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From: tom@wcc.oz (Tom Evans)
Newsgroups: comp.protocols.appletalk
Subject: Re: Localtalk 30 node limit
Message-ID: <450@wcc.oz>
Date: 10 Nov 89 08:19:14 GMT
References: <2548@aecom.yu.edu> <429@wcc.oz> <7054@quick.COM>
Organization: Webster Computer, Melbourne, Australia
Lines: 36

In article <7054@quick.COM>, srg@quick.COM (Spencer Garrett) writes:
> There are 4 reasons I can think of for limiting the number of nodes
> on networks in general, and one additional reason for limiting them
> on a Localtalk net.
> 
> 2) Signal attenuation - Each transmitter on a network has to drive
> 	*every* receiver on the network.  Eventually the signal
> 	levels get so low that they are indistinguishable from the noise.

Only if you're driving a star configuration. The receiver should have
a high-resistance and a low-capacitance. It doesn't "eat" the signal.

> 3) Contention resolution - ...
> 	Binary exponential
> 	backoff is the most efficient, but if you don't know where to
> 	truncate the series then the waits can approach infinity

LocalTalk doesn't use it. It generates a "random" backof with an upper
range determined by the last 8 attempts. It ranges from 0 to 1,500 uS.
Max wait is 400 + 1500, which sounds a lot, but is only 55 byte-times.

> 5) Addressing - Localtalk uses 8 bit addresses, and probes for an
>  	available address each time a node is made active.  As
> 	the fraction of available addresses which are actually
> 	in use rises this process becomes more and more painful.

It probes for the last one it used. It degenerates to statically-addressed.
You'd have to have more than 100 nodes to have this problem, and by
the time this becomes painful, you'd be dead from all the other limits.
			    ---------
Tom Evans  tom@wcc.oz.au	|
Webster Computer Corp P/L	| "The concept of my
1270 Ferntree Gully Rd		|  existence is an
Scoresby VIC 3179    Australia	|  approximation"
Australia			|
61-3-764-1100  FAX ...764-1179	|      D. Conway