TRAFFIC CASES: MS IN ACTIVE MODE

An MS is in active mode when there is a call (speech, fax or data), or a call set up procedure taking place.

Cases which activate an MS and cases when MS is in active mode

CALL FROM AN MS

This section describes what happens when a mobile subscriber wants to set up a voice call to a subscriber in the PSTN. Data and text message calls are described separately.

Call set-up MS to PSTN

1. The MS uses RACH to ask for a signaling channel.

2. The BSC/TRC allocates a signaling channel, using AGCH.

3. The MS sends a call set-up request via SDCCH to the MSC/VLR. Over SDCCH all signaling preceding a call takes place. This includes:

• Marking the MS as “active” in the VLR

• The authentication procedure

• Start ciphering

• Equipment identification

• Sending the B-subscriber’s number to the network

• Checking if the subscriber has the service “Barring of outgoing calls” activated

4. The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The RBS and MS are told to tune to the TCH.

5. The MSC/VLR forwards the B–number to an exchange in the PSTN, which establishes a connection to the subscriber.

6. If the B-subscriber answers, the connection is established.

CALL TO AN MS

The major difference between a call to an MS and a call from an MS is that in a call to an MS the exact location of the mobile subscriber is unknown. Therefore, the MS must be located using paging before a connection can be established. Below is the description of the call set-up procedure for a call from a PSTN subscriber to a mobile subscriber. A call from an MS to a mobile subscriber operates according to the same process, the only difference being that the GMSC is contacted by another MSC/VLR instead of by a PSTN node.

Call to MS from PSTN

1. The PSTN subscriber keys in the MS’s telephone number (MSISDN). The MSISDN is analyzed in the PSTN, which identifies that this is a call to a mobile network subscriber. Aconnection is established to the MS’s home GMSC.

2. The GMSC analyzes the MSISDN to find out which HLR the MS is registered in, and queries the HLR for information about how to route the call to the serving MSC/VLR.

3. The HLR translates MSISDN into IMSI, and determines which MSC/VLR is currently serving the MS. The HLR also checks if the service, “Call forwarding to C–number” is activated, if so, the call is rerouted by the GMSC to that number.

4. The HLR requests an MSRN from the serving MSC/VLR.

5. The MSC/VLR returns an MSRN via HLR to the GMSC.

6. The GMSC analyses the MSRN and routes the call to the MSC/VLR.

7. The MSC/VLR knows which LA the MS is located in. A paging message is sent to the BSC’s controlling the LA.

8. The BSC’s distribute the paging message to the RBS's in the desired LA. The RBS’s transmit the message over the air interface using PCH. To page the MS, the network uses an IMSI or TMSI valid only in the current MSC/VLR service area.

9. When the MS detects the paging message, it sends a request on RACH for a SDCCH.

10. The BSC provides a SDCCH, using AGCH.

11. SDCCH is used for the call set-up procedures. Over SDCCH all signaling preceding a call takes place. This includes:

• Marking the MS as “active” in the VLR

• The authentication procedure

• Start ciphering

• Equipment identification

12. The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The RBS and MS are told to tune to the TCH. The mobile phone rings. If the subscriber answers, the connection is established.