Avaya Nortel Communication Server 1000 Manual page 226

226 Codecs
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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.
Known issues include:
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To solve the issues, another codec selection algorithm was needed. This
algorithm is called the Best Bandwidth codec Selection algorithm and is not
based on the unpredictable Master/Slave information.
The Best Bandwidth codec Selection algorithm is used for virtual trunk
calls between Nortel equipment, because any change to the Master/Slave
algorithm implies a change to the H.323 standard. The H.323 Master/Slave
algorithm is used when there is a virtual trunk call between Nortel equipment
and third-party equipment.
Best Bandwidth Codec Selection algorithm
The Best Bandwidth Codec Selection algorithm was implemented to
solve the issues caused by the H.323 Master/Slave algorithm. The Best
Bandwidth Codec Selection algorithm selects one common codec based on
two codec lists. With this algorithm, every time the selection is done using
the same two lists, the selected codec is always the same.
Copyright © 2003–2008, Nortel Networks
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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.
After an on-hold and off-hold scenario (that triggers Master/Slave
negotiation), the codec used for the restored call can be different than
the codec used before the call was placed on hold. The Master/Slave
information could have been changed when the call was on hold.
Because the terminating end of a call is always the Master, a call from
Telephone1 (Node1) to Telephone2 (Node2) can use a different codec
than a call from Telephone2 (Node2) to Telephone1 (Node1).
For tandem calls, the Master/Slave information is not relevant. That is,
the Master/Slave information is designed to be used only between two
nodes, not among three or more nodes. The Master/Slave algorithm
makes the codec selection for tandem calls more complex and inefficient.
Nortel Communication Server 1000
IP Line Fundamentals
NN43100-500 02.02 Standard
Release 5.5 4 February 2008