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CHAPTER15 Media

Mediant 1000 Gateway & E-SBC | User's Manual

■ 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

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:

■ 'Fax Transport Mode' (FaxTransportMode)

■ 'Vxx ModemTransportType' (VxxModemTransportType)

➢ To configure fax / modem transparent mode:

1. Open the Gateway General Settings page (Setup menu > Signaling & Media tab > Gateway

folder > Gateway General Settings), and then from the 'Fax Signaling Method' drop-down list

(IsFaxUsed), select G.711 Transport:

2. Click Apply.

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:

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AudioCodes E-SBC User Manual

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