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Chapter 12
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Chapter 12

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  1. Chapter 12 Token Bus

  2. Figure 12-1 A Token Bus Network Physical bus, logical ring formed based on MAC (physical) address In descending order next station previous station

  3. Figure 12-2 a, b Token Passing in a Token Bus Network

  4. Figure 12-2 c,d Token Passing in a Token Bus Network

  5. Figure 12-3 Queues for Service Classes lowest priority highest priority A station does not have to classify its data. If it does, it needs to Have 4 queues, one for each class (class 0, 2, 4, 6) 4 timers: TRT0, TRT2, TRT4, and THT THT (token holding timer): set to the max. time a station can send C6 data TRTs (token rotation timers): set to the max. time a token can take to circulate + time to send C0, C2, or C4 data.

  6. Figure 12-4 Ring Management

  7. Removing Stations B C D • * Voluntary leaving • 1.C waits until it receives a token from B • 2. C sends a set-successor frame to B indicating D as the successor to B • 3. B update its record and sets its successor to D • 4. C sends token to D • 5. C leaves the ring • Unexpected leaving • 1.B sends token to C. If there is no activity from C, B sends the token again • to make sure there is no transmission problem • 2. If still no activity from C, B sends a who-follows frame containing the address of C • 3. D (the successor of C) responds with a set-successor frame and • specifies itself as the successor of B • 4. B now changes its records and defines D to be its successor • 5. B sends a token to D

  8. Adding Stations B C D E * Address within the range of those in the ring 1.B (a token-holding station) sends a solicit-successor-1 frame containing the address of B and D (successor) and creates a window set to two times the propagation delay. (This invitation to join is performed by every token-holding station, a periodical testing) 2. If B receives no response, it closes the window and sends the token to D 3. In our example, C (interested in joining the ring) responds with a set-successor frame 4. B changes its successor to C and sends the token to C. C sets its predecessor to B and successor to D (the previous successor of B). Now, C is part of the ring. 5. If more than one response  collision  B sends a resolve-contention frame. 6. Response window is based on the 1st two bits of the physical address of the interested station 00  responds in the 1st response window (no waiting time) 01  2nd (waits for one response window time) 10  3rd (waits for two response window time) 11  4th (waits for three response window time) 7. Still collision  use next two bits  …  until resolved

  9. Adding Stations B C D E * Address outside the range of those in the ring 1.When the station with the lowest address in the ring becomes the token holder, it sends a solicit-successor-2 frame and defines its own address and the largest address in the ring 2. The remainder of the procedure is the same as defined in the first case

  10. Token Recovery • * Situations in which there is no token • 1.When the ring is set up for the first time • 2. When the station holding the token fails and takes the token down with itsels • 3. When a token is corrupted and is discarded by the receiving station • Token-recovery procedure • 1.Token-recovery timer (every station has one) expires & no activity on the line •  token is lost  claim-token frame is sent for generating a new token • 2. More than one timer expires  more than one token claimant •  contention • 3. To resolve this contention, each station pads the data field of the claim-token frame • with a different value, based on the physical address. • 00  0 * time slot • 01  2 * time slot • 10  4 * time slot • 11  6 * time slot • 4. Each station that has sent a claim-token frame continues listening to the bus. • The station that has sent the longest frame has the responsibility to generate • a token. • 5. Still conflict  use next 2 bits  …  until no conflict

  11. Removing Duplicate Tokens • A token-holding station drops the token and moves to the listening state • if it detects some activity on the bus since it indicates some other station • is also holding a token. Ring Initialization • Use two of the previous procedure • 1. Each station sends a token-recovery frame to contend for access to the token • The one that wins becomes the token holder.  a ring with only one station • 2. Use the station-adding procedure to send solicit-successor frame by defining • itself as the predecessor and successor station • 3. After the successor is found, the token is passed to that station and the • process continues until all stations are added to the ring

  12. Figure 12-5 Token Bus Layers Same as that defined for IEEE 802 LANs Use token passing access method over a bus topology 4 specifications: Carrier band, phase continuous Carrier band, phase coherent Broadband Optical fiber

  13. Figure 12-6 Token Bus Frame Added by the physical layer Not part of the frame See Table 12.4, pp.271 SeeTable 12.3, pp.270-271 Indicates the type of a frame

  14. Figure 12-7 SD Field N bit: a specially encoded symbol, not the regular encoding for binary 1 (to be explained in physical layer) used in NN pairs

  15. Figure 12-8 FC Field Format

  16. Figure 12-9 ED Field Format N bit: a specially encoded symbol, not the regular encoding for binary 0 (to be explained in physical layer) used in NN pairs

  17. Figure 12-10 Physical Layer Specification Modulation with multiplexing Modulation without multiplexing

  18. Figure 12-11 Encoding and Signaling for Carrier Band, Phase Continuous HL LH HL L: 3.75 MHz H: 6.25 MHz NN pair: encoded as LLHH padding: encoded as HHLL Data rate: 1 Mbps

  19. Figure 12-12 Encoding and Signaling for Carrier Band, Phase Coherent H L H L: 5 (10) MHz H: 10 (20) MHz Data rate: 5 (10) Mbps NN pair: encoded as ½ periods of H + 1 period of L + ½ periods of H

  20. Figure 12-13 Encoding and Signaling for Broadband Levels 0, 2, 4 Level 0 for binary 0 Level 4 for binary 1 Level 2 for nondata (N bit) A combination of ASK and PSK Data rates: 1, 5, or 10 Mbps N bit (nondata bit): encoded as Level 2

  21. Figure 12-14 Encoding and Signaling for Optical Fiber Zero amp. High amp. Data rates: 5, 10, or 20 Mbps BW: 10, 20, or 40 MHz N bit (nondata bit): encoded as the third level