User`s manual
MP-1xx/H.323
MP-1xx/H.323 User’s Manual 124 Document #: LTRT-00651
C.4 Configuring Fax/Modem ByPass Mode
When VxxTransportType= 2 (FaxModemBypass, Vxx can be one of the following:
V32/V22/V21/Bell/V34/Fax), then on detection of Fax/Modem, the channel automatically switches
from the current voice coder to a high bit-rate coder, as defined by the User, with the
FaxModemBypassCoderType configuration parameter.
If relay is enabled for one of the modes (Fax/Modem), then the Answer Tone mode packets are
relayed as fax relay packets.
During the bypass period, the coder uses the packing factor (by which a number of basic coder
frames are combined together in the outgoing WAN packet) set by the User in the
FaxModemBypassM configuration parameter. 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.
When Fax/Modem transmission ends, the reverse switching, from bypass coder to regular voice
coder, is carried out.
Note: When Fax relay is enabled, V21TransportType must be set to disable (Transparent) mode.
C.5 Supporting V.34 Faxes
V.34 faxes don’t comply with the T.38 relay standard. AudioCodes therefore provides the optional
modes described under sections C.5.1 andC.5.2:
Note that the CNG detector is disabled (CNGDetectorMode=0) in all the following examples.
C.5.1 Using Bypass Mechanism for V.34 Fax Transmission
In this proprietary scenario, the Media Gateway uses a high bit-rate coder to transmit V.34 faxes,
enabling the full utilization of its speed.
Refer to the following configurations:
FaxTransportMode = 2 (Bypass)
V34ModemTransportType = 2 (Modem bypass)
V32ModemTransportType = 2
V23ModemTransportType = 2
V22ModemTransportType = 2
V21ModemTransportType = 2
In this configuration, both T.30 and V.34 faxes work in Bypass mode.
Or
FaxTransportMode = 1 (Relay)
V34ModemTransportType = 2 (Modem bypass)
V32ModemTransportType = 2
V23ModemTransportType = 2
V22ModemTransportType = 2
V21ModemTransportType = 2
In this configuration, T.30 fax uses T.38 Relay mode while V.34 fax uses Bypass mode.