured on the coaxial cable.

Transmitted output asymmetry shall not exceed 2 ns for a 50/50 duty cycle input
on the transceiver cable transmit pair.
7.4.2

Transceiver-to-Transceiver Cable Interface

7.4.2.1 Transmit Pair

The transceiver must present the transceiver cable receive characteristics specified in
7.2.5 to the transmit pair. At the start of a frame transmission, no more than 2 bits
(two 100 ns bit cells) of information may be received from the transmit pair and not
transmitted onto the coaxial cable. The steady-state propagation delay between the
transmit pair input and the coaxial cable output shall not exceed 50 ns. There are
no signal inversion between the transceiver cable transmit pair and the coaxial
cable.

7.4.2.2 Receive Pair

The transceiver must present the transceiver cable transmit characteristics specified
in 7.2.4 to the receive pair. Asymmetry as seen on the receive pair shall not exceed
+ 2 nsec for a + 200 mV peak sinusoidal input from the coaxial cable.
The signal from the coaxial cable shall pass through AC coupling with an
appropriate time constant before proceeding to the receive pair. The time constant
should compensate for the coaxial cable pulse distortion.

At the start of a frame reception from the coaxial cable, no more than 5 bits (five
100 ns bit cells) of information may be received from the coaxial cable and not
transmitted onto the receive pair. In addition, it is permissible for the first bit sent
over the receive pair to contain encoding phase violations or invalid data, however
all successive bits of the frame shall be valid and meet encoding rules. The steadystate propagation delay between the coaxial cable and the receive pair output shall
not exceed 50 ns. There are no signal inversions between the coaxial cable and the
transceiver cable receive pair.

ETHERNET SPECIFICATION: Physical Layer

7.4.2.3Collision Presence Pair
The transceiver must present the transmitter characteristics specified in 7.2.4 to the
collision presence pair.

The signal presented to the collision presence pair shall be

a periodic waveform with a 10 MHz +

15% frequency.

This signal shall be

presented to the collision presence pair no more than 5 bit times (500 nS) after the
average signal on the coaxial cable at the transceiver exceeds either that which
could be produced by two

transceiver outputs in the worst case (if the transceiver

in question is not transmitting), or that which could be produced by that transceiver

alone in the worst case (if that transceiver is transmitting).
7.4.2.4Power Pair
The transceiver cable provides power which may be used for operation of the
transceiver electronics.

The power available shall be as described in 7.2.1.4. The

distribution impedance of the transceiver cable is 4
cable with the resistance specified in 7.2.2.6.

@ maximum, for a 50 meter

In order for the transceiver to derive

its operating power from the power pair, circuitry must be employed to provide the
required electrical isolation specified in 7.4.3.
74.3

Electrical Isolation

The transceiver must provide electrical isolation between the transceiver cable and
the coaxial cable. The isolation impedance shall be greater than 250 K@, measured

between any conductor (including shield) of the transceiver cable and either the
center conductor or shield of the coaxial cable, at 60 Hz. The breakdown voltage of
the isolation means provided shall be at least 250 VAC, rms.
7.4.4

Reliability

No single nor double component failure within the transceiver electronics shall
impede communication among other transceivers on the coaxial cable.

Connectors

and other passive components comprising the means of connecting the transceiver
to the coaxial cable shall be designed to minimize the probability of total network

failure.
7.5

Channel Logic

The following sections describe the functions that must be performed to properly
interface between the data link and the transceiver cable.

They are normally

implemented as logic, typically within the same device implementing the data link
layer.

7.5.1

Channel Encoding

The channel shall use Manchester phase encoding, with a data rate of 10 Mbps, +
.01%, measured at the encoder clock.

Thus, each bit cell is 100 ns long.

The following section describes the requirements for encoding and decoding signals
to be transmitted on, or received from the coaxial or transceiver cables.

ETHERNET SPECIFICATION: Physical Layer

7.5.1.1 Encoder

The

encoder

used

translate

physically

separate

signals

clock

(synchronization) and data into a single, self-synchronizable serial bit stream,
suitable for transmission on the coaxial cable by the transceiver.
During the first half of the bit cell time, the serial signal transmitted is the logical

complement of the bit value being encoded during that cell. During the second half
of the bit cell time, the uncomplemented value of the bit being encoded is
transmitted. Therefore, there is always a signal transition (either positive-going or
negative-going, depending on the bit being encoded) in the center of each bit cell.
A timing diagram for a typical bit stream is given in Figure 7-6.
The encoder output drives the transmit pair of the transceiver cable, and ultimately,
the coaxial cable through the transceiver. The encoder output asymmetry must not
exceed 0.5 ns. The encoder shall provide the defined output for the first (and all
subsequent) bits presented to its input.

All information submitted for encoding

shall appear at the output of the encoder.

Typical data stream

|1|0|0’1’O‘1l1|1[0l

high Jevel
Encoded signal pattern

—

lowlevel

Figure 7-6: Manchester Encoding

7.5.1.2 Decoder

The decoder is used to separate the incoming phase encoded bit stream into a data
stream and a clock signal. The decoder must be able to provide data and clock
signals usable by the data link under the asymmetry imposed by the worst case

system configuration.

The decoder must provide usable output (clock and data)

after no more than 8 bit cell times after reception of an encoded signal.

The first

signals received from the transceiver at the beginning of frame reception may not
constitute a valid, properly encoded bit; it is possible for the time from the first
transition seen to the first true mid-bit cell transition to assume any value from zero
to 100 nS.

The decoder input is normally derived from the coaxial cable, through the
transceiver cable receive pair. It is not necessary for the decoder to provide usable
output when there is a collision on the coaxial cable, regardless of whether the
station using that decoder is involved in the collision.

ETHERNET SPECIFICATION: Physical Layer

7.5.1.3Preamble Generation

Because most of the channel circuitry is allowed to provide valid output some
number of bit times after being presented valid input, it is necessary for a preamble

to be sent before the start of data link information, to allow the channel circuitry to
reach its steady-state, with valid outputs throughout the system.

Upon request by

the data link to transmit the first bit of a new frame, the channel shall first transmit
the preamble; a predetermined bit sequence used for channel stabilization and
synchronization. If, while transmitting the preamble, the channel logic asserts the

collision detect signal as specified in 7.5.2, any remaining preamble bits shall not be
sent. The channel should immediately proceed with the transmission of the bit
submitted by the data link.
The preamble is a 64 bit pattern to be presented to the channel encoder in the same
manner as data link information.

The pattern is:

10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011.
The bits are transmitted in order, from left to right.

The nature of the pattern is

such that when encoded, it appears as a periodic waveform on the cable, with a 5
MHz frequency.

Excepting the final two bits, the only transitions present in the

waveform are in the center of the bit cells. This is depicted in Figure 7-7. The last
two bits of the preamble contain transitions at both the bit cell centers and the

edges, and are used to indicate the end of the preamble, and the beginning of the
data link encapsulation portion of the frame.

The next bit transmitted is the bit

originally submitted by transmission by the data link.
Preamble removal on reception is discussed in 7.5.4.1.

[1lol1]of1]o~1]of1]o]1]
high level

low level

Figure 7-7: Preamble Encoding

ETHERNET SPECIFICATION: Physical Layer

7.5.2

Collision Detect Signal

The channel must indicate to the data link when the signals on the coaxial cable
imply simultaneous transmission attempts by more than one station.

This is

normally indicated through the collision presence pair in the transceiver cable,

described in 7.2.1.3.

The channel logic must assert the collision detect signal within 2 bit times (200 ns),
following the onset of collision presence. This collision detect signal shall be
asserted only when the data link is transmitting.

A functional logic description of

the collision detect signal is shown in Figure 7-8.

Following the loss of collision presence information, the channel must deassert the
collision detect signal within 1.6 bit cell times (160 ns).

Transceiver Cable Collision Pair

Detect*

D—- collisionDetect signal

(to Data Link Layer)

transmitting

*Transition Detect output enabled if ~ (from Data Link Layer)
an input transition has been
detected within the previous
1.6 bit times (160 nS)

Figure 7-8: Functional Logic of collisionDetect

7.5.3

Signal

Carrier Sense Signal

The channel must indicate to the data link the presence of carrier, a signal
transmission attempt on the coaxial cable by a station.

This is normally indicated

through both the receive and collision presence pairs in the transceiver cable,
described in 7.2.1.
The carrier sense signal shall be asserted when one or more station is attempting
transmission on the cable, regardless of whether the station sensing carrier is

transmitting at that time. The channel logic must

assert the carrier sense signal

within 2 bit times (200 ns) following the onset of carrier presence information.

ETHERNET SPECIFICATION: Physical Layer

functional logic description of these signals is shown in Figure 7-9.

Following the loss of carrier presence information (receive transitions and collision
presence information) the channel must deassert the carrier sense signal within 1.6
bit cell times (160 ns).

Transceiver Cable Collision Pair

Detect*

“D— carrierSense signal
Transceiver Cable Receive Pair

(to Data Link Layer)

Detect*

*Transition Detect output enabled if
an input transition has been
detected within the previous
1.6 bit times (160 nS)

Figure 7-9: Functional Logic of carrierSense Signal
754

Channel Framing

During reception, the channel must provide the data link with signals to indicate
beginning and end of frame.
7.5.4.1 Beginning-of-Frame Sequence

The channel logic recognizes the presence of activity on the medium through the
carrier sense signal. This is the first indication that the frame reception process
should begin. However, dependent upon the physical configuration of the system,
there are some number of preamble bits to be received by the channel before the
start of the data link frame as indicated by the double-1 at the end of preamble. In
addition, the first signals received from the decoder may be invalid due to the first
bit allowance of the transceiver (see 7.4.2.2). The channel must wait no less than 8
bit times (800 nS) before monitoring the output of the decoder for the *double-1’
indicating end of preamble, and beginning of data link frame. Upon reception of
the double-1, the channel shall begin passing successive bits to the data link
through the defined receive bit stream interface. If, after waiting the required 8 bit
times, a "double-0" is encountered, the physical channel shall not pass any bits of

ETHERNET SPECIFICATION: Physical Layer

the current frame to the data link.

Normal operation of the data link and channel

shall resume on the subsequent frame.
7.5.4.2 End-of-Frame Sequence
As specified in 7.5.3, the carrier sense signal must be deasserted no later than 1.6 bit
times (160 ns) after the cessation of activity on the coaxial cable as seen by the
channel logic.

The channel ensures that no extraneous bits will appear at the end

of a frame following the last valid bit.
7.6

Channel Configuration Requirements

7.6.1

Cable Sectioning

The 500 meter maximum length coaxial cable segment need not be made from a
single, homogeneous length of cable.

The boundary between two cable sections

(joined by coaxial connectors; two male plugs and a barrel) represents a signal
reflection point due to the impedance discontinuity caused by the batch-to-batch
impedance tolerance of the cable.

Since the worst-case variation from 50 @ is 2

(see 7.3.1.1.1), a possible worst-case reflection of 4% may result from the joining of
two cable sections. The configuration of long

cable segments (up to 500 meters)

from smaller sections must be made with care.

The following recommendations

apply, and are given in order of preference:
1. If possible, the total segment should be made from one homogeneous (no

breaks) cable.

This is feasible for short segments, and results in minimal

reflections from cable impedance discontinuities.
2. If cable segments must be built up from smaller sections, it is highly
desirable to ensure that all the sections are from the same manufacturer and
lot. This is equivalent to using a single cable, since the cable discontinuities
are due to extruder limitations, and not extruder-to-extruder tolerances.
There are no restrictions in cable sectioning if this method is used. However,
if a cable section in such a system is later replaced, it must be replaced either
with another cable from the same manufacturer and lot, or with one of the
standard lengths described below.

3. If uncontrolled cable sections must be used in building up a longer segment,
the lengths should be chosen such that reflections, when they occur, do not
have a high probability of adding in phase.

This can be accomplished by

using lengths which are odd integral multiples of a half-wavelength in the
cable at 5 MHz; this corresponds to using lengths of 23.4, 70.2, and 117
meters (+ 0.5 meters) for all sections.

These are considered to be the

standard lengths for all cable sections.

Using these lengths exclusively, any

mix or match of cable sections may be used to build up a 500 meter segment
without incurring excessive reflections.

ETHERNET SPECIFICATION: Physical Layer
4. As a

last

resort,

arbitrary configuration

of cable

sections may be

employed, if it has been confirmed by analysis or measurement that the

worst-case signal reflection due to the impedance discontinuities at any point

on the cable does not exceed 7% of the incident wave when driven by a
transceiver meeting the specifications of 7.4.
7.6.2

Transceiver Placement

Transceivers and their associated connections to the cable cause signal reflections

due to their non-infinite bridging impedance.

While this impedance must be

implemented as specified in 7.3.1.4 and 7.4.1, the placement of transceivers along

the coaxial cable must also be controlled to insure that reflections from transceiver
do not add in phase to a significant degree.
Coaxial cables marked as specified in 7.3.1.1.6 have marks at regular 2.5 meters
spacing; a transceiver may be placed at any mark on the cable.

This guarantees

both a minimum spacing between transceivers of 2.5 meters, as well as controlling
the relative spacing of transceivers to insure non-alignment on fractional wavelength
boundaries.
The total number of transceivers on a cable segment shall not exceed 100.

7.6.3

System Grounding

The sheath conductor of the coaxial cable shall not make electrical contact with any
earth reference, building structural metal, ducting, plumbing fixture, or other
unintentioned conductor.

Insuldtors may be used to cover any coaxial connectors

used to join cable sections and terminators, to insure that this requirement is met.
A sleeve or boot attached at installation time is acceptable.
The sheath conductor of the transceiver cable shall be connected to the earth
reference or chassis of the device housing the station logic,

ETHERNET SPECIFICATION: Physical Layer

7.64

Repeaters

Repeaters are used to extend the channel length and topology beyond that which
could

achieved

single

configuration model in 7.1.5.)

coaxial

cable

segment.

(See

the

channel

A repeater requires a transceiver on each of the

segments between which it is repeating signals.

These transceivers must be as

specified in 7.4, and must be counted towards the maximum specified in 7.6.2.
A maximum of two repeaters may be in the signal path between any two
transceivers on the channel.

7.6.4.1 Carrier Detect and Transmit Repeat
Repeaters must implement the carrier sense function as specified in 7.5.3 for both
segments between which it is connected.

Upon detection of carrier from one

segment, the repeater must retransmit all received signals from that segment onto
the other segment.

Signals shall be retimed and amplified as specified in 7.6.4.3.

The maximum steady-state propagation delay through the repeater for the repeated
signal (not including startup delays, carrier sense delay or retiming delays) shall not
exceed 800 nS.

7.6.4.2 Collision Detect and Collision Repeat
Repeaters must implement the collision detect function as specified in 7.5.2 for
both segments between which it is connected.

If, while repeating signals as

specified in 7.6.4.1, collision is detected on either side, the repeater must ensure that
all stations involved in the collision recognize the event as a collision, regardless of
which side of the repeater the station is on.

The maximum time between the

recongnition of the collision and the repeating of the collision indication (not
including carrier sense of retiming delays) shall not exceed 200 nS.

7.6.4.3 Repeater Signal Regeneration
7.6. 3.1

Signal Amplification

The repeater (with its associated transceivers) shall ensure that any signals repeated
between segments shall have the same amplitude characteristics at the transceiver
output of the repeated-to segment as they did at the output of the transmitter on
the repeated-from segment, allowing for transceiver output tolerances as specified
in 7.4.1.3.

Any loss of signal-to-noise ratio due to cable loss and noise pickup is

thus regained at the output of the repeater.
7.6.4.3.2

Signal Timing

The repeater must ensure that the symmetry characteristics of the signals at the
transceiver output of the repeated-to segment are the same as those at the output of
the

transmitter on

the

repeated-from

transceiver cable tolerances.

segment,

allowing

for

transceiver

and

Any loss of symmetry due to transceivers and cable

distortion is thus regained at the output of the repeater.

i/
7.7

ETHERNET SPECIFICATION: Physical Layer
Environment Specifications

The following sections specify the physical environment in which all channel
components must operate to be considered compatible.
7.7.1

Electromagnetic Environment

The physical channel hardware shall meet its specifications when operating in the
following ambient plane-wave fields:
2 Volts/Meter from 10 KHz through 30 MHz
5 Volts/Meter from 30 MHz through 1 GHz
7.7.2

Temperature and Humidity

All physical channel hardware, with the possible exception of the channel logic
components shall operate over the ambient temperature range of 5 to 50 degrees
Celsius, and humidity range of 10% to 95% non-condensing. The channel logic
components are normally part of the station hardware, and are thus subject to
individual station product requirements.

Hardware which does not meet the

temperature and humidity requirements specified must state so in its published

product specification.

ETHERNET SPECIFICATION: Appendix A

APPENDIX A:

GLOSSARY

This section defines some of the essential terminology associated with the Ethernet.
baseband coaxial system: A system whereby information is directly encoded and
impressed on the coaxial transmission medium. One information signal at a time
can be present on the medium without distuption (see collision).
binary exponential backoff: The algorithm used to schedule retransmissions after a

collision.

So called because the interval from which the retransmission time is

selected is expanded exponentially with repeated collisions.
broadcast: Describes the class of media for which the Ethernet is designed, in which
all stations are capable of receiving a signal transmitted by any other station.
Also, describes the mode of usage of such a medium by the Data Link Layer in
which all stations are instructed to receive a given frame.
carrier sense: A signal provided by the Physical Layer to the Data Link Layer to

indicate that one or more stations are currently transmitting on the channel.
channel logic: The logical funcuions provided between the transceiver cable and the
Data Link, which support the defined interface between the data link and the
physical layers.

Client Layer: Collective term used to describe any layer of a network architecture,
which use the Ethernet Data Link and Client interface.

coaxial cable: A two-conductor, concentric, constant impedance transmission line.
coaxial cable interface: The electrical, mechanical, and logical interface to the shared
coaxial cable medium.
be correctly

This is a mandatory compatibility interface, which must

implemented by every Ethernet implementation.

coaxial cable section: An unbroken piece of coaxial cable, fitted with coaxial
connectors at its ends, used to build up coaxial cable segments.

coaxial cable segment: A length of coaxial cable made up from one or more coaxial
cable sections and coaxial connectors. terminated at each end in its characteristic
impedance. A 500 meter segment is the longest configuration possible without
repeaters.

collision: The result of multiple transmissions overlapping in the physical channel,
resulting in garbled data and necessitating retransmission.
collision detect: A signal provided by the Physical Layer to the Data Link Layer to

indicate that one or more other stations are contending with the local station’s

ETHERNET SPECIFICATION: Appendix A
transmission.

It can be true only during transmission.

collision enforcement: Transmission of extra,

encoded "jam" bits after a collision is

detected, to insure that the duration of the collision is sufficient to guarantee its
detection by all transmitting stations.

compatibility interfaces: The coaxial cable interface, and the transceiver cable
interface, the two points at which hardware compatibility is defined to allow
connection of independently designed and manufactured components to the
Ethernet.

contention: Interference between colliding transmissions (see collision). Resolution
of occasional contention is a normal part of the Ethernet’s distributed link
management procedure (see CSMA-CD).
controller: The implementation unit which connects a station to the Ethernet,

typically comprising part of the Physical Layer, much or all of the Data Link
Layer, and appropriate electronics for interfacing to the station.
CSM A-CD: Carrier Sense Multiple Access with Collision Detection, the generic term

for the class of link management procedure used by the Ethernet. So called
because it a) allows multiple stations to access the broadcast channel at will, b)
avoids contention via carrier sense and deference, and c) resolves contention via
collision detection and retransmission.

Data Link. Layer: The higher of the two layers in the Ethernet design, which

implements a medium-independent link level communication facility on top of
the physical channel provided by the Physical Layer.

deference: A process by which a data link controller delays its transmission when the
channel is busy to avoid contention with ongoing transmissions.
frame check sequence: An encoded value appended to each frame by the Data Link
Layer to allow detection of transmission errors in the physical channel.
interframe spacing: An enforced idle time between transmission of successive frames
to allow receiving data link controllers and the physical channel to recover.
Jam: An encoded bit sequence used for collision enforcement.
Manchester encoding: A means by which separate data and clock signals can be
combined into a single, self-synchronizable data stream, suitable for transmission
on a serial channel.

multicast: An addressing mode in which a given frame is targeted to a group of
logically related stations.

physical address: The unique address value associated with a given station on the

network.

An Ethernet physical address is defined to be distinct from all other

physical addresses on all Ethernets.

ETHERNET SPECIFICATION: Appendix A

Physical Channel: The implementation of the physical layer.

Physical Layer: The lower of the two layers of the Ethernet design, implemented by
the physical channel using the specified coaxial cable medium.

Layer

insulates

the

Data

Link

Layer

from

The Physical

medium-dependent

physical

characteristics.

preamble: A sequence of 64 encoded bits which the Physical Layer transmits before
each frame to allow synchronization of clocks and other Physical Layer circuitry
at other sites on the channel.

repeater: A device used to extend the length and topology of the physical channel
beyond that imposed by a single segment, up to the maximum allowable end-toend channel length.

round-trip propagation time: In bit times, the time required in the worst-case for a
transmitting station’s collision detect signal to be asserted due to normal

contention for the channel. This delay is the primary component of the slot time.
slot time: A multi-purpose parameter which describes the contention behavior of the
Data Link Layer. It serves as a) an upper bound on the collison vulnerability of

a given transmission, b) an upper bound on the size of the frame fragment
produced by a collision, and c) the scheduling quantum for collision
retransmission.

station: A single addressable site on the Ethernet, generally implemented as a

computer and appropriate peripherals, and connected to the Ethernet via a
controller and a transceiver.

transceiver: The portion of the Physical Layer implementation that connects directly

to the coaxial cable and provides both the electronics which send and receive the
encoded signals on the cable and the required electrical isolation.
transceiver cable: A four pair, shielded cable used for the transceiver cable interface.
transceiver cable interface: The electrical, mechanical and logical interface which
connects the transceiver to the controller. The standard transceiver cable is a
recommended compatibility interface.

ETHERNET SPECIFICATION: Appendix B

APPENDIX B: NOTES ON ADDRESS AND TYPE ASSIGNMENT, AND LICENSING
Address and Type Assignment

The address and type fields will be administered by Xerox Corporation.
A block of addresses will be assigned to each licensee of Ethernet patents (see
below). Others may obtain an address block or type field assignment by request. A
nominal fee to cover administrative costs will be charged.
Submit written requests to:
Xerox Corporation
Ethernet Address Administration Office
3333 Coyote Hill Road
Palo Alto, CA 94304
Licensing
Ethernet incorporates features that are protected by one or more patents assigned to
Xerox Corporation.

Questions on the need for licensing particular uses of this

specification should be directed to:

Xerox Corporation
Director of Licensing
Long Ridge Road
Stamford, CT 06904

ETHERNET SPECIFICATION: Appendix C

APPENDIX C: CRC IMPLEMENTATION

Every frame contains, in its frame check sequence field, a 32-bit cyclic redundancy
check (CRC) code. Because the formal mathematical definition of this code (see
6.2.4) is not suggestive of an appropriate implementation, this appendix outlines one
possible implementation in terms of a feedback shift register. This type of
implementation is likely to be common in practice, but is not a mandatory part of the
specification.

The feedback shift register (see Figure C-1) is used to represent division of the prescaled message by the generating polynomial. The 32-bit register is accessed via the
three signals Inpnt, Output, and Control. When Control = 1, Input bits are shifted
into the feedback shift register and also fed directly back to Output. When Control
= 0, the feedback paths are disabled and the shift register shifts the complement of
its contents to Output.

Before CRC generation at the transmitting end, initialization logic (not shown in
Figure C-1) preloads the shift register to all 1's. ~ Control is then held at 1 while the
address, type and data fields of the outgoing frame are shifted into Input and the
CRC is generated. Meanwhile, the same bits emerging at Output are transmitted over
the network. When the last bit of the data field has been processed, Control is set to

0 and the complemented CRC is shifted out for transmission, starting with the x3!
term (see 6.2.4).

CRC checking at the receiving end also begins with the shift register preloaded to all
1's. Control is then held at 1 while the incoming bits are shifted into Input to
regenerate the CRC. When the last bit of the data field has been processed, the shift
register should contain the CRC whose binary complement is about to arrive on the
network. Since this field boundary cannot be recognized by the receiver, however,
Control remains at 1 and the bits of the CRC continue to feed into the the shift
register until the end of the entire frame is reached. If the two CRCs match, the final
contents of the shift register is the value:

11000111 00000100 11011101 01111011

(where the leftmost bit corresponds to the x31 term of the polynomial and the

rightmost to the x0 term). Any other final value indicates a detected error. (The
extra logic to test for this value is not shown in Figure C-1).

ETHERNET SPECIFICATION: Appendix C

__,?,Xlo_,?_,xu_,?_,xu REIRT xls_,?_,xm T8,

‘*?—nu‘?*x” 24 xzs’_,?_,xzs 2,28

5], 20[ 21]—

5]30],31

® = AND
@ = XOR

Mux

O =NOT
A

L C=0=>A

C—={ Mux = Output: C=1=>B

Figure C-1: CRC Implmentation

)

Control

Input

Output

ETHERNET SPECIFICATION: Appendix C

One potential problem which is avoided in this implementation is insensitivity of the
shift register to incoming zero-bits when it is in the all-zero state.

Following

standard practice, this state is avoided at the beginning and end of the frame by
preloading the shift register with all 1-bits, and by inverting each bit of the final CRC.
Logically, these correspond, respectively, to the complementing of the first 32 bits of
the frame and to the final complementing of the remainder, as specified in the

mathematical definition in 6.24.