Avaya Nortel Communication Server 1000 Manual page 225

nodes. Each node has two codec lists: its own list and the far-end list. To
select the same codec on both nodes, it is essential to use the same codec
selection algorithm on both nodes.
For the codec selection, both the near- and far-end codec lists are retrieved:
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The following conditions are met before codec selection occurs:
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After the above conditions are met, a codec selection algorithm is used
to select the codec to be used for a call. There are two different codec
selection algorithms:
1. H.323 Master/Slave algorithm
2. Best Bandwidth codec Selection algorithm
H.323 Master/Slave algorithm
The codec selection algorithm proposed by the H.323 standard involves a
Master/Slave negotiation, 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/Slave negotiation is not known in advance, it is a random result: one
node could be Master then Slave (or Slave then Master) during the same
call.
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Copyright © 2003–2008, Nortel Networks
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The far-end list is not modified because it is already sorted when it is
received (in FastStart or TCS message).
The near-end list is sorted and then expanded to include lower payloads,
the same way it is done before sending the codec list in FastStart
message.
There are two codec lists:
— The near-end list is the codec list of the local unit.
— The far-end list is the codec list received from the far end.
Each codec list can contain more than one payload size for a given
codec type. The codec list depends on the codec configuration.
Each codec list is sorted by order of preference. The first codec in the
near-end list is the near-end most preferred codec and the first codec in
far-end list is the far end most preferred codec, and so on.
The Master node uses its own codec list as the preferred one. From the
far-end list, it finds the common codec.
The Master gets the first codec in its own list (Codec1). The Master then
checks the far-end list to see if Codec1 is a common codec (that is, is
Codec1 also listed in the far-end list). If Codec1 is common to both lists,
Codec1 becomes the selected codec. Otherwise, the Master obtains
Nortel Communication Server 1000
IP Line Fundamentals
NN43100-500 02.02 Standard
Release 5.5 4 February 2008
Codec selection 225