One-way audio PSTN call

[Mohd. Hidzwan Tamrin]

Hi,

As per title suggest, many would think this is an IP routing/ network issue. However, we have verified that the network is working fine.

The best thing it's only occurred at random time to random phones. It's kinda hard to simulate the issue as well, as we've tried do continuous test call but could not replicate the one-way audio scenario.

The call flow is like this;

Caller -> PSTN -> H.323 VG (PRI group) -> phone

Action take thus far:

1. Reload both Voice Gateways

2. Check with carrier, confirm by them not telco's level caused the problem because DTMF is working

3. Add the "enable voice rtp send-recv" command in global exec config.

4. Verified network was not the problem. RTP tx and rx packets were monitored

Please share your opinion or experience.

Regards,

Hidzwan

[Manish Gogna]

Hi Mohd,

If the issue is random and not easily reproducible you can check the following points first:

1) Check the cucm version and firmware versions of IP phones involved in the call.

2) Check the gateway IOS for any known bugs

3) Try to isolate the issue to a specific site / device pool / swicth / phone model / specific time of the day etc. If the issue is not possible to be narrowed down to any of the above then you will need to set up either continuous packet captures on IP phones / Voice VLAN and the gateway. This may not be easy in case of random issues but one way audio issues do require it for proper RCA.

4) Try upgrading the IP phone firmware and if possible the CUCM version to latest one ( for example if you are on cucm 10.5.1 then try upgrading to latest 10.5.2 available on cisco.com on the 10.x version ).

HTH

Manish

[Mohd. Hidzwan Tamrin]

Hi Manish,

Thanks for your feedback, please find my below  answers for each point

1) Check the cucm version and firmware versions of IP phones involved in the call.

    mohd : CUCM version 8.6.2.25900-8

              : affected phone load : SCCP42.9-3-1SR4-1S

2) Check the gateway IOS for any known bugs

   no open bugs can be relate to this event.

3) Try to isolate the issue to a specific site / device pool / swicth / phone model / specific time of the day etc. If the issue is not possible to be narrowed down to any of the above then you will need to set up either continuous packet captures on IP phones / Voice VLAN and the gateway. This may not be easy in case of random issues but one way audio issues do require it for proper RCA.

  What we know at the moment is, the issue only happened to one physical location where few Call Centre agents reside. The phone configuration as such Device Pool and Media Resource is using the similar config as the other phones at different sites. We haven't narrow down to switches level yet.

4) Try upgrading the IP phone firmware and if possible the CUCM version to latest one ( for example if you are on cucm 10.5.1 then try upgrading to latest 10.5.2 available on cisco.com on the 10.x version ).

 Upgrading the phone firmware is possible however, to upgrade the CUCM version is a bit challenging because our current platform is running on MCS. to Upgrade higher version would requires a virtualized setup altogether.

[Manish Gogna]

You can try upgrading the firmware to 9-4-X version for that site and also check for any MTP related alerts / errors in app logs if MTP is involved in the call. If the issue persists you may open a TAC case to investigate further through captures and traces.

HTH

Manish

[Mohd. Hidzwan Tamrin]

How can we possibly see if there's a problem with the MTP on the VG? Since i read somewhere saying that PSTN always use g.711 while IPT use g729. Could it be the transcoding resource are not sufficient during peak hours hence the call getting one way audio?

Another possibility I'm looking at is QOS on MPLS, but this one I need to sniff the call packet that coming from the phone and see on the jitter value.

Any opinion guys?

Thanks,

Hidzwan

[Manish Gogna]

MTP can be an issue but you need to check cucm traces to identify the same. Transcoder is needed if there is a codec mismatch between the two call legs , it can also be used in case there is no MTP in the MRG. However, you can try setting the region settings between cucm and gateway to g711 and also hardcode g711 on dial-peer to avoid any codec mismatch. If the issue with one way audio is only seen during peak hours then there is a high possibility of MTP leakage.

HTH

Manish

[Mohd. Hidzwan Tamrin]

double post