Nortel Networks NN43001-314 - Manual (Page 57)

Codec negotiation 57
Nortel recommends configuring the same payload size for all codecs in
the same node.
SIP example
If a G.711 20ms codec and G.729 30ms codec are configured, then
codec negotiation uses the minimum payload size of 20 ms. That is, the
G.711 20ms codec and the G.729 20ms codec are used. Instead, Nortel
recommends that both G.711 and G.729 codecs be configured as 20ms.
When a G.729 30ms codec is configured, then the G.729 10ms/20ms/30ms
codecs are supported.
IP Peer Networking performs codec negotiation by providing a list of codecs
that the devices can support. Use CS 1000 Element Manager to configure
the list of codec capabilities. Refer to IP Peer Networking: Installation and
Configuration (NN43001-313) for instructions on configuring codecs.
The codec preference sequence sent over SIP/H.323 depends on the
bandwidth policy selected for the Virtual Trunk zone and the involved
telephones. For "Best Quality," the list is sorted from best to worst voice
quality. For "Best Bandwidth," the list is sorted from best to worst bandwidth
usage.
The G.711 codec delivers "toll quality" audio at 64 kbit/s. This codec is
optimal for speech quality, as it has the smallest delay and is resilient to
channel errors. However, the G.711 codec uses the largest bandwidth.
The G.729A codec provides near toll quality voice at a low delay. The
G.729A codec uses compression at 8 kbit/s. The G.729AB codec also
uses compression at 8 kbit/s.
The G.723.1 codec provides the greatest compression.
Payload default values need to be changed if the customer wants to
communicate with a third-party gateway that does not support the above
default payload sizes. Otherwise, IP Peer calls to or from the third-party
gateway are not successful.
If the payload sizes are set higher than the default values (for example,
to support a third-party gateway), then the local IP calls are affected by
higher latency. This is because the codec configuration applies to both IP
Peer calls and local IP (IP Line) calls.
Nortel Communication Server 1000
Branch Office Installation and Commissioning
NN43001-314 01.02 Standard
Release 5.0 20 June 2007
Copyright © 2007, Nortel Networks
.