Paging and Location Update

By: Abidullah Zarghoon

Some Terms:• MSC/VLR - Mobile Switching Center / Visitor

Location Register

• HLR - Home Location Register

• PLMN - Public Land Mobile Network

• TMSI – Temporary Mobile Subscriber Identity

• LA - Location Area

• LAI - Location Area Identity

• LAC - Location Area Code

• LU – Location Update

• MS - The Mobile Station

• BSS - The Base Station Subsystem

• NSS - The Network Switching Subsystem

• OSS - The Operation Support Subsystem

Overview• A GSM or UMTS network, like all cellular networks, is

basically a radio network of individual cells, known

as base stations.

• Each base station covers a small geographical

area which is part of a uniquely identified location

area.

Location Area• A location area is a set of base stations that are

grouped together to optimize signaling

• Location Area is a group of cells and the subscriber

is paged in this area.

• One or more base station controllers are used to

serve each Location Area but by a single MSC.

• Each Location Area has a unique Location Area

Identity number.

Location Area Update• The mobile station also performs location updating,

in order to indicate its current location, when it

moves to a new Location Area or a different Public

Land Mobile Network (PLMN). This location updating

message is sent to the new MSC/VLR, which gives

the location information to the subscriber's HLR. If

the mobile station is authorized in the new

MSC/VLR, the subscriber's HLR cancels the

registration of the mobile station with the old

MSC/VLR.

Location Update Procedure

• The location update procedure allows a mobile

device to inform the cellular network, whenever it

moves from one location area to the next.

• Mobiles are responsible for detecting location area

codes.

• When mobile finds that location area code is

different from its last update, it performs another

update by sending to the network, a location

update request, together with its previous location

and TMSI (Temporary Mobile Subscriber Identity)

Periodic Location Update• A location updating is also performed periodically.

If after the updating time period, the mobile station

has not registered, it is then deregistered.

• Each mobile is required to regularly report its

location at a set time interval using a periodic

location update.

Random Location Update• Whenever a mobile moves from one location area

to the next while not on a call, a random location

update is required.

• This is also required of a stationary mobile that

reselects coverage from a cell in a different

location area, because of signal fade.

Continued… • When a subscriber is paged in an attempt to deliver

a call or SMS and the subscriber does not reply to

that page then the subscriber is marked as absent

in both the Mobile Switching Center / Visitor

Location Register (MSC/VLR) and the Home

Location Register (HLR) (Mobile not reachable flag

MNRF is set). The next time the mobile performs a

location update the HLR is updated and the mobile

not reachable flag is cleared.

Paging• Paging is the one-to-one communication between

the mobile and the base station

• Paging is a procedure the network uses to find out a

subscriber’s location before actual call

establishment.

• Paging is used to alert the mobile station of an

incoming call.

• Paging is initiated by the NSS (Network Subsystem)

and is based on the Location Registration

information the Mobile Subscriber has supplied

when performing the Location Update.

Paging Strategies• Location Area Splitting in Paging Areas

o Mobile registers only when entering the Location Area; it doesn’t register

when moving between Paging Areas of one Location Area.

o For an incoming call, paging messages are broadcasted in the Paging

Areas according to a sequence determined by different strategies.

Location Area Splitting in Paging Areas

Paging Strategies• Multilayer Location Areas:

o Each mobile is assigned to a given group, and each group is assigned

one or several layers of Location Areas.

o Location Update traffic is distributed over all the cells.

o Multilayer Las solves the problem where Location Update traffic is mainly

concentrated in the cells of the Location Area border.

Multilayer Location Areas:

Differences between Paging and Location Update

• Mobility Management

based on pure Pagingo If a call arrives, terminal is

paged in all cells of the mobile

network

o Location update is not

required

o As paging must be executed

in all cells of the network for

each arriving call/SMS/data-

packet

• high signaling overhead

• high delay in

call/SMS/data-packet

delivery

• Mobility Management

based on pure Location

Update:o Each time the user crosses cell

boundaries a location update

is triggered

o Paging is not required

o As location updates must be

initialized whenever crossing

cell boundaries

• high signaling and

database update

overhead

• high power consumption in

the terminals

• Location Update• Paging

Some Definitions• TMSI:

o The Temporary Mobile Subscriber Identity (TMSI) is a temporary identification number that is used in the GSM network instead of the IMSI

to ensure the privacy of the mobile subscriber

• LAIo A Location Area Identity (LAI) is a globally unique number.

• LACo A Location Area Code (LAC) is only unique in a particular network

• Cell: o Cell is the basic service area: one BTS covers one cell. Each cell is given a

Cell Global Identity (CGI), a number that uniquely identifies the cell.

• MSC/VLR Service Area:o The area covered by one MSC is called the MSC/VLR service area.

• PLMNo The area covered by one network operator is called PLMN. A PLMN can

contain one or more MSCs.

o A network that is established and operated by an administration or by

a recognized operating agency (ROA) for the specific purpose of

providing land mobile telecommunications services to the public

References:• Advances in Network Management

o By Jianguo Ding – 2009