Aastra 9000i Series Administrator's Manual page 255

You can enable the IP phones to use a default "basic" codec set, which consists of the set of
codecs and packet sizes shown above;
or
you can configure a custom set of codecs and attributes instead of using the defaults (see
"Customized Codec Preference List" below).
Customized Codec Preference List
You can also configure the IP phones to use preferred Codecs. To do this, you must enter the
payload value (payload), the packetization time in milliseconds (ptime), and enable or disable
silence suppression (silsupp).
Payload is the codec type to be used. This represents the data format carried within the RTP
packets to the end user at the destination. The default payload setting is to allow all codecs. You
can set payload to use only basic codecs (G.711 u-Law, G.711 a-Law, G.729), or select from up to
14 codecs for the phones AND customize a codec preference list of up to 10 codecs. In the
Aastra Web UI, codecs 2 through 10 can be set to "None" if required (no codecs).
Ptime (packetization time) is a measurement of the duration of PCM data within each RTP
packet sent to the destination, and hence defines how much network bandwidth is used for
transfer of the RTP stream. You enter the ptime values for the customized Codec list in
milliseconds. (See table below).
Silsupp is used to enable or disable silence suppression. Voice Activity Detection (VAD) on the
IP phones is used to determine whether each individual packet contains useful speech data.
Enabling silsupp results in decreased network bandwidth, by avoiding sending RTP packets for
any frame where no voice energy was detected by the VAD.
You must enter the values for this feature in list form as shown in the following example:
payload=8;ptime=10;silsupp=on,payload=0;ptime=10;silsupp=off
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Note: The basic and custom codec parameters apply to all calls, and are
configured on a global-basis only using the configuration files or the
Aastra Web UI.
Note:
In the Aastra Web UI:
- setting Codec 1 to "All" ignores the packetization interval (ptime).
The packetization interval setting defaults to 30, which is the default for
all codecs.
- setting Codec 1 to "All" automatically sets all other codec preference
fields 2 through 10 to "None".
- setting Codec 1 to "Basic" and all other codec preferences in 2
through 10 to "None", forces the phone to use only the basic codecs as in
previous releases (G.711 u-law, G.711 a-law, and G.729). If you select an
additional codec to use in the codec preferences 2 through 10 fields, those
codecs are added to the list of Basic codecs for the phone to use.
41-001343-01 Rev 02, Release 3.2.2