SIP Voice Codec Settings

Fax Voip T.38 Modem supports different Audio Codecs for SIP. This is especially important in terms of some features, such as Voice Modem functionality. It is advisable to ask your SIP provider about supported audio codecs. At least one audio codec must be selected. You should understand that G711 fax mode can only operate when using G.711-ulaw or G.711-Alaw codecs. On the other hand, T38 fax mode allows to use any audio codec that is available in your SIP network. See also chapter T38 and G711 Fax Modes, subsection Voice Codecs requirements of this manual to learn more about fax modes and audio codecs.

In the left panel you can see the codecs available for use. The right panel shows codecs, selected by user. Codecs specified in the right panel can be overridden in the Outgoing Call Routing and Incoming Call Routing. Selected codecs can be arranged in preference order. By default the codec preference order only applies to outgoing calls. During incoming calls Fax Voip T.38 Modem use the preference order given by the remote side.

You can choose from the following Audio Codecs:

G.711-uLaw-64k (default)

G.711-ALaw-64k (default)

G.729* (for compatibility with T.38 only)

G.723.1

G.726-32k

GSM-06.10

iLBC

SpeexNB

SpeexWB

The following commands are available:

<Add> (or double-click selected entry in the Available codecs panel)

Use this button to add codecs for use with Fax Voip T.38 Modem. You must select one of the codecs in the Available codecs panel before using this button.

<Remove> (or double-click selected entry in the Selected codecs panel)

Use this button to remove codecs from the Selected codecs panel. You must select one of the codecs in the Selected codecs panel before using this button.

<Up> and <Down>

Use these buttons to change preference order for the selected audio codecs. You must select one of the codecs in the Selected codecs panel before using these buttons.

Note that you can also do any of the above actions by selecting the appropriate command from the context menu of the list.

Apply codec preference order to incoming calls

Check this option to ignore codec preference order of the other party and use Fax Voip T.38 Modem codec preference order during incoming calls. Note that this option is not in accordance with RFC3264. Nevertheless, we found it useful to add it. It can be used with the following set of codecs (selected): G.711-uLaw-64k, G.711-ALaw-64k, G.723.1, GSM-06.10. Note, that correct work of this option with any other codecs not garanteed.

Notes

• Dummy G.729* codec enables to report G.729 Audio Codec support for Fax T.38 Gateways that do not support G.711 and other supported audio formats; use this codec only for resolving compatibility issue. You are not able to use G.729* with Fax Voip T.38 Modem 'Voice Modem functionality'.
• Typically, if dummy G.729* codec is used, 'T38 Fax with Re-invite' mode for outgoing calls should be used too to ensure CNG tone generation. See in the 'T38 and G711 (audio) Fax Modes => Outgoing Fax Modes => T38 Fax with Re-invite' section of this User Guide.
• The Voice Codec Settings specified in this pane are only applied to SIP calls. To configure similar settings in the case of H.323, you should use the appropriate pane.