From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband

3.8 UMTS CS and PS Call Establishment

To establish a circuit-switched or packet-switched connection, the mobile device has to contact the network and request the establishment of a session. The establishment of the user data bearer is then performed in several phases.

As a first step, the mobile device needs to perform an RRC Connection Setup procedure as shown in Figure 3.37, to establish a signaling connection. The procedure itself was introduced in Figure 3.16. The goal of the RRC connection setup is to establish a temporary radio channel that can be used for signaling between the mobile device, the RNC and a core network node. The RNC can choose either to assign a dedicated channel (Cell-DCH state) or to use the FACH (Cell-FACH state) for the subsequent exchange of messages.

Figure 3.37 Messaging for a mobile-originated voice call (MOC).

If a circuit-switched connection is established as shown in Figure 3.37, the mobile device sends a CM Service Request DTAP message (see Section 1.4) over the established signaling connection to the RNC that transparently forwards the message to the MSC. DTAP messages are exchanged between the RNC and the MSC via the connection-oriented SCCP protocol. Therefore, the RNC has to establish a new SCCP connection before the message can be forwarded.

Once the MSC has received the CM Service Request message, it verifies the identity of the subscriber ...