🚀 go-pugleaf

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

In <1992Nov12.074211.3898@wkuvx1.bitnet> goathunter@wkuvx1.bitnet writes:

>We have a VAX 6320 with a small Pathworks LAN running over Thinwire
>Ethernet; this is in one building.  The 6320 and two DECserver 500s
>are connected to a DELNI that's connected to a DESPR.  I plan to
>eventually remove the DELNI from that configuration, I think.  We'd
>like to extend our Ethernet in two different ways as cheaply as
>possible (of course).

>I have two configuration questions for those of you who know about
>Ethernet.

>   o  The building housing the above LAN is connected to another
>      building via fiber optic cable, but it's not being used yet.
>      We have a Novell LAN in that building, as well as a DECsystem
>      5100 and a VAX 11/785.  We'd like to extend the 6320 LAN to
>      include at least the DECsystem and the 11/785, with the Novell
>      LAN being a nice added bonus.

>      The question: if we buy two Fiber Media Access Units (DEFLM-AA)
>      as advertised on page 105 in the new DECdirect catalog, can we
>      connect the 6320 and the remote systems without purchasing any
>      additional equipment (routers, bridges, etc.)?  If so, do you
>      have any suggestions for configuration?  The remote building is
>      about 500 feet away; the Pathworks LAN is currently about at its
>      length limit.  Approximately 200--300 feet run between the
>      DECsystem and the 11/785; the DECsystem is currently connected
>      to the Novell LAN.

>      It appears that the DEFLM would let us go from Ethernet -> DEFLM
>      -> fiber -> DEFLM -> Ethernet.

>   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.

>Hunter

You will have to look at two kinds of "maximum length", the first is the
maximum lenght of the cable without loosing too much signal strength,
about 100 meters for twisted pair, 180 for thinwire and 500 meters for
the big yellow coax cable. These pieces of cable can be extended with a
repeater, that amplifies the signal.
The second maximum length is not measured in "real" distance, but in
microseconds. Since ethernet is a broadcast type of network, inwich
collisions can occur, it is necessary to be able to detect these
collisions. This means that the transmitter must check the cable to make
sure nothing went wrong. It must check until the signal can travel from
the transmitter to the other side of the network and back. Because of
this, ethernet distances are measured in microseconds. Every unit that
is used has its specific propagation delay, and thus adds to the total
length, although it might only have a physical length of 0.05 meters.
The big problem is how to find these propagation delays, they're not in
the installation guide.
Every repeater you add, adds a rather large propagation delay, and thus
a "large distance". DEC advices not to use more than two repeaters
between any two systems (VAX, PC's, bridges) on the network. (A bridge
does complete retiming on packets, so lenght calculations start all over
if a bridge is passed). This does not mean that you can not have more
than two repeaters as a total: a backbone can have several repeaters, as
long as you don't cascade them, the total number of repeaters betwqeen
systems will be two!

Now in your case, let's look at the possibilities:
Your "backbone will be the fiber between the building, so you would have
to use a bridge, or have a maximum of one repeater in each building, for
instance a DEMPR at the side of the 6320. You can connect the two
thinwire segments, and would have to connect the 6320 with a DESTA.
Don't know if the DECserver 500 has a thinwire, if not, you would have
to use DESTA's for them too. (both DELNI and DEMPR/DESPR are repeaters!)
At the other building you might be able to use a DEMPR to connect the
NOVELL segment, and to connect the 11/785 with a DESTA on thinwire.

Now this might work, but given the maximum length of the cables in use,
i would not make a bet on it. My advise would be to use a bridge in
between, for instance the DECbridge 90, which is rather cheap. Any other
will do, given the right connections. This way you would have to use a
DEMPR on the 6320 side if you want to add the NOVELL network, otherwise
you can still use the DESPR, and connect the AUI output of the DELNI to
the Fiber optic tranciever. At the other side, connect the Fiber optic
tranciever to the bridge, and connect the other side to the network
overthere, for instance with a DEMPR connected to the NOVELL part, and
another thinwire segment to the 11/785 (with a DESTA).

By the way, one of the ways to find out if your ethernet network is to
long, is to boot a terminal server at one side, and see if you get error
messages (on a VAX with DECnet) like "remote node unable to defer,
distance 15384". The distance in meters can be calculated if you know
the delay of the controllers, the tranceivers, the different cable
types, the repeaters etc. This message will show up more often on a busy
network, not if all systems are down. The message means that (at least)
two systems started to transmit, and only one has seen that a collision
has occurred, since the collision was to far away for the second system:
the collision was detected after the second system had concluded that
everything was send correctly. Booting terminal servers in such a
network will almost always work, but somethimes will result in a
communication error.

Hope this helps a little. If someone has a list of propagation delays
for different types of ethernet equiptment, please send it.
>------
>Hunter Goatley, VMS Systems Programmer, Western Kentucky University
>goathunter@WKUVX1.BITNET, 502-745-5251

Lambert Hofstra
lamhof@thijssen.nl