14.7 Global Text Telephony

23.2053GPPBearer-independent circuit-switched core networkRelease 17Stage 2TS

3GPP TS 23.226 [26] describes the high level architecture and functionality of GTT. When text based conversation is needed by a subscriber, the call is established with general call control functions like any other call. Within the call control transactions the UE indicates the need for text conversation (see 3GPP TS 24.008 [4]), which then requires actions in the core network where the pooling mechanism is chosen for GTT feature. This clause describes only the option where the CTM pool is provided in the Media Gateway in the Core Network.

MSC Server receives an indication by the UE about the need of text conversation, allocates terminations in MGW with CTM (Cellular Text telephony Modem) capabilities for the detection of CTM signals from radio access network. The default action of the call path in the CTM-detection/conversion function in MGW is to transfer audio transparently while monitoring for text telephone signals. When valid text telephone signals are detected, the converting action of the channel takes effect. The path converts between the detected CTM at the access termination and PSTN text telephony signalling methods on the network side. This mode of operation continues until text signalling ceases. Then transparent audio transport is re-established, again monitoring for text signals.

The CTM channel is created with Prepare bearer for Iu/ATM terminations, Prepare IP Transport for Iu/IP terminations,Reserve circuit procedure for A interface over TDM terminations or with Reserve RTP Connection Point for A interface over IP by including Cellular Text Telephone package properties. The core network bearer shall be established with default PCM codec to carry the T.140 protocol.

The outcome of CTM negotiation towards the CTM user in the user plane is reported in the CTM report procedure.

At release the MGW may report the number of bits of Global Text Telephony data in accordance with ITU recommendation T.140 sent for the call.