IMG 1010 - SIP Media Baudot Detection and retransmit Call Flow to TTY transcoding server.

Problem Description

In the e911 industry there is still a small segment of the population that uses TTY devices for the hearing impaired.  These devices use specific tones called Baudot tones to communicate letters and characters via key presses on the keyboard of the TTY device. Although this has changed in today’s world using such technologies as SMS, TTY devices are still in use.  One differing capability is that SMS is not assured delivery nor real time whereas a TTY call is similar to a voice call and is real time communication as if you were speaking.

Such type of calls are not frequent and use low port density TTY transcoder servers. Therefore the need to detect the type of call at the Media Gateway level and switch only Baudot calls to the TTY transcoder servers. 

Feature Description

E911 service providers could use the IMG 1010 with Baudot tone detection capability to tell their SIP application server when to route TTY call to TTY Transcoder servers and when to route calls to standard SIP Media Servers.

To allow this notification, the IMG 1010 would be responsible for sending a Re-INVITE to the Application Server, indicating that Baudot tones have been detected.

The Application server would then reply back to the IMG Re_INVITE by negotiating the media stream to the transcoder server.

This would allow TTY calls to be transferred to the transcoder server only when Baudot tones are detected thus solving the scale issue integrators could face.

The Baudot tone detection will be indicated by the custom R-URI parameter “tty-ind” with the SIP Re-INVITE.

INVITE sip:2125551212@dialog.2125551212.sip.example.com:5060;tty-ind SIP/2.0

Via: SIP/2.0/UDP 10.1.1.2:5060;rport;branch=z9hG4bK-a2f-1475867462-4993-484\r\n

Call-ID: 620a-406-97201619112-IMG224-0-10.1.1.2\r\n

CSeq: 1 INVITE\r\n

Max-Forwards: 70\r\n

The following shows the expected call flow.

1. An initial INVITE from IMG to SIP Server, any vocoder can be negotiated.

2. Upon detection of Baudot tone, the IMG shall send a Re-INVITE:

   1. SDP is the same as negotiated initially,

   2. R-URI will carry custom parameter “tty-ind”, indicating Baudot tone has been detected.

3. Application Server will send Re-INVITE to IMG

   1. SDP shall specify G711 codec in order to establish G711 voice path,

   2. SDP will specify information to establish media stream to the T.140/RFC4103 transcoder.

Note: The Baudot detection feature is available on PICASSO voice module only.

Note: The media received by the IMG, between the Baudot tone detection and the reception of the Re-INVITE from Application server, will not be sent to the T.140/RFC4103 transcoder.

The beginning of the TTY transmission may be lost if the caller starts sending TTY messages before the transcoder server connection is established. 

Configuration and Management

The Baudot tone detection feature is configured in the IP Bearer profile and SIP SGP profile panes.

1- ClientView → Profiles →  IP Bearer Profile →  Baudot Mode

This will enable the Baudot tone detection:

• Not supported (default) → The feature is disabled

• Detect → the feature is enabled and will trigger a Re-INVITE upon Baudot tone detection

2- ClientView → Profiles → SIP SGP → R-URI Header Tags

This will trigger the Re-INVITE with "tty-ind" in the R-URI header.

• none selected (default) → IMG will not trigger a Re-INVITE upon detection of Baudot tone.

• tty-ind → adding this tag to the R-URI header will trigger the Re-INVITE to a remote SIP User Agent (TTY transcoder).