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The SBC supports Text over Internet Protocol (ToIP), including the inter-working between ToIP and the Text Telephone (TTY) protocols employed in Code Division Multiple Access (CDMA) wireless networks and wire-line VoIP networks (using Baudot).

The support is applicable to SBC 5400, 7000, SWe (virtual and cloud environments), and CNe.

The T.140 protocol supports the following codecs: AMR-WB, AMR-NB, EVS, G.711, G.722, and Opus.

The following interworkings and related transcodings are also supported:

Note
  • The SBC supports pass-through for all the text transmission protocols mentioned above.
  • The SBC performs T.140 to TTY/Baudot Interworking only when the call is transcoded. However, the SBC does not add DSP/transcoding resource just to perform text interworking.
  • The SBC does not voluntarily insert a "m=text" line, even though the t140Call flag is enabled on the egress trunk group. The SBC only relays the "m=text line", or interworks with TTY/Baudot, if the ingress SDP contains a valid "m=text" line.
Note

The following limitations apply:

  • Ribbon supports only 45.45 bps Baudot.
  • Maximum buffering for T.140 packets is approximately 30 seconds.
Note

To configure this feature, ensure that the t140Call parameter is set to enable, by using the following syntax:

% set profiles media packetServiceProfile <profile_name> flags t140Call enable

RTCP Generation For T.140 Pass-Through Calls

Based on PSP configuration, the SBC has the capability to generate RTCP for T.140 pass-through media streams if RTCP is not received from other peer. The interval for which the SBC monitors the RTCP packets for T140 stream from endpoints are also defined. If RTCP is not received within the interval, the SBC starts RTCP generation. 

For more information, refer to Packet Service Profile - CLI.

Text Transcoding Control

Users can control text transcoding using the flag g711T140BaudotAdapt.

Command Syntax

set profiles media packetServiceProfile <psp_name> flags g711T140BaudotAdapt <disable | enable>
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Command Parameters

Parameter

Default

Description

M/O

g711T140BaudotAdapt

Disable

When this flag is enabled, the SBC can introduce a DSP resource into the call path when the audio codecs are same and while the RTT to Baudot interworking is needed.

  • disable (default)
  • enable

Note

This flag only affects the behavior for G.711-G.711 calls for which one leg has T.140 (one m-line with T.140) with a valid port.
As well, the SBC will ignore this flag when the HDPreferred {*}Flag is enabled under PSP.

O

Configuration Example

set profiles media packetServiceProfile MY_PSP> flags g711T140BaudotAdapt enable
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Support for Issue: "Codec not supporting RTT with DSP insertion but no text adaptation"

In the following example, the Caller (UA-A) provides two streams in its SDP: one for the audio and one for the Real-Time Text media line. Their respective media description is also provided: audio G.729 is specified as the first codec in the intersected Pack Service Profile, and text with t140 subtype on the second stream, but the user could choose ANY codec whose RRT the SBC does not support). A list of codecs could follow G.729. When the Call Recipient rejects the T.140 (using port 0), the SBC knows that it must set the DSP to perform transcoding and that no adaptation can take place (no Baudot to/from T.140 adaptation is possible since G.729 does not support T.140).

For this scenario, the DSP is inserted even though there is no text adaptation. In order to have text adaptation, the offer will always start (first codec in the intersected PSP) with a codec for which the SBC supports T.140 (e.g. AMR, AMR-WB, etc.).