Re: RING vs. ETHER - Theory and practice.


John Leong (leong@andrew.cmu.edu)
Mon, 21 Jul 86 13:12:54 edt


In IEEE802.5 (a.k. IBM token ring), there is two low level acknowlegemnt of
sort in the MAC layer encapsulation - at the end of the frame. When a station
grap a token for transmission, it will set the A and C bits (Address Recognised
and Frame Copied) to 0. As the frame zap round the ring, if all goes well, the
detsination station will receive the frame and set both the A and C bits to
1. When the frame continues its merry way back to the sender for purging, the
sender can deduce from the status of the A and C bit what has happened.

If A and C are both set to 1, all's well.
If A and C are 0, there is a good probability that the destination is not up
or on the net.
If A is 1 and C is 0 then the receiving station has a congestion problem.
If A is 0 and C is 1, we have something really strange going on.

Note that the acknowlegement is all done within one ring rotation as the A and
C bit is flipped on the fly by the receiver and is very efficient. There is
no explicit ACK frame involved.

Furthermore, the IBM token ring has a nifty feature built into the chip set.
If an interface detects a congestion situation, it will send out a special frame
(MAC frame) to tell whoever wants to know (network monitoring station) that
a soft error situation has been detected. It is really useful for network management
and planning.

Leong



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