
The GSM logo is used to identify
compatible handsets and equipment

GSM world coverage as of January
2009
GSM (
Global System for Mobile
communications: originally from
Groupe Spécial
Mobile) is the most popular standard for
mobile phones in the world. Its promoter, the
GSM Association, estimates that 80%
of the global mobile market uses the standard. GSM is used by over
3
billion people across more
than 212 countries and territories.Its ubiquity makes international
roaming very common between
mobile phone operators, enabling
subscribers to use their phones in many parts of the world. GSM
differs from its predecessors in that both signaling and speech
channels are
digital, and thus is considered
a
second generation (
2G) mobile phone
system. This has also meant that data communication was easy to
build into the system.
GSM EDGE is a
3G version of the protocol.
The ubiquity of the GSM standard has been an advantage to both
consumers (who benefit from the ability to roam and switch carriers
without switching phones) and also to network operators (who can
choose equipment from any of the many vendors implementing GSM).
GSM also pioneered a low-cost (to the network carrier) alternative
to voice calls, the
short message
service (SMS, also called "text messaging"), which is now
supported on other mobile standards as well. Another advantage is
that the standard includes one worldwide
emergency telephone number,
112. This makes it easier for international
travellers to connect to emergency services without knowing the
local emergency number.
Newer versions of the standard were backward-compatible with the
original GSM phones. For example,
Release
'97 of the standard added packet data capabilities, by means of
General Packet Radio
Service (GPRS). Release '99 introduced higher speed data
transmission using
Enhanced Data Rates for
GSM Evolution (EDGE).
History
In 1982, the
European Conference of Postal and Telecommunications
Administrations (
CEPT) created the
Groupe Spécial Mobile (GSM) to develop a
standard for a mobile telephone system that could be used across
Europe. In 1987, a
memorandum of understanding was
signed by 13 countries to develop a common cellular telephone
system across Europe. Finally the system created by
SINTEF lead by
Torleiv
Maseng was selected.
In 1989, GSM responsibility was transferred to the
European
Telecommunications Standards Institute (ETSI) and phase I of
the GSM specifications were published in 1990.
The first GSM network
was launched in 1991 by Radiolinja in
Finland
with joint technical infrastructure maintenance
from Ericsson. By the end of 1993,
over a million subscribers were using GSM phone networks being
operated by 70 carriers across 48 countries.
Technical details
Cellular radio network
GSM is a
cellular network, which
means that
mobile phones connect to it
by searching for cells in the immediate vicinity.
There are five different cell sizes in a GSM network—
macro,
micro,
pico,
femto 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 coverage
diameter is a few dozen metres; they are mainly used indoors.
Femtocells are cells designed for use in residential or small
business environments and connect to the service provider’s network
via a broadband internet connection. Umbrella cells are used to
cover shadowed regions of smaller cells and fill in gaps in
coverage between those cells.
Cell horizontal 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 . 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 prerequisite, since indoor coverage is also
provided by in-building penetration of the radio signals from any
nearby cell.
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).
Interference with audio devices
Some audio devices are susceptible to
radio frequency interference
(RFI), which could be mitigated or eliminated by use of additional
shielding and/or bypass capacitors in these audio devices. However,
the increased cost of doing so is difficult for a designer to
justify.
It is a common occurrence for a nearby GSM handset to induce a
"dit, dit di-dit, dit di-dit, dit di-dit" audio output on PAs,
wireless microphones, home stereo systems, televisions, computers,
cordless phones, and personal music devices. When these audio
devices are in the
near field of
the GSM handset, the radio signal is strong enough that the solid
state amplifiers in the audio chain act as a
detector. The clicking noise itself
represents the power bursts that carry the
TDMA signal. These signals
have been known to interfere with other electronic devices, such as
car stereos and portable audio players. This also depends on the
handset's design, and its conformance to strict rules and
regulations allocated by
Title 47
CFR Part 15 regulations in the United States, pertaining to
interference with electronic devices.
GSM frequencies

GSM world coverage as of 2008
GSM networks operate in a number of different frequency ranges
(separated into
GSM frequency
ranges for 2G and
UMTS
frequency bands for 3G). Most
2G GSM networks
operate in the 900 MHz or 1800 MHz bands. Some countries
in the Americas (including Canada and the United States) use the
850 MHz and 1900 MHz bands because the 900 and
1800 MHz frequency bands were already allocated. Most
3G GSM networks in Europe operate in the 2100 MHz
frequency band.
The rarer 400 and 450 MHz frequency bands are assigned in some
countries where these frequencies were previously used for
first-generation systems.
GSM-900 uses 890–915 MHz to send information from the
mobile station to the
base station (uplink) and
935–960 MHz for the other direction (downlink), providing 125
RF channels (channel numbers 1 to 124) spaced at 200 kHz.
Duplex spacing of 45 MHz is used.
In some countries the GSM-900 band has been extended to cover a
larger frequency range. This 'extended GSM', E-GSM, uses
880–915 MHz (uplink) and 925–960 MHz (downlink), adding
50 channels (channel numbers 975 to 1023 and 0) to the original
GSM-900 band.
Time division
multiplexing is used to allow eight full-rate or sixteen
half-rate speech channels per
radio
frequency channel. There are eight radio timeslots (giving
eight
burst periods) grouped into
what is called a
TDMA
frame. Half rate channels use alternate frames in the same
timeslot. The channel data rate for all 8 channels 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.
Voice codecs
GSM has used a variety of voice
codecs to
squeeze 3.1 kHz audio into between 6.5 and 13 kbit/s.
Originally, two codecs, named after the types of data channel they
were allocated, were used, called
Half
Rate (5.6 kbit/s) and
Full Rate
(13 kbit/s). These used a system based upon
linear predictive coding (LPC). In
addition to being efficient with bitrates, these codecs also made
it easier to identify more important parts of the audio, allowing
the air interface layer to prioritize and better protect these
parts of the signal.
GSM was further enhanced in 1997 with the
Enhanced Full Rate (EFR) codec, a
12.2 kbit/s codec that uses a full rate channel. Finally, with
the development of
UMTS, EFR was refactored
into a variable-rate codec called
AMR-Narrowband, which is high quality
and robust against interference when used on full rate channels,
and less robust but still relatively high quality when used in good
radio conditions on half-rate channels.
Network structure
The network behind the
GSM 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.
Subscriber Identity Module (SIM)
One of the key features of GSM is the
Subscriber Identity Module,
commonly known as a
SIM card. The SIM is a
detachable
smart card containing the
user's subscription information and phone book. This allows the
user to retain his or her 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
Australia,
North America and
Europe
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).
In some
countries such as Bangladesh
, Belgium
, Costa Rica
, Indonesia
, Malaysia
, Hong Kong
, Pakistan
and Singapore
, all phones are sold unlocked. However, in
Belgium, 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 1, 2006, when selling subsidized combinations of
handsets and accounts became legal (
3G phones
only), though operators have to unlock phones free of charge after
a certain period (at most 24 months).
In New Zealand, since May 2008, it is illegal for operators to lock
handsets, and any phones purchased locked in the country before
that date can be unlocked for free.
GSM security
GSM was designed with a moderate level of security. The system was
designed to authenticate the subscriber using a
pre-shared key and
challenge-response.
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
authenticates 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.
Serious weaknesses have been found in both algorithms: it is
possible to break A5/2 in real-time with a
ciphertext-only attack, and in
February 2008, Pico Computing, Inc revealed its ability and plans
to commercialize
FPGAs that allow A5/1 to be
broken with a
rainbow table attack.
The system supports multiple algorithms so operators may replace
that cipher with a stronger one.
Standards information
The GSM systems and services are described in a set of standards
governed by
ETSI, where a full list is
maintained.
Example specifications
- GSM 07.07 "AT command set for GSM Mobile Equipment (ME)"
describes the Main AT commands to communicate via a serial
interface with the GSM subsystem of the phone. For more, see
Hayes command set.
- 3GPP TS 27.007 - AT command set for User
Equipment (UE).
- GSM 07.05 has additional AT commands for SMS and CBS.
See also
References
- Siegmund M. Redl, Matthias K. Weber, Malcolm W. Oliphant (March
1995): "An Introduction to GSM", Artech House, ISBN
978-0890067857
- Siegmund M. Redl, Matthias K. Weber, Malcolm W. Oliphant (May
1998): "GSM and Personal Communications Handbook", Artech House,
ISBN 978-0890069578
- Friedhelm Hillebrand, ed. (2002): "GSM and UMTS, The Creation
of Global Mobile Communications", John Wiley & Sons, ISBN 0470
84322 5
- Michel Mouly, Marie-Bernardette Pautet (June 1992): "The GSM
System for Mobile Communications", ISBN 0945592159.
External links