One-way audio - CUCM6.1

Unanswered Question
Feb 11th, 2009

We had the following connection:

PSTN --- Nortel Meridien PBX --QSIG--- Cisco 3845 ---- Checkpoint FW (Allow IP any any) ---- LAN --- IP Phone

When IP Phone A makes a call to a mobile phone @ PSTN, the mobile phone rings, but there is no ringback tone at the IP Phone. When the call is answered, the mobile phone can hear IP phone, but IP phone cannot hear mobile phone.

We verified IP connectivity between the Cisco 3845 and the IP Phone:

- Logon to phone's web page, at stream 1, it is connected to the 3845's loopback address, and the send/recv packet counters are increasing

- Sniff packets at Checkpoint FW, at both interface I can see rtp packets going to both directions.

- Did a debug voip rtp at the 3845, we can see rtp packets sent and recv'd.

3845 is MGCP controlled by the CUCM 6.1.

Any advise would be appreciated

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Nicholas Matthews Wed, 02/11/2009 - 20:05

Hi Dany,

A few things you can check (good work so far):

1) Try busying out the b-channel you're using on the 3845.

interface s0/0/0:23

isdn busy b_channel

See if the problem is specific to your B channel or not.

2) 'show voice dsp group all' and 'show voice dsp detail' - see if there are any errors or bad states

3) Check the input levels on 'show call active voice brief' on the Tele leg of the PRI. It's possible the problem is upstream from the 3845. Ex: i/0:0/0 dBm

This should change with the ringtone

4) If you have any spare DSPs around, try switching them out with the 3845. If you have extras in it, try taking some out (the first ones) and see if that makes a difference.

Hope this clarifies some options for you.

-nick

dany.datacraft Wed, 02/11/2009 - 23:30

Hi Nick,

Thanks for the options.

1) For outgoing call (IP Phone to PSTN or PBX), I can't seem to busy out the channel. For example, if i busy out channel 31, the outgoing call will still use channel 31.

So far our testings show that outgoing call to PBX phones has one-way audio intermittently, while calls to PSTN phones always have one-way audio.

Incoming calls from PSTN and PBX are also intermittent.

2) So far no errors or bad states on DSPs

3) Yes, it does change during a call. I'm attaching a capture logs.

4) We have a spare port as well, but the router is located remotely. Will probably try this as a last resort.

Nicholas Matthews Thu, 02/12/2009 - 05:37

Hi Dany,

You can see in the show call active voice brief this:

i/0:-25/-56 dBm

-25 doesn't appear to be changing on the input dB, which is what we're interested in. This points towards a problem on the PBX.

As well, if it's intermittent for PBX phones and constant for PSTN phones, this points towards the PBX as well. We are going to connect to the PBX exactly the same for both scenarios, and this sounds like the PBX should be investigated further.

This can be proven with a PCM capture, but a PCM capture requires a TAC case.

hth,

nick

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