Location Update (LU)

Introduction (LU)

There are three different types of Location updating. The difference in Signaling between these three types is a parameter in the Location Updating Request Message.

The three types are:
 Normal: used when an MS reaches a new location area; also called Forced Registration
 Periodic: used to prevent mobiles from becoming accidentally detached
from the system
 Attach: used when a mobile is switched on

The steps involve in a location update are RR connection establishment, service request, Authentication (except for periodic registration), ciphering mode setting, location updating and RR connection release.

RR Connection Establishment (LU)

Service Request (LU)

Authentication (LU)

Ciphering (LU)

Update Location (LU)

RR Connection Release (LU)

SCCP, Connection Oriented, Release (LU)

Location Updating Reject (LU)

If location updating cannot be accepted, the network sends a Location Updating Reject message to the MS. This message contains the cause for rejection. For example, IMSI unknown in HLR, illegal MS, congestion, etc.

The RR connection is then released, and if the reject cause is either number 2 "IMSI unknown in HLR" or number 3 "Illegal MS", the MS deletes any TMSI, stored LAI, cipher key and ciphering key sequence number. The MS then enters the "IDLE no IMSI" state. In this state, the MS acts in the following way:

• No starting any normal location updating attempt

• No performing periodic updating

• No performing IMSI detach if powered down

• Only allows emergency calls

• No responding to paging

Signaling Diagram Location Update