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From: phil@amdcad.AMD.COM (Phil Ngai)
Newsgroups: comp.dcom.lans
Subject: Re: transceiver cable lengths
Message-ID: <18136@amdcad.AMD.COM>
Date: Sat, 29-Aug-87 16:19:51 EDT
Article-I.D.: amdcad.18136
Posted: Sat Aug 29 16:19:51 1987
Date-Received: Sun, 30-Aug-87 09:17:06 EDT
References: <14301@hi.UUCP>
Reply-To: phil@amdcad.UUCP (Phil Ngai)
Organization: Advanced Micro Devices
Lines: 48
Keywords: transceiver, specs, lengths, ethernet

In article <14301@hi.UUCP> cyrus@hi.UUCP (Tait Cyrus) writes:
>
>   We all know that there are well defined specifications for an 
>ethernet cable and the placement of transceivers on that cable,
>but I can't find anything which talks about the specifications
>(if any exist) for the transceiver cable.
>   Some transceiver cables that you can get come in 5, 10, 20 and
>40 meter lengths.  Are these the ONLY lengths possible?  If not,
>can I make one that is 0 meters long, 2 meters long or 2.1 meters

The nominal maximum transceiver cable length is 50 meters, based on a
loss/meter spec and a maximum loss spec. This includes everything,
like the cable from the interface board to the computer's backpanel.

Cables are sometimes rated in terms of equivalent cable length which
is different from physical cable length; this reflects a higher
loss/meter than the spec calls for. 

The Interlan NI1010A board cable, for example, falls in this category
(although I don't think Interlan actually specifies its equivalent
cable length), as do DEC's office transceiver cables. Most people
don't sell 50 meter cables to allow for the loss in the board cable.

DEC's office transceiver cables are rated at 4X, that is, a 5 meter
cable has the loss of 20 meters. As you can see, they are not useful
for long runs, what you can't see without holding one in your hands is
that they are very convenient, nice solid shells with flexible wire.
We love to use them in our racks, going from DELNIs to internet
routers, for example. 

There is no integer multiple of anything restriction on transceiver
cable lengths as there is on the trunk cable. Although it can be
viewed as a transmission line, there are no stubs to worry about and
thus no problems with reflections reinforcement. 

This kind of stuff is in the blue book, although it helps to know what
they're trying to tell you already. 

If you're making your own cables, be aware that the Ethernet 2.0 spec
and the 802.3 spec is different. I also recommend you look in the
Belden catalog for the wire; they make the best stuff, bare none.  (I
own no stock in Belden, I just want the best wire possible for my
company.)

-- 
I speak for myself, not the company.

Phil Ngai, {ucbvax,decwrl,allegra}!amdcad!phil or amdcad!phil@decwrl.dec.com