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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:

1 H.323’s Master/Slave algorithm

2 Best 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.

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