AudioCodes Mediant 500 MSBR User Manual Download

Page: 167 / 1248

Version 7.2

167

Mediant 500 MSBR

User's Manual

17. Media



T.38 fax relay (see T.38 Fax Relay Mode on page 167)



G.711 Transport: switching to G.711 when fax/modem is detected (see G.711 Fax /
Modem Transport Mode on page 170)



Fax fallback to G.711 if T.38 is not supported (see Fax Fallback on page 170)



Fax and modem bypass: a proprietary method that uses a high bit rate coder (see
Fax/Modem Bypass Mode on page 171)



NSE Cisco’s Pass-through bypass mode for fax and modem (see Fax / Modem NSE
Mode on page 172)



Transparent with events: passing the fax / modem signal in the current voice coder
with adaptations (see Fax / Modem Transparent with Events Mode on page 173)



Transparent: passing the fax / modem signal in the current voice coder (see Fax /
Modem Transparent Mode on page 173)



RFC 2833 ANS Report upon Fax/Modem Detection (see RFC 2833 ANS Report upon
Fax/Modem Detection on page 174)

‘Adaptations’ refer to automatic reconfiguration of certain DSP features for handling
fax/modem streams differently than voice.

17.2.2.1 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 167)



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 168)

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.

17.2.2.1.1 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.

AudioCodes Mediant 500 MSBR, User Manual

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