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GSM(GLOBAL
SYSTEM FOR MOBILE COMMUNICATIONS)
GSM (Global System for Mobile Communications) is
the most popular standard for mobile phones in
the world. GSM phones are used by over a billion
people across more than 200 countries. The
ubiquity of the GSM standard makes international
roaming very common with "roaming agreements"
between mobile phone operators. GSM differs
significantly from its predecessors in that both
signalling and speech channels are digital,
which means that it is seen as a second
generation (2G) mobile phone system. This fact
has also meant that data communication was built
into the system from very early on. GSM is an
open standard which is currently developed by
the 3GPP.
From the point of view of the consumer, the key
advantage of GSM systems has been higher digital
voice quality and low cost alternatives to
making calls such as text messaging. The
advantage for network operators has been the
ability to deploy equipment from different
vendors because the open standard allows easy
inter-operability. Also, the standards have
allowed network operators offer roaming services
which mean subscribers can use their phone all
over the world.
GSM retained backward-compatibility with the
original GSM phones as the GSM standard
continued to develop, for example packet data
capabilities were added in the Release '97
version of the standard, by means of GPRS.
Higher speed data transmission have also been
introduced with EDGE and UMTS in the Release '99
version of the standard.
History
In spite of its current popularity,
the history of mobile phones began long before
GSM was conceived. The GSM group ("Groupe
Spécial Mobile" (French) 1, 2, 3 and 4) was
founded in 1982. The name of the system comes
from the name of this group, though later the
decision was made to keep the initials but to
change what they stood for. Originally the group
was hosted by CEPT. The technical fundamentals
of the GSM system were defined in 1987. In 1989,
ETSI took over control and by 1990 the first GSM
specification was completed, amounting to over
6,000 pages of text. Commercial operation began
in 1991 with Radiolinja in Finland.
In 1998, the 3rd Generation Partnership Project
(3GPP) was formed. Originally it was intended
only to produce the specifications of the next
(third, 3G) generation of mobile networks.
However, 3GPP also took over the maintenance and
development of the GSM specification. ETSI is a
partner in 3GPP.
Market
situation
More than one billion people use GSM
phones as of 2005, making GSM the dominant
mobile phone system worldwide with about 70% of
the world's market. GSM's main competitor,
CDMA2000, is used primarily in the United
States, Canada and Korea, although it was seeing
increased, but limited, worldwide adoption as a
stepping stone to a 3G standard when WCDMA did
not appear to be fully functional. It is
important to note that there is no such thing as
a standard CDMA handset, so roaming across CDMA
networks from different operators is difficult
or impossible, depending on the handset and
operators concerned. As WCDMA networks have
begun to take off, at least in high density
markets, GSM's rate of expansion may slow. This
seems likely to take some time, however.
A major reason for the growth in GSM usage,
particularly between 1998 to 2002, was the
availability of prepaid calling from mobile
phone operators. This allows people who are
either unable or unwilling to enter into a
contract with an operator to have mobile phones.
Prepaid also enabled the rapid expansion of GSM
in many developing countries where large
sections of the population do not have access to
banks or bank accounts and countries where there
are no effective credit rating agencies.
Radio
interface
GSM is a cellular network, which
means that mobile phones connect to it by
seaching for cells in the immediate vicinity.
GSM networks operate at various different radio
frequencies.
There are four different cell sizes in a GSM
network - Macro, micro, pico and umbrellacells.
The coverage area of each cell is different in
different environments. Macro cells can be
regarded as cells where the base station antenna
is installed in a mast or a building above
average roof top level. Micro cells are cells
whose antenna height is under average roof top
level; they are typically used in urban areas.
Picocells are small cells whose diameter is a
few dozen metres; they are mainly used indoors.
On the other hand, umbrellacells are used to
cover shadowed regions of smaller cells and fill
in gaps in coverage between those cells.
Cell radius varies depending on antenna height,
antenna gain and propagation conditions from a
couple of hundred meters to several tens of
kilometres. The longest distance the GSM
specification supports in practical use is 35
km. There is also a concept of an extended cell,
where the cell radius could be double or even
more.
Indoor coverage is also supported by GSM and is
achieved by using power splitters to deliver the
radio signal from the antenna outdoors to a
separate indoor antenna distribution system.
This is typically deployed when a lot of call
capacity is needed indoors, for example in
shopping centres or airports. However, this is
not a pre-requisite, since indoor coverage is
also provided by in-building penetration of the
radio signal.
Network
structure
The structure of a GSM network
The network behind
the GSM system seen by the customer is large and
complicated in order to provide all of the
services which are required. It is divided into
a number of sections and these are each covered
in separate articles.
the Base Station Subsystem (the base stations
and their controllers).
the Network and Switching Subsystem (the part of
the network most similar to a fixed network).
This is sometimes also just called the core
network.
the GPRS Core Network (the optional part which
allows packet based Internet connections).
all of the elements in the system combine to
produce many GSM services such as voice calls
and SMS.
Subscriber
Identity Module
One of the key features of GSM is the
Subscriber Identity Module (SIM), commonly known
as a SIM card. The SIM is a detachable smartcard
containing the user's subscription information
and phonebook. This allows the user to retain
his information after switching handsets.
Alternatively, the user can also change
operators while retaining the handset simply by
changing the SIM. Some operators will block this
by allowing the phone to use only a single SIM,
or only a SIM issued by them; this practice is
known as SIM locking, and is illegal in some
countries.
In the USA and Europe, most operators lock the
mobiles they sell. This is done because the
price of the mobile phone is usually subsidised
with revenue from subscriptions and operators
want to try to avoid subsidising competitor's
mobiles. A subscriber can usually contact the
provider to remove the lock for a fee (which
operators sometimes try to claim to be ignorant
of), utilize private services to remove the
lock, or make use of ample software and websites
available on the Internet to unlock the handset
themselves. Some providers in the USA, such as
T-Mobile and Cingular, will unlock the phone for
free if the customer has held an account for a
certain period. Third party unlocking services
exist that are often quicker and lower cost than
that of the operator. In most countries removing
the lock is legal.
GSM security
GSM was designed with a moderate
level of security. The system was designed to
authenticate the subscriber using shared-secret
cryptography. Communications between the
subscriber and the base station can be
encrypted. The development of UMTS introduces an
optional USIM, that uses a longer authentication
key to give greater security, as well as
mutually authenticating the network and the user
- whereas GSM only authenticated the user to the
network (and not vica versa). The security model
therefore offers confidentiality and
authentication, but limited authorization
capabilities, and no non-repudiation.
GSM uses several cryptographic algorithms for
security. The A5/1 and A5/2 stream ciphers are
used for ensuring over-the-air voice privacy.
A5/1 is a stronger algorithm used within Europe;
A5/2 is weaker and used in other countries.
Serious weaknesses have been found in both
algorithms, and it is possible to break A5/2 in
real-time in a ciphertext-only attack. The
system supports multiple algorithms so operators
may replace that cipher with a stronger one.
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