Fax / Modem Transport Mode - AudioCodes Mediant 3000 User Manual

User's Manual
RTP method is used, whereby the device encapsulates the entire T.38 packet
(payload with all its headers) in the sent RTP. For T.38 over RTP, AudioCodes
devices use the proprietary identifier "AcUdptl" in the 'a=ftmp' line of the SDP. For
example:
v=0
o=AudiocodesGW 1357424688 1357424660 IN IP4 10.8.6.68
s=Phone-Call
c=IN IP4 10.8.6.68
t=0 0
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:
call parties, the device uses T.38 Over RTP.
15.2.2.2 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).
Version 6.8
If both T.38 (regular) and T.38 Over RTP coders are negotiated between the
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15. Media
Mediant 3000