Mobility Management and Call Processing

This section provides an introductory overview of mobility management (i.e., allowing a subscriber to roam) and call processing (the setting up and clearing down of calls) in GSM networks.

Mobility management entails keeping track of the MS while it is on the move. The mobility management procedures vary across three distinct scenarios, namely:

• MS is turned off

• MS is turned on but is idle

• MS has an active call

In the first scenario, when it cannot be reached by the network because it does not respond to the paging message, the MS is considered to be in the turned-off state. In this scenario, the MS obviously fails to provide any updates in relation to changes in Location Area (LA), if any exist. In this state, the MS is considered detached from the system (IMSI detached).

In the second scenario, the MS is in the ready state to make or receive calls. The system considers it attached (IMSI attached), and it can be successfully paged. While on the move, the MS must inform the system about any changes in LA; this is known as location updating.

In the third scenario, the system has active radio channels that are allowed to the MS for conversation/data flow. The MS is required to change to new radio channels if the quality of current channels drops below a certain level; this is known as handover. The MSC (sometimes BSC) makes the decision to handover an analysis of information that is obtained real-time from the MS and BTS.

All operations revolve around the three scenarios presented above. The rest of this chapter examines these operations in more detail, beginning with simple operations: paging, IMSI detach/attach. Following, more complex operations are presented, such as location update, call handover, mobile terminated call, mobile originated call, and mobile-to-mobile call.

Location Update

Location updating is the mechanism that is used to determine the location of an MS in the idle state. The MS initiates location updating, which can occur when:

• The MS is first switched on

• The MS moves within the same VLR area, but to a new LA

• The MS moves to a new VLR area

• A location updated timer expires

Mobile Terminated Call (MTC)

In the case of an MTC, a subscriber from within the PSTN dials the mobile subscriber's MSISDN. This generates an ISUP IAM message (it also could potentially be TUP as Level 4) that contains the MSISDN as the called party number. The ISDN (i.e., PSTN) routes the call to the GMSC in the PLMN, based on the information contained in the MSISDN (national destination code and the country code).

The GMSC then identifies the subscriber's HLR based upon the MSISDN and invokes the MAP/C operation Send Routing Information (SRI) towards the HLR to locate the MS. The SRI contains the MSISDN. The HLR uses the MSISDN to obtain the IMSI.

Because of past location updates, the HLR already knows the VLR that currently serves the subscriber. The HLR queries the VLR using the MAP/D operation Provide Roaming Number (PRN) to obtain the MSRN. The PRN contains the subscriber's IMSI.

The VLR assigns a temporary number known as the mobile station roaming number (MSRN), which is selected from a pool, and sends the MSRN back in an MAP/D MSRN Acknowledgement to the HLR.

The HLR then passes the MSRN back to the GMSC in a MAP/C Routing Information Acknowledgement message. To the PSTN, the MSRN appears as a dialable number.

Since the GMSC now knows the MSC in which the MS is currently located, it generates an IAM with the MSRN as the called party number. When the MSC receives the IAM, it recognizes the MSRN and knows the IMSI for which the MSRN was allocated. The MSC then returns the MSRN to the pool for future use on another call.

The MSC sends the VLR a MAP/B Send Information message requesting information, including the called MS's capabilities, services subscribed to, and so on. If the called MS is authorized and capable of taking the call, the VLR sends a MAP/B Complete Call message back to the MSC.

The MSC uses the LAI and TMSI received in the Complete Call message to route a BSSMAP Page message to all BSS cells in the LA.

Air interface signaling is outside the scope of this book.

Figure 12-9 shows the sequence of events involved in placing an MTC.

In Figure 12-9, the sequence of events involved in placing an MTC is as follows:

If the calling subscriber were in the same PLMN as the called party (internal MS-to-MS call), steps 2 and 3 would not be required.

Chapter 13 describes GSM-MAP operations in more detail. Appendix F, "GSM and ANSI MAP Operations," provides a list of GSM-MAP operations.