SUMMARY: Sunlink X.25 problem....

From: Gregory Bond (
Date: Thu Nov 24 1994 - 22:19:55 CST

(Original query, somewhat abridged)

> Since then, we have been unable to hold a connection open for more than about
> 30 seconds. The first 3-6 (varies) packets each way go no problems, but then
> it hangs. I've had the PSDN trace stuff on the wire, and they are sending us
> packets that the local machine is simply ignoring. The incoming data does
> _not_ appear in x25trace, and eventually after 30 seconds the vitrual circuit
> gets shut down because the PSDN gets sick of retansmitting the packet that the
> Sun insists it is not receiving. I don't think its a line problem, because
> all this time the underlying hdlc acks are being sent and acknowledged.

Many thanks to those who replied, especially Mike Daffey. Thanks to those who
pointed out there is a patch for X25 V7 (100328-22), which, while it improved
things, didn't actually solve this problem. The problem was resolved this
morning in an unusual manner. We managed to get the PSDN people to send
someone onsite to put a monitor on our end of the modem link, as every
indication (including a bunch of CRC errors) indicated we were getting line
hits, but testing from their end or ours indicated a perfect line. The tech
walked in the door, took one look at the NTU and spotted the problem.

The NTU supplied by the PSDN was an old Ericsson model. That model used two
pins on the X.21 interface (18 & 21) to put the NTU into local loop. We were
using a 25pin cable, and the crosstalk in the cable between the data lines
(when long packets were sent or received) or noise from the electrical system
put enough voltage on these pins to put the NTU into test mode for an instant.

This is a well-known (to the techs!) problem with these NTUs and the answer is
to cut pins 18 & 21 on the NTU end of the cable. 30 seconds onsite and it's
working. I had mentioned the symptoms (i.e. test light on the NTU flashing
dimly occasionally), but they didn't connect it as these NTUs are old and
somewhat rare.

A lesson for those with Austpac!


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