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From: normt@cbnewsc.att.com (Norman R Tiedemann)
Newsgroups: comp.dcom.lans
Subject: Re: Novice LAN Questions - Cheapernet
Summary: T's very short
Message-ID: <1990Oct1.131140.14177@cbnewsc.att.com>
Date: 1 Oct 90 13:11:40 GMT
References: <313@gandp>
Distribution: na
Organization: AT&T Bell Laboratories
Lines: 74

In article <313@gandp>, rg@gandp (Dick Gill) writes:

> I am now into another new world (aka "learning experience" !-).  I
> am familiar with the mechanics of coax from my years as a TV
> broadcast technician but need a little help in the physical
> aspects of building a cheapernet LAN.
> 1. Where (aside from friends on the net) should I look for
> information about physically setting up a cheapernet lan?  Any
> practical publications to recommend?

There are some Tech notes that come with Cheapernet <-> Ethernet
transceivers which help. Also if you are buying cheapernet PC
cards, they should have a decent description of the physical aspects.


> 2. At the most basic level it appears that RG-58 (50 ohm) coax
> is used with BNC connectors; are there any traps or potential
> problems here?  What should I look out for in buying the pieces?

No, as long as you get good quality connectors. The T's must be
sturdy and good connections must be made with the cable.


> 3. In terms of design, I understand that this is a bus topology
> characterized by a single spine to which individual nodes are
> attached.  I guess the simplest model is a coax connecting a
> group of PC-NIC's via a "T" in the bus.
>       T-----------//---------T--------//---------T------->>--<50ohm
>       |                      |                   |
>   ----I----              ----I----           ----I----
>     PC A                   PC F                PC W
>   ---------              ---------           ---------

Almost. You will also need a 50ohm terminator on the end of a piece
of cable on the left side of PC A.
Each transceiver must see a 25ohm load (i.e. 50ohms at each end.)

> 4. How long can a line to a node be from the bus?  Can, for example,
> PC-D below be 30 feet from the bus?  How about 100 feet?
> 

Here is where I'm going to break your heart. The length that this
drop cable can be is on the order of Centimeters not Meters. Since
the transceiver must identify collision on the bus within the 64
bit preamble time, this is very different from Ethernet. Normally
you connect the T directly to the PC board connector and run both
cables to this. You should not think of putting another piece of
cable between the T and the actual termination device.


> 5. If the answer to 4 above is yes, can I further split that line
> to create nodes PC-D(1) and PC-D(2)? 
> 
>       T---------T-//---------T--------//---------T------->>--<50ohm
>       |         |            |                   |
>   ----I----     |        ----I----           ----I----
>     PC A        |          PC F                PC W
>   ---------   /  \       ---------           ---------
>             /      \
>           /          \
>       ----I----  ----I----
>        PC D(1)    PC D(2)
>       ---------  ---------

Sorry, this obviously won't work either, for reasons given above.
Hope this helps.

	Norm Tiedemann		AT&T Bell Labs IH 2G-419
	att!ihlpy!normt		2000 Naperville Rd.
	normt@ihlpy.att.com	Naperville, IL	60566
-- 
	Norm Tiedemann		AT&T Bell Labs IH 2G-419
	att!ihlpy!normt		2000 Naperville Rd.
	normt@ihlpy.att.com	Naperville, IL	60566