I.4 MGCF and IM-MGW interactions

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

I.4.1 Introduction

This clause describes requirements for extensions to the Mn interface protocol in 3GPP TS 29.332 [15] needed to support the Interworking of Real-Time Text Telephony media in IMS with text/modem calls in CS domain.

I.4.2 Mn signalling interactions

I.4.2.1 Introduction

The following clauses describe the Mn interface procedures.

All message sequence charts in these clauses are examples.

I.4.2.2 H.248 Context Model

The H.248 context model depicted in figure I.4.2.2.1 shall be applied for interworking Real-Time Text media in IMS to text/modem in CS.

Figure I.4.2.2.1: H.248 Context Model for GTT support

The H.248 context contains one IP termination (IMS side) with two streams (one stream for Real-Time Text/RTP, another stream for audio/RTP), plus a TDM, AAL2 or IP termination (CS side) with one single stream carrying both voice via PCM and V.18 text telephony.

I.4.2.3 Specific Mn signalling for GTT

The following information shall be provided from the MGCF towards the IM-MGW:

– SDP with audio and Real-Time Text m-lines for the IMS termination;

– PCM (G.711) codec for the CS side termination.

This shall trigger the IM-MGW to insert an IWF to support RTT to PSTN text telephony conversion. If the topology is incoming (or bothway) from the CS network the IM-MGW shall monitor for text telephony and convert to RTT towards the IMS network. If the topology is outgoing (or bothway) to the CS network the IM-MGW shall multiplex RTT payload into the PCM stream.

I.4.2.4 IM CN subsystem originated session between the MGCF and IM-MGW

I.4.2.4.1 Initial INVITE with an SDP offer including Real-Time Text

Figure I.4.2.4.1.1: Example Mn signalling interactions for IMS originated session with audio and text

The Mn signalling interactions specified in clause 9.2.2.3 shall apply, with the following differences.

When reserving the IMS termination, the MGCF shall configure two streams; one for the audio and one for the Real-Time Text media line, with their respective media description (signal 3 in figure I.4.2.4.1.1). Monitoring of text telephony at the CS side termination is indicated by the inclusion of Real-Time Text media at the IMS side termination and by the topology being bothway or incoming from the CS side.

The text telephone mode(s) for which the CS side termination is monitored shall be provisioned in the IM-MGW.

I.4.2.5 CS network originated session

I.4.2.5.1 Initial INVITE with an SDP offer only including audio

Figure I.4.2.5.1.1: Mn signalling interactions for CS originated session – Initial INVITE with audio only

The Mn signalling interactions specified in clause 9.2.3.3 shall apply, with the following differences.

Upon receipt of a subsequent SDP offer from the IMS terminal including the audio and Real-Time Text media lines, the MGCF shall add an additional text stream to the IMS termination and configure it with the remote media description (signal 13 in figure I.4.2.5.1.1).

I.4.3 Mn Signalling procedures

I.4.3.1 Overview

This clause describes the logical signalling procedures (i.e. message identifiers are not part of the protocol) between the MGCF and IM-MGW. The procedures within this clause are intended to be implemented using the standard H.248 procedure as defined in ITU recommendation H.248.1 [2] with appropriate parameter combinations.

The procedures "Reserve IMS connection point" and "Configure IMS resources" and "Reserve IMS Connection point and configure remote resources" shall allow the configuration of two media streams; one for audio and one for Real-Time Text on the IMS termination. See clause 9.3.1.

Annex J (informative):
Call Flow for Customized Alerting Tone (CAT) interworking between CS network and IMS