3 Network functions for supporting cellular operation

03.013GPPNetwork FunctionsTS

3.1 Location registration

3.1.1 Definitions

Location registration means that the PLMNs keep track of where MSs are located in the system area. The location information is stored in functional units called location registers. Functionally there are four types of location registers:

– the home location register (HLR) where the current location and all subscriber parameters of a MSare permanently stored;

  • the visitor location register (VLR) where all relevant parameters concerning a MS are stored aslong as the station is within the area controlled by that visitor location register.
  • the serving GPRS support node (SGSN) where all relevant parameters concerning a MS using packet switched services are stored as long as the MS is controlled by that SGSN,

– the GGSN node where routing information for all static PDP addresses related to a mobile subscriber is stored.

See also Technical Specification GSM 03.02 where the network architecture is described.

3.1.2 Procedures

Procedures related to location registration are specified in Technical Specification GSM 03.12.

They comprise:

i) location updating which enables the MS to inform the network that its location area has to be updated, i.e. the MS has received a location area identity which is different from that contained in its memory. In order to avoid unnecessary updating, the current location area identity should be stored in a non-volatile memory in the MS;

ii) location cancellation which is used to delete an MS from a VLR or from an SGSN;

iii) periodic location updating which enables the location of stationary MSs to be confirmed at a rate determined by the operator;

  1. as a network option, IMSI detach/attach operation. When a MS has informed the network that it is IMSI detached, the network is aware that the subscriber is not active;
  2. routing updating which enables the MS to inform the network that its routing area has to be updated;
  3. periodic routing updating which enables the location of stationary MSs to be confirmed at a rate determined by the operator;
  4. GPRS detach/attach operation. When a MS has informed the network that it is GPRS detached, the network is aware that the subscriber is not active;

viii) cell updating which enables the MS to inform the network that its cell has been updated.

The procedures i) to iv) are defined for circuit switched services, and ii) and v) to viii) are defined for packet switched services. Combined procedures for circuit and packet switched services are also defined in GSM 03.60 for GPRS. The procedure i) may be combined with v), iii) may be combined with vi) and iv) may be combined with vii).

The procedures also include mechanisms for restoration of location registers after failure. These procedures are defined in Technical Specification GSM 03.07.

3.1.3 Information stored in location registers

Information to be stored in location registers is listed in Technical Specification GSM 03.08.

3.2 Handover

3.2.1 Definitions

The following case are required:

i) handover between physical channels of the same Base Station System (BSS). This capability could be used in the following situations:

– when the physical channel carrying the call is subject to interference or other disturbances;

– when a physical channel or channel equipment carrying a call has be to be taken out of service for maintenance or other reasons;

ii) handover between Base Station Systems (BSSs) of the same MSC;

iii) handover between BSSs of different MSCs of the same PLMN.

Cases ii) and iii) are used in order to ensure continuity of the connection when an MS moves from one BSS area to another.

For case iii) two procedures are defined:

a) basic handover procedure where the call is handed over from the controlling MSC (MSC-A) to another MSC (MSC-B);

b) subsequent handover procedure where the call is handed over from MSC-B to MSC-A or to a third MSC (MSC-B’).

3.2.2 Procedures

The procedures are described in Technical Specification GSM 03.09.

3.3 Call re-establishment

Re-establishment of a call may be performed when a traffic channel (TCH) has been lost during the call, e.g. when a handover could not be completed in time.

Call re-establishment is only performed on TCHs and when the call is in the connected state (see Technical Specification GSM 04.08). The procedures are specified in Technical Specification GSM 04.08.