Codec negotiation 59
Codec selection
For every Virtual Trunkcall, a codec must be selected before the media
path can be opened. When a call is set up or modified (that is, media
redirection), one of two processes occurs:
•The terminating node selects a common codec and sends the selected
codec to the originating node.
•The codec selection occurs on both nodes.
Each node has two codec lists: its own list and the far end’s list. To select
the same codec on both nodes, it is essential to use the samecodec
selection algorithm on both nodes. Before the codec selection occurs, the
following conditions are met:
•Each codec list contains more than one payload size for a given codec
type (it depends on the codec configuration).
•Each codec list is sorted by order of preference (the first codec in the
near end’s list is the near end’s most preferred codec, the first codec in
the far end’s list is the far end’s preferredcodec).
Codec selection algorithms
When the codec lists meet the aboveconditions, one of the following codec
selection algorithms selects the codec to be used:
•H.323 Master/Slave algorithm
•SIP Offer/Answer model
•"Best Bandwidth" codec selection algorithm
If a SIP trunk call is between a CS 1000 system and other third-party
gateway/SIP clients (forexample, MCS 5100), then the codec selection
does not guarantee that the same codec is selected for a call from endpoint
A to endpoint B and for a call from endpoint B to endpoint A. This different
codec selection makes it difficult for bandwidth management. However, calls
between two CS 1000 systems have the same codecselection decision
regardless of who originated the call.
H.323 Master/Slave algorithm
In the case of a Virtual Trunkcall betweenNor tel and third-party equipment,
the H.323 Master/Slave algorithm is used.
The codec selection algorithm proposed by the H.323 standard involves a
Master/Slave negotiation. This is initiated each time two nodes exchange
their capabilities (TCS message). The Master/Slave information decides
that one node is Master and the other node is Slave. The outcome of the
Master/Slavenegotiation is not known in advance; it is a random result. One
node could be Master then Slave (or vice versa) during the same call.
Nortel Communication Server 1000
Branch Office Installation and Commissioning
NN43001-314 01.02 Standard
Release 5.0 20June 2007
Copyright© 2007, Nor tel Networks
.