Mobile-originated circuit-switched calls
The steps to establish an MOC are as follows:
Step 1: RRC connection setup between UE and SRNC
Step 2: Authentication and ciphering
Step 3: Radio access bearer establishment and call setup
Step 4: Call and Iu release
Step 1: RRC connection setup between UE and SRNC. Figure 5-11 illustrates the interaction within UTRAN to establish an RRC connection between the UE and the RNC. The process to set up a call begins with the UE sending an RRC connection request over a CCCH (which is a RACH in the uplink direction). This message contains several information elements, including IMSI or TMSI, LAI, RAI, and the reason for requesting the RRC connection.
Figure 5-11 Step 1: RRC connection setup.
The RNC analyzes the reason for the request in order to decide the appropriate resources, i.e., dedicated or common. The RNC then initiates the process to establish an Iub bearer by sending the NBAP radio link setup message to Node B. This message contains information elements such as the transaction ID, communication ID, scrambling code, transport format set, and FDD-DL channelization code number. The Node-B acknowledges this message by sending an NBAP RL setup response. This message contains the information related to Transport Layer addressing information, i.e., AAL2 address. The SRNC uses ALCAP in the Transport Network Layer to establish an Iub bearer, using the information received from the Node B, i.e., AAL path and channel ID. The Iub bearer is bound together with the DCH assigned to the transaction. The SRNC then synchronizes the frame protocol (FP) connection by sending an FP downlink sync message. The RNC responds to the UE, indicating a successful RRC connection by sending an RRC connection setup message. This message contains information elements such as transport format, power control, and scrambling code. The UE responds with the RRC connection setup complete to confirm the RRC connection establishment.
Step 2: Authentication and ciphering. On successful connection setup with the RNC, the UE sends the RRC initial direct transfer message. This message is destined to the core network. However, the RNC processes this partially, adds some more information needed to set up a call and map it to the RANAP UE initial message. and sends it to the 3G MSC. The information elements within this message carry information on UE identity, location, and connection setup requirements. This message also indicates to the MSC and the RNC that a new signaling relationship between the UE and CN needs to be established.
On receiving the service request from the UE, the MSC initiates the security procedures. This includes the UE authentication and exchange of the encryption key. The MSC sends an authentication request within the RANAP direct transfer message. The RNC maps and forwards the authentication request message using RRC direct transfer to UE. The UE executes the authentication algorithm and sends the result back in an authentication response message to the MSC. As shown in Figure 5-12, this message is carried over as payload in the RRC direct transfer and RANAP direct transfer messages. The RNC merely acts as a relay. Assuming that the UE is successfully authenticated, the MSC then sends a security mode command to the RNC indicating that the further transactions between the UE and the UTRAN should be encrypted. The RNC in turn sends an RRC security mode command message to UE. The security mode command message conveys the encryption algorithm and the encryption and integrity keys.
Figure 5-12 Step 2: Authentication and ciphering.
The UE starts encrypting any further transaction toward UTRAN and informs the RNC, using a RRC security mode complete message. The RNC in turn informs the MSC. Note that encryption is applied only on the transaction between the UTRAN and the UE.
Step 3: Radio access bearer establishment and call setup. After the successful authentication and security procedures, the UE sends a call control setup message to the MSC. The MSC verifies that the UE is authorized for the requested services. If yes, the MSC starts a process to set up a bearer for the user data (speech in this case). This is achieved by the MSC by sending an RAB assignment request to the RNC (Figure 5-13). The MSC includes the RAB ID and the QoS parameters to be set up. The RNC, on receiving this message, checks the resources and sets up a bearer at Iu. The actual bearers are set up by using the ALCAP in the Network Transport Layer. The ALCAP procedures are not shown in the figure. The RNC in turn sets up a radio bearer between the RNC and the UE by sending a radio bearer setup message. This message contains the information on bearer allocation, i.e., a radio bearer identifier. The UE responds with the radio bearer setup complete message. The RNC then sends an RAB assignment response to the MSC. With this procedure successfully executed, there exists a bearer to transport used data from the UE to the MSC.
Figure 5-13 Step 3: RAB establishment and call setup.
From this point onward, the call proceeds in a normal way, using call control messages as in GSM call setup.
Step 4: Call and RAB release. Once the call is released by any of the parties, the resources need to be released. As shown in Figure 5-14, on receiving a disconnect message from the UE (in this example, the calling party releases the call) and transfer of subsequent call clearing messages, the MSC issues an Iu release command to the RNC. On receiving this message, the RNC releases the radio bearer over Iub interface and informs the MSC by sending an Iu release complete message. Now the RNC takes charge to clear the RRC connection by sending an RRC connection release message to the UE. The UE acknowledges with a connection release complete message.
The last action for the RNC is to clear the Iub interface resources. The procedure is illustrated in Figure 5-15. The MSC sends an NBAP radio link deletion message to the Node B. The Node B responds with a radio link deletion response message to indicate the release of Iub interface resources.
Figure 5-14 Step 4(a): Call clearing.
Figure 5-15 Step 4(b): Iu bearer release.
Mobile-originated packet-switched calls
In general, the steps defined in the previous section to establish a circuit-switched call are also followed to establish a packet-switched call. However, as one can understand, the procedures used are somewhat different.
Step 1: RRC connection setup between UE and SRNC. The same procedures are followed as in the case of a circuit-switched call except that the reason indicated in the RRC connection request message is a data call.
Step 2: Authentication and ciphering. The same procedures are followed as in the case of circuit-switched call except that the authentication and security procedures are invoked with the serving SGSN, as shown in Figure 5-16
Figure 5-16 Authentication and ciphering.
Figure 5-17 RAB and PDP context establishment.
Figure 5-18 PDP context deactivation and Iu resource release.
Step 3: Radio access bearer establishment and PDP context activation. As shown in Figure 5-17, the main difference in a packet-switched call is that the session management (SM) protocol is used instead of the call control protocol.
Step 4: PDP context deactivation and Iu release. Figure 5-18 shows the Iu release procedure when the UE deactivates the PDP context.
