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Administer CODECs
The IP CODEC Set screen is used to establish an audio CODEC preference list, to associate
silence suppression, and to assign frame and packet size attributes to each CODEC. You can
specify up to 7 sets of different CODECs. By default, all the sets have one CODEC G.711
(µ-law) with no silence suppression and packet size 20ms in Communication Manager.
CODEC bandwidth usage
The bandwidth usage of the available CODECs help determine which CODEC(s) you
administer. The signaling information between the G150 Media Gateway and the C-LAN board
(CSI, SI, S8500, S8700, or S8710) or processor interface (S8300) consists of H.323 compatible
messages, which are exchanged over TCP/IP control links. The control links for the analog
phones are permanently established for the length of time that a phone is registered. There are
two to four additional control links, one that is a shared signaling connection for the analog
trunks, and one signaling connection for each digital trunk. With rough estimation, an active
signaling channel consumes about 3 Kbps bandwidth.
Calculating the amount of bandwidth that voice-encoding-over-IP requires is a little more
complex. The G150 Media Gateway offers a choice of CODECs from the G.711, G.723 (not
recommended), and G.729 family at the point of call registration. The media server selects a
particular CODEC at the point of call establishment.
Not including overhead, the CODEC bandwidth required for each call is as follows:
For G.711: 64 Kbps
For G.729: 8 Kbps
You multiply the bandwidth for each CODEC by the various packet sizes (expressed in ms) to
obtain the number of bits of payload per packet for various packet sizes.
Table 5: The Number of Bits of Payload per Packet for Various Packet Sizes
Packet "size"
10 ms
20 ms
30 ms
Number of bits of
payload/packet (G.711)
640
1280
1920
Administer CODECs
Number of bits of
payload/packet (G.729)
80
160
240
Issue 1 June 2005
57
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