SIP User's Manual 248 Document #: LTRT-65415
MediaPack Series
8.2.6 Fax and Modem Capabilities
This section describes the device's fax and modem capabilities, and includes the following
main subsections:
îš„ Fax and modem operating modes (see ''Fax/Modem Operating Modes'' on page 248)
îš„ Fax and mod
em transport modes (see ''Fax/Modem Transport Modes'' on page 248)
îš„ V.152 sup
port (see ''V.152 Support'' on page 253)
8.2.6.1 Fax/Modem Operating Modes
The device supports two modes of operation:
îš„ Fax/modem negotiation that is not performed during the establishment of the call.
îš„ Voice-band data (VBD) mode for V.152 implementation (see ''V.152 Support'' on page
253): fax/mod
em capabilities are negotiated between the device and the remote
endpoint at the establishment of the call. During a call, when a fax/modem signal is
detected, transition from voice to VBD (or T.38) is automatically performed and no
additional SIP signaling is required. If negotiation fails (i.e., no match is achieved for
any of the transport capabilities), fallback to existing logic occurs (according to the
parameter IsFaxUsed).
8.2.6.2 Fax/Modem Transport Modes
The device supports the following transport modes for fax per modem type
(V.22/V.23/Bell/V.32/V.34):
îš„ T.38 fax relay (see ''T.38 Fax Relay Mode'' on page 248)
îš„ G.711 Tran
sport: switching to G.711 when fax/modem is detected (see ''G.711 Fax /
Modem Transport Mode'' on page 250)
îš„ Fax fallback to G.711 if T.38 is not supp
orted (see ''Fax Fallback'' on page 250)
îš„ Fax and mod
em bypass: a proprietary method that uses a high bit rate coder (see
''Fax/Modem Bypass Mode'' on page 250)
 NSE Cisco’
s Pass-through bypass mode for fax and modem (see ''Fax / Modem NSE
Mode'' on page 251)
îš„ Tran
sparent with events: passing the fax / modem signal in the current voice coder
with adaptations (see ''Fax / Modem Transparent with Events Mode'' on page 252)
îš„ Tran
sparent: passing the fax / modem signal in the current voice coder (see ''Fax /
Modem Transparent Mode'' on page 252)
îš„ RFC 2
833 ANS Report upon Fax/Modem Detection (see ''RFC 2833 ANS Report
upon Fax/Modem Detection'' on page 253)
‘Adaptation
s’ refer to automatic reconfiguration of certain DSP features for handling
fax/modem streams differently than voice.
8.2.6.2.1 T.38 Fax Relay Mode
In Fax Relay mode, fax signals are transferred using the T.38 protocol. T.38 is an ITU
standard for sending fax across IP networks in real-time mode. The device currently
supports only the T.38 UDP syntax.
T.38 can be configured in the following ways:
îš„ Switching to T.38 mode using SIP Re-INVITE messages (see ''Switching to T.38
Mode using SIP Re-INVITE'' on page 249)
îš„ Automatically
switching to T.38 mode without using SIP Re-INVITE messages (see
''Automatically Switching to T.38 Mode without SIP Re-INVITE'' on page 249)
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