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CS 635 Lecture 5, Part 2

Fiber Distributed Data Interface (FDDI)

Technical Specifications


Binary 4B/5B Code Symbol
0000 11110 0
0001 01001 1
0010 10100 2
0011 10101 3
0100 01010 4
0101 01011 5
0110 01110 6
0111 01111 7
1000 10010 8
1001 10011 9
1010 10110 A
1011 10111 B
1100 11010 C
1101 11011 D
1110 11100 E
1111 11101 F

Optical-fiber cable

These limits are designed to minimize latency

See table 8.1 page 253 for diff between FDDI and 802.5
MAC Token Frame


The preamble contains 16 or more idle symbols that provide a pattern to establish and maintain clock synchronization among stations on the ring (idle = 11111). The starting delimiter consists of a specific pattern, The J symbol (J=11000) is the first symbol of the starting delimiter sequence; the K symbol (K=10001) is the second. No frame or token is considered valid unless it starts with this explicit sequence. The ending delimiter consists of two consecutive T-symbols to indicate the end of the token frame (T=01101).

SD (8 bits) Starting delimiter

FC (8 bits) frame control

DA (16/48 bits) Destination Address

SA (16/48) Source Address

Two points

  1. After the station seizes the token the ring is made idle for a brief period of time while the packet is being set up. This allows for more time between the DTE (a user station) and the ring interface to structure and move the packet across the interface. Less expensive and simpler interface
  2. The token is transmitted immediately after the packet. Another station down the line could also use the token. Provides for much more efficient utilization for large rings which might have much latency in transfer around them.

The status field FS allows the originating station to detect three conditions:

MAC does not retransmit. Error is passed to LLC, which takes corrective actions.

Capacity Allocation

XXX picture of the FDDI back bone

Synchronous Traffic

To support a mixture of stream and bursty traffic FDDI defines two types of traffic:

            DMAX + FMAX +TokenTime + SUM(SAi) <= TTRT

Each station also stores:

Each station is initialized with TRT = TTRT and LC=0

If the token is early, the station saves the remaining time from TRT in THT, resets TRT and enables TRT. The station then transmits according to the following rules:

  1. It may transmit synchronous frames for a time SAi
  2. After transmitting synchronous frames, or if there were no synchronous frames to transmit, THT is enabled. The station may transmit asynchronous frames for time as long as THT > 0

If the token is late, LC=0, TRT continues to run. The station can transmit synchronous frames for a time SAi. The station may not transmit any asynchronous frames.

See example on page 260. TTRT = 100 milliseconds, SAi=30milliseconds

Ring Maintenance

Claim Token Process

Arbitration Hierarchy

The process completes when one station receives its own claim frame, which has made a complete circuit of the ring without being preempted.

Initialization Process

Beacon Process

FDDI Data Link Algorithm

Specific application to DEC implementation
Issue: removing unwanted frames from the ring and stability and reliability of the ring

Frame Content Independent Stripping (FCIS) Algorithm
Ring Purging Algorithm

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