Codec Selection Algorithms - Nortel SRG200 Configuration Manual

Page 112 of 254

Codec selection algorithms

NN43001-308 Standard 01.02
Bandwidth Management
When the codec lists meet the above conditions, one of the following codec
selection algorithms selects the codec to be used:
• H.323 Master/Slave algorithm
• Best Bandwidth codec selection algorithm
H.323 Master/Slave algorithm
In the case of a Virtual Trunk call between Nortel 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/
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 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 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 during a Virtual Trunk call.
May 2007