Patton electronic SmartNode 4110 Series manual No-Signal Retransmission, Fax bypass method

SmartWare Software Configuration Guide 47 • VoIP profile configuration

T.38 No-Signal Retransmission

Some SIP gateways change their port number when switching from audio to T.38. This behavior causes prob- lems if the SmartNode is located on the A-Side behind a NAT. Due to T.30 is a unidirectional. protocol and the B-Side is normally the initiator of the T.30 handshaking, the SmartNode never receives the initial packets of the B-Side because the NAT ports are not yet opened.

To open the NAT ports SmartWare no sends T.38 ‘no-signal’ packets when a codec change is detected. By default SmartNode sends 3 such packets. To adjust the number of ‘no-signal’ packets, use the following config- uration command.

Mode: Configure/profile voip

Step	Command	Purpose
1	[name] (pf-voip)[name]#fax nosig-	Sets how many times a T.38 ‘nosignal’ is retrans-
	nal-retransmission [1...5]	mitted. Default: 3

Fax bypass method

This command specifies the method for notifying the remote device that the RTP Stream has switched to a voice-band FAX transmission. This feature is only available on the SIP protocol. If the command option ‘v150- vbd’ is selected, a re-invite is sent even if the current voice coder is configured the same as the fax bypass coder. Furthermore the re-invite contains a gpmd-attribute line with the value ‘vbd=yes’ in the media description part. It signals the remote device of the new media transmission. If the command option ‘default’ is selected, the system behavior is the same as before.

For a fax transmission over a VoIP (SIP or H.323) network, the Cisco NSE standard uses events defined by RFC2833. These events are used for the setup of the fax transmission starting between the calling- and called- peer. Upon detecting a fax transmission, the called-peer issues NSE Event 192. NSE Event 192 indicates the data stream is via a voice band, and it forces the calling-peer to do two things—deactivate voice activity detec- tion and reconfigure the de-jitter buffer for data reception. The option ‘nse’ enables this fax transmission stan- dard.

Mode: profile voip

Step Command Purpose

1[name] (pf-voip)[name]#fax bypass- Specifies the fax bypass signaling method.

method {default v150-vbdnse} Default: default

Configuring fax failover

When using fax transmission in SIP, it is possible configure the SIP gateway to first try to use T.38 and to fall back to a high-rate codec, if the remote gateway does not support T.38. This can be configured as follows:

Mode: profile voip <pf-name>

Step	Command	Purpose
1	[name] (pf-voip)[pf-name]# fax	Define T.38 UDP as the first fax transmission
	transmission 1 relay t38-udp	method to try

VoIP profile configuration task list 588