[asterisk-ss7] KNK SS7-27 - first experiences - part 2

Pavel Troller patrol at sinus.cz
Thu Jun 27 00:25:41 CDT 2013

Hi there!
  I would like to report another issue, which was actually the first I've
found when trying to put my gateway into operation, but once solved it didn't
appear anymore, so it's not as important as other ones. But it's there and
it should be fixed.
  As already discussed in my first post, I have an Asterisk box connected to
two EWSD exchanges. Every one has its own, well-separated linkset, with one
separate physical signalling link. So, it should work well even with very
limited MTP3 support in libss7. EWSDs have differnt PCs, Asterisk uses the
same OPC for both of them.
  When I tried to start the gateway for the first time, a very strange things
were occuring: MTP2 came up on both links as expected, and then EWSDs started
to send an ISUP traffic immediately to the Asterisk. It means, that MTP3 
also successfully opened on the EWSD side, BUT NOT ON THE ASTERISK ONE! The
linksets were down! And Asterisk just reported tons of messages about 
unexpected ISUP when linksets are not up.
  Please note that we don't play the TRA game here in Czech Republic (and
probably in other European countries), so the only verification tool that
the linkset is up is sending SLTM and verifying correctness of the returned
SLTA. And it passed on EWSD and didn't pass on Asterisk side.
  Finally I found that my cables are swapped (the new server assigned the
order of E1 cards differently than the old one) and that I'm trying to start
linksets against the opposite exchanges! Such a stupid error! But a very
strange is, that EWSDs didn't find it and they promptly tried to use the
linksets. So I'm afraid that the SLTAs returned as a response to their SLTMs
were somewhat "fake", containing data, which they liked, while they would not
definitely like the real data (a different PC would not make the linkset to
activate). EWSDs are very strict in this. Isn't it possible that when forming
the SLTA response, we just reverse the PCs from incoming SLTM instead of
putting our own data here ?
  I tried to find the problem in the libss7 code but on the contrary to the
code in isup.c, I almost cannot understand the code in mtp3.c - it's not
commented well and I can't even find a place, where we construct an SLTA
to respond to the partner's SLTM.
  With regards,
    Pavel Troller

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