Nortel VoIP Gateway, Configuration Manual

Page: 204 / 220

204 Fax/Modem Settings
P0606298 02
Fax/Modem Settings
You can choose to use for fax, and for each modem type (V.21/V.22/V.23/Bell/V.32/V.34), one of
the following transport methods:
• Fax relay mode (demodulation / remodulation, not applicable for Modem)
• Bypass (using a high bit rate coder to pass the signal)
• Transparent (passing the signal in the current voice coder)
When any of the relay modes are 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
decision is made The packets sent to the network at this stage are fax relay packets (The packets
can be either T.38-complaint, or FRF.11-based proprietary syntax, selected by setting the channel’s
configuration parameter UseT38orFRF11.)
Configuring Fax Relay Mode
When FaxTransportType= 1 (relay mode) and a fax is detected the channel automatically switches
from the current voice coder to answer tone mode, and then to fax relay mode. The
UseT38orFRF11 configuration parameter defines either T.38-compliant network packets or
proprietary FRF.11-based packets. The last mode is used mostly for backward-compatibility with
older software versions.
When fax transmission ends, the reverse is carried out, and fax relay switches to voice. This mode
switch occurs automatically, both at the local and remote endpoints.
Users can limit the fax rate using the FaxRelayMaxRate parameter and can enable/disable Error
Correction Mode (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.
When using FRF.11 mode, only “FaxRelayRedundancyDepth” parameter can be used.
Note: For interoperability with other Nortel Networks endpoints, use only T.38 fax relay.
Note: T.38 mode currently supports only the T.38 UDP syntax.