Mediant 2000 SIP User’s Manual D. Fax and Modem Transport ModesVersion 4.4 203 July 2005Appendix D Fax and Modem Transport ModesD.1 Fax/Modem SettingsUsers can choose to use for fax, and for each modem type (V.22/V.23/Bell/V.32/V.34), one of thefollowing transport methods:• Fax relay mode (demodulation / remodulation, not applicable to Modem),• Bypass (using a high bit rate coder to pass the signal), or• Transparent (passing the signal in the current voice coder).When any of the relay modes are enabled, distinction between fax and modem is not immediatelypossible at the beginning of a session. The channel is therefore in “Answer Tone” mode until adecision is made The packets sent to the network at this stage are T.38-complaint fax relaypackets.D.1.1 Configuring Fax Relay ModeWhen FaxTransportType= 1 (relay mode), then when fax is detected the channel automaticallyswitches from the current voice coder to answer tone mode, and then to T.38-complaint fax relaymode.When fax transmission ends, the reverse is carried out, and fax relay switches to voice. Thismode switch occurs automatically, both at the local and remote endpoints.Users can limit the fax rate using the FaxRelayMaxRate parameter and can enable/disable ECMfax mode using the FaxRelayECMEnable parameter.When using T.38 mode, the User can define a redundancy feature to improve fax transmissionover congested IP network. This feature is activated by “FaxRelayRedundancyDepth” and“EnhancedFaxRelayRedundancyDepth” parameters. Although this is a proprietary redundancyscheme, it should not create problems when working with other T.38 decoders.Note: T.38 mode currently supports only the T.38 UDP syntax.D.1.2 Configuring Fax/Modem ByPass ModeWhen VxxTransportType=2 (FaxModemBypass, Vxx can be either V32/V22/Bell/V34/Fax), thenwhen fax/modem is detected, the channel automatically switches from the current voice coder toa high bit-rate coder, as defined by the User, with the FaxModemBypassCoderType configurationparameter.During the bypass period, the coder uses the packing factor (by which a number of basic coderframes are combined together in the outgoing WAN packet) set by the User in theFaxModemBypassM configuration parameter. The network packets to be generated and receivedduring the bypass period are regular voice RTP packets (per the selected bypass coder) but witha different RTP Payload type.When fax/modem transmission ends, the reverse is carried out, and bypass coder is switched toregular voice coder.
