RTP Payloads

[russell.sage]

I have a 3825 router acting as an E1 gateway. This is under Call Manager control using MGCP. The RTP packetization is configured for 10mS. Using a G.729 codec. My customers are complaining of "choppy/robotic" voice intermittently. When I analyse the RTP stream from the gateway I notice that the RTP payload changes from 20 bytes to 2.Am I correct to assume that the RTP payload should remain constant.

[thisisshanky]

On the phones, do you see the packetization as 10 or 20 ? Hit i twice to see the status of packets coming in and out of the phone

[russell.sage]

On the phones I see it set for 20. If I trace the call on CM and look at the MGCP CRCX message that has the p=20. What I don't get is that the packet size has to be a mutliple of the codec sample which has a minimum value of 10ms so the twenty value is just two samples per packet, so how come I get an RTP payload of 2 bytes? May raise a TAC. Thanks

[Chris Deren]

You are correct, the sample size should be constant. However with 10 ms sample interval you should be seeing 10 byte packets not 20 byte.

Is this on WAN? Make sure there is not Payload compression enabled.

Good reference link:

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml

Chris

[russell.sage]

Thanks for the confirmation. Question remains how I am getting RTP payloads of 2 bytes? Will continue to look. Thanks again.