SIP User's Manual 446 Document #: LTRT-12804
Mediant 800 MSBG
For G.711A-law: a=gpmd:0 vbd=yes;ecan=on
For G.711 µ-law: a=gpmd:8 vbd=yes;ecan=on
In this mode, the parameter FaxTransportMode is ignored and automatically set to
‘transparent’.
To configure fax fallback mode, set IsFaxUsed to 3.
8.3.7.2.4 Fax/Modem Bypass Mode
In this proprietary mode, when fax or modem signals are detected, the channel
automatically switches from the current voice coder to a high bit-rate coder (according to
the parameter FaxModemBypassCoderType). In addition, the channel is automatically
reconfigured with the following fax / modem adaptations:
Disables silence suppression
Enables echo cancellation for fax
Disables echo cancellation for modem
Performs certain jitter buffering optimizations
The network packets generated and received during the bypass period are regular voice
RTP packets (per the selected bypass coder), but with a different RTP payload type
(according to the parameters FaxBypassPayloadType and ModemBypassPayloadType).
During the bypass period, the coder uses the packing factor, which is defined by the
parameter FaxModemBypassM. The packing factor determines the number of coder
payloads (each the size of FaxModemBypassBasicRTPPacketInterval) that are used to
generate a single fax/modem bypass packet. When fax/modem transmission ends, the
reverse switching, from bypass coder to regular voice coder is performed.
To configure fax / modem bypass mode, perform the following configurations:
IsFaxUsed = 0
FaxTransportMode = 2
V21ModemTransportType = 2
V22ModemTransportType = 2
V23ModemTransportType = 2
V32ModemTransportType = 2
V34ModemTransportType = 2
BellModemTransportType = 2
Additional configuration parameters:
• FaxModemBypassCoderType
• FaxBypassPayloadType
• ModemBypassPayloadType
• FaxModemBypassBasicRTPPacketInterval
• FaxModemBypasDJBufMinDelay
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