FDDI (Fiber Distributed Data Interface)

1
FDDI (Fiber Distributed Data Interface)
Standardized by ASI and ITU-T (ITU-T X.3
2
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)
3
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)

4
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

5
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

6
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

7
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

8
Figure 12-22
FDDI Example
Round 0 The token travels around the ring each
station sets its TRT timer to 0
9
FDDI Example
Figure 12-22-continued
10
FDDI Encoding
Figure 12-23
NRZ-I
11
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
0000 11110
0001 01001
0010 10100
0011 10101
0100 01010
0101 01011
0110 01110
0111 01111
Data Sequence Encoded Sequence
1000 10010
1001 10011
1010 10110
1011 10111
1100 11010
1101 11011
1110 11100
1111 11101

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

12
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
13
FDDI Layers
Figure 12-24
14
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
15
Figure 12-26
Physical Medium Dependent Layer
Dual Ring secondary ring make s FDDI
self-healing
16
FDDI Ring Failure
Figure 12-27
17
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