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From: ch@gipsy.UUCP (Christian Huitema)
Newsgroups: net.dcom
Subject: Re: Re: Re: Standards for commercial pac
Message-ID: <231@gipsy.UUCP>
Date: Mon, 9-Sep-85 22:46:50 EDT
Article-I.D.: gipsy.231
Posted: Mon Sep  9 22:46:50 1985
Date-Received: Thu, 12-Sep-85 12:14:25 EDT
References: <678@wdl1.UUCP> <513@petrus.UUCP>
Organization: GIPSI, Rocquencourt. France
Lines: 42

Please don't try to revive the old VC/D.gram polemic! Refer to the
litterature instead.

The big difference between VC & D.grams, from a commercial point of view, is
the possibility to guarantee a certain level of "quality of service". During 
the call set-up phase, buffers can be reserved in the intermediate nodes for 
the virtual circuit; it is even possible to reserve some part of the
transmission ressource (An extreme case is the simulation of "physical
circuits" by TDMA satellite). Obviously, the "per duration" charge derives
from the amount of ressources that were reserved. This charge is generally
*small*. (Transpac charges FF0.02, i.e. $0.0018, per minute of connection,
for a 1200bit/s national connection). The worst "per duration" charges are
encountered on international connections.

The other difference is the ability to avoid congestion due to
"transmission" overload. A typical PSDN operates at a load of 40-50% per
link during the peak hours. It is possible to block the incoming packets; a
user that would try to ignore the window limitations would get his packets
rejected (reinitialisation cause "remote protocol error"). It is also
possible to choose the "best route" at call set up time, thus avoiding the
"congested areas". The same behaviour is not recommanded on a Datagram
network, as it tends to propagate congestion.

Still, VC networks can be congested, just like Telephone networks, by an
excess of calls. That was the reason of Transpac "black friday", last june.
However, flow control procedure have been already implemented on some
telephone networks, and could be ported onto PSDNs.

At INRIA we experimented with LAN-satellite connections; the first
design used a datagram based gateway for "transparent" interconnection.
However we found out that the efficiency was poor, as the gateway had to
trow away packets when the load increased. Thus, in the next design, we have
implemented X25 on top of Ethernet, which allows for an easy interconnecton
to the outer world, and for a much more efficient usage of external
connection. This will not cause an undue overload for local communications,
as we can use the "class 0" transport protocol, i.e. no end to end
acknowledgments. X25 allows you to optimize windows and packet sizes on each
subnetworks.

The last, but not the least, advantage of X25 is that all public data
networks are interconnected, and that one can establish a direct connection
with virtually any computer in the world.