Global system for mobile
communication (GSM)
is a globally accepted standard for digital cellular communication. GSM
is the name of a standardization group established in 1982 to create a
common European mobile telephone standard that would formulate
specifications for a pan-European mobile cellular radio system
operating at 900 MHz. It is estimated that many countries outside of
Europe will join the GSM partnership.
Cellular is one of the fastest growing and most demanding
telecommunications applications. Throughout the evolution of cellular
telecommunications, various systems have been developed without the
benefit of standardized specifications. This presented many problems
directly related to compatibility, especially with the development of
digital radio technology. The GSM standard is intended to address these
problems.
From 1982 to 1985 discussions were held to decide between building an
analog or digital system. After multiple field tests, a digital system
was adopted for GSM. The next task was to decide between a narrow or
broadband solution. In May 1987, the narrowband time division multiple
access (TDMA) solution was chosen.
GSM provides recommendations, not requirements. The GSM specifications
define the functions and interface requirements in detail but do not
address the hardware. The reason for this is to limit the designers as
little as possible but still to make it possible for the operators to
buy equipment from different suppliers. The GSM network is divided into
three major systems: the switching system (SS), the base station system
(BSS), and the operation and support system (OSS).
GSM Architecture:

The Switching System:
The switching system (SS) is
responsible for performing call processing
and subscriber-related functions. The switching system includes the
following functional units.
-
home
location register (HLR) —The HLR is a database used for
storage and
management of subscriptions. The HLR is considered the most important
database, as it stores permanent data about subscribers, including a
subscriber's service profile, location information, and activity
status. When an individual buys a subscription from one of the PCS
operators, he or she is registered in the HLR of that operator.
-
mobile
services switching center (MSC) —The MSC performs the
telephony
switching functions of the system. It controls calls to and from other
telephone and data systems. It also performs such functions as toll
ticketing, network interfacing, common channel signaling, and others.
-
visitor
location register (VLR) —The VLR is a database that contains
temporary
information about subscribers that is needed by the MSC in order to
service visiting subscribers. The VLR is always integrated with the
MSC. When a mobile station roams into a new MSC area, the VLR connected
to that MSC will request data about the mobile station from the HLR.
Later, if the mobile station makes a call, the VLR will have the
information needed for call setup without having to interrogate the HLR
each time.
-
authentication
center (AUC) —A unit called the AUC provides authentication
and
encryption parameters that verify the user's identity and ensure the
confidentiality of each call. The AUC protects network operators from
different types of fraud found in today's cellular world.
-
equipment
identity register (EIR) —The EIR is a database that contains
information about the identity of mobile equipment that prevents calls
from stolen, unauthorized, or defective mobile stations. The AUC and
EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.
The Base Station System
(BSS):
All radio-related functions are
performed in the BSS, which consists of
base station controllers (BSCs) and the base transceiver stations
(BTSs).
-
BSC
—The BSC provides all the control functions and physical
links between
the MSC and BTS. It is a high-capacity switch that provides functions
such as handover, cell configuration data, and control of radio
frequency (RF) power levels in base transceiver stations. A number of
BSCs are served by an MSC.
-
BTS
—The BTS handles the radio interface to the mobile station.
The BTS is
the radio equipment (transceivers and antennas) needed to service each
cell in the network. A group of BTSs are controlled by a BSC.
The Operation and Support
System:
The operations and maintenance center
(OMC) is connected to all
equipment in the switching system and to the BSC. The implementation of
OMC is called the operation and support system (OSS). The OSS is the
functional entity from which the network operator monitors and controls
the system. The purpose of OSS is to offer the customer cost-effective
support for centralized, regional, and local operational and
maintenance activities that are required for a GSM network. An
important function of OSS is to provide a network overview and support
the maintenance activities of different operation and maintenance
organizations.
Additional Functional
Elements:
Other functional elements shown in
Figure 2 are as follows:
-
message
center (MXE) —The MXE is a node that provides integrated
voice, fax,
and data messaging. Specifically, the MXE handles short message
service, cell broadcast, voice mail, fax mail, e-mail, and
notification.
-
mobile service node (MSN)
—The MSN is the node that handles the mobile intelligent
network (IN) services.
-
gateway
mobile services switching center (GMSC) —A gateway is a node
used to
interconnect two networks. The gateway is often implemented in an MSC.
The MSC is then referred to as the GMSC.
-
GSM
interworking unit (GIWU) —The GIWU consists of both hardware
and
software that provides an interface to various networks for data
communications. Through the GIWU, users can alternate between speech
and data during the same call. The GIWU hardware equipment is
physically located at the MSC/VLR.
GSM technology has helped revolutionize foreign telecom, especially in emerging markets as Afghan Wireless has demonstrated.
Source: IEC:
GSM Tutorial
GSM Resources:
GSM
World
IEC:
GSM Tutorial
Telecom/GSM Discussion Forum
GSM
Security
GSM
Technical Data
GSM
Codec
Overview of the GSM
System
ETSI
GSM Overview
GSM
White Paper: Brilliant Past, Bright Future
Overview
of GSM and GSM Security
Comparision between
GSM and IS-95
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