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, allowing calls, SMS and other mobile phone services to be
delivered to them. Location update procedure.
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 which is part of a uniquely identified location area. By integrating the
coverage of each of these base stations, a cellular network provides a radio
coverage over a much wider area. A group of base stations is named a location area,
or a routing area.
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 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 its Temporary Mobile Subscriber Identity (TMSI).
several reasons why a mobile may provide updated location information to the
network. Whenever a mobile is switched on or off, the network may require it to
perform an IMSI attach or IMSI detach location update procedure. Also, each
mobile is required to regularly report its location at a set time interval
using a periodic location update
procedure. 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. Thus a subscriber has reliable access to the network and may be
reached with a call, while enjoying the freedom of mobility within the whole
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.
Temporary Mobile Subscriber Identity (TMSI) is the identity that is most
commonly sent between the mobile and the network. TMSI is randomly assigned by
the VLR to every mobile in the area, the moment it is switched on. The number
is local to a location area, and so it has to be updated each time the mobile
moves to a new geographical area.
network can also change the TMSI of the mobile at any time. And it normally
does so, in order to avoid the subscriber from being identified, and tracked by
eavesdroppers on the radio interface. This makes it difficult to trace which
mobile is which, except briefly, when the mobile is just switched on, or when
the data in the mobile becomes invalid for one reason or another. At that
point, the global "international mobile subscriber identity" (IMSI)
must be sent to the network. The IMSI is sent as rarely as possible, to avoid it
being identified and tracked.
A key use
of the TMSI is in paging a mobile. "Paging" is the one-to-one
communication between the mobile and the base station. The most important use
of broadcast information is to set up channels for "paging". Every
cellular system has a broadcast mechanism to distribute such information to a
plurality of mobiles. Size of TMSI is 4 octet with full hex digits and can't be
all 1 because the SIM uses 4 octets with all bits equal to 1 to indicate that
no valid TMSI is available.
is one of the fundamental mobility management procedures of all cellular
networks. Roaming is defined as the ability for a cellular customer to
automatically make and receive voice calls, send and receive data, or access
other services, including home data services, when travelling outside the
geographical coverage area of the home network, by means of using a visited
network. This can be done by using a communication terminal or else just by
using the subscriber identity in the visited network. Roaming is technically
supported by mobility management, authentication, authorization and billing
"location area" is a set of base stations that are grouped together
to optimise signalling. Typically, tens or even hundreds of base stations share
a single Base Station Controller (BSC) in GSM, or a Radio Network Controller
(RNC) in UMTS, the intelligence behind the base stations. The BSC handles
allocation of radio channels, receives measurements from the mobile phones,
controls handovers from base station to base station.
location area, a unique number called a "location area code" is
assigned. The location area code is broadcast by each base station, known as a
"base transceiver station" BTS in GSM, or a Node B in UMTS, at
regular intervals. In GSM, the mobiles cannot communicate directly with each
other but, have to be channeled through the BTSs. In UMTS networks, if no Node
B is accessible to a mobile, it will not be able to make any connections at
location areas are very large, there will be many mobiles operating
simultaneously, resulting in very high paging traffic, as every paging request
has to be broadcast to every base station in the location area. This wastes
bandwidth and power on the mobile, by requiring it to listen for broadcast
messages too much of the time. If on the other hand, there are too many small
location areas, the mobile must contact the network very often for changes of
location, which will also drain the mobile's battery. A balance has therefore
to be struck.
routing area is the PS domain equivalent of the location area. A "routing
area" is normally a subdivision of a "location area". Routing
areas are used by mobiles which are GPRS-attached. GPRS is optimized for
"bursty" data communication services, such as wireless
internet/intranet, and multimedia services. It is also known as GSM-IP
("Internet Protocol") because it will connect users directly to
Internet Service Providers (ISP).
bursty nature of packet traffic means that more paging messages are expected
per mobile, and so it is worth knowing the location of the mobile more
accurately than it would be with traditional circuit-switched traffic. A change
from routing area to routing area (called a "Routing Area Update") is
done in an almost identical way to a change from location area to location
area. The main differences are that the "Serving GPRS Support Node"
(SGSN) is the element involved.
The tracking area is the LTE counterpart of the
location area and routing area. A tracking area is a set of cells. Tracking
areas can be grouped into lists of tracking areas (TA lists), which can be
configured on the User equipment. Tracking area updates are performed
periodically or when the UE moves to a tracking area that is not included in
its TA list.
can allocate different TA lists to different UEs. This can avoid signaling
peaks in some conditions: for instance, the UEs of passengers of a train may
not perform tracking area updates simultaneously. On the network side, the
involved element is the Mobility Management Entity.
Handover means handing over the mobile from one
cell to another cell. There are two reasons for handover.
When a mobile station moves out of the range of BTS
the signal level decreases continuously and falls below the minimal
requirements for communication.
The error rate increases due to interference.
The quality of radio link decrease.
The traffic in one cell is too high, shifting of some MS to other cells
with lower load.
This is called load balancing.
The number of handover will be more when the cell
size is small.
handover the calls should not get to cutoff which is called as call drop.
(1) INTRA CELL HANDOVER:
With in a
cell, narrow band interference can cause transmission at a certain frequency
decides to change the carrier frequency.
(2) INTER CELL, INTRA BSC HANDOVER:
The mobile station moves from one cell to
another but remains with in the same BSC.
The BSC performs a handover, assigns a new
radio channel in the new cell and releases the old one.
(3) INTER BSC, INTRA MSC HANDOVER:
The BSC controls only limited cells.
Handover needs to be done between different
This is controlled by MSC.
(4) INTER MSC HANDOVER:
A handover is needed between 2 cells which
belong to Difference MSC.
performs the handover together.