Why is a transcoder getting involved?

From: cisco-voip-bouncespuck.nether.net [mailto:cisco-voip-bouncespuck.nether.net] On Behalf Of Dave Wolgast
Sent: Tuesday, March 04, 2008 12:18 PM
To: Cisco VOIP Newsletter - puck.nether.net
Subject: [cisco-voip] Why is a transcoder getting involved?

I have a semi-complex call flow which at some point, is invoking a transcoding resource.  The problem is, I am not expecting to need a transcoder in this situation, and it's exhausting my transcoder pool for calls that actually need them.

Call flow involves a 3rd-party H.323 IVR behind an IP-IP-GW configured as an H.323 GW with MTP in CUCM 4.1(3).  This was necessary because the IVR's only useful transfer method is H.450.2, and we were able to use the IPIPGW to convert that to something that CUCM could deal with.

PSTN --> MGCP PRI (3845 ISR) --> IP-IP-GW/hairpin (H323-to-H323) --> IVR --> IP-IP-GW --> IP Phone (SCCP)

I believe that all legs of this call are configured for g.711ulaw, including all dial-peers on the IPIPGW, which is why I don't know why the transcoder is getting invoked.

The one thing I noticed, when looking at what the transcode resources were doing, was the following:

#show sccp connections detailed

sess_id        conn_id     ;  call-id    codec  ;  pkt-period  ;  type         ;       bridge-info(bid, cid)   ;  mmbridge-info(bid, cid)

117552198    -      ;           352499  N/A     ;  N/A           ;  transmsp ;        All RTPSPI Calllegs   N/A

117552198   126766065   352500  g711u    20      ;         rtpspi         ;     (255454,352499)      ;   N/A

117552198    126766049  352498  g711u    20      ;         rtpspi         ;     (255453,352499)      ;   N/A

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Not sure if this is relevant or not.

If anyone has any insight as to where the transcoder is creeping in, or can give me a clue how to track it down better, I will appreciate it very much.

Thanks!!!
--
Dave Wolgast
Livonia, NY