eric@unmvax.UUCP (10/29/85)
Folks, Does a standard exist for the so call "thin Ethernet" which is getting more and more press these days. This has the same signal and protocol as the standard Ethernet, but uses a smaller cable (RG58), and as such cannot support as many stations, and the segment length is less. It also uses T connectors and BNC connections in place of Transceivers. I have seen several companys talk of it (e.g. 3com) but also none of them have quoted the same segment length, number of supported stations per segment, etc. Hence, my question, Is there a standard for this "thin ethernet" or a group that backs it with published specs, such as DEC-INTEL-XEROX group that backed the standard Ethernet? Eric Engquist ...ucbvax!unmvax!eric University of New Mexico College of Engineering Office of the Dean Farris Engineering Center Albuquerque, NM 87131 (505) 277-5501 or 6254
drk@well.UUCP (Doug Kaye) (10/31/85)
In article <943@unmvax.UUCP>, eric@unmvax.UUCP writes: > ... Is there a standard for this "thin ethernet" > or a group that backs it with published specs, such as DEC-INTEL-XEROX > group that backed the standard Ethernet? As this is probably of interest to many, I've posted this article. To the best of my knowledge, there is no published spec for RG-58. However, after months of research, here's what we've come up with: A "segment" (Thick or RG-58) can support a mximum of 100 nodes of MAUs. A segment may be composed of one or more lengths of cable. Using thick cable, each segment has a maximum length of 500 meters (1640 feet). With RG-58 this maximum is reduced to 152 meters (500 feet[!]). The difference is capacitance and signal loss. The formula to use is: T * 3.28 + E = 500 meters where T= length of <T>hin and E= length of <E>thernet (thick). Transceivers are ALWAYS required, it's just that using RG-58 it is reasonable to use on-board transceivers such as 3Com, etc. It is just as legal to connect thick cable to a thick-to-thin adaptor, through the T, and back to thick cable again...but the big yellow stuff would probably bust the T! Likewise, you can use a transceiver with BNC connectors and run a transceiver cable from the RG-58 net to the standard 15-pin IEEE 802.3 connector. (Another difference between Thick and RG-58 is the velocity, of course. Thick cable is 0.77c while RG-58 is somewhat slower, perhaps 0.65c or less.) Now, how come some folks (3Com in particular say "run 1000' with RG-58"? They cheat! As best we can tell, their transceviers put out a signal level greater than that of the IEEE802.3 standard. This means that a network with ALL 3Com can be 1000' per segment. BUT (!) if you put any other transceiver on the net, look out! We never go beyond the 802.3 specs in our installations. Anyone needing more info can reach me through the net. Doug Kaye Rational Data Systems 5725 Paradise Drive Corte Madera, CA 94925 ihnp4!hplabs!well!drk <doug kaye>
pjk@ascvax.UUCP (Pat Keziah) (02/21/86)
A number of previous articles have pointed out various good strategies
for success with thin Ethernet. But two critical points have been left
unclear, at least in my mind:
1) The cable must be 50 ohm coax. (in our shop at least,
75 ohm cable is commonly used for video signals)
2) The cable MUST be terminated with 50 ohm resistors.
(some previous discussions have alluded to
leaving the cable open at the ends)
We have only used thin cable so far and we are getting good service
from it, but we are starting to see 2-3% of the packets have errors
now that we are running 400 feet of cable. Our other big lesson so
far is that connector construction is extremely critical. It must
be done well.
--
Pat Keziah Ampex Switcher Company
{hao,boulder,avsdS}!ascvax!pjk 10604 West 48th Avenue
Wheatridge, CO 80033
USA
(303)423-1300 x226