TOKEN BUS AND TOKEN RING

1. TOKEN BUS AND TOKEN RING RIZWAN REHMAN CENTRE FOR COMPUTER STUDIES DIBRUGARH UNIVERSITY

2. IEEE 802.4 Token Ring • CSMA/CD – probabilities – MAC model – bad link – station wait for infinitely a long time! – no priorities • not useful for real time system.

3. Token Bus >> stations take time sending frames. – n frame , nT sec to wait

5. Message Exchange in Token Bus

6. Token Bus Organisation • Linear tree shaped cable on to which stations are attached. • Each station knows the address of its left and right neighbours. • Ring is first initialized – coordinator to initialize ring. – stations inserted in the order of station address

7. Token Bus Organisation • Token passing from higher to lower order station address • Token acquired station transmits for certain amount of time • Hand over token either at end of time or no frame to transmit • prioritize tokens

8. Token Bus • each maintains a queue of frames • each has timers • handover token from higher priority to lower priority. • fraction of token holding time allocated to each priority. • useful for implementing real – time traffic.

9. Token Bus Frame Format

10. Token Bus Frame Format • Preamble – clock synchronization • Starting and ending delimiter • frame boundaries • no need of length field

11. Token Bus: Issues • Frame Control – Successors, – predecessors – Entry of new station – Clain token • Token lost, station with token dead – Protocols to handle all issues – Useful for real time traffic

12. Problems with Token Bus

14. TOKEN RING

16. SENDING AND RECEIVING

17. ACCESSING WITH TOKEN RING

18. Frame Format for Token Ring Token Frame Format ED SD AC • If the ring is inactive, only the 3-Byte-Token (SD, AC, ED) circulates. If a station wants to send, it sets a certain bit in this token from 0 to 1. • SD and ED serve for marking the frame. They contain invalid sequences of the differential Manchester code. • Access control contains the token bit, further a monitor bit, priority bits and reservation bits. • Frame control marks the kind of the frame: Data, control,… Data Frame Format 1 1 2 or 6 4 1 1 2 or 6 1 Destination Address Source Address AC Information FS SD FC FCS ED

19. • Frame status contains confirmation bits A and C. If a frame arrives at the station with the destination address, bit A is set. If the station processes the frame, also bit C is set. When the sending station gets the frame back, it can see whether the receiving station is not working (A = 0, C = 0), if the frame was not accepted (A = 1, C = 0), or whether the frame was received correctly (A = 1, C = 1). To protect against bit errors, both bits are doubly present. • The addresses and the checksum are identical to Ethernet. Bits of access control: • The monitor bit serves for recognition of a second frame circulation • The priority bits make possible several priorities. They indicate the priority of the token. If a station wants to send with priority n, it must wait for a token of priority n or higher. • The reservation bits permit a station to reserve the next frame for itself. If a station wants to do this, it registers its priority into the reservation bits. This is only possible, if not already a higher priority is registered. During the next token generation, the priority is copied into the priority bits.

20. RING MAINTAINANCE

21. THANK YOU