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Published byNelson Rodgers Modified over 9 years ago
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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 Mbps Access method – token passing CDDI – 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 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
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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 SequenceEncoded Sequence 000011110 000101001 001010100 001110101 010001010 010101011 011001110 011101111 Transforms each 4-bit data into a 5-bit unit that contains no more than two consecutive 0s Data SequenceEncoded Sequence 100010010 100110011 101010110 101110111 110011010 110111011 111011100 111111101
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4B/5B Encoding Control SymbolEncoded 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 codes FC – 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 Nodes SAS – Single Attachment Station DAS – Dual Attachment Station DAC – Dual Attachment Concentrator
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