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FDDI (Fiber Distributed Data Interface). Standardized by ASI and ITU-T (ITU-T X.3. FDDI (Fiber Distributed Data Interface). Data rate – 100 Mbps Access method – token passing CDDI – copper version. S-frames – synchronous (real time data) A-frame – asynchronous (not real time).

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fddi fiber distributed data interface

FDDI (Fiber Distributed Data Interface)

Standardized by ASI and ITU-T (ITU-T X.3


FDDI (Fiber Distributed Data Interface)

Data rate – 100 Mbps

Access method – token passing

CDDI – copper version

S-frames – synchronous (real time data)

A-frame – asynchronous (not real time)

access method
Access Method
    • Synchronous allocation (SA)
    • Target token rotation time (TTRT)
    • Absolute maximum time (AMT)
  • Access is limited by time
  • Priority – real time data
  • Steps
    • A station captures the token
    • Send S-frames first
    • Any remaining time may then be used to send A-frames
time registers
Time Registers

Synchronous Allocation (SA)

  • Length of time allowed for S-frames
  • Different for each station
  • Ring initialization

Target Token Rotation Time (TTRT)

  • Average time required for a token to circulate around the ring exactly once
  • Absolute Maximum Time (AMT)
    • Twice the TTRT
    • To avoid monopolizing the network

Token Rotation Timer (TRT)

  • Runs continuously
  • Measures the actual time taken by the token to complete a cycle
  • Incrementing or (decrementing) TRT

Token Holding Timer (THT)

  • Begins running as soon as the token is received
  • Shows how much time remains for sending asynchronous frames
  • Decrementing or (incrementing ) THT
station procedure
Station Procedure
  • Set the values of timers
    • THT = TTRT – TRT
    • TRT = 0
  • Sends synchronous data
  • Sends asynchronous data as long as the value of THT is positive
fddi example
FDDI Example


  • TTRT = 30 time units
  • Traveling token from one station to another = 1 time unit
  • Each station is allowed to send 2 S-frames per turn
  • Each station has a lot of A-frames to send

Figure 12-22

FDDI Example

Round 0 – The token travels around the ring: each station sets its TRT timer to 0


FDDI Example

Figure 12-22-continued


FDDI Encoding

Figure 12-23


4b 5b encoding
4B/5B Encoding

None of the five bit patterns start with more than one zero

Transforms each 4-bit data into a 5-bit unit that contains no more than two consecutive 0s

4b 5b encoding1
4B/5B Encoding

Unused five-bit codes are used for control


FDDI Layers

Figure 12-24


Figure 12-25

FDDI Frames

Each frame is preceded by 16 idle symbols (1111), a total of 64 bits

ED – Full byte in token frame, T control code(s)

FS – same to that of token ring

SD – J and K control codes

FC – Identify the frame type


Figure 12-26

Physical Medium Dependent Layer

Dual Ring

– secondary ring make s FDDI self-healing


FDDI Ring Failure

Figure 12-27


FDDI Nodes

Figure 12-28

MIC – Media Interface Connector

Three Types of Nodes

SAS – Single Attachment Station

DAS – Dual Attachment Station

DAC – Dual Attachment Concentrator