Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!nbires!hao!hplabs!ucbvax!ANDREW.CMU.EDU!leong From: leong@ANDREW.CMU.EDU (John Leong) Newsgroups: mod.protocols.tcp-ip Subject: Re: RING vs. ETHER - Theory and practice. Message-ID: Date: Mon, 21-Jul-86 13:12:54 EDT Article-I.D.: andrew.MS.leong.0.leong.139.3 Posted: Mon Jul 21 13:12:54 1986 Date-Received: Tue, 22-Jul-86 03:39:07 EDT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The ARPA Internet Lines: 26 Approved: tcp-ip@sri-nic.arpa 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