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