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Codec negotiation 99
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 vice versa) during the same call.
Algorithm details The H.323 Master/Slave algorithm operates in the
following manner:
•
The Master node uses its own codec list as the preferred one and finds
a common codec in the far end’s list. In other words, the Master gets the
first codec in its list (for example, C1), checks in the far end’s list if it is a
common codec; if it is, C1 is the selected codec. Otherwise, it gets the
second codec in its list and verifies it against the far end, and so on.
•
The Slave node uses the far end’s list as the preferred one and finds
in its own list the common codec.
Issues caused by the H.323 Master/Slave algorithm The issues
caused by the Master/Slave algorithm are due to the random nature of the
Master/Slave information. In other words, one cannot predetermine the
codec that is used dur ing a Virtual Trunk call.
The following are the issues associated with the H.323 Master/Slave
algorithm:
•
After an on-hold and off-hold scenario (which triggers Master/Slave
negotiation), the codec used for the restored call might be different than
the one used before on-hold, because the Master/Slave information
could have been changed.
•
When using Fast Start codec selection, a call from Telephone 1 (node1)
to Telephone 2 (node2) can use a different codec than a call from
Telephone 2 (node2) to Telephone 1 (node1), because the terminating
end is always Master.
•
For tandem calls, the Master/Slave information is not relevant. The
Master/Slave information is designed for use between two nodes only,
not between three or more nodes. It makes the codec selection for
tandem calls more complex and inefficient.
To solve the issues, another codec selection algorithm, not based on the
unpredictable Master/Slave information, is needed. Since any change to the
Master/Slave algorithm implies a change to the H.323 standard, the new
codec algorithm is used for Virtual Trunk calls between Nortel equipment.
SIP Offer/Answer model
The SIP codec negotiation is based on the Offer/Answer model with Session
Description Protocol (SDP).
Nortel Communication Server 1000
Main Office Configuration Guide for SRG 50
NN43001-307 02.02 Standard
Release 5.0 3 December 2007
Copyright © 2005-2007, Nortel Networks
.