From: zrepachol@cc.curtin.edu.au
Subject: Re: Q: Extending Ethernet over fiber
Organization: Curtin University of Technology
Date: Mon, 16 Nov 1992 17:24:10 GMT

In article <1992Nov12.074211.3898@wkuvx1.bitnet>, goathunter@wkuvx1.bitnet writes:
...
>    o  We have another Novell LAN (Thinwire Ethernet) in the same building
>       as the 6320.  It too has reached its maximum length.  Naturally,
>       we'd like to connect the Novell LAN and the Pathworks LAN.  Will
>       that work if the DESPR is replaced with a DEMPR?  How about just
>       connecting a gateway to the DELNI now?  Would that work?
> 
> Thanks in advance for any help you can provide.  No one around here
> (including our DEC reps) has had much experience with the limits of
> Ethernet.
> 

First off, how many fibres are in the cable between buildings? You will need
2 to link Ethernets via repeaters or bridges or 4 for FDDI. Idealy you will
have 2+4+n with n spares, n > 0. the costs for a fibre are, Installing,
Terminating the 'pigtails', everthing else ( including the cable ).

To your config. A assume it is

		-----------------
		|		|
		| DELNI		|
		|---------------|
		  |   |    |
		  |   \    -----------------6xxx
		  |    \
		  |     \
		  |     other vax?
		  |
		|---------------|
		|		|
		|  DESPR	|
		|		|
		|---------------|
		 thin wire segments


plus some other thin-wire not conected to the above.


plus of course, the second building.

First you need to consider if you want to link ALL your protocols, or just
some. Note that you have DEcnet, Novell, LAT, MOP, LAD and possibly NI-cluster
protocols to consider. The interface you mention would function at the decnet
transport level I think, so terminal servers etc would still be partitioned
onto their own LAN. A repeater would merge all the LANs into one multi-
segment LAN. A bridge would connect the Ethernet segments into as many 'LANs'
as you wish.

As to the length bit. There are 2 length limits in Ethernet. The first is
based on signal loss, noise, cable resistance and loading. This is where
the 180 metre thin-wire limit bites. After that, the increased noise and
decreased signal start to bite into your error margin.
The other length limit is time. This limit defines your 'slot-time' of the
Ethernet, and can't be exceded. Note that the different media have differing
length -> time functions, called propagation constant. It is ~0.8 for thick,
.75 for thin, and about .62 fotr fibre. Each repeater or DELNI also adds
some constant time delay ( depending on the device type ) and chops off some
of the 64 bit packet pre-amble. If we shove our trafic through a bridge, all
of the above falls in the bit-bucket. The bridge recieves, buffers, and xmits
a NEW packet. There are now 2 physical nets forming one logical net. Because
the bridge apeares as one node on each physical net it allows the total
propagation time to increase to what ever you or your network software will
tollerate, 10s to 100s of msec rather than usecs. In any reasonable config
the lengths you refer to will not be an issue, as the total 'length' is
about 300+300(thin-wire segments)+1000(500m fibre @.6 plus a fudge for repeater
delay)+200(DELNI)=1800 metres end to end 'length'. this is still 1Km inside
the Ethernet 2.8Km limit, so you will be right for the moment.

The last sentence is the trap. You may soon need to expand further and belt
your head hard on the wall. :=( Plan *WELL* ahead!! And make sure you can
change your plans radicaly without major disruption or de-instalation.

~Paul