Page 300 of 910 Codecs
553-3001-365 Standard 4.00 August 2005
Once 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:
1H.323’s Master/Slave algorithm
2Best Bandwidth codec Selection algorithm

H.323’s 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.
• 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 the
second codec from its own list and repeats the search in the far-end list,
and so on.
• The Slave node uses the far-end list as the preferred list. The Slave
selects a codec from the far-end list and then searches in its own list to
find the common codec.
The issues caused by the Master/Slave algorithm are due to the random nature
of the Master/Slave information. The codec that is selected and used during a
virtual trunk call cannot be pre-determined. This can make bandwidth usage
calculations and bandwidth management difficult.