Version 5.2 341 September 2007
SIP User's Manual 7. Telephony Capabilities
7.2.2 Fax/Modem Transport Modes
The gateway supports the following transport modes for fax and each modem type
(V.22/V.23/Bell/V.32/V.34):
îš„ T.38 fax relay (refer to 'Fax Relay Mode' on page 341)
îš„ Fax and modem bypass: a proprietary method that uses a high bit rate coder (refer to
'Fax/Modem Bypass Mode' on page 342)
 NSE Cisco’s Pass-through bypass mode for fax and modem (refer to 'Fax / Modem
NSE Mode' on page 343)
îš„ Transparent: passing the fax / modem signal in the current voice coder (refer to 'Fax /
Modem Transparent Mode' on page 345)
îš„ Transparent with events: passing the fax / modem signal in the current voice coder
with adaptations (refer to 'Fax / Modem Transparent with Events Mode' on page 345)
îš„ G.711 Transport: switching to G.711 when fax/modem is detected (refer to 'G.711 Fax
/ Modem Transport Mode' on page 344)
îš„ Fax fallback to G.711 if T.38 is not supported (refer to 'Fax Fallback' on page 344)
‘Adaptations’ refer to automatic reconfiguration of certain DSP features to treat fax/modem
streams differently than voice.
7.2.2.1 T.38 Fax Relay Mode
In this mode, fax signals are transferred using T.38 protocol. T.38 is an ITU standard for
sending fax across IP networks in a real-time mode. The gateway currently supports only
the T.38 UDP syntax.
T.38 can be configured in the following two ways:
îš„ Switching to T.38 mode using SIP Re-INVITE messages (refer to 'Switching to T.38
Mode using SIP Re-INVITE' on page 341)
îš„ Automatically switching to T.38 mode without using SIP Re-INVITE messages (refer to
'Automatically Switching to T.38 Mode without SIP Re-INVITE' on page 342)
When fax transmission has ended, the reverse switching from fax relay to voice is
automatically performed at both the local and remote endpoints.
You can change the fax rate that is declared in the SDP using the parameter
FaxRelayMaxRate (this parameter doesn’t affect the actual transmission rate) and can
enable/disable Error Correction Mode (ECM) fax mode using the FaxRelayECMEnable
parameter.
When using T.38 mode, you can define a redundancy feature to improve fax transmission
over congested IP networks. This feature is activated using the
FaxRelayRedundancyDepth and FaxRelayEnhancedRedundancyDepth parameters.
Although this is a proprietary redundancy scheme, it should not create problems when
working with other T.38 decoders.
7.2.2.1.1 Switching to T.38 Mode using SIP Re-INVITE
In this mode, the terminating gateway on detection of a fax signal, negotiates T.38
capabilities using a Re-INVITE message. If the far-end gateway doesn't support T.38, the
fax fails.
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