I.2 Control plane interworking

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

I.2.1 General

Before text conversation can begin, a call shall first be established, from PSTN to IMS or vice-versa. The ISUP or BICC signalling to/from the PSTN indicates a "speech" call, without any GTT indication.

The IMS user may request a text connection from the beginning of a call, or add request for Real-Time Text media at a later stage in a call that was originally established with audio only.

I.2.2 Functionalities required in the MGCF for GTT calls support

In addition to the control plane interworking between SIP and ISUP or BICC (see clause 7), the MGCF shall support the negotiation of the Real-Time Text payload type (T.140 Text Conversation MIME media type as specified by IETF RFC 4103 [124]) in a distinct SDP m-line.

I.2.3 IM CN subsystem originated session

I.2.3.1 Initial INVITE with an SDP offer including a text media line

Figure I.2.3.1.1 shows an example call flow where the IMS terminal requests Real-Time Text telephony by sending an SDP offer including one audio line and one text media line within an initial INVITE message.

Figure I.2.3.1.1: IM CN subsystem originated session – Initial INVITE offering audio and text

Upon receipt of a SIP INVITE request offering text media (possibly combined with audio media) (signal 1 in figure I.2.3.1.1) the Interworking Node (MGCF) starts the call set-up at the CS side by sending an IAM requesting a speech with G.711 codec only or 3.1 Khz bearer (signal 2 in figure I.2.3.1.1), and completes the call setup on IMS and CS side using the procedures specified in clause 7, but returning an SDP answer including Real-Time Text media (possibly combined with audio media if audio media has been offered) (signal 3 in figure I.2.3.1.1).

The MGCF triggers the insertion an Interworking function in the IM-MGW for the duration of the call if a Real-Time Text media stream is established.

The MGCF reserves corresponding Real-Time Text media resources in the IM-MGW and activate the Interworking function, and if resources were available, return an SDP answer with audio and Real-Time Text.

I.2.4 CS network originated session

I.2.4.1 General

When starting the session setup signalling from a CS based network towards the IMS, the MGCF has no knowledge whether the CS side terminal supports and will accept to use text telephony.

I.2.4.2 Initial INVITE with an SDP offer including audio only

The MGCF offers only audio media when setting up a call towards the IMS and waits for IMS terminals desiring Real-Time Text media to send a new offer adding Real-Time Text media attribute prior to inserting an Interworking function in the IM-MGW. If the MGCF receives a new offer adding Real-Time Text media and applies signalling with OoBTC on the CS CN, the MGCF shall apply appropriate OoBTC procedures to only select G.711 audio codec on the CS call leg.

Figure I.2.4.2.1 shows an example call flow.

Figure I.2.4.2.1: CS originated session – Initial INVITE offering audio only

Upon receipt of an IAM request for a speech or 3.1 Khz audio call (signal 1 in figure I.2.4.2.1) the Interworking Node (MGCF) starts the call set-up at the IM CN subsystem side by sending an INVITE request (signal 2 in figure I.2.4.2.1) offering audio media applying the interworking procedures in Clause 7.

IMS terminals supporting GTT and configured to use it will send a new SDP offer including an audio and a Real-Time Text media line within a subsequent UPDATE or re-INVITE request.

When GTT interworking between IMS and CS networks is required, the MGCF shall reserve corresponding Real-Time Text media resources in the IM-MGW and thereby request the insertion of the Interworking function, and if resources were available, return an SDP answer with audio and Real-Time Text media attributes.

I.2.5 Subsequent SDP offer/answer exchange adding text to an audio session

If only audio media has been offered in the initial SDP offer, the IMS terminal can also request GTT support by sending a new SDP offer including audio and Real-Time Text when a SIP dialogue (early or confirmed) has already been established.

The MGCF shall then trigger the insertion of an Interworking function in the IM-MGW following the same principles as those specified in clause I.2.3.14. However, if OoBTC signalling is supported and a codec other than PCM (default G.711) has been selected at the CS network, then a codec modification is required to G.711. Otherwise No additional ISUP or BICC signalling towards the CS network is required.