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From: phil@amdcad.UUCP
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet : Broadband vs Baseband
Message-ID: <14863@amdcad.UUCP>
Date: Thu, 19-Feb-87 13:15:20 EST
Article-I.D.: amdcad.14863
Posted: Thu Feb 19 13:15:20 1987
Date-Received: Fri, 20-Feb-87 06:44:39 EST
References: <2334@sunybcs.UUCP> <18500001@clio>
Reply-To: phil@amdcad.UUCP (Phil Ngai)
Organization: Advanced Micro Devices, Sunnyvale, California
Lines: 58

In article <18500001@clio> berger@clio.Uiuc.ARPA writes:
>
>Briefly, the length of the cable is limited by capacitance and
>the number of terminations.  Splices will change the SWR and
>also affect performance and maximum length.

This is sort of correct but not really. Maximum cable length is
limited by several things but most notably, by *attenuation*.
Capacitance does have something to do with this but so does the
resistance of the copper.  If you've ever looked at Thin Wire Ethernet
cable vs regular Ethernet trunk cable, you'll see that there's a lot
more copper in the regular trunk cable. That's why you can use 500
meters vs 185 meters. 

Other considerations in setting Ethernet physical dimensions include
timing jitter and collision propagation times. 

An Ethernet cable should only have two terminations, one at each end.
The transceivers are *taps*, not terminations. (by Ethernet cable, I
mean the trunk cable. Transceiver cables are either transceiver cables
or AUI cables.)

>  If broadband networks are longer
>than baseband networks, it's only because broadband repeaters are
>easier and cheaper to build.

In the case of Ethernet, the limit is imposed by collision propagation
times. Ethernet repeaters are not particularly difficult to build.

>Data transfer rates, as noted above, are based on a number of factors.
>A typical intra-city television cable may carry 150 channels, each of
>which is several megahertz wide.  150 devices can thus use the cable
>concurrently.  Only one device can use a broadband cable at a time,
>thus, each of 150 devices would have to wait their turn (and possibly
>arbitrate to decide who grabs the line, wasting more bandwidth).  Slow
>devices tie up the bus for longer periods.

I've stayed away from broadband systems like the plague (who needs
modems that stop working because the temperature changed from 75
degrees to 70 degrees) but I believe you are wrong about broadband
cable usage. 

The way I understood it, broadband bandwidth is divided up into a
number of channels, much like television. Each group of devices which
need to communication with each other gets a channel. I think they are
about 6 MHz. If this seems like television, you're right. There is a
reuse of cable TV components, due to the great economy of scale those
components have achieved, as well as the large base of people who know
how to work with it. 

I could be wrong about the broadband stuff but I'm pretty sure about
the baseband stuff.
-- 
 How can I be Asian when I like milk so much?

 Phil Ngai +1 408 982 7840
 UUCP: {ucbvax,decwrl,hplabs,allegra}!amdcad!phil
 ARPA: amdcad!phil@decwrl.dec.com