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AS5400 HPX one-way audio on TDM-IP call leg


Hi Experts, I've a strange issue and any help is appreciated.

I've used three AS5400 in different ways:
- H.323 gateway
- SIP gateway
- MGCP gateway controlled by PGW (current scenario)

I've loaded different IOS during years but in all these environments I had always same SPE version (np. and same issue.

In some calls my customers have one-way audio.

This problem is present only during incoming calls.
I've no NAT and IP routing is ok.
I've no codec negotiation problem.
The problem is indipended by customer CPEs. Is present with devices of different vendors (cisco ATA18x, Linksys SPA2102, Innovaphone, etc).

I've captured a problematic call with wireshark.

I notice two RTP flows, one from AS5400 to IP network and one in the opposite direction. 
RTP stream from IP device to AS5400 is correct (in wireshark you can listen the audio), but the RTP from AS5400 doesn't contain voice (wireshark plays only silence).

I've saved the audio trace and I've amplified the volume with Audacity.
After this manipulation you can hear the voice.

In summary:

- after many years I've changed the TDM provider without fix the problem

- the problem regards about 2% of both incoming and outgoing calls, so it is indipendent from direction of the call

- the audio issue is always in the call leg from TDM to IP side

- we observed this strange issue on all AS5400HPX gateway (3 different gateway with different IP address and bought in different period)

- a bidirectional RTP flow is always estabilished

- the problematic RTP flow from TDM to IP have always the same volume level: -24,4 dBm

- the wireshark audio trace shows only noise but if I increase the volume I can hear the voice (see original attachments)

- it happens on different DSP slot

So the problem is in the AS5400. Can be a configuration problem or an IOS - DSP bug?

Do you have any suggestions?

Best Regards.

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