Speech codec settings

The following parameters are defined for the supported speech codecs (CODEC):

Media subtype name (MEDIASUBTYPE)

• Number range 0 to 10.

Priority index (PRIORITYINDEX)

• Number range 0 to 99.

Octet-align (OCTET-ALIGN)

When this parameter is defined, octet-aligned framing is used according to RFC 3267 and when not defined, bandwidth efficient framing is employed. Relevant for AMR codecs only.

• Default: Parameter is defined.

Mode Set (MODE-SET):

Requested AMR mode set. Restricts the active codec mode set to a subset of all modes. For the AMR-NB the possible value is a space separated list of modes from the set: 0,...,7:

• 0 — AMR 4,75 kbit/s

• 1 — AMR 5,15 kbit/s

• 2 — AMR 5,90 kbit/s

• 3 — AMR 6,70 kbit/s (PDC-EFR)

• 4 — AMR 7,40 kbit/s

• 5 — AMR 7,95 kbit/s

• 6 — AMR 10,2 kbit/s

• 7 — AMR 12,2 kbit/s (GSM-EFR)

For the AMR-WB the possible value is a space separated list of modes from the set: 0,...,8:

• 0 — AMR-WB 6,60 kbit/s

• 1 — AMR-WB 8,85 kbit/s

• 2 — AMR-WB 12,65 kbit/s

• 3 — AMR-WB 14,25 kbit/s

• 4 — AMR-WB 15,85 kbit/s

• 5 — AMR-WB 18,25 kbit/s

• 6 — AMR-WB 19,85 kbit/s

• 7 — AMR-WB 23,05 kbit/s

• 8 — AMR-WB 23,85 kbit/s

The list of numbers shall be in ascending order, for example <parm name="MODE-SET" value="0 2 3 5 7"/>

If a mode is specified by the decoder, the encoder must abide by the request and must not use modes outside the subset. If not present, all codec modes are allowed for the session. Relevant for the AMR codecs only.

• Default: Parameter is not defined.

Mode Change Period (MODE-CHANGE-PERIOD)

Specifies a number of frame-blocks, N, that is the interval at which codec mode changes are allowed. The initial phase of the interval is arbitrary, but changes must be separated by multiples of N frame-blocks. If this parameter is not present, mode changes are allowed at any time during the session. Relevant for the AMR codecs only.

• Default: Parameter is not defined.

Mode Change Neighbor (MODE-CHANGE-NEIGHBOR)

The following two values are possible:

• 0 — Change between any two modes in the active codec mode set is allowed.

• 1 — Mode changes shall only be made to the neighboring modes in the active codec mode set.

Neighboring modes are the ones closest in bit rate to the current mode, either the next higher or next lower rate. Relevant for the AMR codecs only.

• Default: Parameter is not defined.

Ptime (PTIME)

The length of time in milliseconds represented by the media in a packet. The ptime may have values up to maxptime. However, FEC multiplied with ptime must not exceed 220. Possible values are multiples of the sampling intervals. 10 ms, 20 ms, and 30 ms sampling intervals are supported. This parameter has an affect for the encoding (uplink media) only.

• Default: Parameter is not defined. The following table shows the default ptime values used when ptime is not defined by this parameter.

Voice Activation Detection (VAD)

When this parameter is defined, VoIP Discontinuous Transmission (DTX) is enabled. That is, RTP packets are not sent during silent periods but:

• AMR generates Silence Description (SID) packets during inactivity with reduced packet frequency and

• iLBC, G.711, and G.726 codecs generate Comfort Noise (CN) frames. For that, CN codec needs to be defined.

This parameter is provided to maximise the number of concurrent VoIP calls per WLAN AP and reduce data traffic over 3G/HSPA. This parameter is not relevant for the G.729 codec that uses a built-in mechanism to reduce packets during the silent periods (Annex b).

• Default: Parameter is defined.

Annex b (ANNEXB)

When this parameter is defined, annex-b enhancements of IETF RFC 3555 are enabled. Relevant for the G.729 codec only.

• Default: Parameter is defined.

G.726 RFC Payload Format (G726RFCPAYLOADFORMAT)

When this parameter is defined, RTP payload format used for the G.726 codecs is in accordance with RFC 3551 and when not included, RTP payload format is in accordance with ITU-T I.366.2.

• Default: Parameter is not defined.

iLBC operating frame mode (MODE)

The iLBC operating frame mode (20 or 30 ms) that will be encapsulated in each RTP packet. Values can be 20 and 30 (where 20 stands for preferred 20 ms frame size and 30 stands for preferred 30 ms frame size). This parameter has an affect for the encoding (uplink media) only. If the parameter is not defined, or has been set to ‘0’, then the preferred mode is not advertised in the initial SDP offer. Relevant for the iLBC codec only.

• Default: Parameter is not defined.

Default values of the parameters for the codecs are presented in the following table. In the table, parameters that do not have a value associated and are recommended to be defined, are marked with ‘D’. Parameters that are not defined by default are marked with ‘ND’. A value with a ‘ND’ parameter is the default used since no value is provided by the parameter. Not applicable parameters for a certain codec have been grayed. The G.726 codec has a Media Subtype Name reserved for each of its data rates, that is, for 40, 32, 24, and 16 kbit/s.