In order to establish a circuit-switched or packet-switched connection, the terminal 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 terminal needs to perform an RRC connection setup procedure as shown in Figure 3.39 to establish a signaling connection. The procedure itself was introduced in Section 3.4.5 and Figure 3.18. The goal of the RRC connection setup is to establish a temporary radio channel that can be used for signaling between the terminal, 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.
If a circuit-switched connection is established as shown in Figure 3.39, the terminal sends a CM service request DTAP message (see Section 1.4.2) 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 (see Section 1.4.1). 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 via the attached TMSI or IMSI. This is done in a challenge and response procedure similar to GSM. In addition to the terminal authentication already known from GSM, a UMTS network has to authenticate itself to the user to protect against air interface eavesdropping with a false base station. Once the authentication procedure has been performed, the MSC activates the ciphering of the radio channel by issuing a security mode command. Optionally, the MSC afterwards assigns a new TMSI to the subscriber which, however, is not shown in Figure 3.39 for clarity.
After successful authentication and activation of the encrypted radio channel, the terminal then proceeds to inform the MSC of the exact reason of the connection request. The call control (CC) setup message contains among other things the telephone number (MSISDN)
Figure 3.39 Messaging for a mobile originated voice call (MOC)
of the destination. If the MSC approves the request, it returns a call proceeding message to the terminal and starts two additional procedures simultaneously.
At this point, only a signaling connection exists between the terminal and the radio network, which is not suitable for a voice call. Thus, the MSC requests the establishment of a speech path from the RNC via an RAB assignment request message. The RNC proceeds by reserving the required bandwidth on the Iub interface and instructs the Node-B to allocate the necessary resources on the air interface. Furthermore, the RNC also establishes a bearer for the speech path on the Iu(cs) interface to the MSC. As a dedicated radio connection was already established for the signaling in our example, it is only modified by the radio resource allocation procedure (radio link reconfiguration). The reconfiguration includes for example the allocation of a new spreading code as the voice bearer requires a higher bandwidth connection than a slow signaling connection. If the RNC has performed the signaling via the FACH (Cell-FACH state), it is necessary at this point to establish a dedicated channel and to move the terminal over to a dedicated connection. Figure 3.40 shows the necessary messages for this step of the call establishment.
Simultaneous to the establishment of the resources for the traffic channel in the radio network, the MSC tries to establish the connection to the called party. This is done for example via ISUP signaling to the gateway MSC for a fixed-line destination as described in Section 1.4. If the destination is reachable, the MSC informs the caller by sending call control ‘altering’ and ‘connect’ messages.
Figure 3.40 Radio resource allocation for a voice traffic channel
The establishment of a packet-switched connection is also called packet data protocol (PDP) context activation (see Figure 3.41). From the users’ point of view, the activation of a PDP context means getting an IP address in order to be able to communicate with the Internet or another IP network. Further background information on the PDP context activation can be found in Chapter 2. As shown for a voice call in the previous example the establishment of a packet-switched connection also starts with an RRC connection setup procedure.
Once the signaling connection has been established successfully, the terminal continues the process by sending an ‘activate PDP context request’ message via the RNC to the SGSN. As shown in the previous example, this triggers the authentication of the subscriber and activation of the air interface encryption. Once encryption is in place, the SGSN continues the process by establishing a tunnel to the GGSN which in turn assigns an IP address to the user. Furthermore, the SGSN requests the establishment of a suitable bearer from the RNC based on the QoS parameters (e.g. minimal bandwidth, latency, etc.) for the new connection which have been given to the SGSN at the beginning of the procedure in the ‘activate PDP context request’ message. However, these values can be modified by the SGSN or GGSN in the event that the user is not subscribed to the requested QoS or if the connection requires a different QoS setting. The establishment of the radio bearer is done in the same way as shown in Figure 3.39 for a circuit-switched channel. However, as the bearer for a packet- switched connection uses other QoS attributes, the parameters inside the messages will be different.
Figure 3.41 PDP context activation