FDDI

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 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) – Target token rotation time (TTRT) – Absolute maximum time (AMT)

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

Encoded Sequence

Data Sequence

Encoded Sequence

0000

11110

1000

10010

0001

01001

1001

10011

0010

10100

1010

10110

0011

10101

1011

10111

0100

01010

1100

11010

0101

01011

1101

11011

0110

01110

1110

11100

0111

01111

1111

11101

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

4B/5B Encoding Unused five-bit codes are used for control 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

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 codes FC – 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 Nodes SAS – Single Attachment Station DAS – Dual Attachment Station DAC – Dual Attachment Concentrator