Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!yale!cmcl2!phri!roy From: roy@phri.UUCP (Roy Smith) Newsgroups: comp.dcom.lans Subject: Re: Req For Info - Ethernet Electrical Rules/Specs Message-ID: <4040@phri.UUCP> Date: 7 Oct 89 17:58:19 GMT References: <188.2526de30@acci.com> <580@trwind.UUCP> <850@maxim.erbe.se> Reply-To: roy@phri.UUCP (Roy Smith) Organization: Public Health Research Inst. (NY, NY) Lines: 43 In article <850@maxim.erbe.se> prc@erbe.se (Robert Claeson) writes: > When there were a need to connect a workstation, a Mac or whatever to the > network, they just connected a few meter long cable to the T connector and > to the thin ethernet connector on the Ethernet board in the machine. > > Judging from prior knowledge, this just should't work, yet still it does. > Can anyone explain this? To mis-quote Abraham Lincoln, "You can violate some of the rules some of the time. You can even violate some of rules all of the time. But you can't violate all of the rules all of the time." In any well-designed system, the configuration rules are conservative and are designed to guarantee that your system will always work if you follow all the rules. They don't guarantee the converse; that your system will be broken if you violate any of the rules. Ethernet is no exception. You can probably get away with cheating in one area or another, but eventually, if you cheat enough, it will catch up with you. By installing the "pigtails", you are introducing reflections into the system. If your network is small enough, you can probably get away with it, but eventually you will discover that you can't put the full number of stations on your net, or you can't run the full number of meters of cable, and still have things work right. Perhaps if you looked carefully, you might discover that even though your net appears to work fine, you really have an abnormally high number of corrupted packets or collisions but your software is silently correcting the problem. I'll give you a down-home example of what I am talking about. When we first installed our ethernet backbone, I misinterpreted the instructions about tranciver tap spacing. What you are supposed to do is make sure the taps are on 2.5m(?) spacings, i.e. right on the black stripes marked on the cable. I read it as not allowing taps to be *any closer than* 2.5m and made sure that the taps were between stripes, with at least 2 stripes between every two taps. I've since realized my mistake, and all new taps go on stripes, but we still have a dozen or so mis-placed taps. So far, everything stil works fine, but I'm constantly aware that if our net grows enough, some day I may have to pull out the ethernet trunk cable and replace it with one done correctly. -- Roy Smith, Public Health Research Institute 455 First Avenue, New York, NY 10016 {att,philabs,cmcl2,rutgers,hombre}!phri!roy -or- roy@alanine.phri.nyu.edu "The connector is the network"