Bandwidth Management Page 111 of 254Main Office Configuration for SRG200/400 Release 1.5 Configuration GuideBandwidth usage for Virtual Trunks is accumulated in its zone in order toblock calls that exceed the bandwidth availability in a specific zone.However, the amount of bandwidth that is required to complete a given callis not known until both call endpoints have negotiated which codec to use.The bandwidth used for calculating the usage of a Virtual Trunk call isdetermined by the preferred codec of the device that connects to the VirtualTrunk. If the device is an IP Phone, the bandwidth calculations use thepreferred codec of the IP Phone, based on the codec policy defined for thezones involved (that is, Best Bandwidth or Best Quality). Likewise, thebandwidth calculations use the preferred codec of the Voice Gateway MediaCard for connections between a circuit-switched device (for example, a PRItrunk) and a Virtual Trunk.Codec selectionFor every Virtual Trunk call, a codec must be selected before the media pathcan 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 selectedcodec 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 toselect the same codec on both nodes, it is essential to use the same codecselection algorithm on both nodes. Before the codec selection occurs, thefollowing conditions are met:• Each codec list contains more than one payload size for a given codectype (it depends on the codec configuration). Payload size must be set to30 msec for proper functionality between the CS1000 and the SRG.• Each codec list is sorted by order of preference (the first codec in the nearend’s list is the near end’s most preferred codec, the first codec in the farend’s list is the far end’s preferred codec).
