User`s manual

MediaPack SIP
MediaPack SIP User’s Manual 176 Document #: LTRT-65405
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-complaint
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
‘EnhancedFaxRelayRedundancyDepth’ 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, 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.