AudioCodes Mediant 800 - Fax;Modem Transport Modes; T.38 Fax Relay Mode

CHAPTER15 Media
Mediant 800 Gateway & E-SBC | User's Manual
■ Voice-band data (VBD) mode for V.152 implementation (see V.152 Support): fax/modem
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).
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 (see T.38 Fax Relay Mode)
■ G.711 Transport: switching to G.711 when fax/modem is detected (see G.711 Fax / Modem
Transport Mode)
■ Fax fallback to G.711 if T.38 is not supported (see Fax Fallback)
■ Fax and modem bypass: a proprietary method that uses a high bit rate coder (see Fax/Modem
Bypass Mode)
■ NSE Cisco’s Pass-through bypass mode for fax and modem (see Fax / Modem NSE Mode)
■ Transparent with events: passing the fax / modem signal in the current voice coder with
adaptations (see Fax / Modem Transparent with Events Mode)
■ Transparent: passing the fax / modem signal in the current voice coder (see Fax / Modem
Transparent Mode)
■ RFC 2833 ANS Report upon Fax/Modem Detection (see RFC 2833 ANS Report upon
Fax/Modem Detection)
‘Adaptations’ refer to automatic reconfiguration of certain DSP features for handling fax/modem
streams differently than voice.
T.38 Fax Relay Mode
In Fax Relay mode, fax signals are transferred using the T.38 protocol. T.38 is the 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 (see Switching to T.38 Mode using
SIP Re-INVITE)
■ Automatically switching to T.38 mode without using SIP Re-INVITE messages (see
Automatically Switching to T.38 Mode without SIP Re-INVITE)
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 'Fax Relay Max Rate' parameter
(FaxRelayMaxRate). The parameter does not affect the actual transmission rate. You can also
enable or disable Error Correction Mode (ECM) fax mode using the 'Fax Relay ECM Enable'
parameter (FaxRelayECMEnable).
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 'Fax Relay Redundancy Depth'
parameter (FaxRelayRedundancyDepth) and the 'Fax Relay Enhanced Redundancy Depth'
parameter (FaxRelayEnhancedRedundancyDepth). Although this is a proprietary redundancy
scheme, it should not create problems when working with other T.38 decoders.
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