Fddi fiber distributed data interface
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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)

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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

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

  • TIME REGISTERS

    • 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


Timers

Timers

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

ASSUMPTIONS:

  • 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


Fddi fiber distributed data interface

Figure 12-22

FDDI Example

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


Fddi fiber distributed data interface

FDDI Example

Figure 12-22-continued


Fddi fiber distributed data interface

FDDI Encoding

Figure 12-23

NRZ-I


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 fiber distributed data interface

FDDI Layers

Figure 12-24


Fddi fiber distributed data interface

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


Fddi fiber distributed data interface

Figure 12-26

Physical Medium Dependent Layer

Dual Ring

– secondary ring make s FDDI self-healing


Fddi fiber distributed data interface

FDDI Ring Failure

Figure 12-27


Fddi fiber distributed data interface

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


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