GSM Today

Today, GSM is the first digital cellular mobile communication system to enable international
roaming and ISDN service characteristics.

GSM is an open standard for services, infrastructure and communication - independent of
the individual countries, network operators and producers, and flexible to the requirements of
the individual user.
All this has boosted the development of GSM:
• Now, over 300 network operators in 130 countries are offering GSM services.
• Over 150 million customers use GSM.
• A considerable share of the world-wide telecommunication market goes to GSM
products and -services.

The cellular structure in GSM provides an almost complete radio coverage. The system
allows a maximum distance of 35 kilometers between the mobile station and the base
station. Therefore, the geographical area where GSM is used must be subdivided into
smaller areas, which are known as cells.

The cell size must be adjusted to the subscriber density and to the environment. If a
subscriber moves from one cell to another during a call, the connection is handed over from
the old radio station to the new radio station, without any interruptions. This procedure is
called "handover".

GSM distinguishes different channels on the basis of their frequency. Each cell is assigned
one or more frequencies, which it uses to serve active subscribers. An uplink/downlink
frequency pair can be used by up to 8 mobile stations practically simultaneously. This is
done with the help of the Time Division Multiple Access (TDMA). Since there is only a limited
amount of available traffic channels, high subscriber numbers require that the frequencies be
used several times. This is not an insoluble problem, since several cells can use the same
frequency, provided that the distance between the cells is sufficient. But it sometimes makes
network planning a very complex task.

Thus GSM offers capacity for more subscribers than one would expect in view of the very
limited supply of frequency resources.

To guarantee flexibility, open interfaces are specified in GSM between particular network
elements. This way, network operators can be supplied by different producers. Nevertheless,
the interfaces' functionality is very well specified, to guarantee a smooth data transmission.
In GSM, there are two truly open interfaces.

The first one is the air interface, located between the mobile and the base station.
The other one is the A-interface between the Base Station Subsystem (BSS) and the
Network Subsystem (NSS). The GSM network structure is decentralized, and consists of
three separate subsystems communicating with each other over a series of interfaces. Apart
from the Network Subsystem for routing, and subscriber localisation, and the Base Station
Subsystem (BSS) for radio coverage and radio resource management, there is also the
Operation & Maintenance Subsystem (OMS), which guarantees network management and
administration.