Fax/Modem Transport Modes; Fax Relay Mode - AudioCodes Mediant 500L User Manual

User's Manual
any of the transport capabilities), fallback to existing logic occurs (according to the
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 187)

G.711 Transport: switching to G.711 when fax/modem is detected (see ''G.711 Fax /
Modem Transport Mode'' on page 190)

Fax fallback to G.711 if T.38 is not supported (see ''Fax Fallback'' on page 190)

Fax and modem bypass: a proprietary method that uses a high bit rate coder (see
''Fax/Modem Bypass Mode'' on page 191)

NSE Cisco's Pass-through bypass mode for fax and modem (see ''Fax / Modem NSE
Mode'' on page 192)

Transparent with events: passing the fax / modem signal in the current voice coder
with adaptations (see ''Fax / Modem Transparent with Events Mode'' on page 193)

Transparent: passing the fax / modem signal in the current voice coder (see ''Fax /
Modem Transparent Mode'' on page 193)

RFC 2833 ANS Report upon Fax/Modem Detection (see ''RFC 2833 ANS Report
upon Fax/Modem Detection'' on page 194)
'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 187)

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 188)
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
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-
Version 7.2
(FaxRelayRedundancyDepth)
187
and the 'Fax Relay
Mediant 500L Gateway & E-SBC
14. Media
Enhanced