User`s manual

MP-1xx/H.323 User’s Manual 10. Appendices
Version 4.2 Beta 123 June 2003
Appendix C - DTMF, Fax & Modem Modes
C.1 DTMF Relay Settings
Users can control the way DTMF digits are transported to the remote endpoint, using the
DTMFTransport configuration parameter. The following four modes are supported:
DTMFTransportType= 0 (MuteDTMF). 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= 1 (Proprietry DTMF Relay). In this mode, DTMF digits are erased
from the audio stream and are relayed to the remote side using a proprietary RTP syntax.
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.
C.2 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 fax relay
packets. (The packets can be either T.38-complaint, or FRF.11-based proprietary syntax,
selected by setting the channel’s configuration parameter UseT38orFRF11.)
C.3 Configuring Fax Relay Mode
When FaxTransportType= 1 (relay mode), then on detection of fax the channel automatically
switches from the current voice coder to answer tone mode, and then to fax relay mode. The
UseT38orFRF11 configuration parameter defines either T.38-compliant network packets orr
proprietary FRF.11-based packets (the last mode should be used mostly for backward-
compatibility with previous software versions).
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 select between two protection strategies – redundancy
packets or forward error correction (FEC). This selection is made using the
T38FaxRelayProtectionMode configuration parameter. The User can also control a special
(proprietary) redundancy mode that was specially designed to improve protection against packet
loss using the EnhancedFaxRelayRedundancyDepth parameter. Although this is a proprietary
redundancy scheme, it is compatible with other T.38 decoders. When using FRF.11 mode, only
redundancy packets are supported. The depth of the redundancy in both protocols (that is, the
number of repetitions) is defined by the FaxRelayRedundancyDepth configuration parameter.
Note: T.38 mode currently supports only the T.38 UDP syntax.