gnu@hoptoad.UUCP (04/23/86)
We just finished installing a Sun-3/50 as a client of our Sun-3/160.
When it arrived with just a BNC-to-BNC cable and a BNC T-connector,
I figured it would take a while to get the hardware connection (to a
3Com "Big Ethernet" transceiver) going.
Turns out that the local Zack Electronics store stocks BNC-to-Type N
(big ether barrel connector) adapters, as well as 50-ohm terminating
resistors with BNC connectors. They also had a 15-pin straight thru
cable for hooking up the 3Com transceiver to the 160. I plugged it all
in and it worked!
I can see why 3Com pushed Thin Ethernet -- it's easy and the parts
are available locally. For all I know, Radio Shack has what you need.
[I will note that a local audio supplier which carries the 3Com product
line wanted to sell me the same stuff for twice the price.]
Why did xeroX go with expensive custom cabling originally? They figured
that standard TV style thin cables wouldn't permit a big enough network?
--
John Gilmore {sun,ptsfa,lll-crg,ihnp4}!hoptoad!gnu jgilmore@lll-crg.arpaed%mtxinu@mtxinu.UUCP (04/24/86)
In article <735@hoptoad.uucp> gnu@hoptoad.uucp (John Gilmore) writes: >I can see why 3Com pushed Thin Ethernet -- it's easy and the parts >are available locally. For all I know, Radio Shack has what you need. I didn't check radio shack since I didn't think they would have the stuff. TV cable is *not* Ethernet cable. TV cable is 75 ohm and uses F series connectors, Ethernet is 50 ohm. A local electronics store (Al Lasher's in Berkeley) had most of the parts I needed. They didn't stock the BNC-to-N adapters, though. Neither did they have RG-58/CU cable, which is what 3Com recommends. They did have RG-58/AU, which is slightly less flexible, has lower capacitance and a higher propogation speed. It works, too, at least for our small network (Vax, microvax, sun, PC). >Why did xeroX go with expensive custom cabling originally? They figured >that standard TV style thin cables wouldn't permit a big enough network? There are differences in the physical and electrical characteristics of the cables. The thin stuff is *much* more fragile than the thick, and stinger-tap transcievers (like everyone but 3Com sells) are easier to install on the thick stuff. On the other hand, thin cable cost me about $80 for 500 feet (of the aforementioned RG-58/AU); Inmac sells RG-58/CU for $.23/foot ($115/500 ft) in bulk and thick cable for $1.40/foot ($700/500 ft - both prices are for 500-1000 foot lengths). The electrical differences relate to things like total network length and distance between nodes. I don't know the details. -- Ed Gould mt Xinu, 2910 Seventh St., Berkeley, CA 94710 USA {ucbvax,decvax}!mtxinu!ed +1 415 644 0146 "A man of quality is not threatened by a woman of equality."
dougm%ico@ico.UUCP (04/24/86)
> Turns out that the local Zack Electronics store stocks BNC-to-Type N > (big ether barrel connector) adapters, as well as 50-ohm terminating > resistors with BNC connectors. They also had a 15-pin straight thru > cable for hooking up the 3Com transceiver to the 160. I plugged it all > in and it worked! We've been getting our BNC to N connectors (and the coax) from a local hardware store rather than an electronics store. BNC connectors are fairly common, so thin ethernet is a big win. > > Why did xeroX go with expensive custom cabling originally? They figured > that standard TV style thin cables wouldn't permit a big enough network? The thin cables do put more of a length restriction on your network as well as a reduction in total number of stations. Maximum segment lengths are reduced to 185 meters vs. 500 for the thick cable. It is also recommended that the total number of stations on a cable segment not exceed 30. Now if only more vendors would put the BNC connectors on their controllers the way 3COM did... Doug McCallum Interactive Systems {cbosgd, hao, ima}!ico!dougm
phil%portal@portal.UUCP (04/25/86)
> Why did xeroX go with expensive custom cabling originally? They figured > that standard TV style thin cables wouldn't permit a big enough network? > -- > John Gilmore {sun,ptsfa,lll-crg,ihnp4}!hoptoad!gnu jgilmore@lll-crg.arpa In part you are right. The thick Ethernet cable was developed especially for its characteristics like loss and propagation delay. The delay is important as it along with the end-to-end span of the network, clock frequency, and encoding method determine things like the minimum packet size, time to backoff after collision, jam time, etc. The bottom line is the higher the wave velocity in the cable and the lower the loss, the longer you can make the Ethernet installation limit and the higher you can make the data rate, both desirable attributes. If you are really curious, try getting a copy of the version 1.0 Ethernet "blue book" that I think came out in 1980. (I have no idea where you would get one now. Maybe an engineering lib.) Thin Ethernet works because as long as the impedance of the cable is matched (50ohm) and there are no "bumps" at the connectors or bends to cause funkie reflections in the wire (just like regular EN) the protocol can't tell the difference below a certain network size and intertransceiver (?) minimum distance. I think "Cheapernet" as it has been called is a good idea for situations where EN is overkill. Yours sounds like a case in point. Ours probably is too since we have a pair of 3/160's in a closet connected by a big honkin' coil of cable. - Phil
henry@utzoo.UUCP (Henry Spencer) (04/28/86)
> > Why did xeroX go with expensive custom cabling originally? They figured > > that standard TV style thin cables wouldn't permit a big enough network? > > ...The thick Ethernet cable was developed especially > for its characteristics like loss and propagation delay... Thick Ethernet cable also has enough shielding to work well in truly blood- curdling RFI environments, e.g. radio broadcast transmitters at unfortunate frequencies. Or so I have been told. Whether all this shielding is really justified for the usual case is another matter... -- Support the International League For The Derision Henry Spencer @ U of Toronto Zoology Of User-Friendliness! {allegra,ihnp4,decvax,pyramid}!utzoo!henry
jbn@glacier.UUCP (05/05/86)
Thin Ethernet has its uses, but there are a few things to watch
for. First, of course, just because there's a T-connector at each
station does NOT mean that you can have a section of cable between the
side of the T and the tranceiver; the T connector has to be right next
to the tranceiver. Any length of cable beyond an inch or two introduces
out-of-phase reflections.
Second, after extending your net, make sure that the two most distant
stations can still talk. When you have too much loss in the cable, the
observed symptoms are that some pairs of stations cannot communicate and
that undetected collisions occur, resulting in high retransmit counts.
You're only allowed 8db loss across the whole cable. If the two most
distant stations can talk, the cable is probably OK.
John Nagle