TOKEN BUS AND TOKEN RING

RIZWAN REHMANCENTRE FOR COMPUTER STUDIES

DIBRUGARH UNIVERSITY

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.

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

Message Exchange in Token Bus

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

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

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.

Token Bus Frame Format

Token Bus Frame Format

• Preamble – clock synchronization • Starting and ending delimiter • frame boundaries • no need of length field

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

Problems with Token Bus

TOKEN RING

SENDING AND RECEIVING

ACCESSING WITH TOKEN RING

Frame Format for Token Ring

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,…

SD DestinationAddress

Source Address

Information FCS

1 4

EDFC

2 or 6 2 or 61 1

AC

1

FS

1

SD AC EDToken Frame Format

Data Frame Format

• 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.

RING MAINTAINANCE

THANK YOU