Fax Fallback; Fax/Modem Bypass Mode - AudioCodes Mediant 2000 User Manual

SIP User's Manual

Dynamic Jitter Buffer Optimization Factor = 13
After a few seconds upon detection of fax V.21 preamble or super G3 fax signals, the
device sends a second Re-INVITE enabling the echo canceller (the echo canceller is
disabled only on modem transmission).
A 'gpmd' attribute is added to the SDP according to the following format:

For G.711A-law: a=gpmd:0 vbd=yes;ecan=on (or off, for modems)

For G.711 µ-law: a=gpmd:8 vbd=yes;ecan=on (or off for modems)
The parameters FaxTransportMode and VxxModemTransportType are ignored and
automatically set to the mode called 'transparent with events'.
To configure fax / modem transparent mode, set IsFaxUsed to 2.

12.2.2.3 Fax Fallback

In this mode, when the terminating device detects a fax signal, it sends a Re-INVITE
message to the originating device with T.38. If the remote device doesn't support T.38
(replies with SIP response 415 'Media Not Supported'), the device sends a new Re-INVITE
with G.711 VBD with the following adaptations:

Echo Canceller = on

Silence Compression = off

Echo Canceller Non-Linear Processor Mode = off

Dynamic Jitter Buffer Minimum Delay = 40

Dynamic Jitter Buffer Optimization Factor = 13
When the device initiates a fax session using G.711, a 'gpmd' attribute is added to the SDP
according to the following format:

For G.711A-law: a=gpmd:0 vbd=yes;ecan=on

For G.711 µ-law: a=gpmd:8 vbd=yes;ecan=on
In this mode, the parameter FaxTransportMode is ignored and automatically set to
'transparent'.
To configure fax fallback mode, set IsFaxUsed to 3.

12.2.2.4 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

Disables echo cancellation for modem

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 performed.
To configure fax / modem bypass mode, perform the following configurations:

IsFaxUsed = 0
Version 6.4 137
12. Media
November 2011