Location updating
A powered-on mobile is informed of an incoming call by a paging message sent over the PAGCH channel of a cell. One extreme would be to page every cell in the network for each call, which is obviously a waste of radio bandwidth. The other extreme would be for the mobile to notify the system, via location updating messages, of its current location at the individual cell level. This would require paging messages to be sent to exactly one cell, but would be very wasteful due to the large number of location updating messages. A compromise solution used in GSM is to group cells into location areas. Updating messages are required when moving between location areas, and mobile stations are paged in the cells of their current location area.
In conventional cellular location messages are sent to the exact cell a mobile is in.
To review, the VLR Data Base, or Visited or Visitor Location Register, contains all the data needed to communicate with the mobile switch. Levine says this data includes:
* Equipment identity and authentication-related data
* Last known Location Area (LA)
* Power Class and other physical attributes of the mobile or handset
* List of special services available to this subscriber
* More data entered while engaged in a Call
* Current cell
* Encryption keys
The location updating procedures, and subsequent call routing, use the MSC and two location registers: the Home Location Register (HLR) and the Visitor Location Register (VLR). When a mobile station is switched on in a new location area, or it moves to a new location area or different operator's PLMN, it must register with the network to indicate its current location. In the normal case, a location update message is sent to the new MSC/VLR, which records the location area information, and then sends the location information to the subscriber's HLR. The information sent to the HLR is normally the SS7 address of the new VLR, although it may be a routing number. The reason a routing number is not normally assigned, even though it would reduce signalling, is that there is only a limited number of routing numbers available in the new MSC/VLR and they are allocated on demand for incoming calls. If the subscriber is entitled to service, the HLR sends a subset of the subscriber information, needed for call control, to the new MSC/VLR, and sends a message to the old MSC/VLR to cancel the old registration.
All of these abbreviations are covered on this page.
For reliability reasons, GSM also has a periodic location updating procedure. If an HLR or MSC/VLR fails, to have each mobile register simultaneously to bring the database up to date would cause overloading. Therefore, the database is updated as location updating events occur. The enabling of periodic updating, and the time period between periodic updates, is controlled by the operator, and is a trade-off between signalling traffic and speed of recovery. If a mobile does not register after the updating time period, it is deregistered.
SIM: Subscriber identify module.
BSC: Base station controller.
MSC: Mobile services switching center.
UM: Represents the radio link.
ME: Mobile equipment.
HLR: Home location register.
EIR: Equipment identity register.
BTS: Base transceiver station.
VLR: Visitor location register.
AuC: Authentication Center.
Abis: Represents the interface between the base stations and base station controllers.
"A": The interface between the base station subsystem and the network subsystem.
PSTN and PSPDN: Public switched telephone network and packet switched public data network.
Figure 1. General architecture of a GSM network
A procedure related to location updating is the IMSI (International Mobile Subscriber Identity) attach and detach. A detach lets the network know that the mobile station is unreachable, and avoids having to needlessly allocate channels and send paging messages. An attach is similar to a location update, and informs the system that the mobile is reachable again. The activation of IMSI attach/detach is up to the operator on an individual cell basis.