Re: Linking LAN's via Public X.25


Tim Symchych (symchych@skl-crc.arpa)
Thu, 2 Jun 88 18:59:43 edt


Perhaps another view of IP over X.25 might help. While the original
question was asked about Sun X.25, there are a number of networks within the
ARPA/Internet that use IP over X.25.

The experience that Phil Karn described at Bellcore does not describe
our experience in establishing the DRENET and XDRENET here in Canada.
There is no doubt in my mind that some implementations of packet
switching using CCITT X.25 are poor. We did some testing of various
networks about two years ago including ARPANET (1822 Links), Telenet
in the USA, SATNET, and the public packet switching networks in the
UK and Canada (Datapac). Packets were timed in some great loops, and
by network segment to allow us to determine level of service over the
various segments. Generally, the packets through Telenet and SATNET
suffered the most delay. The X.75 gateways didn't seem to work all that
well then either.

I'm not sure that X.25 was designed for slow speed terminal multiplexing.
NO packet switching network works well at that task.

In our own case, use of TCP/IP over X.25 has been shown effective from
both points of view: cost and throughput. In Canada, we have at least
one first rate X.25 packet switching network. Through eight years
of using Datapac for applications, including five years of TCP/IP over
X.25, we have found no problems in service or reliability. By the way,
I don't work for any of the Bell franchises, nor do I own any of their
stock. I just pay the phone bill.

We have several hosts and gateways that connect LANs using X.25. I
don't agree that X.25 is overly complex, or managing virtual circuits
is a big problem. Our gateways and hosts usually do the work in software
on board level products. We did some preliminary testing with SunLink
using our hosts and one of their software implementations. Everything
worked but the Sun stuff has some minor rough edges. For instance,
the Sun would only allow one virtual circuit between two hosts even
thought our side would allow one per user process. Imagine sharing
your TELNET session with an FTP with each IP packet waiting it turn in
in the X.25 queue. TELNET over X.25 is bad, but Sun made it worse. I
hope they worked on that "kludge".

Our tariff structure for communication services is almost the reverse
to that in the U.S. Our leased lines cost us dearly, but out packet
switching cost much less. If you look at our telco infrastructure, its
easy to see why. Our population is spread across the country, and
leased lines are nearly always new services.

In the way of cost and performance, we have this kind of experience:

Rental of 9600 bps X.25 modem with 20 virtual circuits is $390.00 U.S. per
month.
Traffic charges range from $35.00 to $300.00 U.S. per month per X.25 interface
depending on usage and distance. The later is for about 50MB per month
to a site 1500 miles away using the Datapac cost formula.
On most of our X.25 legs, we get between 50 and 75 per cent of the of the
9600 bps. But this does depend a great deal on the HOST implementation
of TCP/IP. In contrast, our 9600 bps HDH line gives us about 80 to 85
per cent of the line speed. Our 56 kbps line gives about 20 to 30 percent
of the line speed, but we are not sure that we have hosts at either
end to drive it faster.
To send a 10 MB file across the country, it would cost about $75.00 U.S..
Thats about the same as it would cost to run the data on to a mag new tape
and send it FEDEX.

Some of our sites will be running 19.2 bps X.25 service which will double
the monthly modem cost, and increase throughput but will be limited by
the TCP/IP on the hosts.
We expect that these 19.2 links will get us about 12000 bps or just about
what we get out of our very expensive 56k bps line between our core gateway
and the U. of Rochester. When our core gateway was replaced in Feb/88 with
a Butterfly, the measured throughput went up slightly even though the HDH
interface was replaced with an X.25 interface. I've tried to figure that one
out.

I will be the first to agree that our needs and network structure is
different than Bellcore. However, I view X.25 packet switches as a low cost
backbone that will allow us to operate until we have sufficient traffic
levels to support leased lines.

If there is a target needing a thumper, it sounds like the implementation
of X.25 in Telenet will do. When it comes to bad implementations of TCP/IP,
the cry is "burn them at the stake". There are good and bad X.25
implementations, and I recommend burning where it is due.



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