For µ-law: 'a=gpmd:0 vbd=yes;ecan=on'
When this parameter is set to 1, 2, or 3, the parameter
FaxTransportMode is ignored.
When this parameter is set to 0, T.38 might still be used without the
control protocol's involvement. To completely disable T.38, set
FaxTransportMode to a value other than 1.
This parameter can also be configured per IP Profile (using the
IPProfile parameter).
For more information on fax transport methods, see 'Fax/Modem
Transport Modes' on page 147.
[HandleG711asVBD]
Enables the handling of G.711 as G.711 VBD coder.
[0] = (Default) Disable. The device negotiates G.711 as a regular
audio coder and sends an answer only with G.729 coder. For
example, if the device is configured with G.729 and G.711 VBD
and “regular” G.711 coders, it sends an SDP answer containing only
the G.729 coder.
[1] = Enable. The device assumes that the G.711 coder received in
the INVITE SDP offer is a VBD coder. For example, if the device is
configured with G.729 and G.711 VBD coders and it receives an
INVITE with an SDP offer containing G.729 and “regular” G.711
coders, it sends an SDP answer containing G.729 and G.711 VBD
coders, allowing a subsequent bypass (passthrough) session if
fax/modem signals are detected during the call.
Note: This parameter is applicable only if G.711 VBD coder(s) with
regular G.711 payload types 0 or 8 are configured for the device (using
the CodersGroup parameter).
[FaxVBDBehavior]
Determines the device's fax transport behavior when G.711 VBD coder
is negotiated at call start.
[0] = (Default) If the device is configured with a VBD coder (see the
CodersGroup parameter) and is negotiated OK at call start, then
both fax and modem signals are sent over RTP using the bypass
payload type (and no mid-call VBD or T.38 Re-INVITEs occur).
[1] = If the IsFaxUsed parameter is set to 1, the channel opens with
the FaxTransportMode parameter set to 1 (relay). This is required to
detect mid-call fax tones and to send T.38 Re-INVITE messages
upon fax detection. If the remote party supports T.38, the fax is
relayed over T.38.
Notes:
If VBD coder negotiation fails at call start and if the IsFaxUsed
parameter is set to 1 (or 3), then the channel opens with the
FaxTransportMode parameter set to 1 (relay) to allow future
detection of fax tones and sending of T.38 Re-INVITES. In such a
scenario, the FaxVBDBehavior parameter has no effect.
This feature can be used only if the remote party supports T.38 fax
relay; otherwise, the fax fails.
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