AudioCodes Mediant 800B - Fax;Modem Transport Modes; Fax Relay Mode

User's Manual 194 Document #: LTRT-10632
Mediant 800B Gateway & E-SBC
parameter IsFaxUsed).
14.2.2 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 (see 'T.38 Fax Relay Mode' on page 194)
 G.711 Transport: switching to G.711 when fax/modem is detected (see 'G.711 Fax /
Modem Transport Mode' on page
196)
 Fax fallback to G.711 if T.38 is not supported (see 'Fax Fallback' on page 197)
 Fax and modem bypass: a proprietary method that uses a high bit rate coder (see
'Fax/Modem Bypass Mode' on page
198)
 NSE Cisco’s Pass-through bypass mode for fax and modem (see 'Fax / Modem NSE
Mode' on page
199)
 Transparent with events: passing the fax / modem signal in the current voice coder
with adaptations (see 'Fax / Modem Transparent with Events Mode' on page
200)
 Transparent: passing the fax / modem signal in the current voice coder (see 'Fax /
Modem Transparent Mode' on page
200)
 RFC 2833 ANS Report upon Fax/Modem Detection (see 'RFC 2833 ANS Report upon
Fax/Modem Detection' on page
201)
‘Adaptations’ refer to automatic reconfiguration of certain DSP features for handling
fax/modem streams differently than voice.
14.2.2.1 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' on page
194)
 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
195)
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.
14.2.2.1.1 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 'Fax Transport Mode'
parameter (FaxTransportMode) is ignored.