 the following fields: - Message Type Indicator: this field indicates the type of message (RP-DATA-ACK RP, RP-ERROR); - Message Reference: this field contains a sequence number that associates the RP-DATA message and the RP-ACK or RP-ERROR response; - Originator Address: this field indicates the E.164 phone number of the SMS SC entity issuing the SMS message; - Destination address: this field indicates the E.164 phone number of the SMS SC entity receiving the SMS message. The RTP-DATA header contains SMS-SUBMIT, SMS-DELIVER or SMS-STATUS-REPORT messages from the SM-TL layer. The RP-ACK header is an acknowledgment of the receipt of RP-DATA header and contains the SMS-SUBMIT-REPORT or SMS-DELIVER- REPORT messages from the SM-TL layer. 10.1.3. SM-CL layer The header of the SM-LC layer contains the following fields: - Protocol Discriminator: this field indicates the nature of the data transmitted. It takes the value "SMS message"; - Transaction Identifier: this field identifies the number of the transaction; - Message Type: this field shows the type of message (CP-DATA, CP-ACK, CP-ERROR). 276 VoLTE and ViLTE The CP-DATA header contains the headers RP-DATA or RP-ACK and the associated messages of the SM-TL layer. The CP-ACK header is an acknowledgment of receipt of the message CP-DATA. 10.2. SMS over SGsAP 10.2.1. Functional architecture Short message service over SGsAP is a feature of the circuit-switched fallback (CSFB) mechanism which allows a mobile connected to a 4G network to use the services of the 2G/3G network in CS mode. The functional architecture of the short message service over SGsAP is described in Figure 10.3. Outgoing SMS IWMSC UTRAN Server Incoming SMS Figure 10.3. Functional architecture for SMS over SGsAP When attaching the mobile, the MME entity attaches the mobile to the MSC server entity, selected by the MME entity. The MME entity must perform a conversion of the tracking area identity (TAI) of the mobile on the 4G network in a location area identity (LAI) on the 2G or 3G network. SMS message is