I.3 User plane interworking

29.1633GPPInterworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networksTS

I.3.1 Functionalities required in the IM-MGW for GTT support

An IM-MGW supporting GTT interworking between IMS and CS network shall support:

– Real-Time Text as specified in 3GPP TS 26.114 [104] for a Multimedia Telephony Service Indicator (MTSI) compliant IM-MGW; this includes:

– requirement to support ITU-T Recommendation T.140 [123] Text Conversation MIME media type and RTP-text transport as specified by IETF RFC 4103 [124] and ITU-T Recommendation T.140 [123] Text Conversation presentation coding;

– recommended support of redundancy coding variant specified in IETF RFC 4103 [124] for error resilience;

– PSTN based text telephony using ITU‑T Recommendation V.18 [4] or any of its specific sub-modes.

NOTE: One specific text telephone protocol of the ITU-T Recommendation V.18 standard, called "Baudot Code" (Baudot modulation at 45.45 baud), is particularly important for support of emergency calls in US.

I.3.2 Monitoring of text/modem signals on the CS side

When the IMS session is setup with text media (possibly combined with audio media), the MGCF thereby requests the IM-MGW to monitor the CS termination for possible receipt of text telephone signals, in order to detect whether the CS user wishes to transit from voice to text during a voice call. The IM-MGW shall be provisioned with the text telephone mode(s) for which the termination should be monitored. The presence of the text media in the IMS session is an explicit indication that monitoring of text telephone signals is required.

When monitoring for text telephony signals the IM-MGW should listen on the CS side voice line for text telephone signals, and shall not transmit modem tones from the CS side termination to IMS side termination until one of the following occurs:

1) If the PSTN terminal is a carrier-based device and transmits answer tone. The IM-MGW shall then resolve the text telephone mode according to V.18 procedures and then operate in that mode;

2) If the PSTN terminal is a carrier-less device, the end user starts sending characters such that these may be detected by the IM-MGW. The IM-MGW shall then detect the text telephone mode and then operate in that mode.

If the IMS terminal transmits text characters first the IM-MGW shall initiate a text connection toward the CS network and resolve the textphone type, so that communication can continue in text mode.

See ITU-T Recommendation V.18 [125] for a detailed description of those procedures.

I.3.3 Multiplexing between the CS and IMS streams

The IM-MGW should remain in speech mode until such time that text is detected from either user. The IM-MGW shall detect modem signals on the CS termination and decide based on this detection if the media received from the CS side is interworked to the speech or the text media stream on the IMS side. In the media direction towards the CS network, the IM-MGW needs to multiplex both media streams into the PCM signal.

The following procedures provide further information on the procedures to be followed by the IM-MGW, for normative behaviour the ITU-T Recommendation V.18 [125] shall be followed.

– Carrier text phone:

– After a carrier mode text connection is established, loss of carrier can be taken as the indication that the audio stream on the IMS termination shall be connected with the (PCM) interface of the line on the CS termination.

– When the text carrier reappears on the CS termination, or text is received from the text media stream on the IMS termination, the IM-MGW connects the text stream on the IMS termination with the (PCM) interface of the line on the CS termination.

– Carrierless text phone:

– When the V.18 modem detects text, the textphone CS termination stops feeding the audio stream of the IMS termination, and instead inserts the detected and T.140 converted characters into the text stream of the IMS termination. This mode is continued as long as characters keep coming from the PSTN textphone. When no more characters arrive, and no textphone signal is received within 1 second, the audio channel is again fed to the audio stream of the IMS termination. If new text comes from the V.18 side, the process is repeated.

– If text is received from the text stream of the IMS termination when V.18 is not actively receiving text, no media shall be applied to the IMS audio stream, and the characters are sent to the V.18 modem for transmission. When all text is transmitted and no more is received for two seconds, the audio channels are enabled again. Since the carrierless systems are one-way alternate transmission systems, transmission of characters is possible only in one direction at a time. Once started, reception is given priority. Characters received from text stream of the IMS termination while V.18 is busy receiving should be buffered (up to a reasonable limit).

I.3.4 Conversion between text/modem and Real-Time Text over RTP

The legacy PSTN textphone modes have limited character sets. For all legacy modes, the text received through the V.18 modem shall be converted if necessary in the RTP/T.140 format for the text stream on the IMS termination, according to the rules in ITU-T Recommendation. T.140 [123] and IETF RFC 4103 [124], and vice-versa.

Redundancy may be used as specified in IETF RFC 4103 [124]. Use or not of redundancy is derived from the text media description configured on the IMS termination (RTP/RED/T.140).