Hybrid ECHO using CLEARMODE

[Daniele Giordano]

Hi experts, we have implemented a TCCS (Transparent Clear Channel Signaling) between cisco as5400 and third party SIP device. The session is established using SIP and CLEARMODE codec and connects two PBX SIEMENS Hipath.

SIEMENS PBX <===> ISDN PRI <===> SIP DEVICE <===> SIP/CLEARMODE <===> CISCO AS5400 <===> SIEMENS PBX

All works fine. I've only one issue. When a call is originated or terminated from analog phone, the user hear echo.

Now, we know that CLEARMODE doesn't provide echo cancellation but the echo is very annoying.

Before IP link, two Siemens PBXs were connected via ISDN TDM Trunk and no echo occured.

We know that echo can be caused by network dealy but two sites are connected with a dedicated 100 Mbit/s full duplex fiber link.

This is the report of a fast ping test without any problems:

Ping statistics

        Packets: Sent = 54027, Received = 54027, Lost = 0 (0% loss)

Approximate round trip times in milli-seconds:

        Minimum = 0.1 ms, Maximum = 8.4 ms, Average = 0.7 ms

We have already tried to reduce volume level without succes.

Lower levels can be more problematic.

Do you have any suggestions for us and our customers?

The echo is clearly an hybrid echo.

Best regards.

[paolo bevilacqua]

You should not use clearchannel codec for voice traffic, because is not meant for that.

Use a voice codec instead.

[Daniele Giordano]

Thank you for the quick answer.

You have right, but unfortunately it is a forced choice.

The ISDN Q.931 setup message of Siemens PBX uses Unrestricted Digital Information Bearer Capability and I don't have the possibility to change SIEMENS configuration.

In this situation the third party SIP device sends an INVITE/SDP that contains only clearmode codec.

Now I've a doubt.

I could replace the third party SIP device with a cisco media gateway. In this way I would a cisco in both sites.

In this new scenario, can I establish a session using G.711 even if the bearer cap sent from SIEMENS is UDI?

Can G.711 transports UDI without problems?

Best regards.

[paolo bevilacqua]

No, G.711 cannot transport digital data reliably.

You can try have the cisco force a G.711, but I'm not sure if that is possible.

You should fix the PBX so that it requests a voice bearer for voice call.

[Daniele Giordano]

Thanks for the help.