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GSM
stands for Global
System for Mobile
Communication. GSM is a digital technology used for transmitting
mobile voice and data services. The concept of GSM emerged at Bell
Laboratories in 1970s. GSM is also a name of a standardization group
which was responible for the creation of Eurpean Mobile Telephone
Standard. GSM is used to accomodate more calls and tranfer more data
within the limited amount of radio spectrum available.

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GSM
is a circuit-switched system that divides each 200 kHz channel into
eight 25 kHz time-slots. GSM operates on the mobile communication
bands 900 MHz and 1800 MHz in most parts of the world. In the US, GSM
operates in the bands 850 MHz and 1900 Mhz. GSM
uses Time Division Multiple Access (TDMA) for transmitting signals.

GSM
is capable of carrying 64 kbps to 120 Mbps of data rates. GSM
provides basic to advanced voice and data services including roaming
service. Roaming is the ability to use your GSM phone number in
another GSM network.

Features
of GSM include the following :-

International roaming

Low-cost mobile sets and base stations
(BSs)

High-quality speech

Compatibility with Integrated Services
Digital Network (ISDN) and other telephone company services

Support for new services

Improved
spectrum efficiency

2.
Block Diagram of GSM Model

3.
Components Of GSM

A
GSM network comprises of many functional units. The GSM network can
be broadly divided into:

The Mobile Station (MS)

The Base Station Subsystem (BSS)

The Network Switching Subsystem (NSS)

The Operation Support System (OSS)

Pictorial
view of GSM Architecture :

1. Mobile Station (MS):
A mobile station communicates across the air
interface with a base station transceiver in the same cell in which
the mobile subscriber unit is located. The MS communicates the
information with the user and modifies it to the transmission
protocols if the air-interface to communicate with the BSS. The
user’s voice information is interfaced with the MS through a
microphone and speaker for the speech, keypad, and display for short
messaging, and the cable connection for other data terminals. The MS
has two elements. The Mobile Equipment (ME) refers to the physical
device, which comprises of transceiver, digital signal processors,
and the antenna. The second element of the MS is the GSM is the
Subscriber Identity Module (SIM). The SIM card is unique to the GSM
system. It has a memory of 32 KB.

2. Base Station Subsystem (BSS):
A base station subsystem consists of a base
station controller and one or more base transceiver station. Each
Base Transceiver Station defines a single cell. A cell can have a
radius of between 100m to 35km, depending on the environment. A Base
Station Controller may be connected with a BTS. It may control
multiple BTS units and hence multiple cells. There are two main
architectural elements in the BSS – the Base Transceiver Subsystem
(BTS) and the Base Station Controller (BSC). The interface that
connects a BTS to a BSC is called the A-bis interface. The interface
between the BSC and the MSC is called the A interface, which is
standardised within GSM.

3. Network and switching subsystem (NSS)
The NSS is responsible for the network
operation. It provides the link between the cellular network and the
Public switched telecommunicates Networks (PSTN or ISDN or Data
Networks). The NSS controls handoffs between cells in different BSSs,
authenticates user and validates their accounts, and includes
functions for enabling worldwide roaming of mobile subscribers. In
particular the switching subsystem consists of:

Mobile switch
center (MSC)

Home location
register (HLR)

Visitor location
Register (VLR)

Authentications
center (Auc)

Equipment
Identity Register (EIR)

Interworking Functions (IWF)

The NSS has one hardware, Mobile switching
center and four software database element: Home location register
(HLR), Visitor location Register (VLR), Authentications center (Auc)
and Equipment Identity Register (EIR). The MSC basically performs the
switching function of the system by controlling calls to and from
other telephone and data systems. It includes functions such as
network interfacing and common channel signalling.

4. Operation
Support Subsystem
(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
components included in NSS also have various features and perform
various functions which
are as follows –

HLR:

The HLR is database software that handles the
management of the mobile subscriber account. It stores the
subscriber address, service type, current locations, forwarding
address, authentication/ciphering keys, and billings information. In
addition to the ISDN telephone number for the terminal, the SIM card
is identified with an International Mobile Subscribes Identity
(IMSI) number that is totally different from the ISDN telephone
number. The HLR is the reference database that permanently stores
data related to subscribers, including subscriber’s service
profile, location information, and activity status.
VLR:

The VLR is temporary database software similar
to the HLR identifying the mobile subscribers visiting inside the
coverage area of an MSC. The VLR assigns a Temporary mobile
subscriber Identity (TMSI) that is used to avoid using IMSI on the
air. The visitor location register maintains information about
mobile subscriber thatis currently physically in the range covered
by the switching center. When a mobile subscriber roams from one LA
(Local Area) to another, current location is automatically updated
in the VLR. When a mobile station roams into anew MSC area, if the
old and new LA’s are under the control of two different VLRs, the
VLR connected to the MSC will request data about the mobile stations
from the HLR. The entry on the old VLR is deleted and an entry is
created in the new VLR by copying the database from the HLR.
AuC:
The AuC database holds different algorithms that
are used for authentication and encryptions of the mobile
subscribers that verify the mobile user’s identity and ensure the
confidentiality of each call. The AuC holds the authentication and
encryption keys for all the subscribers in both the home and visitor
location register.
EIR:
The EIR is another database that keeps the
information about the identity of mobile equipment such the
International mobile Equipment Identity (IMEI) that reveals the
details about the manufacturer, country of production, and device
type. This information is used to prevent calls from being misused,
to prevent unauthorised or defective MSs, to report stolen mobile
phones or check if the mobile phone is operating according to the
specification of its type.
White list:
This list contains the IMEI of the phones who
are allowed to enter in the network.
Black list:
This list on the contrary contains the IMEI of
the phones who are not allowed to enter in the network, for example
because they are stolen.

Grey list:
This list contains the IMEI of the phones
momentarily not allowed to enter in the network, for example because
the software version is too old or because they are in repair.

Interworking Function-

Interworking
Function: It is a system in the PLMN that allows for
non speech communication between the GSM and the other networks. The
tasks of an IWF are particularly to adapt transmission parameters
and protocol conversions. The physical manifestations of an IWF may
be through a modem which is activated by the MSC dependent on the
bearer service and the destination network. The OSS (Operational
Support Systems) supports operation and maintenance of the system
and allows engineers to monitor, diagnose, and troubleshoot every
aspect of the GSM network.

4.
Services Of GSM
GSM offers much more than just voice telephony.
Contact your local GSM network operator to the specific services that
you can avail.
GSM offers three basic types of services:

Telephony services or
teleservices

Data services or bearer services

Supplementary services

Teleservices
The abilities of a Bearer Service are used by a Teleservice to
transport data. These services are further transited in the
following ways:
Voice
Calls
The most basic Teleservice supported by GSM is telephony. This
includes full-rate speech at 13 kbps and emergency calls, where the
nearest emergency-service provider is notified by dialing three
digits.
Videotext
and Facsmile
Another group of teleservices includes Videotext access,
Teletex transmission, Facsmile alternate speech and Facsmile
Group 3, Automatic Facsmile Group, 3 etc.
Short
Text Messages
Short Messaging Service (SMS) service is a text messaging
service that allows sending and receiving text messages on your
GSM mobile phone. In addition to simple text messages, other text
data including news, sports, financial, language, and
location-based data can also be transmitted.
Bearer
Services
Data services or Bearer Services are used through a GSM phone.
to receive and send data is the essential building block leading to
widespread mobile Internet access and mobile data transfer. GSM
currently has a data transfer rate of 9.6k. New developments that
will push up data transfer rates for GSM users are HSCSD (high speed
circuit switched data) and GPRS (general packet radio service) are
now available.
Supplementary
Services
Supplementary services are additional services that are provided
in addition to teleservices and bearer services. These services
include caller identification, call forwarding, call waiting,
multi-party conversations, and barring of outgoing (international)
calls, among others. A brief description of supplementary services
is given here:

Conferencing : It allows a mobile subscriber to
establish a multiparty conversation, i.e., a simultaneous
conversation between three or more subscribers to setup a conference
call. This service is only applicable to normal telephony.

Call Waiting : This service notifies a mobile
subscriber of an incoming call during a conversation. The subscriber
can answer, reject, or ignore the incoming call.

Call Hold : This service allows a subscriber to put
an incoming call on hold and resume after a while. The call hold
service is applicable to normal telephony.

Call Forwarding : Call Forwarding is used to divert
calls from the original recipient to another number. It is normally
set up by the subscriber himself. It can be used by the subscriber
to divert calls from the Mobile Station when the subscriber is not
available, and so to ensure that calls are not lost.

Call Barring : Call Barring is useful to restrict
certain types of outgoing calls such as ISD or stop incoming calls
from undesired numbers. Call barring is a flexible service that
enables the subscriber to conditionally bar calls.

Number Identification : There are following
supplementary services related to number identification:

Calling Line Identification Presentation : This
service displays the telephone number of the calling party on your
screen.

Calling Line Identification Restriction : A person
not wishing their number to be presented to others subscribes to
this service.

Connected Line Identification Presentation : This
service is provided to give the calling party the telephone number
of the person to whom they are connected. This service is useful in
situations such as forwarding’s where the number connected is not
the number dialled.

Connected Line Identification Restriction : There
are times when the person called does not wish to have their number
presented and so they would subscribe to this person. Normally,
this overrides the presentation service.

Malicious Call Identification : The malicious call
identification service was provided to combat the spread of obscene
or annoying calls. The victim should subscribe to this service, and
then they could cause known malicious calls to be identified in the
GSM network, using a simple command.

Advice of Charge (AoC) : This service was designed
to give the subscriber an indication of the cost of the services as
they are used. Furthermore, those service providers who wish to
offer rental services to subscribers without their own SIM can also
utilize this service in a slightly different form. AoC for data
calls is provided on the basis of time measurements.

Closed User Groups (CUGs) : This service is meant
for groups of subscribers who wish to call only each other and no
one else.

Unstructured supplementary services data (USSD) :
This allows operator-defined individual services.