Expert Documentation

Chapter 6
Alcatel-Lucent OmniPCX Office Communication Server only allows T.38 sessions over UDP,
see Ports
In order to ensure the reliability of the UDP transmission, the packets are sent several times to
ensure that the information reaches its destination. This operation is called "UDP
Redundancy". In order to reduce bandwidth use, an operation (framing) allows the
concatenation of packets of the same type: see Configuring H323 Gateway - Hardware
configuration - Configuration of T38 parameters for Fax over IP (optional)
As of R9.1, the Error Correction Mode (ECM) is supported for fax over IP using T.38. The ECM
procedure is an end-to-end process, that is to say it is managed by the fax machines involved
in the call. By default, the ECM mode is enabled, but it is possible to disable it through a
noteworthy address (disable_fax_ecm), refer to Public SIP Trunking - Configuration procedure
- Fax ECM .
Fax over G711
When Fax over G711 is selected, RTP voice channels are maintained but if current codec is
G729A or G723, it is renegotiated to G711.
Fax over G711 is not compatible with Voice Activity Detection (VAD) which is a mechanism
used to save bandwidth during VoIP calls. Therefore, the PCX supports the "silenceSupp"
attribute in the SDP part of the SIP message in order to dynamically disable VAD during the
Fax over G711 media session.
This attribute is used to request the other side to disable VAD. It works as follows:
- Case of an incoming fax call: the PCX detects the fax tone in the RTP stream and sends
Re-Invite to renegotiate the media session characteristics with "silenceSupp" attribute set
to "off" in order to ask the remote side to disable VAD
• If the PCX receives a response containing "silenceSupp" attribute set to "off", it means
that remote side has disabled VAD and then the PCX disables VAD also. This is the
normal mode of operation
• If the PCX receives a response which does not contain "silenceSupp" attribute set to
"off, then VAD remains active on the PCX side. In that case fax over G.711 is not
guaranteed when VAD on both ends is not disabled
- Case of an outgoing fax call: remote side (called side) should detect fax tone. The PCX
expects to receive a Re-Invite from the remote side with "silenceSupp" attribute set to "off"
• If the PCX receives a Re-Invite request containing "silenceSupp" attribute set to "off", it
means that remote side requests to disable VAD, then the PCX disables VAD and
sends a response containing "silenceSupp" attribute set to "off" to acknowledge the
request. This is the normal mode of operation
• If the PCX receives a request which does not contain "silenceSupp" attribute set to "off,
then VAD remains active on the PCX side. In that case, fax over G.711 is not
guaranteed when VAD on both ends is not disabled
Recommendation for deployment of Fax over G711:
- If remote side supports "silenceSupp" attribute => this is the expected mode of operation
- If remote side does not support "silenceSupp" attribute => disable VAD on both the remote
side and the PCX => Fax over G711 works but VoIP calls will not benefit from bandwidth
optimization
- If remote side does not support "silenceSupp" attribute and it is not possible to disable
VAD on remote side => fax over G.711 is not guaranteed as VAD on both ends is not
disabled
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