The Ethernet A Local Area Network Data Link Layer and Physical Layer Specifications

This document, "The Ethernet: A Local Area Network Data Link Layer and Physical Layer Specifications," Version 1.0, published on September 30, 1980, is a foundational technical specification developed jointly by Digital Equipment Corporation, Intel Corporation, and Xerox Corporation.

Key aspects covered:

1. Scope: It defines the lowest two layers of the Ethernet network architecture: the Data Link Layer and the Physical Layer. It is intended as a detailed design reference, not an introductory tutorial. Higher-level network management, error recovery, and security functions are explicitly outside its scope.
2. Core Characteristics (as of 1980):
   • Data Rate: 10 Megabits per second (Mbps).
   • Medium: Shielded coaxial cable, utilizing base-band signalling.
   • Topology: Branching non-rooted tree.
   • Station Limits: Maximum 1024 stations.
   • Distance Limits: Maximum station separation of 2.5 kilometers.
   • Link Control: Fully distributed peer protocol with statistical contention resolution, known as CSMA/CD (Carrier Sense Multiple Access with Collision Detection).
   • Message Protocol: Variable-size frames with "best-effort" delivery.
3. Data Link Layer Specification:
   • Functions: Handles data encapsulation (framing, 48-bit addressing for single, multicast, or broadcast destinations, and CRC-32 error detection) and link management (channel allocation and contention resolution).
   • Frame Format: Consists of Destination Address (6 octets), Source Address (6 octets), Type (2 octets), Data (46-1500 octets), and Frame Check Sequence (4 octets). Valid frames range from 64 to 1518 octets.
   • Collision Handling: Incorporates Carrier Deference (deferring transmission when the channel is busy), a minimum Interframe Spacing (9.6 µsec), and Collision Enforcement (transmitting a "jam" of 32-48 bits after collision detection). Retransmission attempts use a "truncated binary exponential backoff" algorithm, with a maximum of 16 attempts. The "slot time" (512 bit times) is a critical parameter for collision handling.
4. Physical Layer Specification:
   • Functions: Manages the physical channel, including data encoding (Manchester encoding), timing, preamble generation/removal for synchronization, bit transmission/reception, carrier sensing (detecting network traffic), and collision detection.
   • Components: Coaxial cable segments, transceivers (connecting stations to the coax), and repeaters (for extending network length and topology).
   • Physical Configuration Limits: Details maximum cable lengths (e.g., 500m per coaxial segment, 1500m total coaxial cable length), transceiver cable lengths (max 50m), and the use of repeaters (max 2 in any signal path between stations). It defines strict parameters for cable impedance, attenuation, signal levels, and environmental conditions to ensure compatibility.
   • Interfaces: Defines compatibility interfaces at the coaxial cable level (mandatory) and the transceiver cable level (recommended for flexibility), specifying electrical, mechanical, and logical characteristics.
5. Goals: Emphasizes simplicity, low cost, high speed, low delay, stability, maintainability, a layered architecture, and crucially, compatibility among all implementations at the data link level to enable data exchange.
6. Intellectual Property: Notes that the specification includes subject matter related to patents held by Xerox Corporation, which also administers address and type field assignments.