Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 (MU) 9/23/84; site mulga.OZ Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!genrad!decvax!mulga!ian From: ian@mulga.OZ (Ian Richards) Newsgroups: net.dcom,net.lan Subject: Re: DEC's broadband Ethernet tranceivers Message-ID: <605@mulga.OZ> Date: Sun, 6-Jan-85 18:25:35 EST Article-I.D.: mulga.605 Posted: Sun Jan 6 18:25:35 1985 Date-Received: Sun, 13-Jan-85 08:50:55 EST References: <1634@sdcrdcf.UUCP> Distribution: net Organization: Comp Sci, Melbourne Uni, Australia Lines: 33 Xref: watmath net.dcom:779 net.lan:597 > DEC has announced Ethernet tranceivers for use on broadband networks. They > implement the standard 10 M bps in 18MHz of bandwidth. There are > configurations for either single or double cable broadband. Maximum > distance is 3800 meters. Available in the spring. > Darrel J. Van Buer, PhD Is the 3800 metres correct? If so, how do they do it? If they are still using CSMA/CD then the maximum distance between nodes is limited by the contention period (46.4 microseconds) which in turn dictates the minimum frame size (512 bits). The DEC/Xerox/Intel "standard" suggests that after you allow for all kinds of switching and hardware delays you have about 23 of the 46 microseconds left over for propogation time which at 0.7c is pretty close to 5000 metres. This of course is the "round trip" time which means a maximum node separation of 2500 metres. Now if you use a broadband cable surely this ought be halved again. This is because the worst case contention problem is two nodes near to each other but furthest from the head end. One begins transmission. Its signal propogates via the head end and all the way back until it almost reaches the other node which then begins a transmission. This signal (which is now a collision) must propogate all the way via the head end and back to the first station so it can detect the collision. The first station must continue transmitting for all of this time. Now perhaps they can avoid the double propogation delay by doing the collision detection on the transmission channel/cable. That brings it back to the baseband situation. (But how does a station upstream from a transmitting station find out that if it transmits it will cause a collision?) Perhaps they can get rid of some of those aforementioned delays or perhaps their signals go faster than 0.7c. I would be interested to hear the answer if anyone knows. Ian Richards decvax!mulga!ian