G.711 Fax / Modem Transport Mode - AudioCodes Mediant 4000 SBC User Manual

CHAPTER 15 Media
■ Call Parties belong to AudioCodes Devices: T.38-over-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 the 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 the proprietary T.38-
over-RTP mode; otherwise, the standard mode is used.
If both T.38 (regular) and T.38 Over RTP coders are negotiated between the call parties,
the device uses T.38 Over RTP.

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
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Mediant 4000 SBC | User's Manual