ASSIGNMENT#

Mobile communication

SHAHBAZ JADOON(SP12-BTN-010)

Discuss the call setup process in 2G and 3G Networks? Draw a model by yourself in which call setup process should be depicted (Step 1 to step N)?

Call setup process in 2G

How a Call Gets to a GSM Mobile PhoneThe call is routed through the telephone network to the closest MSC to the called GSM subscriber. Next, Gateway MSC checks with HLR, asking “Where is the GSM subscriber?” After that, the call is established to the actual MSC/VLR (Visiting MSC) either directly or through the fixed or international telephone network. Finally, the request for a mobile call is transmitted over all BTSs in the actual location area of the called GSM subscriber. Mobile recognizes its own identity, and the call begins.

Call from a GSM Phone to a GSM PhoneHow does it work for GSM, calling from mobile device to mobile device? Well, aia the radio path and the base station network a call request for a GSM subscriber is sent from a mobile phone to MSC/VLR). The MSC/VLR then collects authentication data from HLR (if such data has not been collected earlier). Typically, a bulk of such data is transferred, thus avoiding repeating authentication requests to HLR. Next, the MSC/VLR requests HLR of the actual location of GSM subscriber, and the call is established as described earlier.

Call SetupDifferent procedures are necessary depending on the initiating and terminating party:

o Mobile Originating Call MOC: Call setup, which are initiated by an MSo Mobile Terminating Call MTC: Call setup, where an MS is the called partyo Mobile mobile Call MMC: Call setup between two mobile subscribers; MMC thus consists of

the execution of a MOC and a MTC one after the other.o Mobile Internal Call MIC: a special case of MMC; both MSs are in the same MSC area,

possibly even in the same cell.

 

 Mobile Originating Call MOC

1. Channel Request: The MS requests for the allocation of a dedicated signaling channel to perform the call setup.

2. After allocation of a signaling channel the request for MOC call setup, included the TMSI (IMSI) and the last LAI, is forwarded to the VLR

3. The VLR requests the AC via HLR for Triples (if necessary).

4. The VLR initiates Authentication, Cipher start, IMEI check (optional) and TMSI Re-allocation (optional).

5. If all this procedures have been successful, MS sends the Setup information (number of requested subscriber and detailed service description) to the MSC.

6. The MSC requests the VLR to check from the subscriber data whether the requested service an number can be handled (or if there are restrictions which do not allow further proceeding of the call setup)

7. If the VLR indicates that the call should be proceeded, the MSC commands the BSC to assign a Traffic Channel (i.e. resources for speech data transmission) to the MS

8. The BSC assigns a Traffic Channel TCH to the MS

9. The MSC sets up the connection to requested number (called party).

Remark: This MOC as well as the MTC described in the following describes only the principles of an MOC / MTC, not the detailed signaling flow.

Call setup process in 3G

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.