GSM - Global System for
Mobile Communications 
Overview
GSM (Global System for Mobile Communications) is
a European digital communications standard which provides full
duplex data traffic to any device fitted with GSM capability, such
as a phone, fax, or pager, at a rate of 9600 bps using the TDMA
communications scheme. Since GSM is purely digital, it can easily
interface with other digital communications systems, such as ISDN,
and digital devices, such as Group 3 facsimile machines. Unlike any
other service, GSM products such as cellular phones, require the use
of a Subscriber Identity Module, or SIM card. These small electronic
devices are aproximatly the size of a credit card and record all of
the user information it. This includes data such as programmed
telephone numbers and network security features which identify the
user. Without this module, the device will not function. This allows
for greater security and also greater ease of use as this card may
be transported from one phone to another, while maintaining the same
information available to the user. GSM is also present outside of
Europe but known by different names. In North America it is known as
PCS 1900 and elsewhere are DCS 1800 (also known as PCS). The only
difference between these systems is the frequency at which operate.
The number stands for the operating frequency in megahertz. While
each system uses the GSM standard, they are not compatible with each
other.
Technology
Services provided by GSM
From the beginning, the planners of GSM wanted ISDN compatibility in
terms of the services offered and the control signalling used.
However, radio transmission limitations, in terms of bandwidth and
cost, do not allow the standard ISDN B-channel bit rate of 64 kbps
to be practically achieved.
Using the ITU-T definitions, telecommunication services can be
divided into bearer services, teleservices, and supplementary
services. The most basic teleservice supported by GSM is telephony.
As with all other communications, speech is digitally encoded and
transmitted through the GSM network as a digital stream. There is
also an emergency service, where the nearest emergency-service
provider is notified by dialing three digits (similar to 911).
A variety of data services is offered. GSM users can send and
receive data, at rates up to 9600 bps, to users on POTS (Plain Old
Telephone Service), ISDN, Packet Switched Public Data Networks, and
Circuit Switched Public Data Networks using a variety of access
methods and protocols, such as X.25 or X.32. Since GSM is a digital
network, a modem is not required between the user and GSM network,
although an audio modem is required inside the GSM network to
interwork with POTS.
Other data services include Group 3 facsimile, as described in ITU-T
recommendation T.30, which is supported by use of an appropriate fax
adaptor. A unique feature of GSM, not found in older analog systems,
is the Short Message Service (SMS). SMS is a bidirectional service
for short alphanumeric (up to 160 bytes) messages. Messages are
transported in a store-and-forward fashion. For point-to-point SMS,
a message can be sent to another subscriber to the service, and an
acknowledgement of receipt is provided to the sender. SMS can also
be used in a cell-broadcast mode, for sending messages such as
traffic updates or news updates. Messages can also be stored in the
SIM card for later retrieval [2].
Supplementary services are provided on top of teleservices or bearer
services. In the current (Phase I) specifications, they include
several forms of call forward (such as call forwarding when the
mobile subscriber is unreachable by the network), and call barring
of outgoing or incoming calls, for example when roaming in another
country. Many additional supplementary services will be provided in
the Phase 2 specifications, such as caller identification, call
waiting, multi-party conversations.
Architecture of the GSM network
A GSM network is composed of several functional entities, whose
functions and interfaces are specified. Figure 1 shows the layout of
a generic GSM network. The GSM network can be divided into three
broad parts. The Mobile Station is carried by the subscriber. The
Base Station Subsystem controls the radio link with the Mobile
Station. The Network Subsystem, the main part of which is the Mobile
services Switching Center (MSC), performs the switching of calls
between the mobile users, and between mobile and fixed network
users. The MSC also handles the mobility management operations. Not
shown is the Operations and Maintenance Center, which oversees the
proper operation and setup of the network. The Mobile Station and
the Base Station Subsystem communicate across the Um interface, also
known as the air interface or radio link. The Base Station Subsystem
communicates with the Mobile services Switching Center across the A
interface.
Figure 1. General architecture of a GSM network
Mobile Station
The mobile station (MS) consists of the mobile equipment (the
terminal) and a smart card called the Subscriber Identity Module (SIM).
The SIM provides personal mobility, so that the user can have access
to subscribed services irrespective of a specific terminal. By
inserting the SIM card into another GSM terminal, the user is able
to receive calls at that terminal, make calls from that terminal,
and receive other subscribed services.
The mobile equipment is uniquely identified by the International
Mobile Equipment Identity (IMEI). The SIM card contains the
International Mobile Subscriber Identity (IMSI) used to identify the
subscriber to the system, a secret key for authentication, and other
information. The IMEI and the IMSI are independent, thereby allowing
personal mobility. The SIM card may be protected against
unauthorized use by a password or personal identity number.
Base Station Subsystem
The Base Station Subsystem is composed of two parts, the Base
Transceiver Station (BTS) and the Base Station Controller (BSC).
These communicate across the standardized Abis interface, allowing
(as in the rest of the system) operation between components made by
different suppliers.
The Base Transceiver Station houses the radio tranceivers that
define a cell and handles the radio-link protocols with the Mobile
Station. In a large urban area, there will potentially be a large
number of BTSs deployed, thus the requirements for a BTS are
ruggedness, reliability, portability, and minimum cost.
The Base Station Controller manages the radio resources for one or
more BTSs. It handles radio-channel setup, frequency hopping, and
handovers, as described below. The BSC is the connection between the
mobile station and the Mobile service Switching Center (MSC).
Network Subsystem
The central component of the Network Subsystem is the Mobile
services Switching Center (MSC). It acts like a normal switching
node of the PSTN or ISDN, and additionally provides all the
functionality needed to handle a mobile subscriber, such as
registration, authentication, location updating, handovers, and call
routing to a roaming subscriber. These services are provided in
conjuction with several functional entities, which together form the
Network Subsystem. The MSC provides the connection to the fixed
networks (such as the PSTN or ISDN). Signalling between functional
entities in the Network Subsystem uses Signalling System Number 7
(SS7), used for trunk signalling in ISDN and widely used in current
public networks.
The Home Location Register (HLR) and Visitor Location Register (VLR),
together with the MSC, provide the call-routing and roaming
capabilities of GSM. The HLR contains all the administrative
information of each subscriber registered in the corresponding GSM
network, along with the current location of the mobile. The location
of the mobile is typically in the form of the signalling address of
the VLR associated with the mobile station. The actual routing
procedure will be described later. There is logically one HLR per
GSM network, although it may be implemented as a distributed
database.
The Visitor Location Register (VLR) contains selected administrative
information from the HLR, necessary for call control and provision
of the subscribed services, for each mobile currently located in the
geographical area controlled by the VLR. Although each functional
entity can be implemented as an independent unit, all manufacturers
of switching equipment to date implement the VLR together with the
MSC, so that the geographical area controlled by the MSC corresponds
to that controlled by the VLR, thus simplifying the signalling
required. Note that the MSC contains no information about particular
mobile stations --- this information is stored in the location
registers.
The other two registers are used for authentication and security
purposes. The Equipment Identity Register (EIR) is a database that
contains a list of all valid mobile equipment on the network, where
each mobile station is identified by its International Mobile
Equipment Identity (IMEI). An IMEI is marked as invalid if it has
been reported stolen or is not type approved. The Authentication
Center (AuC) is a protected database that stores a copy of the
secret key stored in each subscriber's SIM card, which is used for
authentication and encryption over the radio channel.
GSM - Global System for
Mobile Communications
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