Mobility Management

In order to make a mobile terminated call the GSM network should know the location of the MS, despite of its movement. For this purpose the MS periodically reports its location to the network using the Location Update procedure. The Location Update procedure is performed:
· When the MS has been switched off and wants to become active, or
· When it is active but not involved in a call, and it moves from one location area to another, or
· After a regular time interval.

Network Attachment
Network attachment is the process of selecting an appropriate cell (radio frequency) by the mobile station to provide the available services, and making its location known to the network. The process starts when the mobile station is switched on, and ends when the mobile station enters the idle mode. In idle mode the mobile station does not have a traffic channel allocated to make or receive a call, but the Public Land Mobile Network (PLMN) is aware of the existence of the mobile station within the chosen cell.

Network attachment process

The network attachment process consists of the following tasks:

· Cell identification
When a mobile station is switched on, it attempts to make contact with a GSM PLMN by performing the following actions:
Ø Measure the BCCH channels
Ø Search for a suitable cell
The mobile station measures the signal strength of the BCCH (Broadcast Control Channel) channels received. It stores in list information about 30 of these BCCH channels, such as the signal strength and the frequency corresponding to these BCCH channels.

· PLMN selection
A suitable PLMN is chosen.

· Cell selection
Cell selection is the process of selecting an appropriate cell (radio frequency) by the mobile station to provide the available services.

· Location update
In order to initiate a call or to receive a call, the mobile station tunes to the control channel (BCCH plus CCCH) of the chosen cell. Then, it registers its presence in this cell (registration process) by means of a location updating procedure.

No suitable cell found
If the mobile station is unable to find a suitable cell to access, it attempts to access a cell irrespective of the PLMN identity, and enters a "limited service" state in which it can only attempt to make emergency calls.

PLMN selection mechanism
The particular PLMN to be contacted can be selected either in one of the following modes:
· Automatic mode
In automatic mode, the mobile station will choose which PLMNs to try all by itself. The automatic mode is based on the existence of the preferred list, which is stored in a non-volatile memory in the SIM. This list includes a number of PLMN identities in order of preference and is under control of the user. The most preferred is usually the home PLMN. The list is filled in by the user through a mechanism to be specified by the mobile station manufacturer.
· Manual mode
In manual mode, the user is presented a list containing all found PLMNs.
The user chooses one of the PLMNs from the list.
Cell selection criteria
The mobile station attempts to find a suitable cell by passing through the list in descending order of received signal strength; the first BCCH channel which satisfies a set of requirements is selected. The requirements that a cell must satisfy before a mobile station can provide service from it, are:
· It should be a cell of the selected PLMN
The mobile station checks whether the cell is part of the selected PLMN.
· It should not be "barred"
The PLMN operator may decide not to allow mobile stations to access certain cells. These cells may, for example, only be used for handover traffic. Barred cell information is broadcast on the BCCH to instruct mobile stations not to access these cells.
· The radio path loss between the mobile station and the selected BTS must be below a threshold set by the PLMN operator

PLMN area
A Public-Land-Mobile-Network (PLMN) area is the geographical area in which a particular PLMN operator provides land mobile communication services to the public. From any position within a PLMN area, the mobile user can set up calls to another user of the same network, or to a user of another network. The other network may be a fixed network, another GSM PLMN, or another type of PLMN. Other network users, and users of other networks, can also call a mobile user who is active in the PLMN area.
When there are several PLMN operators, the geographical areas covered by their networks may overlap. National borders normally limit the extent of a PLMN area.

Location area identity
Every radio transmitter in the PLMN broadcasts, via a control channel, a Location Area Identity (LAI) code to identify the location area that it serves.
When an MS is not engaged in a call, it automatically scans the control channel broadcasts transmitted by the base stations in the locality and selects the channel that is delivering the strongest signal. The LAI code broadcast by the selected channel identifies the location area in which the MS is currently situated. This LAI code is stored in the Subscriber Identity Module (SIM) of the mobile equipment.
As the MS moves through the network area the signal received from the selected control channel gradually diminishes in strength until it is no longer the strongest. At this point the MS re-tunes to the channel that has become dominant and examines the LAI code that it is broadcasting. If the received LAI code differs from that stored on the SIM, then the MS has entered another location area and initiates a location update procedure to report the change to the MSC. At the end of the procedure the LAI code in the SIM is also updated.
Location area identity format

A Location Area Identity (LAI) code identifies the location area in a PLMN. The LAI code has three components:

· Mobile Country Code (MCC)
The MCC is a 3-digit code that uniquely identifies the country of domicile of the mobile subscriber (for example, Germany is 262, and Brunei is 528). It is assigned by the ITU-T.

· Mobile Network Code (MNC)
The MNC is a 2-digit code that identifies the home GSM PLMN of the mobile subscriber. If more than one GSM PLMN exist in a country, a unique MNC is assigned to each of them. The government of each country assigns it.

· Location Area Code (LAC)
The LAC component identifies a location area within a PLMN; it has a fixed length of 2 octets and can be coded using hexadecimal representation. An operator assigns it.

Location Registration
Two databases are used by Location Management to store MS location related data:

· Visitor Location Register
A VLR contains a data record for each of the MS that are currently operating in its area. Each record contains a set of subscriber identity codes, related subscription information, and a Location Area Identity (LAI) code. This information is used by the MSC when handling calls to or from an MS in the area.
When an MS moves from one area to another, the responsibility for its supervision passes from one VLR to another. A new data record is created by the VLR that has adopted the MS, and the old record is deleted.

· Home Location Register
The HLR contains information relevant to mobile subscribers who are fee-paying customers of the organization that operates the PLMN. Two types of information are stored in the HLR:
Ø Subscription information
The subscription information includes the identity code and directory number allocated to the subscriber, the type of service(s) provided, and any related restrictions.
Ø Location information
The location information includes the address of the VLR in the area where the subscriber's MS is currently located, and the address of the associated MSC.
The location information enables incoming calls to be routed to the MS. The absence this information indicates that the MS is inactive and cannot be reached.
When an MS moves from one VLR area to another, the location information in the HLR is updated with the new VLR and MSC addresses. The VLR then creates a new entry for the MS, using subscription data copied from the HLR.
Provided that an inter working agreement exists between the network operators concerned, data transactions can cross both network and national boundaries.

Types of identification numbers
During the performance of the Location Update procedure and the processing of a mobile call different types of numbers are used
· Mobile Station ISDN Number (MSISDN)
· Mobile Subscriber Roaming Number (MSRN)
· International Mobile Subscriber Identity (IMSI)
· Temporary Mobile Subscriber Identity (TMSI)
· Local Mobile Station Identity (LMSI)

Mobile Station ISDN Number
The MSISDN is the directory number allocated to the mobile subscriber. It is dialed to make a telephone call to the mobile subscriber.

The number consists of Country Code (CC) of the country in which the mobile station is registered (for example, Germany is 49, and Brunei is 673), followed by national mobile number which consists of Network Destination Code (NDC) and Subscriber Number (SN). A Network Destination Code is allocated to each GSM PLMN.

The composition of the MSISDN is such that it can be used as a global title address in the Signaling Connection Control Part (SCCP) for routing messages to the HLR of the mobile subscriber.

Mobile Station Roaming Number
The MSRN is the number required by the gateway MSC to route an incoming call to a MS that is not currently under the gateway's control.

Using the MSISDN a mobile-terminated call is routed to the gateway MSC. Based on this MSISDN the gateway MSC requests for a MSRN to route the call to the current visited MSC.

International Mobile Subscriber Identity
A MS is identified by its IMSI. The IMSI is embodied in the SIM of the mobile equipment. The MS provides it anytime it accesses the network.

An IMSI code has three components:

· Mobile Country Code (MCC)
The MCC component of the IMSI has the same meaning and format as those of the LAI. It is assigned by the ITU-T.

· Mobile Network Code (MNC)
The MCC component of the IMSI has the same meaning and format as those of the LAI. The government of each country assigns it.

· Mobile Subscriber Identification Number (MSIN)
The MSIN is a code that identifies the mobile subscriber within a GSM PLMN. An operator assigns it.

The overall number of digits in an IMSI code does not exceed 15.

Temporary Mobile Subscriber Identity
The TMSI is an identity alias which is used instead of the IMSI when possible. The use of a TMSI ensures that the true identity of the mobile subscriber remains confidential by eliminating the need to transfer an IMSI code enciphered over a radio link.

A VLR allocates a unique TMSI code to each mobile subscriber that is operating in its area. This code, which is only valid within the area supervised by the VLR, is used to identify the subscriber in messages to and from the MS. When a change of location area also involves a change of VLR area, a new TMSI code is allocated and communicated to the MS. The MS stores the TMSI on its SIM.

Local Mobile Station Identity
The LMSI is temporary subscriber data. Note that the use of the LMSI is optional.

In order to speed up the search for subscriber data in the VLR a supplementary Local Mobile Station Identity (LMSI) can be defined. The LMSI is allocated by the VLR at location updating and is sent to the HLR together with the IMSI. The HLR makes no use of it but includes it together with the IMSI in all messages sent to the VLR concerning that MS.