User`s manual

SIP User's Manual 400 Document #: LTRT-83303
Mediant 1000 & Mediant 600
7.2.2.1.1 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 parameter
FaxTransportMode is ignored.
To configure T.38 mode using SIP Re-INVITE messages, set IsFaxUsed to 1. Additional
configuration parameters include the following:
 FaxRelayEnhancedRedundancyDepth
 FaxRelayRedundancyDepth
 FaxRelayECMEnable
 FaxRelayMaxRate
7.2.2.1.2 Automatically Switching to T.38 Mode without SIP Re-INVITE
In the Automatically Switching to T.38 Mode without SIP Re-INVITE mode, when a fax
signal is detected, the channel automatically switches from the current voice coder to
answer tone mode, and then to T.38-compliant fax relay mode.
To configure automatic T.38 mode, perform the following configurations:
 IsFaxUsed = 0
 FaxTransportMode = 1
 Additional configuration parameters:
• FaxRelayEnhancedRedundancyDepth
• FaxRelayRedundancyDepth
• FaxRelayECMEnable
• FaxRelayMaxRate
7.2.2.2 Fax/Modem Bypass Mode
In this proprietary mode, when fax or modem signals are detected, the channel
automatically switches from the current voice coder to a high bit-rate coder (according to
the parameter FaxModemBypassCoderType). In addition, the channel is automatically
reconfigured with the following fax / modem adaptations: disables silence suppression,
enables echo cancellation for fax and disables it for modem, and performs certain jitter
buffering optimizations. The network packets generated and received during the bypass
period are regular voice RTP packets (per the selected bypass coder), but with a different
RTP payload type (according to the parameters FaxBypassPayloadType and
ModemBypassPayloadType).
During the bypass period, the coder uses the packing factor, which is defined by the
parameter FaxModemBypassM. The packing factor determines the number of coder
payloads (each the size of FaxModemBypassBasicRTPPacketInterval) that are used to
generate a single fax/modem bypass packet.
When fax/modem transmission ends, the reverse switching, from bypass coder to regular
voice coder is carried out.