AudioCodes Mediant 600 - Fax;Modem Operating Modes; Fax;Modem Transport Modes; Fax Relay Mode

SIP User's Manual 144 Document #: LTRT-83310
Mediant 600 & Mediant 1000
12.2.1 Fax/Modem Operating Modes
The device supports two modes of operation:

Fax/modem negotiation that is not performed during the establishment of the call.

Voice-band data (VBD) mode for V.152 implementation (see 'V.152 Support' on page
150): 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).
12.2.2 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 144)

G.711 Transport: switching to G.711 when fax/modem is detected (see 'G.711 Fax /
Modem Transport Mode' on page 145)

Fax fallback to G.711 if T.38 is not supported (see 'Fax Fallback' on page 146)

Fax and modem bypass: a proprietary method that uses a high bit rate coder (see
'Fax/Modem Bypass Mode' on page 146)

NSE Cisco’s Pass-through bypass mode for fax and modem (see 'Fax / Modem NSE
Mode' on page 147)

Transparent with events: passing the fax / modem signal in the current voice coder
with adaptations (see 'Fax / Modem Transparent with Events Mode' on page 148)

Transparent: passing the fax / modem signal in the current voice coder (see 'Fax /
Modem Transparent Mode' on page 148)

RFC 2833 ANS Report upon Fax/Modem Detection (see 'RFC 2833 ANS Report upon
Fax/Modem Detection' on page 149)
‘Adaptations’ refer to automatic reconfiguration of certain DSP features for handling
fax/modem streams differently than voice.
12.2.2.1 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 (see 'Switching to T.38 Mode
using SIP Re-INVITE' on page 145)

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 145)
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.