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Document:	Fiber Distributed Data Interface Network Config Guidelines
Order Number:	EK-DFDDI-CG
Revision:	001
Pages:	34
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OCR Text

Fiber Distributed
Data Interface

Network Configuration
Guidelines

Order Number:

EK-DFDDI-CG-001

Fiber Distributed
Data Interface

Network Configuration
Guidelines

November 1991
This manual describes the guidelines
for connecting devices to an FDDI network. It also includes network configuration examples.
Supersession/Update Information:
This is a new manual.

Order Number: EK-DFDDI-CG-001

EK–DFDDI–CG–001

November 1991

The information in this document is subject to change without
notice and should not be construed as a commitment by Digital Equipment Corporation. Digital Equipment Corporation
assumes no responsibility for any errors that may appear in
this document.

The following are trademarks of Digital Equipment Corporation:
DEBET

DECnet

TURBOchannel

DEC
DEC FDDIcontroller

DECstation
DECsystem

ULTRIX
UNIBUS

DECbridge

DECUS

VAX

DECconcentrator
DECconnect

DECwindows
LAN Traffic Monitor

VAXcluster
VAXstation

DECelms

PDP

VMS

the Digital logo

ThinWire

This manual was produced by Telecommunications
and Networks Publications.

Contents
Preface
1

FDDI Network Connection Rules

1.1
1.2
1.3

PHY Port Connection Rules . . . . . . . . . . . . . 1
FDDI Station Connection Guidelines . . . . . . 4
Optical Bypass Relay . . . . . . . . . . . . . . . . . . . 6

FDDI Network Configurations

2.1

2.3
2.4
2.5

Standalone Configurations Using
Concentrators . . . . . . . . . . . . . . . . . . . . . . . . . 9
Tree Configurations Using Concentrators
and Bridges . . . . . . . . . . . . . . . . . . . . . . . . . 10
Dual Ring Configuration . . . . . . . . . . . . . . . . 13
FDDI Dual Homing . . . . . . . . . . . . . . . . . . . . 14
Single-Mode Fiber Configurations . . . . . . . 15

FDDI Specifications

3.1
3.2

FDDI and Cable Specifications . . . . . . . . . . 17
Optical Bypass Link Loss . . . . . . . . . . . . . . . 22

Related Documents

2.2

Index
Figures
1–1
1–2
1–3
1–4
2–1
2–2
2–3
2–4
2–5

PHY-Port Types . . . . . . . . . . . . . . . . . . . . . . 2
FDDI Connection Rules Matrix . . . . . . . . . 2
Installed Optical Bypass Relay . . . . . . . . . 6
Optical Bypass Connector Signals . . . . . . 7
Standalone Workgroup Installation . . . . . . 9
Dedicated Network in a Dual Ring
Installation . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Tree of Concentrators in a Campus . . . . 11
Tree of Concentrators in a Building . . . . . 12
Dual Ring of Concentrators Installed in a
Building . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
iii

2–6
2–7
2–8
2–9

Concentrators Installed for Dual Homing 14
DAS Bridge Installed for Dual Homing . . 14
Connecting to the Dual Ring Through a
Single-Mode Link . . . . . . . . . . . . . . . . . . . . 15
Dual Ring with Mixed Link Lengths . . . . . 15

Tables
3–1
3–2
3–3
3–4
3–5
3–6
3–7
3–8

FDDI General Specifications . . . . . . . . . . 17
Optical System General Specifications . 18
Multimode Fiber Optic Cable
Specifications . . . . . . . . . . . . . . . . . . . . . . . 19
Single-Mode Fiber Optic Cable
Specifications . . . . . . . . . . . . . . . . . . . . . . . 19
FDDI 100/140-µm Multimode Fiber Optic
Cable Specifications . . . . . . . . . . . . . . . . . 20
FDDI 50/125-µm Multimode Fiber Optic
Cable Specifications . . . . . . . . . . . . . . . . . 21
ThinWire and Shielded Twisted-Pair
Cable Specifications . . . . . . . . . . . . . . . . . 22
Calculating Allowable Link-Loss Budget . 23

Preface
This manual provides the rules for connecting Fiber Distributed Data Interface (FDDI) devices to the network. This
manual also contains examples of FDDI network configurations built on Digital’s implementation of the EIA/TIA 568
wiring standard for commercial buildings.

Intended Audience
This manual is for network system managers, network
planners, installers, and others who set up, manage, and
troubleshoot FDDI networks and network devices.

Document Structure
This manual contains three chapters and an appendix:
Chapter 1

Provides rules for connecting FDDI devices
to the network.

Chapter 2

Provides FDDI network configuration examples.

Chapter 3

Lists FDDI and cable specifications.

Appendix A

Lists related documents.

The postage-paid Reader’s Comments card in this document requests your critical evaluation to help us prepare
future documentation.

1
FDDI Network Connection Rules
The FDDI standards specify four physical port (PHY port)
types: A, B, M, and S.

1.1 PHY Port Connection Rules
Figure 1–1, a simplified diagram of a concentrator, shows
the various PHY ports. FDDI defines the PHY ports as
follows:

•

•
•

PHY A — Connects to the incoming primary ring and
the outgoing secondary ring of the FDDI dual ring.
This port is part of a dual attachment station (DAS) or
a dual attachment concentrator (DAC). This port also
connects to PHY M of a concentrator as the backup
path in a dual homing topology.
PHY B — Connects to the outgoing primary ring and
the incoming secondary ring of the FDDI dual ring.
This port is part of a DAS or a DAC. This port also
connects to PHY M of a concentrator as the primary
path in a dual homing topology.
PHY M — Connects a concentrator to a single attachment station (SAS), DAS, or another concentrator.
This port is implemented only in the concentrator.
PHY S — Connects a SAS to a PHY M of a concentrator. PHY S also connects to a DAS or another SAS,
but these configurations are not always recommended.

Figure 1–1:

PHY-Port Types

Secondary
ring
PHY A

PHY B

Primary
ring

Concentrator
PHY M

PHY M

PHY B

PHY S

DAS or DAC

SAS or SAC

LKG-5579-91I

The FDDI standards specify connection rules to prevent
illegal topologies. Connections between stations have the
status of in use or not in use. When a cable is connected,
the system decides whether to use the cable in the token
path. Figure 1–2 shows the ring-connection rules for Digital devices. This Node is the reference point for all connection decisions.
Figure 1–2:

FDDI Connection Rules Matrix

This Node

Other
Node

PHY port

Rule 1

Yes

Rule 2

Yes

Rule 3

Yes
Note 1

Yes

Yes
Note 3

Yes

Yes = Accepts this connection.
No = Does not accept this connection.

LKG-5580-91I

FDDI Network Configuration Guidelines

Rule 1:

PHY B of This Node accepts connection to
PHY A of Other Node if:

• PHY A of This Node is not connected, or
• PHY A of This Node is connected to PHY
B or PHY S of another node.
Rule 2:

PHY A of This Node accepts connection to
PHY B of Other Node if:

• PHY B of This Node is not connected, or
• PHY B of This Node is connected to
PHY A or PHY S of another node.
See also Note 2.
Rule 3:

PHY A of This Node accepts the connection to
PHY M of Other Node if:

• PHY B of This Node is not connected, or
• PHY B of This Node is connected to PHY A
or PHY S of another node.
See also Note 2.

Note 1: The following conditions apply for a backup tree
connection (dual homing).
The connection of PHY B of This Node to PHY M of Other Node always supersedes the connection of PHY A of
This Node to PHY M of another node.
Any connection between PHY A of This Node and a PHY
M or PHY B of another node is disconnected when PHY B
of This Node connects to PHY M of Other Node.
If the connection breaks between PHY B of This Node
and PHY M of Other Node, the system reestablishes the
connection between PHY A of This Node and PHY M of
another node.

Note 2: Tree connections come before dual ring connections when only one of two connections may be accepted.
The connection of PHY A or B of This Node to PHY M of
Other Node always supersedes the connection of the
remaining PHY (A or B) of This Node to PHY A or PHY B
of another node.

FDDI Network Connection Rules

The system disconnects the connection between the remaining PHY (A or B) of This Node and PHY A or PHY B
of another node when PHY A or PHY B of This Node connects to PHY M of Other Node.

Note 3: The shielded twisted-pair and ThinWire PMD
specifications support only connections from PHY M to
PHY S. The specifications do not support connections
from PHY S to PHY S.
1.2 FDDI Station Connection Guidelines
Use the following guidelines when connecting a SAS or
DAS concentrator, bridge, bridge/router or adapter into an
FDDI network.
Distance requirements:

•

Maximum distance between FDDI stations must not
exceed:
–

2 km (1.2 mi) for ANSI multimode

–

1 km (0.62 mi) for low-power multimode

–

40 km (24.8 mi) for single-mode

–

100 m (330 ft) for single segment point-to-point
ThinWire and shielded twisted-pair links.

Distances include cables connected between the
patch panel and the device. However, the maximum
distance for ThinWire is reduced when patch cables
are added.

•

Total length of the network must not exceed 200 km
(124.2 mi), regardless of media type. This distance
must include the length of the secondary ring, if you
have implemented a dual ring.

Attenuation:

•

Maximum attenuation between an FDDI port and its
neighbor is:

–

11 dB for ANSI multimode.

–

7 dB for low-power multimode.

–

22 dB for single-mode connections. Minimum attenuation for single-mode is 12 dB.

–

11/5 dB @ 62.5 MHz for ThinWire.

–

12 dB @ 62.5 MHz for shielded twisted-pair.

FDDI Network Configuration Guidelines

Number of stations and connections:

•

Maximum number of stations in an FDDI dual ring is
500 (1000 Physical layer entities). Digital recommends at most 100 stations in a dual ring or dual ring
of trees. A station can have one Physical layer entity
(SAS), two Physical layer entities (DAS), or multiple
Physical layer entities (concentrator).
Maximum number of Physical layer entities that can
connect to the ring limits both the number of stations
in an FDDI network (dual ring of trees and dual ring)
and the maximum number of concentrator sublevels
in a tree structure.

Bridges and adapters:

•

•
•

SAS bridges or bridge/routers should connect to a
PHY M port on an FDDI concentrator to work in an
FDDI ring. You can connect DAS bridges or bridge/
routers directly to the dual ring or dual home them
through PHY M ports on two concentrators.
The extended LAN can support up to seven bridges
between stations.
SAS adapters should connect to a concentrator PHY
M port to work in an FDDI ring. In a standalone concentrator configuration, you can connect an adapter
PHY S port to a concentrator PHY A or PHY B port.
(Digital does not recommend this configuration.)

FDDI Network Connection Rules

1.3 Optical Bypass Relay
Figure 1–3 shows an optical bypass relay which, can
maintain connectivity of the FDDI ring if there is no power
or if something is wrong in a node. The bypass relay allows the light to bypass the optical transmitter and receiver
in the faulty node.
Figure 1–3:

Installed Optical Bypass Relay

Secondary ring
Optical
bypass

(Dual ring)

Primary ring

PHY A
PHY B
Dual attachment station

LKG-5581-91I

Use of optical bypass relays may cause the maximum
allowable distance or maximum loss between stations to
become greater than the system allows. When operating
in bypass mode, optical bypass relays serially link two
cableplant segments and adds loss.
Each sequential relay adds another cableplant segment. A
loss is attributed to the segment and to the addition of the
relay. This limits the number of serially connected relays in
the ring.
The total loss of the serially linked cableplant must not
exceed 11 dB. The total length of the serially linked cableplant must not exceed 2 km (1.2 mi). See section 3.2 for
additional information.
Other considerations when using optical bypass relays
include:

•

Bypass relays do not amplify or restore the bit stream.

•

Bypass relays can be less than reliable because they
are mechanical devices.

FDDI Network Configuration Guidelines

•

Bypass relay technology provides for only a few consecutively bypassed stations. The exact number depends on the configuration and the station. In a building, this number ranges from zero to three.

Refer to the optical bypass connector signal information in
Figure 1–4 when selecting optical bypass devices.
Figure 1–4:

Optical Bypass Connector Signals
Pins
654 321

Modular jack (RJ12)
Pin

Description

1,2
3,4,6
5

Relay drive; +5V @ 200 mA (max)
Return; grounded internally
Bypass present; must be externally grounded to pin
3,4, or 6

LKG-5582-91I

FDDI Network Connection Rules

2
FDDI Network Configurations
The EIA/TIA 568 wiring standard for commercial buildings
provides an infrastructure for FDDI networks. This chapter
contains examples of FDDI network configurations built on
Digital’s implementation of that wiring standard.
These configurations allow the use of star wiring, which
can use existing structured cabling.

2.1 Standalone Configurations Using Concentrators
Figure 2–1 shows a concentrator installed in a standalone
workgroup. While SAS devices are preferred for this configuration, the stations can be either SAS or DAS devices.
This configuration is useful for compute-intensive applications shared by a limited number of users in a small geographical area, or where security is paramount. This
configuration can have up to 12 connections through fiber
optic cable, or up to 18 connections through ThinWire or
shielded twisted-pair cable.
Figure 2–1:

Standalone Workgroup Installation

(SAS)
File
server

(SAS)
Workstation

Concentrator

Workstation
(SAS)

File
server
(SAS)

SAS = single attachment station
LKG-5583-91I

Figure 2–2 shows a dedicated network on a dual ring. The
attached workstations, file server, minicomputer, and
mainframe are SAS devices. This configuration serves the
same purpose as the standalone configuration and extends the geographical area.
Figure 2–2:

Dedicated Network in a Dual Ring
Installation

Workstation
Workstation

100-Mb/s
FDDI ring
Concentrator

Workstation
File
server

Concentrator

Workstation
Minicomputer
Main
frame

LKG-5584-91I

2.2 Tree Configurations Using Concentrators and
Bridges
Use tree configurations when wiring large groups of user
devices together. Wire concentrators and bridges (singleport or multiport) in a star topology with one or more concentrators serving as the root of the tree.

FDDI Network Configuration Guidelines

Figure 2–3 shows a tree of concentrators with bridges and
a bridge/router installed in a campus configuration wired
with Digital’s implementation of the EIA/TIA 568 wiring
standard.
Figure 2–3:

Tree of Concentrators in a Campus

Building 1

Building 2

(HDF)

B
B/R

B
Bridge

Concentrator

(IDF)

Concentrators
(MDF)

Building 3
(HDF)

(IDF)

Building 4
(IDF)

Bridge

(HDF)

Concentrator

B
B
B

IDF = Intermediate distribution frame
HDF = Horizontal distribution frame
MDF = Main distribution frame
B/R = Bridge/Router
B
= Bridge
LKG-5585-91I

FDDI Network Configurations

Figure 2–4 shows a tree of concentrators installed in a
building with multiple horizontal distribution frames
(HDFs).
Figure 2–4:

Tree of Concentrators in a Building

802.3/Ethernet
10-Mb/s LAN

802.3/Ethernet 802.3/Ethernet
10-Mb/s LAN 10-Mb/s LAN

Bridge

IDF
Concentrator

Workstation

Workstation
Minicomputer

HDF
Concentrator

Mainframe

HDF
Concentrator

Bridge

802.3/Ethernet
10-Mb/s LAN

Workstation
Workstation
File
server

HDF= Horizontal distribution frame
IDF = Intermediate distribution frame

LKG-5586-91I

FDDI Network Configuration Guidelines

2.3 Dual Ring Configuration
A dual ring of concentrators allows multiple devices to be
attached to the dual ring through the concentrators.
Figure 2–5 shows a dual ring of concentrators installed in
a building. The concentrators can be located in the MDF or
IDF of a building.
Figure 2–5:

Dual Ring of Concentrators Installed in a
Building

VAX

Concentrator
Workstation
Minicomputer

100 Mb/s
FDDI ring
Concentrator

Concentrator

Workstation
Workstation
Workstation

Mainframe

Concentrator

VAX

File
server

Workstation

LKG-5587-91I

FDDI Network Configurations

2.4 FDDI Dual Homing
Figure 2–6 shows three concentrators installed in a dual
homing configuration. Note that, if concentrator 2 or the
primary link to concentrator 3 fails, the backup link to concentrator 3, through concentrator 1, takes over. This ensures that the devices connected to concentrator 3 have
uninterrupted service.
Figure 2–6:

Concentrators Installed for Dual Homing

PHY A
PHY B
Concentrator 1

PHY A
PHY B
Concentrator 2

PHY M

Backup
link

PHY M

FDDI
dual
ring

Primary
link
PHY A
PHY B
Concentrator 3
PHY M

PHY M
LKG-5589-91I

Figure 2–7 shows a DAS bridge installed in a dual homing
configuration. Note that, if concentrator 2 or the primary
link to the bridge fails, the backup link to the bridge,
through concentrator 1, takes over. This ensures that the
devices on the LANs connected to the bridge have uninterrupted service.
Figure 2–7:

DAS Bridge Installed for Dual Homing

PHY A
PHY B
Concentrator 1

PHY A
PHY B
Concentrator 2

PHY M

Backup
link

PHY M

FDDI
dual
ring

Primary
link
PHY A
PHY B
DAS Bridge

LKG-5590-91I

FDDI Network Configuration Guidelines

2.5 Single-Mode Fiber Configurations
Figure 2–8 shows a SAS bridge connected to a dual ring
through a long-distance single-mode fiber cable. This is
the recommended method to connect remote sites through
FDDI.
Figure 2–8:

Connecting to the Dual Ring Through a
Single-Mode Link

PHY A

Concentrator

PHY B

Link length from 0 km or mi
up to 40 km (24 mi)
SAS bridge

LKG-5591-91I

Figure 2–9 shows a dual ring that requires some links to
be longer than 2 km (1.2 mi). In this configuration, the
total length of all links must not exceed 100 km (62 mi).
Figure 2–9:

Dual Ring with Mixed Link Lengths
< 2 km
Bridge

< 2 km

Bridge
B

Concentrator

Concentrator
A
< 2 km

> 2 km
singlemode fiber

B
B

Concentrator A

Bridge

< 2 km

> 2 km single-mode fiber
LKG-5592-91I

FDDI Network Configurations

3
FDDI Specifications
The following tables list the FDDI general specifications,
multimode and single-mode fiber optic cable specifications,
and copper cable specifications. This chapter also contains
a table that helps calculate the loss associated with optical
bypass relays.

3.1 FDDI and Cable Specifications
Table 3–1 lists the FDDI general specifications.
Table 3–1:

FDDI General Specifications

Attribute

Value

Transmission rate

125 megabaud
(100 Mb/s at the data link)

Physical layer entities1

1000 (max)

Total Ring length

200 km (124 mi) (max)

Transmission medium

Fiber optic or copper cable

Network topology

Dual ring of trees

Media access method

Timed-token passing

1Digital recommends 100 stations, where a station can have one

Physical layer entity (SAS), two Physical layer entities (DAS), or
multiple Physical layer entities (concentrators).

Table 3–2 lists the optical system general requirements.
Table 3–2:

Optical System General Specifications

Attributes

Values

ANSI multimode 1
Link length

2.0 km (1.2 mi) (max)

Transmit launch signal

–20 dBm to –14 dBm (max)

Receive input signal

–31 dBm to –14 dBm (max)

Link-loss range

0 dB to 11 dB

Low-power multimode1
Link length

1.0 km (0.62 mi) (max)

Transmit launch signal

–22 dBm to –14 dBm (max)

Receive input signal

–29 dBm to –14 dBm (max)

Link-loss range

0 dB to 7 dB

Single-mode
Link length

40 km (24.8 mi) (max)

Transmit launch signal

–8.0 dBm to –2.5 dBm (max)

Receive input signal

–30 dBm to –14 dBm (max)

Link-loss range

12 dB to 22 dB

µm multimode fiber optic cable. See
Table 3–5 and Table 3–6 for information about alternate µm fiber
optic cable.
1Assumes the use of 62.5-

Table 3–3 and Table 3–4 list multimode and single-mode
fiber optic cable specifications.

FDDI Network Configuration Guidelines

Table 3–3:

Multimode Fiber Optic Cable
Specifications

Attributes

Values

Core diameter

62.5 ± 3.0 µm

Cladding diameter

125.0 ± 2.0 µm

Numerical aperture

0.275 ± 0.015

Optical power attenuation 1.5 dB/km @ 1300 nm,
3.5 dB/km @ 850 nm
Modal bandwidth (min)

500 MHz• km @ 1300 nm,
160 MHz• km @ 850 nm

Chromatic dispersion
requirements

ZDW1

Dispersion Slope

1295 nm 0.105 ps/(nm2• km)
1300 nm 0.110 ps/(nm2• km)
1348 nm 0.110 ps/(nm2• km)
1365 nm 0.093 ps/(nm2• km)

1The zero dispersion wavelength (ZDW) and dispersion slope
must fall within the bounds shown in the table when plotted as
wavelength (x axis) and dispersion slope (y axis) on a graph.

Table 3–4:

Single-Mode Fiber Optic Cable
Specifications

Attributes

Values

Mode Field diameter

8.2 µm to 10.5 µm

Cladding diameter

125 µm ± 2 µm

Fiber cladding noncircularity

2% (max)

Core to cladding concentricity error 1 µm (max)
Nominal operating wavelength

1300 nm

Fiber cutoff wavelength

1270 nm (max)

Zero dispersion wavelength

1300 to 1322 nm

Zero dispersion slope

0.095 ps/(nm2km)
(max)

Optical power attenuation

≤ 0.40 dB per km
@ 1310 nm

FDDI Specifications

Table 3–5 lists the specifications for 100/140-µm multimode fiber optic cable.
Table 3–5:

FDDI 100/140-µm Multimode Fiber Optic
Cable Specifications

100/140-µm Attributes

Values

Core diameter

100 ± 4.0 µm

Cladding diameter

140 µm ± 6.0 µm

Numerical aperture

0.290 ± 0.02

Modal bandwidth (min)

500 MHz• km @ 1300 nm

ANSI multimode
1.6 km (0.96 mi)1

Link length

Transmit launch signal –20 dBm to –11 dBm (max)
Receive input signal

–31 dBm to –14 dBm (max)

Link-loss range

3 dB to 11.0 dB2

Low-power multimode
1 km (0.62 mi)3

Link length

Transmit launch signal –22 dBm to –11 dBm (max)
Receive input signal

–29 dBm to –14 dBm (max)

Link-loss range

3 dB to 7.0 dB2

1The link length can be 1.6 km (0.96 mi) if the cable meets

the minimum modal bandwidth. If the modal bandwidth of
the fiber is unknown, then a 500-m distance can be supported.
2The use of 100-µm fiber may launch excessive light into
the fiber. This fiber requires a guaranteed minimum of 3 dB
of link loss for proper operation.
3The link length can be 1.0 km (0.62 mi) if the cable meets

the minimum modal bandwidth. If the modal bandwidth of
the fiber is unknown, then a 500-m distance can be supported.

FDDI Network Configuration Guidelines

Table 3–6 lists the specification for 50/125-µm multimode
fiber optic cable.
Table 3–6:

FDDI 50/125-µm Multimode Fiber Optic
Cable Specifications

50/125-µm Attributes

Values

Core diameter

50 µm ± 3.0 µm

Cladding diameter

125 µm ± 2.0 µm

Numerical aperture

0.200 ± 0.015

Modal bandwidth (min)

500 MHz• km @ 1300 nm

ANSI multimode
Link length

2 km (1.2 mi)1

Transmit launch signal –25 dBm to –14 dBm (max)
Receive input signal

–31 dBm to –14 dBm (max)

Link-loss range

0 dB to 6.0 dB2

Low-power multimode
Link length

1 km (0.62 mi)

Transmit launch signal –26 dBm to –14 dBm (max)
Receive input signal

–29 dBm to –14 dBm (max)

Link-loss range

0 dB to 3.0 dB2

1The link length can be 2 km (1.2 mi) if the cable meets the

chromatic dispersion requirements in Table 3–3. If the
chromatic dispersion characteristics of the cable plant are
unknown, then a 1.6 km (0.96 mi) distance can be supported
2The minimum values shown are worst-case values.

FDDI Specifications

Table 3–7 lists the specifications for ThinWire and shielded
twisted-pair cable. Note: ThinWire refers to ThinWire coax
cable supplied by Digital.
Table 3–7:

ThinWire and Shielded Twisted-Pair Cable
Specifications

Attributes

Values

Characteristic Impedance
ThinWire

Z0 = 50 Ω ± 2 Ω

Shielded twisted-pair

Z0 = 150 Ω ± 15%

Supported Cable
ThinWire

Digital ThinWire Coax

IEEE 807.5 Token Ring Types 1 & 2 behind the wall
shielded twisted-pair
Type 6 for patch cables
Interstation link length (behind the wall cable)1
ThinWire

90 m (315 ft) (max)2

Shielded twisted-pair

90 m (315 ft) (max)

Cross-connect configuration1
ThinWire

99 m (325 ft) (max)2

Shielded twisted-pair

99 m (325 ft) (max)

Link-loss budget (cross-connect configurations)
ThinWire

11.5 dB (max loss) @ 62.5
MHz

Shielded twisted-pair

12 dB (max loss) @ 62.5
MHz

1Refer to the EIA/TIA 568 Commercial Building Wiring Standard

for details.
2The maximum lengths obtained for ThinWire are configu-

ration sensitive.

3.2 Optical Bypass Link Loss
Installed optical bypass relays operating in bypass mode introduce additional loss to the cableplant. Use the following
procedure to determine the allowable link loss per cableplant
segment.

FDDI Network Configuration Guidelines

1. Obtain from the vendor the worst-case loss (Lwc) for
the optical bypass relay when the device is in bypass
mode.
2. Multiply the number of serially linked optical bypass
relays by the known Lwc.
3. Subtract the calculated Lwc from the available 11-dB
link-loss budget.
4. Divide the result by the number of intervening cableplant segments.
The result equals the allowable link loss per cableplant
segment.
Table 3–8 shows the formula to determine allowable link
loss as a function of the number of bypass relays.
Table 3–8:

Calculating Allowable Link-Loss Budget

Number of Serially Allowable Link Loss
Bypassed Stations Calculation
1

11 dB – Lwc
–––––––––––––– = link loss
2

11 dB – (2• Lwc)
–––––––––––––– = link loss
3

11 dB – (3• Lwc)
–––––––––––––– = link loss
4

11 dB – (4• Lwc)
–––––––––––––– = link loss
5

If the exact link loss is known for the cable segments:
1. Calculate the total Lwc for the concatenated cableplant.
2. Add the known link loss for each segment to the total
Lwc.

FDDI Specifications

The link-loss budget for the concatenated cableplant must
be less than or equal to the available link-loss budget of 11
dB.

FDDI Network Configuration Guidelines

A
Related Documents
For additional information about FDDI products offered by
Digital, refer to the following documents:

•
•
•
•

Networks Buyer’s Guide
Fiber Distributed Data Interface System Level Description (Order No. EK-DFSLD-SD)
Bridge and Extended LAN Reference
(Order No. EK-DEBAM-HR)
DECnet/OSI Routing Overview
(Order No. AA-PCU9A-TE)

The following documents provide information concerning
other Digital network offerings:

•
•
•
•
•

DECconnect System Fiber Optic Planning and Configuration
(Order No. EK-DECSY-FP)
DECconnect System Fiber Optic Installation (Order
No. EK-DECSY-FI)
Networks and Communications Product
Documentation (Order No. EK-NACPD-RE)
Network Wiring and Application Guidebook
(Order No. EB-K2411-4290)
Network Solutions Guidebook
(Order No. EB-32600-78)

For information on miscellaneous networking topics, refer
to the following documents:

•
•

Guide to DECnet-VAX Networking for DECnet node
information
Introduction to Network Performance for network performance information

FDDI Network Configuration Guidelines

Index
A
Adapter, guidelines, 4, 5
Attenuation, 4, 19

E
EIA building standard, 9,
11, 22

Bridge
dual homing, 14
guidelines, 4, 5
multiport, 10, 12, 14
Bridge/router, 4, 5, 11

Multimode fiber. See
Specifications

C
Concentrator
dual homing, 14
guidelines, 4, 5
Configurations
dedicated network, 10
dual homing, 3, 14
dual ring, 13
limited area, 9
single-mode fiber, 15
tree, 11, 12
Connection rules
guidelines, 4
matrix, 2
PHY-port types, 1
Copper. See Specifications

D
Dispersion
slope, 19
wavelength, 19
Dual attachment station
dual homing, 14
guidelines, 5
link length, 4
Dual homing
configuration, 14
guidelines, 3

O
Optical bypass relay
control signals, 7
description, 6
limitations, 6
link loss budgets, 22

P
PHY-port types
description, 1
matrix, 2

S
Single attachment station
guidelines, 5
link length, 4
Single-mode fiber. See
Specifications
Specifications
100/140-micron fiber, 20
50/125-micron fiber, 21
FDDI, 17
multimode fiber, 18, 19
optical system, 18
shielded twisted-pair, 22
single-mode fiber, 18,
19
ThinWire, 22

HOW TO ORDER ADDITIONAL DOCUMENTATION
DIRECT TELEPHONE ORDERS
In Continental USA
and Puerto Rico
call 800–258–1710

In Canada
call 800–267–6146

In New Hampshire
Alaska or Hawaii
call 603–884–6660

DIRECT MAIL ORDERS (U.S. and Puerto Rico*)
DIGITAL EQUIPMENT CORPORATION
P.O. Box CS2008
Nashua, New Hampshire 03061

DIRECT MAIL ORDERS (Canada)
DIGITAL EQUIPMENT OF CANADA LTD.
940 Belfast Road
Ottawa, Ontario, Canada K1G 4C2
Attn: A&SG Business Manager

INTERNATIONAL
DIGITAL
EQUIPMENT CORPORATION
A&SG Business Manager
c/o Digital’s local subsidiary
or approved distributor

Internal orders should be placed through Publishing and
Circulation Services (P&CS),
Digital Equipment Corporation, 10 Forbes Road,
Northboro, Massachusetts 01532–2597

*Any prepaid order from Puerto Rico must be placed
with the Local Digital Subsidiary:
809–754–7575

Fiber Distributed Data Interface
Network Configuration Guidelines
EK-DFDDI-CG-001
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