Nortel Networks Remote Gateway 50 - Manual (Page 53)

Bandwidth Management Page 53 of 258
Main Office Configuration for Survivable Remote Gateway 50 Configuration Guide
zones involved (that is, Best Bandwidth or Best Quality). Likewise, the
bandwidth calculations use the preferred codec of the Voice Gateway Media
Card for connections between a circuit-switched device (for example, a PRI
trunk) and a Virtual Trunk.
Codec selection
For every Virtual Trunk call, 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 same codec
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 for proper functionality between the CS1000 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 preferred codec).
Codec selection algorithms
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