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From: medin@orion.arpa (Milo S. Medin, NASA ARC Code ED)
Newsgroups: mod.protocols.tcp-ip
Subject: Re: Looking for DDN/X.25 to TCP/IP router
Message-ID: <8701220629.AA23169@orion.arpa>
Date: Thu, 22-Jan-87 01:29:07 EST
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Posted: Thu Jan 22 01:29:07 1987
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I can certainly believe that.  But under certain circumstances, such as
a gateway with many simultaneous VC's open to PSNs across the network,
I think you may run into the 64Kb/s X.25 host access line speed bottleneck,
a bottleneck that you wouldn't hit with 1822.  Granted the PSN's only have
56 Kb/s lines, but the NASA gateway to the DDN (for example) may be pushing 
traffic through multiple modem port lines and might actually be able to use that
added bandwidth.  I'm not saying using X.25 the way DDN is using it in
Standard service is a bad idea, in fact, its certainly more elegant
than ECU's and the like.  And even for hosts connected to a PSN, 
I think it's quite reasonable because you'll generally not have a 
lot of open VC's to other PSN's, and thus won't hit the 64 Kb/s
bottleneck since the X.25 flow control should block you when you
have lots of outstanding data to a given PSN (just like RFNM's). 
What I am saying is that for certain applications, especially gateways
with large numbers of hosts behind them, which are in many cases 
colocated (or in 1822DH range) with the PSN, that 1822 may provide
better performance with a much simpler interface.  1822 is far from
perfect, but it works, and it's been figured out pretty well
over the years.  I think that in a year or two, the X.25 connections
to PSNs will be pretty well figured out and shaken down.  But
the issue of performance in certain cases may still side with
1822.  If you could plug into a PSN with a 112 Kb/s X.25 line, then I don't
think you'd have that problem.  

Has anyone here done any benchmarking with X.25 connections from PSNs to
gateways in a situation where many VC's are open to multiple PSNs
all at once and plenty of data to send through all of them?  You should
be able to monitor the host access line and see if you are pushing
the limits of line utilization.

In addition, I understand from some folks at BBN that PSN 7.0 will
have a better end-to-end protocol that should greatly improve
X.25 performance.

					Thanks,
					  Milo