[cisco-voip] H323/MGCP T.38 Issue & CCM Trace

[Brian Meade (brmeade)]

And just a little more detail on SDL analysis for everyone out there:

The process status is the status prior to that message being sent.  This is why the message is sent again to see if the previous message changed the process status.  The second time it says "SdlSig-S".  That means it is saving that message to the queue since the destination process isn't ready.  You'll normally later see the same line printed with "SdlSig-Q" where that message ends up being de-queued and tried to be sent again.

Brian

From: cisco-voip [mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Brian Meade (brmeade)
Sent: Thursday, October 17, 2013 8:55 AM
To: Daniel Pagan; cisco-voip at puck.nether.net
Subject: Re: [cisco-voip] H323/MGCP T.38 Issue & CCM Trace

And your trace interpretation is correct.  MediaExchange is sending a MXInterfaceOpenLogicalChannelIncoming message to MGCPInterface while MGCPInterface is currently in a state of waiting for an MGCP Response.

Brian

From: cisco-voip [mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Daniel Pagan
Sent: Wednesday, October 16, 2013 10:04 PM
To: cisco-voip at puck.nether.net<mailto:cisco-voip at puck.nether.net>
Subject: [cisco-voip] H323/MGCP T.38 Issue & CCM Trace

Folks:

Thanks ahead of time for reading this. I'm looking at an issue involving protocol based T.38 between an H323 fax server and MGCP gateway. The call starts as normal: The CRCX/200 contains the appropriate FXR package and caps, audio cut-through occurs successfully while first negotiating G711ulaw, the fax server sends CUCM an H245 mode request for T.38, a CLC on the existing channel and then a new OLC for T.38. CUCM ACKs the CLC but never ACKs the new OLC. Instead, after the new OLC is received, it appears MediaExchange is waiting for MGCP to proceed (I assume for a MDCX to the gateway w/ a new SDP media header... which I never see); immediately after the OLC is received, traces show "waitForMGCPResponse" for MGCPInterface and transactions for both CI's go completely cold. Only one node is processing this call - screenshot below.

[cid:image013.jpg at 01CECAB5.7B1B6F90]
Has anyone seen this before? CUCM not ACKing the OLC and never sending the appropriate MDCX to the gateway? The fax server is attempting to perform Fast Start at the beginning of the call, but not configured for inbound Fast Start support at the trunk level. I doubt this is related but figured it wouldn't hurt to mention.

Follow-up question: The "MGCPInterface" and "MediaExchange" processes listed above on the same trace line... The way I interpreted this entry, based on the fact that each corresponding process number is listed (1209 & 10771), is that MGCPInterface is receiving a request from the MediaExchange process. Is this description correct?

Thanks again.

- Daniel
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