98 Bandwidth Management
call is not known until both call endpoints have negotiatedwhich codec to
use. The bandwidth used for calculating the usage of a Virtual Trunk call
is determined by the preferred codec of the device that connects to the
Virtual Trunk. If the device is an IP Phone, the bandwidth calculations use
the preferred codec of the IP Phone, based on the codec policy defined for
the zones involved(that is, Best Bandwidth or Best Quality). Likewise, the
bandwidth calculations use the preferred codec of the VoiceGateway Media
Card for connections between a circuit-switcheddevice (for example, a PRI
trunk) and a Virtual Trunk.
Codec selection
For every Virtual Trunkcall, a codec must be selected before the media
path can be opened. When a call is set up or modified (that is, media
redirection), one of two processes occurs:
•The terminating node selects a common codec and sends the selected
codec to the originating node.
•The codec selection occurs on both nodes.
Each node has two codec lists: its own list and the far end’s list. In order to
select the same codec on both nodes, it is essential to use the samecodec
selection algorithm on both nodes. Before the codec selection occurs, the
following conditions are met:
•Each codec list contains more than one payload size for a given codec
type (it depends on the codec configuration). Payload size must be set
to 30 msec forproper functionality between the CS 1000 and the SRG.
•Each codec list is sorted by order of preference (the first codec in the
near end’s list is the near end’s most preferred codec, the first codec in
the far end’s list is the far end’s preferredcodec).
Codec selection algorithms
When the codec lists meet the aboveconditions, one of the following codec
selection algorithms selects the codec to be used:
•H.323 Master/Slave algorithm
•SIP Offer/Answer model
•Best Bandwidth codec selection algorithm
H.323 Master/Slave algorithm
In the case of a Virtual Trunkcall betweenNortel and third-par ty 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
Nortel Communication Server 1000
Main Office Configuration Guide forSRG 50
NN43001-307 02.02 Standard
Release 5.0 3 December 2007
Copyright© 2005-2007, Nor tel Networks
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