fddi (fiber distributed data interface) standardized by asi and itu-t (itu-t x.3
TRANSCRIPT
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FDDI (Fiber Distributed Data
Interface)
Standardized by ASI and ITU-T (ITU-T X.3
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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)
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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
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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
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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
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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
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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
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Figure 12-22
FDDI Example
Round 0 – The token travels around the ring: each station sets its TRT timer to 0
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Figure 12-22-continued FDDI Example
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Figure 12-23 FDDI Encoding
NRZ-I
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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
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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
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Figure 12-24 FDDI Layers
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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
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Figure 12-26
Physical Medium Dependent Layer
Dual Ring – secondary ring make s FDDI self-healing
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Figure 12-27 FDDI Ring Failure
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Figure 12-28 FDDI Nodes
MIC – Media Interface Connector
Three Types of NodesSAS – Single Attachment StationDAS – Dual Attachment StationDAC – Dual Attachment Concentrator