Cnetralized CCM call process design with two routers in central site, router A for WAN and toll bypass, router B has T1 PSTN access. My question is : If I setup DSP farm on the B router and do transcoder, remote office G.729 ip phone call to HQ G.711 device(like uone or unity running G.711), will that DSPfarm do transcoder? If it will, does each packet have to send to router B do transcode first, which apparently will increase latency and performance load? Right?
Yes, each packet will go to routerB. Actually, to make it more exact, the remote end device will be connected via a G.729 RTP session to routerB and the VM system will be connected via G.711 RTP session to routerB. This will increase latency on those calls and load on routerB, but as routerB is local to routerA, unless you are already red-lining, the extra hop should not make a difference. The fact that the converstaion has to be transcoded at all will have a bigger inpact, but still typically not enough to prevent it from working. Particularly in the primarily one-way communication with voicemail.
IP Phones are capable of either G711 or G729, and during the call setup both endpoints go through a compatibilities exchange to determine the required codec.
In this instance an IP phone at location A requires G729 across the WAN, and depending on the capabilities of the endpoint at the remote location will negotiate eith G711 or G729. If the remote device can support both codecs the media stream will be established using G729, and transcoding will not be necessary.
Unity (v4.01) is capable of supporting both compression formats, and therefore the mediastream from the remote office to the centralized voice-mail does not requires transcoding session. Unfortunately, I'm not clear as to whether earlier versions of Unity have this support.
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