User`s manual

MediaPack
SIP User's Manual 220 Document #: LTRT-65408
8.3 Fax & Modem Transport Modes
8.3.1 Fax/Modem Settings
Users may choose to use one of the following transport methods for fax and for each
modem type (V.22/V.23/Bell/V.32/V.34):
 Fax relay: demodulation / modulation
 Bypass: using a high bit rate coder to pass the signal
 Transparent: passing the signal in the current voice coder
When the fax relay mode is enabled, distinction between fax and modem is not
immediately possible at the beginning of a session. The channel is therefore in ‘Answer
Tone’ mode until a distinction is determined. The packets being sent to the network at this
stage are T.38-compliant fax relay packets.
8.3.2 Configuring Fax Relay Mode
When FaxTransportMode = 1 (relay mode), then on detection of fax the channel
automatically switches from the current voice coder to answer tone mode, and then to
T.38-compliant fax relay mode.
When fax transmission has ended, the reverse switching from fax relay to voice is
performed. This mode switching automatically occurs at both the local and remote
endpoints.
Users can limit the fax rate using the FaxRelayMaxRate parameter and can enable/disable
ECM fax mode using the FaxRelayECMEnable parameter.
When using T.38 mode, the user can define a redundancy feature to improve fax
transmission over congested IP network. This feature is activated by
‘FaxRelayRedundancyDepth’ and ‘FaxRelayEnhancedRedundancyDepth’ parameters.
Although this is a proprietary redundancy scheme, it should not create problems when
working with other T.38 decoders.
Note: T.38 mode currently supports only the T.38 UDP syntax.
8.3.3 Configuring Fax/Modem Bypass Mode
When VxxTransportType= 2 (FaxModemBypass, Vxx can be one of the following:
V32/V22/Bell/V34/Fax), then on detection of fax/modem, the channel automatically
switches from the current voice coder to a high bit-rate coder (G.711 or G.726), as defined
by the user, with the FaxModemBypassCoderType configuration parameter.
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.