AudioCodes Mediant 2000, Руководство пользователя

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Version 5.6 351 November 2008
SIP User's Manual 7. IP Telephony Capabilities
7.5.2 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):
„ T.38 fax relay (refer to ''Fax Relay Mode'' on page 351 )
„
Fax and modem bypass: a proprietary method that uses a high bit rate coder (refer to
''Fax/Modem Bypass Mode'' on page
352 )
„ NSE Cisco’s Pass-through bypass mode for fax and modem (refer to ''Fax / Modem
NSE Mode'' on page
353 )
„ Transparent: passing the fax / modem signal in the current voice coder (refer to ''Fax /
Modem Transparent Mode'' on page
354 )
„ 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
355 )
„
G.711 Transport: switching to G.711 when fax/modem is detected (refer to ''G.711 Fax
/ Modem Transport Mode'' on page
355 )
„ Fax fallback to G.711 if T.38 is not supported (refer to ''Fax Fallback'' on page 356 )
‘Adaptations’ refer to automatic reconfiguration of certain DSP features for handling
fax/modem streams differently than voice.
7.5.2.1 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:
„ Switching to T.38 mode using SIP Re-INVITE messages (refer to ''Switching to T.38
Mode using SIP Re-INVITE'' on page
351 )
„ 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
352 )
When fax transmission ends, the reverse switching from fax relay to voice is automatically
performed at both the local and remote endpoints.
You can change the fax rate declared in the SDP, using the parameter FaxRelayMaxRate
(this parameter doesn’t affect the actual transmission rate). In addition, you can enable or
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.
7.5.2.1.1 Switching to T.38 Mode using SIP Re-INVITE
In the Switching to T.38 Mode using SIP Re-INVITE mode, upon detection of a fax signal,
the terminating device negotiates T.38 capabilities using a Re-INVITE message. If the far-
end device doesn't support T.38, the fax fails. In this mode, the parameter
FaxTransportMode is ignored.