Post on 02-Jan-2016
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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