T.140 relay and Interworking to TTY functionality

Introduction

This page documents support for T.140 and TTY functionality across Ribbon SBC Core products (SBC 5400, SBC 7000, SBC SWe, and SBC CNe).

IMS defines a real-time text communication service using the ITU T.140 presentation protocol over RTP transport. This real-time text service (RTT), or Text Over IP (ToIP), is used for general communication purposes. RTT is also useful for the hearing and/or speech impaired since it is designed as a replacement for TDD (Telecommunications Device for the Deaf) devices.  

IETF RFC 4103 describes the formatting of real-time text conversation content within RTP packets. ITU T.140 is the real-time text presentation layer protocol, and is based on the ISO 10646-1 character set used by most IP text specifications. ITU T.140 utilizes the UTF-8 format, and operates character-by-character transmission (buffering is allowed to optimize bandwidth usage). Some countries, including the U.S., have mandated T.140 relay in mobile portions of their network. For NG911 services, it is recommended to support T.140 in conjunction with G.711, G.722, and Opus for interfacing with a PSAP.

Telecommunication services for the hearing and/or speech impaired use a Text Telephone (TTY) device, which is designed to transport real-time text over the PSTN using a range of low-speed modem technologies (as defined in annexes A – G of ITU V.18), including the 5-bit Baudot operational mode. Baudot tones are a common protocol in the U.S. for TDD. As the PSTN network has migrated to VoIP based technologies, TTY services are carried like other modem modulations. This involves detecting the Baudot modulation and falling back to the G.711 CODEC.

TTY service is also available on mobile/cellular telephone networks. CDMA networks use a TTY-based extension to the EVRC/EVRC-B codecs in order to transport the Baudot codes in-band through the encoder/decoder. 3GPP networks use AMR, AMR-WB, and EVS codecs, and transport text using T.140.

The interworking between the ToIP (T.140), CDMA TTY, and PSTN TTY services are performed in an interworking node that is hosted on a Ribbon MRFP or Ribbon SBC.

Interworking Support on Ribbon SBC Core

The Ribbon SBC supports the ToIP protocols Real Time Text (RTT) relay, along with the interworking between ToIP and the Text Telephone (TTY) protocols employed in Code Division Multiple Access (CDMA) wireless networks and wire-line VoIP networks. This allows the SBC/MRFP to act as a "Text Gateway" and adapts to different forms of text transport methods. 

The two kinds of supported interworkings are:

• For wire-line VoIP: the SBC/MRFP provides TTY modem tone detection and switching to the Voice Band Data (VBD) mode on the VoIP side, while providing the protocol translation between the 5-bit Baudot used on the wire-line VoIP bearer and the T.140/RFC 4103 protocols.
• For CDMA: the SBC/MRFP supports the TTY extension to the EVRC/EVRC-B codec and interworking to ToIP.

The following figure illustrates the possible forms of interworking (interworking between any of these permutations is supported):

SBC Support Across Various Releases

The following sections describe the support of the TTY RTT interworking across SBC Core releases. 

Even before the interworking support was added in the 8.2 release, the SBC supported the T.140 m= line relay.

Release 8.2 (SBC VNF only)

Feature Summary

The functionality added in this release is intended to support customers with interworking needs between mobile networks and non-mobile networks.

The support for interworking of T.140 to G.711 Baudot and EVRC TTY was added in the SBC SWe. A configuration is added to selectively enable the T.140 on a particular call leg.  

The following interworking scenarios are supported by the SBC:

1. Passthrough for T.140 and audio
2. Interworking of T.140 in AMR and AMR-WB codecs with Baudot over G.711
3. Interworking of T.140 to TTY over EVRC, EVRC-B
4. Mid call modification to add or remove m=text lines

There is a constraint that the SBC does not add the transcoding resource for completing T.140 interworking if the audio codecs in the call do not need transcoding. 

Configuration Details

G.711 Baudot is supported without any configuration. EVRC TTY is supported without any configuration.

Note that if the m= line is not supported (T.140 disabled), then the SBC will relay all the m= lines from ingress to egress with a port=0 (T.140 disabled). Please also note that the SBC relays all the m= lines from ingress to egress even if some of those streams have port=0.

Release 10.1.5 (SBC VNF and SBC 5000, SBC 7000)

Feature Summary

Features added in the 10.1.5 release support customers with interworking needs. Focus is given to transit carriers that may transit calls with a VoLTE endpoint but are otherwise not directly a mobile network.

The SBC is enhanced to support the T.140 to TTY interworking for additional codecs:

• G.711
• G.722
• OPUS

The following interworking scenarios are additionally supported by the SBC:

1. T.140 interworking within the same codec (G.711). For example: G.711-G.711 calls for which one leg has T.140 and the other has Baudot (m=text line rejected). The SBC adds the transcoding resource just for the T.140 interworking.
2. A mid-call introduction of text without codec change

Also, GW-GW support is added for RTT and supports both SBC-SBC and SBC-GSX scenarios. This allows the GSX to act as one leg of an interworked call where the GSX has PSTN trunking and requires TTY support while the SBC can get connected to a network supporting T.140.

Configuration Details

Limitations

• Audio pass-through with T.140/TTY interworking is not supported on the SBC. The call would have to get transcoded for the interworking to work.
  • Introducing a DSP for only T.140 to TTY interworking is supported using the g711T140BaudotAdapt configuration for G.711 codec alone.
• Call recording when using a T.140 stream is not supported. SIPREC does not support multiple streams.
• The SBC will disable the text stream (port=0) in the case of conflicts between a Payload type used by the audio stream and the Text stream (both using the same PT).
• 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.
• The SBC supports only 45.45 bps Baudot when interworking to/from T.140.
• Maximum buffering for T.140 packets is approximately 30 seconds.

Call Scenarios

Release 8.2

The following interworking scenarios are supported by the SBC in the 8.2 release:

• Passthrough for T.140 and audio
• Interworking of T.140 in any codec to Baudot over G.711
• Interworking of T.140 to TTY over EVRC
• Mid call modification adding or removing m=text lines

Some examples of call scenarios are listed below.

In the above scenario, both side codecs support RTT. Both parties also support m=text. Therefore, the SBC relays the RTT packets without any transcoding.

In the above scenario, the ingress Codec (AMR) supports RTT. However, the egress party rejects the m=text line and the selected codecs are different and need transcoding. Therefore, the SBC performs T.140 to Baudot interworking in the DSP.

In the above scenario, the codecs negotiated are AMR and EVRC. Therefore, the SBC performs interworking with the T.140 with EVRC inband TTY.

In the above scenario, the SBC supports interworking G.711 Baudot tones to the EVRC inband TTY.

In the above scenario, the m=text line is added in the re-INVITE that involves a codec change. Therefore, the SBC adds the DSP for the changed codecs to perform the transcoding and the RTT-to-Baudot interworking appropriately.

Note that the DSP addition will only occur if the codec change warrants transcoding (Release 10.1.4 supports addition of DSP for TTY interworking).

Release 10.1.5

T.140 support is added for G.711, G.722, and OPUS. The g711T140BaudotAdapt configuration flag is added for dynamic DSP insertion of text alone.

In the above scenario, the SBC introduces a DSP in the path for transcoding for the T.140 to Baudot interworking. This is possible only when the g711T140BaudotAdapt flag is enabled.

In the above scenario, the midcall modification Ingress re-INVITE does not make any codec change but m=text line is added. When the egress side rejects the text line, the SBC adds a DSP to the media path for interworking the T.140 with Baudot. This is possible only when the g711T140BaudotAdapt flag is enabled.

The following scenario is supported in the 10.1.5 release:

• GW-GW scenario

Interworking is also supported over GW-GW call flows.

In the GW-GW scenarios, the codec used in the GW-GW link will determine if interworking occurs on the ingress SBC or egress SBC.

The Caller (UA-A) provides two streams within its SDP: one for the audio and one for the Real-Time Text media line. The streams' respective media descriptions are also provided: an audio codec for the first stream such as AMR supporting RTT is specified as the first codec in the intersected PSP, and text with the T.140 subtype on the second stream.

As shown in the scenario above, if the egress SBC supports an in-band text audio codec (G.711 or EVRC/B), then the ingress SBC can reply with a 200OK with T.140 enabled and it must set the DSP to perform Baudot to/from T.140 adaptation.