C.2 Routing of regular user calls

23.2263GPPGlobal Text Telephony (GTT)Release 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

Regular user calls that may contain text are routed to the CTM-SRF by a CAMEL service.

Text users are identified by a Text Telephony CAMEL Service Key (SK) and other CAMEL information stored in the HLR. The CTM part of this service modifies the Called Party Address (see following clauses). The call is routed through the CTM-SRF server and is then routed to the original destination.

C.2.1 Mobile Terminating calls

Figure C.2: Paths for routing of mobile terminating text calls with call path shown in thick arrows

The GMSC discovers that the user is a subscriber of the text telephony service, by the Camel Subscription Information information received from the HLR. The text Service Key is present and the Detection Point = Terminating_Attempt_Authorized.

The SCP with the text telephony service application is connected and the routing of the call through the CTM-SRF node is performed.

To control the activation of CAMEL service invocations an indication is sent from the CAMEL service via the CTM-SRF server back to the CAMEL service. This information is used by the CAMEL CTM-SCP service (2^nd invocation) to do nothing but just to continue the call.

The indication is carried in the Calling Party Category parameter of ISUP.

This ISUP parameter is supported in CAP V1. InitalDP, and Connect. The CTM-SRF node and the CAMEL CTM-SCP service form an integrated application, and this ISUP parameter handling is regarded internal application signalling.

Figure C.3: Mobile Terminating Text Call

For the support of text users, the CAMEL CTM-SCP service will return the textphone-number with a CTM-SRF server prefix in CAP_CONNECT. The GMSC creates a new O-BCSM for this CAMEL based forwarding leg. It uses the Destination Routing Address in CAP_CONNECT to do the routing and to send the ISUP_IAM. The routing is based on the CTM-SRF server prefix part.

The CTM-SRF server links in the CTM/textphone conversion function and extracts Text subscriber address, Nature of Address, and the Numbering Plan Indicator from the Destination Routing Address parameter. It stores the original CPC value in Additional Calling Party Number parameter. If Called Party’s Category is not received, then this information as such is encoded. It writes this information to the appropriate IAM parameters and routes the call to a GMSC. It could be the same GMSC as before or a different one.

The GMSC regards this incoming IAM as a new terminating call to a text subscriber. A second time a dialogue for the "same text call" to a CAMEL service is invoked. The service realises this fact and does nothing but connecting the call to the Text subscriber (Called Party Number, respectively Destination Routing Address has not been changed by the CTM-SRF server).

C.2.2 Mobile originating calls

Figure C.4: Mobile originating call routing overview

The following sequence chart shows the mobile originating traffic case.

Figure C.6: Mobile Originating Text Call

For normal originating calls the MSC finds the CAMEL text telephony Service Key in the VLR, together with information on the Detection Point = Collection of dialled digits. All non-emergency calls for the user will cause a connection to the text telephone service application in the CTM-SCP. The A-category value from the subscriber data in the VLR is used as CPC value.

The CAMEL service knows that it is an originating call. In this case it just forwards the Calling Party’s Category parameter untouched.

The CTM-SRF removes its own address digits from the Called Party Address. The CTM/Text telephone conversion function is inserted before the call is routed towards the B-subscriber.

C.2.3 Mobile to Mobile Calls

Two CTM-SRF servers are linked in, one on the originating leg and one on the terminating leg. The originating ISUP_IAM sent from the CTM-SRF server on the outgoing side is received by the GMSC as normal ISUP_IAM.