AudioCodes Mediant 1000B - Fax; Modem Transport Mode

User's Manual 192 Document #: LTRT-27055
Mediant 1000B Gateway & E-SBC
m=audio 6080 RTP/AVP 18 100 96
a=ptime:20
a=sendrecv
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
a=rtpmap:100 t38/8000
a=fmtp:100 T38FaxVersion=0
a=fmtp:100 T38MaxBitRate=0
a=fmtp:100 T38FaxMaxBuffer=3000
a=fmtp:100 T38FaxMaxDatagram=122
a=fmtp:100 T38FaxRateManagement=transferredTCF
a=fmtp:100 T38FaxUdpEC=t38UDPRedundancy
a=fmtp:100 AcUdptl
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
 AudioCodes Call Party with non-AudioCodes Party: The device uses the standard
T.38-over-RTP method, which encapsulates the T.38 payload only, without its headers
(i.e., includes only fax data) in the sent RTP packet (RFC 4612).
The T.38-over-RTP method also depends on call initiator:
 Device initiates a call: The device always sends the SDP offer with the proprietary
token "AcUdpTl" in the 'fmtp' attribute. If the SDP answer includes the same token, the
device employs AudioCodes proprietary T.38-over-RTP mode; otherwise, the
standard mode is used.
 Device answers a call: If the SDP offer from the remote party contains the 'fmtp'
attribute with "AcUdpTl", the device answers with the same attribute and employs
AudioCodes proprietary T.38-over-RTP mode; otherwise, the standard mode is used.
Note: If both T.38 (regular) and T.38 Over RTP coders are negotiated between the
call parties, the device uses T.38 Over RTP.
14.2.2.2 G.711 Fax / Modem Transport Mode
In this mode, when the terminating device detects fax or modem signals (CED or AnsAM),
it sends a Re-INVITE message to the originating device, requesting it to re-open the
channel in G.711 VBD with the following adaptations:
 Echo Canceller = off
 Silence Compression = off
 Echo Canceller Non-Linear Processor Mode = off
 Dynamic Jitter Buffer Minimum Delay = 40
 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.711 A-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 following parameters are ignored and automatically set to Events Only: