Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde!uunet!cos!howard
From: howard@cos.com (Howard C. Berkowitz)
Newsgroups: comp.dcom.lans
Subject: Re: X.25
Summary: No upper limit!
Keywords: X.25 OS
Message-ID: <24802@cos.com>
Date: 4 Dec 89 18:32:16 GMT
References: <1100@sumax.UUCP>
Organization: Corporation for Open Systems, McLean, VA
Lines: 49

In article <1100@sumax.UUCP>, woodman@sumax.UUCP (David Woodman) writes:
>  Does anyone know the maximum number of operating systems "hung" off of
>  an X.25 network? 7 is the high water mark so far.
> 
>  P.S. I am posting this for a friend, so if I mangle the terms into
>       utter nonsense, please don't flame me.

I am not completely clear what you mean by "hung," so I will assume
you mean what is the maximum number of operating systems containing
X.25 implementations which can connect to a single X.25 network.

It should be clarified that X.25 is a CCITT Recommendation which
has physical, frame, and packet protocols.  Clearly, only one
machine can plug into each physical layer connector.   At the
frame layer, the "base" standard allows 127 addresses (or more
using an address extension feature), but implementor agreements
for the frame layer specify that only two stations will be
on each link.  Many more users, however, can be multiplexed onto
it using packet protocols.

At the packet layer, there is a 14-digit address space into
which packet layer entities are placed; each address could 
be for a machine running a different operating system!  There
are techniques (e.g., upper layer multiplexing, address 
extension) for getting beyond this limit.

There is a 12-bit address space for the number of virtual
circuits active on one link.  Practical implementations rarely use
more than 255 virtual circuits.

When I was at TELENET in the early 1980's, I remember generally that
there were perhaps 100,000 ports on various hosts of the network;
we had around 1000 switches and concentrators inside the public 
network.  These numbers have grown significantly.  Even then, there
were several private networks using TELENET components which also
had thousands of ports (and presumably lots of operating systems).

At COS, we routinely do interoperability testing with X.25 networks 
containing around 8-12 different operating systems.  The limit here
is primarily the number of computers we can physically put into the
lab, and other logistics such as the number of participating companies.

I'll be glad to clarify further if you can define your question 
a bit more clearly.
-- 
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