23.0123GPPLocation management proceduresRelease 17TS
2.1 Location management
Location management means that the PLMNs keep track of where the MSs are located in the system area. The location information for each MS is stored in functional units called location registers. Functionally, there are two types of location registers:
– the Home Location Register where all subscriber parameters of an MS are permanently stored, and where the current location may be stored;
– the Visitor Location Register where all relevant data concerning an MS are stored as long as the station is within the area controlled by that visitor location register;
– the CSG Subscriber Server where the CSG subscription data are stored in the visited PLMN for inbound roaming MS, and where the current location may be stored.
See also 3GPP TS 23.002  where the network architecture is described, and 3GPP TS 23.008  where the data stored in the location registers are described.
The action taken by a MS in order to provide location information to the PLMN will be referred to as location updating.
2.2 Location area and MSC area
The MSC area is composed of the area covered by all base stations controlled by the MSC. An MSC area may consist of several location areas. A location area is an area in which, after having performed a location update once, MSs may roam without being required to perform subsequent location updates for reason of location change. A location area consists of one or more cells.
For further details of the network architecture, see 3GPP TS 23.002 .
2.3 Location area identification
The Location Area Identification (LAI) plan is part of the base station identification plan. The base stations are identified uniquely (see 3GPP TS 23.003 ).
2.4 IMSI detach/attach operation
The support of IMSI detach/attach operation is mandatory in MSs. The facility is optional in the fixed infrastructure of the PLMN.
2.4.1 Explicit IMSI detach/attach
Explicit IMSI detach operation is the action taken by an MS to indicate to the PLMN that the station has entered an inactive state (e.g. the station is powered down). Explicit IMSI attach operation is the action taken by an MS to indicate that the station has re-entered an active state (e.g. the station is powered up).
2.4.2 Implicit IMSI detach
Implicit IMSI detach operation is the action taken by the VLR to mark an MS as detached when there has been no successful contact between the MS and the network for a time determined by the implicit detach timer. The value of the implicit detach timer is derived from the periodic location updating timer; when the MSC/VLR applies Mobility Management Congestion Control to a MS, the MSC/VLR may need to adjust the Implicit Detach timer as specified in clause 3.7.2. During an established radio contact, the implicit detach timer shall be prevented from triggering implicit detach. At the release of the radio connection, the implicit detach timer shall be reset and restarted. Implicit IMSI detach shall also be performed in the case of a negative response to an IMEI check.
2.5 Use of the term mobile station (MS) in the present document
In order to simplify the text the term Mobile Station (MS) as used in relation to location management refers to the entity where the IMSI is stored, i.e., in card operated MSs the term Mobile Station (MS) refers to the card.
2.6 Paging area
As an option, and for paging optimization purpose, the VLR may control Paging Areas. A Paging Area (PgA) is composed of up to 5 Location Areas, and the MSC area is composed of several Paging Areas. Paging areas may overlap each other. The Paging Area is stored in the HLR and updated at each paging area change. The Paging Area is sent by the HLR to the VLR at roaming number request and may be used by the MSC/VLR for paging (e.g. when LAI is not known, after MSC/VLR restart) (see 3GPP TS 23.018 [5a]).