The document discusses location management for roaming mobile subscribers in 2G and 3G cellular networks. It describes how in 2G networks, location is managed using home and visitor location registers (HLR and VLR). In 3G networks, an additional node called the gateway location register (GLR) is introduced, which acts as an HLR in the visited network to reduce signaling costs associated with location updates of roaming subscribers between the visited and home networks. The GLR downloads subscriber data from the HLR and handles location updates from the VLR without involving the HLR directly.

1. SHAHZAD HAMID 2006-02-27
hshahzad@kth.se
Royal Institute of technology Stockholm - Sweden
1
Location Management of
Roaming Mobile Subscribers in
2G and 3G Cellular Network
Abstract
To have the records of the mobile
terminals moving form one location to
another location has been and will always
be an important issue in mobile
communication. This report discusses
some scenarios of location management
for the roaming mobile subscribers in 2G
and 3G cellular networks. It also describes
through scenarios how deployment of
GLR in 3G cellular network helps to
reduce the associated signaling cost for
the location management of roaming
mobile subscribes.
Keywords
Mobile terminal, Mobile communication,
Location management, Roaming, 2G, 3G ,
Cellular network, signaling cost, Mobile
subscriber.
Introduction
Today millions of people around the
world are using cellular phones and every
one expects that his/ her cellular phone
should be able to make/ receive calls, send
SMS, and utilize other cellular services any
time and anywhere around the globe. To
facilitate mobile subscribers accessing to
cellular network anywhere in the world.
Mobile operators form different countries
have signed roaming agreements. Based
upon these roaming agreements the
mobile subscriber of one operator can use
the cellular services in the cellular network
of another operator nationally or
internationally.
In 2G cellular networks such as GSM the
location management of the roaming
mobile subscriber is handled with two
location registers HLR (home location
register) and VLR(visitor location
register). HLR stores the data permanently
associated with the mobile subscriber and
VLR temporary stores the data for a
visiting mobile subscriber. In the GSM
network an incoming call for the roaming
mobile subscriber first goes to the HPLM
(home public land mobile network) of the
roaming mobile subscriber and after
confirming the current location of the
roaming mobile subscriber from the HLR
then the call is redirecting to the roaming
mobile subscriber. In some special cases if
the caller and called mobile subscribers
happen to be in the same visited network
the result is considerable extra signaling
cost.
When 3G cellular networks were
introduced, they deployed an extra node
in the visited cellular network for the
roaming mobile subscriber which is called
GLR (gateway location register). This
GLR is deployed at the edge of the visited
network between the HLR and VLR.
GLR acts as an HLR for the roaming
mobile subscriber and it downloads all the
necessary data regarding the roaming
mobile subscriber form the HLR that is
needed for using the cellular services in
this visited network. Furthermore, in the
2G/ 3G roaming scenario there are still
some troubles for the 3G mobile
subscribers roaming in 2G cellular
networks due to more limited capabilities
of 2G cellular networks as compared to
3G technology. But the 3G mobile
subscriber with the help of 2G/ 3G inter-
network roaming can have the same
coverage as the 2G mobile subscriber
while enjoying good quality the voice calls.
Location Management

2. SHAHZAD HAMID 2006-02-27
hshahzad@kth.se
Royal Institute of technology Stockholm - Sweden
2
Mobility Management is one of the major
functions of a GSM or a UMTS network
that allows mobile phones to work. The
aim of mobility management is to track
where the subscribers are, so that calls,
SMS and other mobile phone services can
be delivered to them. Location
management is an important problem in
mobile computing since wireless mobile
computers can change location while
connected to the network. Location
management is a key factor for mobile
computing. Without a good strategy for
location management, mobile computing
cannot exist.
A GSM or UMTS network, like all cellular
networks, is a radio network of individual
cells, known as base stations. Each base
station covers a small geographical area,
and is uniquely identified by a location
area code. By integrating the coverage of
each of these base stations, a cellular
network provides radio coverage over a
very much wider area. A group of base
stations is called a location area
The location update procedure allows a
mobile device to inform the cellular
network, whenever it moves from one
area to the next. Mobiles are responsible
for detecting location area codes. When a
mobile finds that the 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 it s Temporary
Mobile Subscriber Identity (TMSI)
Cellular Network Roaming
Roaming is the feature of the cellular
networks which enables subscribers to use
their mobile phone outside the coverage
area of their respective mobile operator
nationally or internationally [1]. While
traveling, roaming enables the subscriber
to make and receive calls, and send and
receive SMS inside their visited area
(where their home service provider has no
coverage). Depending on the visited area
and roaming type available, it may also be
possible for the mobile subscriber to use
their existing handset, mobile number and
the ease to dial and receive calls or
sent/receive SMS freely [1].
If the visited network by the mobile
subscriber is in the same country where
the home mobile operator resides then the
form of roaming is called national
roaming and if the visited network is
outside their home country then the
roaming is called international roaming.
Roaming between the different technical
standards is called inter standard roaming
[2].
When one is roaming in another country
then one can only use the service of the
mobile operator in the visited country
with which one s home mobile operator
has a roaming agreement. A roaming
agreement can be described as a
(commercial) agreement between the
mobile operators from different locations
to cooperate in providing services to each
others customers [1]. These roaming
agreements list the terms and conditions
these roaming partner mobile operators
give the access of the cellular services to
each others subscribers. Roaming
agreements are most valuable in those
areas where no one mobile operator can
provide full coverage either nationally or
internationally [4].
How to Access Roaming

3. SHAHZAD HAMID 2006-02-27
hshahzad@kth.se
Royal Institute of technology Stockholm - Sweden
3
If mobile subscriber wants to use his/ her
mobile phone outside the coverage area of
their mobile operator (for example in
another country) then he / she has to
check following conditions concerning
his/her mobile operator
1. If his/ her home mobile operator
has a roaming agreement with the
mobile operator in his/ her visited
country.
2. He/she should also check if the
roaming partner of his/ her home
mobile operator provides the
coverage in the specific area of the
visited country.
3. He/she should also confirm that
his/ her handset will be technically
effective in the visited area.[1]
The methods of accessing the visited
network when roaming in a visited area
mainly depends on the roaming type and
may also vary from a mobile operator to
mobile operator. So a user should confirm
the availability of access from their home
mobile operator before moving to another
area whether his/ her home mobile
operator has roaming agreements in
his/ her visited country and how roaming
will work in his/ her visited area.
Moreover his/ her mobile phone number
should not change and to make a call
she/ he has to ask his/her home mobile
operator about the procedures how to
make call and to use other services.
There are two types of roaming: Plastic
Roaming, & Automatic Roaming. In the
plastic roaming you have to insert a new
SIM in your mobile phone to access the
roaming services and you have to take this
SIM from your home mobile operator
before coming to the visited area. In
automatic roaming there is no need to
insert new SIM in the mobile phone in the
visited area, you just have to ask your
home mobile operator to activate this
service for you before coming to visited
area or foreign country.
Cost for accessing the roaming
internationally / nationally vary from
mobile operator to mobile operator and
from country to country [1]. But one thing
is for sure that customer has to pay high
charges while on roaming as compared to
the normal charges in the home network.
GSM Roaming
The GSM standard is the most widely
used standard for mobile communication
throughout the world. Most of the mobile
subscribers who travel along with their
GSM phone ask for roaming service from
their respective mobile operator so that
they can make and receive call in any
GSM network worldwide. Therefore they
should be able to enjoy the same user
specific services using the same phone
number everywhere. To achieve this
roaming agreements are required between
pairs of mobile operators worldwide.
Theses roaming agreements have also
encouraged the phone manufactures to
make dual and triple band phones which
mean service converge is now longer
limited to for example Europe, but
includes Asia, Pacific Islands, Africa,
Middle East and even North America. In
coming sections we will see how GSM
roaming works and what mechanisms are
involved in the GSM roaming. As the
mobile users may change their location we
will examine some procedures for how the
mobile users can be located in the GSM
world and how the call is forward to a
specific mobile station within this GSM
network.
The most important mechanism to find a
mobile station within the GSM network is
location updating which inform the
network about the mobile station s

4. SHAHZAD HAMID 2006-02-27
hshahzad@kth.se
Royal Institute of technology Stockholm - Sweden
4
current location. This location is identified
by the digits like MCC (which is a three
digits value used to identify the country
where the network exist). The MNS which
is a two digit value used to identify
individual network within a country
(MNC has three digit values in North
America), and LAI is used to identify the
physical area where the mobile station can
be found [6].
A GSM network uses two location
registers as shown in figure-1 to store the
information regarding the location of the
mobile stations. These are HLR and
VLR. The HLR is used to store the
permanent data concerning a mobile
subscription the subscription details
specify which services this mobile
subscriber is authorized to use, the IMSI
and MSISDN number along with current
address of VLR address of the mobile
station (if it is roaming). A HLR may be
associated with multiple MSCs at the same
time. The VLR is used to temporary store
data associated with the mobile subscriber
for the period of time. During the location
update procedure the VLR obtains the
mobile subscriber s data from subscriber s
HLR and this data remains in the VLR as
long as the mobile subscriber remains in
the VLR s associated area.
An incoming call to the subscriber s
causes the check of the HLR, which
returns the current location of the mobile
subscriber. During the location updating
procedure the mobile station sends a
message that indicates its location to the
VLR. The VLR checks if the mobile
subscriber is already registered or not and
if it is registered then the VLR simply
indicates if its location area should be
updated or not. If the mobile subscriber is
not registered in the VLR then the VLR,
requests the mobile subscriber s data from
its home network and from that HLR it
gets all necessary data about the mobile
subscriber so that that it can provide
services to the roaming mobile subscriber.
As a result the HLR learns which VLR
should receive subsequent incoming calls
for this subscriber, despite the fact that
they are roaming. Additionally the
roaming mobile subscriber wants to make
a call the VLR has the data about the
subscription details of this subscriber.
Call Routing for a Roaming Mobile
Subscriber in GSM
When a mobile subscriber is roaming in
another country then the call of the caller
from the home country will be routed as
explained in the following scenario, see
figure-2.
When a Mobile subscriber A calls Mobile
subscriber B while the later is currently
roaming in a foreign country (of the
mobile network associated with India) the
call is routed to the GMSC of the mobile
network associated subscriber B.
1. GMSC queries the HLR for the
current location of mobile
subscriber B.

5. SHAHZAD HAMID 2006-02-27
hshahzad@kth.se
Royal Institute of technology Stockholm - Sweden
5
2. HLR replies with the current
location of this subscriber.
3. GMSC routes the call to the
GMSC of the visited network.
4. GMSC of this visited network asks
its VLR about the location of
mobile subscriber B.
5. VLR replies with the location of
the mobile subscriber B.
6. GMSC now routes the call to the
mobile station being used by
mobile subscriber B.
Throughout this whole process the calling
party (mobile subscriber A) remains
unaware that the called is roaming in
another network. In a special case in
which both the mobile subscribers A and
B are roaming in same country, and
mobile subscriber A makes a call to
mobile subscriber B then the call first
routed to the home country of mobile
subscriber B then back to the originating
country because the HLR of the mobile
subscriber knows that B is currently
roaming in this particular country. In this
case the call will be routed with two
international links and becomes even
more expensive even though both
subscribers are in same network. In the
coming sections we will examine that how
we can overcome these extra signaling.
GSM phase 2 + support for Optimal
Routing
GSM phase 2+ provides a solution to
overcome the extra signaling especially
when the mobile subscribers are roaming,
this solution is implemented through the
SOR (support for optimal routing) [6].
SOR is a feature of the network which
forces the call to be directly routed to the
current location of the mobile subscriber
especially when mobile subscriber is
roaming rather than first routing the call
to the mobile subscriber s home network
and back [7].
To understand this more optimal routing
considers a scenario depicted in figure-3.
Given two mobile subscribers C and D.
Assume that mobile subscriber C is
presently roaming in the home network of
mobile subscriber D. If mobile subscriber
D calls mobile subscriber C without the

6. SHAHZAD HAMID 2006-02-27
hshahzad@kth.se
Royal Institute of technology Stockholm - Sweden
6
support of optimal roaming the call will
be routed to the home network of mobile
subscriber C and then back to the
network where the roaming mobile
subscriber C is because of this, the mobile
subscribers have to bear high call charges.
When support for optimal routing is
implemented if mobile subscriber C calls
mobile subscriber D; mobile subscriber
C s home network tries to route the call to
the mobile subscriber D s home network
the home network of mobile subscriber C
will report that mobile subscriber C
currently happens to be in mobile
subscriber s D home network. So mobile
subscriber C home network will cancel the
standard GSM routing to mobile
subscriber s D home network and will
directly route the call to the current
location subscriber D within the same
network.
The main drawback of supporting for
optimal routing is that when in above
scenario subscriber C is the subscriber of
a fixed network then optimal routing will
no longer function [6].
Towards 3G (third generation)
Cellular Network Roaming
These days the numbers of roaming users
are gradually increasing due to
international travel by t mobile
subscribers. 3G introduced a new
database node, GLR (gateway location
register) between the database nodes HLR
and VLR as a standard within the UMTS
core network. GLR handles location
management of the roaming mobile
subscriber in the visited network.
GLR does not inform the user s home
HLR of every location update. Thus GLR
helps to reduce the associated signaling
cost between the visited network and
home network of the roaming mobile
subscriber. The GLR is located in the
visited network and is used for location
updating and it stores a subscriber profile
of the roaming mobile subscriber. The
GLR is invisible to the HLR because it
has the same interface as the interface
between the VLR and HLR [8].
If a GLR is deployed at the edge of the
network in which the mobile subscriber is
roaming, then when a roaming mobile
subscriber turns on his/ her mobile in the
visited network and the first location
update procedure starts and the subscriber
profile data is downloaded from the HLR
to GLR. After this data the GLR handles
the all location updates of roaming mobile
subscriber from the VLR and act as if it
were the HLR of the mobile subscriber
(after first location update). The GLR
stores the roaming subscriber profile until
it is asked by the HLR to delete this
subscriber profile through a cancel
location message.
Figure-4 depicts the network architecture
for a 3G cellular network following the
deployment of a GLR at the edge of the
visited cellular network. This GLR
controls multiple VLRs which are under
the control of an MSC in the visited
network and receives the location
management messages and handles these
updates without involving the subscriber s
HLR.

7. SHAHZAD HAMID 2006-02-27
hshahzad@kth.se
Royal Institute of technology Stockholm - Sweden
7
Location updating can be of three types
for the roaming mobile subscriber in the
visited 3G cellular network [8]: GLR
location update, VLR location update, and
HRL location update.
The location update occurs in the HLR of
the roaming mobile subscriber when the
roaming mobile subscriber moves from
one visited mobile network to another
visited mobile network where it would be
served by another GLR because each
mobile network has its own GLR. An
HLR location update causes updates in
the GLR and an location update in VLR.
The location update in the database of
GLR is performed when a roaming
mobile subscriber moves from one
location area to another location area
within the visited network. The GLR
location update also causes a location
update in the relevant VLR. When a
roaming mobile subscriber moves
between cells of a location area, then a
VLR location update is performed.
Now consider some scenarios showing
location updates are performed in 3G
cellular networks with respect to the
movement of the roaming mobile
subscriber in the visited network when a
roaming mobile subscriber is moving
within an area served by a specific
MSC/ VLR in this subscriber enter in
another cell of this area then the MSC will
update its VLR including that this
subscriber now resides in the new cell.
Consider the scenario (depicted in figure-
5) when a roaming mobile subscriber
moves from one VLR to another VLR
(i.e., the subscriber has moved to another
location area served by another
MSC/VLR).
1. The MSC updates its associated
VLR with the location of the new
subscriber in this area.

8. SHAHZAD HAMID 2006-02-27
hshahzad@kth.se
Royal Institute of technology Stockholm - Sweden
8
2. The MSC also informs the GLR
of this location update. Here we
also assume that the GLR already
known that this roaming mobile
subscriber resides in its service
area.
3. GLR informs the old serving MSC
of the subscriber to cancel the
outdate location record.
4. The old MSC performs a
cancellation in its associated VLR
and sends a confirmation message
to the GLR.
5. The GLR updates its database
indicating the current serving MSC
of the roaming mobile subscriber
and sends a confirmation message
to the current serving MSC.
Now consider a scenario (depicted in
figure-6 in which the roaming mobile
subscriber moves from the area serving by
one GLR to the area serving by another
GLR, in other words the roaming mobile
subscriber moves from one visited
network to another visited. The location
updates for this roaming mobile
subscriber will be performed in the
following way
1- The MSC in the new visited
network discovers the new
roaming mobile subscriber and
updates it s associated VLR with
the information that roaming
mobile subscriber is now residing
in its area.
2- The MSC informs the GLR about
the new roaming mobile
subscriber in its area.
3- The GLR finds that it has no
record of this roaming mobile
subscriber in its database. The
GLR asks the HLR of the
roaming mobile subscriber to
inform it about the subscription
profile of this subscriber.
4- The HLR informs the old GLR to
cancel its data about this
subscriber which was previously in
its network.
5- An old GLR asks the MSC to
delete the data concerning this
subscriber.
6- MSC deletes the data from its
associated VLR and sends
confirmation to the GLR of its
network.
7- Old GLR also sends a
confirmation to the subscriber s
HLR.
8- The HLR now informs the GLR
in new visited network where
subscriber currently resides, about
the subscription details of this
subscriber.
9- The new GLR now updates all the
data about this subscriber and also
sends a confirmation to the
serving MSC.

9. SHAHZAD HAMID 2006-02-27
hshahzad@kth.se
Royal Institute of technology Stockholm - Sweden
9
So now when a roamer is called by the
mobile subscriber of the visited network
the call will be routed through the GLR
and there will be no need that the first call
will route to HLR and then will come
back to the location of the roamer. This
will avoid the situation of the extra
signaling problem. This situation is
depicted in figure-7.
1. When MS calls the roamer the
network will check the location of
the roamer.
2. Network will ask GLR about the
location of the roamer.
3. The GLR will return the current
location of roaming subscriber.
4. The network will forward the call
to the current MSC/VLR.
5. Here call will be forward to the
current location of the roaming
subscriber.
So in this scenario there will be no need to
extract information form the home
network of the roaming subscriber.
2G/3G Inter-Roaming
One of the main differences between 2G
and 3G cellular networks is the handover
procedure. 2G cellular network (such as
GSM) implement hard handover whereas
3G cellular network (UMTS) implements
as soft handover scheme. In a soft
handover scheme the mobile station
remains connected with more that one
base stations whereas in a hard handover
scheme the mobile station any time can
only communicate with one base station.
In a soft handover scheme the total
promised throughput of the system might
not be equal to the throughput which is
observed in real time. Moreover in a soft
handover scheme the link of the mobile
station with base station remains
connected [11].
To enable roaming between the 3G and
2G cellular technologies the 3GPP (third
generation partnership project) core
network was base on the GSM core
network. Today GSM subscribers are
greater in number than the 3G mobile
subscribers and 3G network is being
introducing slowly, but by using the 2G as
a fall back technology 3G users can have
the same coverage as the 2G users but
with reduced 3G functionality.
When 3G mobile subscribers roams into a
GSM network many problem can occur.
Consider the Like the handover problem
when a 3G mobile subscriber switches to
the GSM network due to the loss of 3G

10. SHAHZAD HAMID 2006-02-27
hshahzad@kth.se
Royal Institute of technology Stockholm - Sweden
10
network coverage, what should be the
criteria for the 3G mobile subscriber to
roam back to the 3G network? To avoid
oscillation between the 3G and GSM
network, the 3G mobile subscribers
should not switch back to the 3G network
until its finds a stable 3G coverage area,
but this is not easy when the GSM and
3G networks belong to different
operators. Another issue is the price
model when the 3G mobile subscriber
roaming in the GSM network gets limited
services.
3G mobile subscribers roaming in 2G
networks enjoy only 2G network services
because the services that use the
capabilities of 3G network are not
available in the 2G network. For example
video calls are not offered in the GSM
and although web surfing is available over
GPRS in GSM but performance is much
worse than in a 3G network.
Conclusion
As the numbers of roaming mobile
subscriber are increasing at a tremendous
rate this paper has discussed the different
roaming scenarios for the mobile
subscribers in 2G and 3G technologies. It
is observed when the mobile subscriber of
2G technology is roaming in another
foreign 2G network then for the location
update of mobile subscriber the access to
HLR and associated signaling is costly
because the mobile subscriber has to
inform again and again its HLR whenever
the mobile subscriber changes its location
area in the visited network. Hence the 3G
technology was introduced. The 3G
standard introduced another database
node GLR in between the HLR and the
VLR to reduce the associated signaling
cost for the roaming mobile subscriber.
GLR is deployed at the edge of visited
network which acts as HLR for the
roaming mobile subscriber in 3G
technology. When the inter-roaming
between 2G and 3G technologies is
observed, there are many problem for the
3G roamer in the 2G network due to the
limited capabilities in 2G as compare to
the 3G technology. but the main
advantage for the 3G roamer in 2G
network is that he/ she can still enjoys
voice calls despite 3G cellular network
coverage not being available in every
country. Thus using 3G/ 2G roaming the
3G roamer can have the same coverage
area as the 2G mobile subscribers.
Abbreviations
SMS Short message service
SIM Subscriber identity module
2G Second generations of mobile
communication
GSM Global system for mobile
MCC Mobile country code
MNC Mobile national code
LAI Location area identity
HLR Home location register
VLR Visitor location registers
IMSI International mobile subscriber
Identity
IMISDN Mobile subscriber ISDN
Number
ISDN Integrated services digital
Number
MSC Mobile switching center
LAI Location area identity
GMSC Gateway mobile switching
Centre
SOR Support for optimal routing
PLMN Public land mobile network
HPLMN Home public land mobile
Network
VPLMN Visited public land mobile
Network
3G Third generations for mobile c
Communication

11. SHAHZAD HAMID 2006-02-27
hshahzad@kth.se
Royal Institute of technology Stockholm - Sweden
11
GLR Gateway locations register
UMTS Universal mobile
Telecommunication system
SGSN Serving GPRS support node
GPRS General Packet radio service
MS Mobile station
MS Mobile Subscriber
BTS Base station transceiver
BSC Base station controller
RNC Radio network controller
RBS Radio base station
LA Location area
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(2006-02-09)
[2]
http://www.gsmworld.com/roam
ing (2006-02-09)
[3] http://www.vodafone.com/au
(2006-02-09)
[4] http://www.mobilemedia.com
(2006-02-09)
[5]http://www.imit.kth.se/courses
2G1330 (2006-02-11)
[6] GSM and personal
communication handbook by
Siemund M. Redl, Mitthias
K.Webbeer and Malkolm
W.Oliphant Artech House
Boston (London)
[7]http://www.mpirical.com/com
panion/mpirical_companion.html
#http://www.mpirical.com/comp
anion/Multi_Tech/SORRouting.h
tm (2006-02-13)
[8] Cache-Enhanced Dynamic
Movement-Based Location
Management Schemes for 3G
Cellular Networks. A Thesis
Presented in Partial Fulfillment of
Requirements for the Degree of
Master of Science in the College
of Arts and Sciences, Georgia
State University, 2003 by Krishna
Priya Patury
http://ww.academicdb.com
(2006-02-12)
[9] 3rd Generation Partnership
Project; Technical Specification
Group Core Network;
Gateway Location Register (GLR)
- Stage2 (Release 1999)
[10] Anders Roos, Magnus
Hartman, And Stephen Dutnall,
Northstream Ab Critical Issues for
Roaming In 3G
http://www.ieee.org (2006-02-22)
[11] Effect of soft handover on
Throughput in UMTS network
http://www.its.bth.se/exjobb/
InbjudanXjob050901.html

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