Mobility Management is one of the features of the GSM
network that allows the mobile phones to make or receive calls while moving one
place to another.
Mobile device uses the location update procedure to update
the current location information to the cellular network. Mainly there are
three different reasons for a mobile station to do a location update.
- Normal location update
- IMSI attach
- Periodic registration
Whenever a mobile station moves from one location to another
or comes to an active state (IMSI attach), the mobile station will check the
Location Aread Identity (LAI) stored in its SIM against the LAI of the cell.
When both LAIs are different, mobile station will initiate a location update
process. The network also has a provision for periodic registration to avoid
the unnecessary paging of the mobile if MSC never receives the IMSI detach
message.
Normal location update
Mobile station will initiate a normal loaction update
procedure as when it finds a change of location area. As soon as a mobile
station changes the location ares, it will compare the LAI stored in the SIM
with the current LAI. If the two differs, then mobile station has to update the
new location information to ensure that the mobile station will be paged to
corret location.
The location update procedure will start with the mobile
station sending the new location information to the network. After the
authentication, the MSC will update its VLR. If the mobile station is new to
this MSC, then MSC will interact with the HLR to get all the necessary
information about the subscriber and then updates its VLR.
IMSI attach
When a mobile station switched on, ie, moved from an idle state
to active state, it will try to attach to the network. This process of
attachment is called IMSI attachment. During the IMSI atatch procedure, if the
MSC finds that the mobile station is new, then MSC will query the corresponding
HLR and then it will update the VLR with status “IMSI Attached”. During the
process, the HLR will update its own data base and sends a message to the old
MSC where the mobile station was located before to update its VLR.
Similarly, when a mobile station is switched off or sim card
is removed, the mobile station will sends a message to the VLR and the VLR will
mark the mobile station status as detached. Once the status is updated in the
VLR, the incoming call to the mobile station will not be paged.
Periodic Registration
It is not necessary that the MSC will always gets the IMSI
detach request from the mobile station whenever the later is switched off. So
there will be a possibility of unnecessay paging of the mobile whenever there
is an incoming call. The periodic registration is provisioned to avoid such unnecessay
paging.
After each registration, the mobile station and MSC will reset
their timers and when the timer in mobile station expires, the mobile station
will perform a location update. If the mobile station does not perform a
location update within the time, then the MSC will update the VLR and HLR with
status a “detached”.
