What is GSM?
The Global System for Mobile Communications
(GSM) is the most popular standard for mobile phones
in the world. GSM service is used by over 2 billion
people across more than 210 countries and territories
[1] [2]. The ubiquity of the GSM standard makes international
roaming very common between mobile phone operators,
enabling subscribers to use their phones in many parts
of the world. GSM differs significantly from its predecessors
in that both signaling and speech channels are Digital
call quality, which means that it is considered 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 3rd Generation
Partnership Project (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.
Like other cellular standards GSM allows network operators
to offer roaming services which mean subscribers can
use their phones all over the world.
As the GSM standard continued to develop, it retained
backward compatibility with the original GSM phones;
for example, packet data capabilities were added in
the Release '97 version of the standard, by means
of GPRS. Higher speed data transmission has also been
introduced with EDGE in the Release '99 version of
the standard.
Radio interface
GSM is a cellular network, which means that mobile
phones connect to it by searching for cells in the
immediate vicinity. GSM networks operate in four different
radio frequencies. Most GSM networks operate in the
900 MHz or 1800 MHz bands. Some countries in the Americas
(including the USA and Canada) use the 850 MHz and
1900 MHz bands because the 900 and 1800 MHz frequency
bands were already allocated.
The rarer 400 and 450 MHz frequency
bands are assigned in some countries, notably Scandinavia,
where these frequencies were previously used for first-generation
systems.
In the 900 MHz band the uplink frequency band is 890-915
MHz, and the downlink frequency band is 935-960 MHz.
This 25 MHz bandwidth is subdivided into 124 carrier
frequency channels, each spaced 200 kHz apart. Time
division multiplexing is used to allow eight speech
channels per radio frequency channel. There are eight
radio timeslots (giving eight burst periods) grouped
into what is called a TDMA frame. The channel data
rate is 270.833 kbit/s, and the frame duration is
4.615 ms
The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.
GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter, leaving behind a residual signal. Speech is encoded at 13 kbit/s.
There are four different cell sizes in a GSM network - macro, micro, pico and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on 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 meters; they are mainly used indoors. On the other hand, umbrella cells 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 kilometers. The longest distance the GSM specification supports in practical use is 35 km or 22 miles. There are also several implementations of the concept of an extended cell, where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.
Indoor coverage is also supported by
GSM and may be achieved by using an indoor picocell
base station, or an indoor repeater with distributed
indoor antennas fed through power splitters, to deliver
the radio signals from an antenna outdoors to the
separate indoor distributed antenna system. These
are typically deployed when a lot of call capacity
is needed indoors, for example in shopping centers
or airports. However, this is not a pre-requisite,
since indoor coverage is also provided by in-building
penetration of the radio signals from nearby cells.
The modulation used in GSM is Gaussian minimum shift
keying (GMSK), a kind of continuous-phase frequency
shift keying. In GMSK, the signal to be modulated
onto the carrier is first smoothed with a Gaussian
low-pass filter prior to being fed to a frequency
modulator, which greatly reduces the interference
to neighboring channels (adjacent channel interference).
Network structure
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 smart card 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 United States, Europe and Australia, many operators lock the mobiles they sell. This is done because the price of the mobile phone is typically 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, utilize private services to remove the lock, or make use of ample software and websites available on the Internet to unlock the handset themselves. While most web sites offer the unlocking for a fee, some do it for free. The locking applies to the handset, identified by its International Mobile Equipment Identity (IMEI) number, not to the account (which is identified by the SIM card). It is always possible to switch to another (non-locked) handset.
Some providers in the USA and Europe,
such as T-Mobile, Cingular and the three French Operators,
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.
A curious exception to this rule is Belgium, where
all phones are sold unlocked. However, it is unlawful
for operators there to offer any form of subsidy on
the phone's price. This was also the case in Finland
until April the 1st 2006, when selling subsidized
combinations of handsets and accounts became legal
though operators have to unlock phone free of charge
after a certain amount of time (at most 24 months)!
GSM security
GSM was designed with a moderate level of security.
The system was designed to authenticate the subscriber
using sharedsecret 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 vice 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 was developed first and is a stronger algorithm used within Europe and the United States; A5/2 is weaker and used in other countries. A large security advantage of GSM over earlier systems is that the Ki, the crypto variable stored on the SIM card that is the key to any GSM ciphering algorithm, is never sent over the air interface. 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.
Patent issues
In 2005, a number of companies (including Cisco Systems
and Ericsson) were sued for infringement of U.S. Patent
No. 5,561,706, for offering products alleged to be
compliant with the GSM 3.60 standard.
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