User`s manual

H.323 User's Manual 8. Telephony Capabilities
Version 5.0 261 December 2006
8.2.1 Overview of In-Band DTMF Transport Types
The Mediant 1000 gateway controls the way DTMF digits are transferred in-band (as part
of the audio stream) using the DTMFTransport configuration parameter.
Note that this parameter is automatically updated in the first four methods described in the
previous section and therefore shouldn’t be changed, while, for the fifth method (in-band
DTMF) it should normally be set to Transparent.
The following four modes are supported:
 DTMFTransportType = 0 (Mute DTMF). In this mode DTMF digits are erased from the
audio stream and are not relayed to the remote side. Instead silence is sent in the
RTP stream.
 DTMFTransportType = 2 (Transparent DTMF). In this mode DTMF digits are left in the
audio stream and the DTMF relay is disabled.
 DTMFTransportType = 3 (RFC 2833 DTMF Relay). In this mode, DTMF digits are
relayed to the remote side using the RFC 2833 Relay syntax.
8.3 Fax and Modem Transport Modes
8.3.1 Fax / Modem Settings
Users may choose to use one of the following transport methods for fax and for each
modem type (V.21/V.22/V.23/Bell/V.32/V.34):
 Fax relay: demodulation / modulation
 Bypass: using a high bit rate coder to pass the signal
 Transparent: passing the signal in the current voice coder
When the fax relay mode is enabled, distinction between fax and modem is not
immediately possible at the beginning of a session. The channel is therefore in ‘Answer
Tone’ mode until a distinction is determined. The packets being sent to the network at this
stage are T.38-complaint fax relay packets.
8.3.2 Configuring Fax Relay Mode
When FaxTransportMode = 1 (relay mode), then on detection of fax the channel
automatically switches from the current voice coder to answer tone mode, and then to
T.38-compliant fax relay mode.
When fax transmission has ended, the reverse switching from fax relay to voice is
performed. This mode switching automatically occurs at both the local and remote
endpoints.
Users can limit the fax rate using the FaxRelayMaxRate parameter and can enable/disable
ECM fax mode using the FaxRelayECMEnable parameter.
When using T.38 mode, the user can define a redundancy feature to improve fax
transmission over congested IP network. This feature is activated by
‘FaxRelayRedundancyDepth’ and ‘FaxRelayEnhancedRedundancyDepth’ parameters.
Although this is a proprietary redundancy scheme, it should not create problems when
working with other T.38 decoders.
Note: T.38 mode currently supports only the T.38 UDP syntax.