FDDI (Fiber Distributed Data

Interface)

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

FDDI (Fiber Distributed Data Interface)

Data rate – 100 MbpsAccess method – token passingCDDI – copper version

S-frames – synchronous (real time data)A-frame – asynchronous (not real time)

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

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

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

1. Set the values of timersa. THT = TTRT – TRT

b. TRT = 0

2. Sends synchronous data

3. Sends asynchronous data as long as the value of THT is positive

FDDI Example

ASSUMPTIONS:

1. TTRT = 30 time units

2. Traveling token from one station to another = 1 time unit

3. Each station is allowed to send 2 S-frames per turn

4. 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

Figure 12-22-continued FDDI Example

Figure 12-23 FDDI Encoding

NRZ-I

4B/5B Encoding

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

Data Sequence Encoded Sequence

0000 11110

0001 01001

0010 10100

0011 10101

0100 01010

0101 01011

0110 01110

0111 01111

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

Data Sequence Encoded Sequence

1000 10010

1001 10011

1010 10110

1011 10111

1100 11010

1101 11011

1110 11100

1111 11101

4B/5B Encoding

Control Symbol Encoded Sequence

Q (Quiet) 00000

I (Idle) 11111

H (Halt) 00100

J (used in start delimiter)

11000

K (used in start delimiter)

10001

T (used in end delimiter)

01101

S (Set) 11001

R (Reset) 00111

Unused five-bit codes are used for control

Figure 12-24 FDDI Layers

Figure 12-25

FDDI Frames

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

SD – J and K control codesFC – Identify the frame type

ED – Full byte in token frame, T control code(s)FS – same to that of token ring

Figure 12-26

Physical Medium Dependent Layer

Dual Ring – secondary ring make s FDDI self-healing

Figure 12-27 FDDI Ring Failure

Figure 12-28 FDDI Nodes

MIC – Media Interface Connector

Three Types of NodesSAS – Single Attachment StationDAS – Dual Attachment StationDAC – Dual Attachment Concentrator