We recently upgraded our primary voice gateway from an MGCP 2821 running 12.4(7e) to an H.323 3845 running 12.4(11)T2. The same port on the switch, same cables, and same PRI lines were used. Immediately afterwards, we began experiencing high jitter for outbound calls. Before it was 0 - 2 ms, now it is 14 - 16 ms on average. Call quality is still for the most part fine, however rarely a user will sometimes have a call cut in and out of silence. This seems to happen usually on conference calls. Pulling up their phone stats I see a high number of Rcvr Discarded packets (like 100+ per call) and a low MoS score of 2.00. On incoming calls, all stats are great and MoS is 4.5. G.711 is used in both directions.
This happens whether the phone is a 7940 or 7941. However, if I do the exact same call using IP Communicator, Jitter stats are much better. This I can't explain.
Also, I took the old 2821, converted it to H.323 and upgraded IOS to 12.4(11)T2, and placed it on the same switch. Its average jitter for an outbound call was 1 - 2 ms in all test calls.
The 3845 has 4 PVDM-64s installed, while the 2821 has 3 PVDM-32s. Ideas?
Probably the VAD algorithm wasn't detecting start and end of speech correctly.
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