PRI Channel History

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Jul 13th, 2010
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We have a site with a full channelized T1 PRI - all channels in one group.  The site does not seem to have a high number of calls going through but they complain that people calling are saying they are getting busy signals.  I've watched the circuit during busy call times and noticed that the channel selection seems to be fairly random.

In CUCM - the channel selection is set to top-down, as the service provider has told me they use bottom-up.

Is there a way to see historical channel usage, or to track over time which channels are being selected for calls?



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kelvin.blair Tue, 07/13/2010 - 17:44
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One way you can track calls coming into a gateway is by using a syslog server. On the gateway setup a syslog server and issue "debug isdn q931".  This will send all debugging information to the server for later retrieval.  Keep in mind, this debug will give alot of information.

If you are runnnig Cisco Communications Manager as your PBX, then you can run a CDR report on the gateway.  This will give you a utilization report over a period of time.


rcoote5902_2 Wed, 07/14/2010 - 07:49
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Syslog would be nice but I don't have one in place right now.

CDR does give utilization information but I need to see each channel.  The utilization is well below the 24 channels, but I'm trying to see if there's a pattern to which channels are being used in what order, and if there are some channels that are not being used at all, or some that are causing issues.


Aaron Harrison Wed, 07/14/2010 - 08:03
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Generally each end of the connection (i.e. CCM or the SP) will assign channels sequentially.

So CCM will start with channel 1, then if channel 1 is busy, the next call will use channel 2.

SP in that example woudl start with channel 24 (T1) or 30 (E1) and would next use the next lower numbered e.g. 23 or 29 if the end channel is used.

What you find is that on a busy circuit, calls are all random lengths - so for example if  12 channels are in use, and the caller on the 4th channel ends the call, then CCM will use channel 4 next before channel 13.

So it appears random, but it's fairly logical - it's actually the people that behave randomly by talking for different lengths of time. Don't get too hung up on it :-)


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rcoote5902_2 Wed, 07/14/2010 - 08:11
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Thanks for the explanation Aaron, that makes sense and I sort of assumed that's how it worked.

That being said, the secretaries at this site insist that people calling in are reporting busy signals, which shouldn't happen if utilization is as low as it is.  I just don't know how to try and isolate a bad channel without busying them all out (except for one) and testing manually.  I figured if there was some way I could see historical usage/calls by channel it might help identify where the issue (if there is one) exists.

Aaron Harrison Wed, 07/14/2010 - 08:16
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Well - assuming you are running MGCP, all the PRI sig is backhauled to CCM.

I would enable detailed logging for the CCM service, and then pull logs for a period when busy signals are being reported (or just sample a few hours).

Drop the traces into translatorx and select the 'calls with abnormal disconnect' option from the dropdown- this might give you a quick view of any problem calls (i.e. terminations for non 16 cause codes).




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