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Introduction:

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GSM
(Global System for Mobile Communications) is a standard
developed by the European Telecommunications Standards Institute
(ETSI) to describe the protocols for second-generation digital
cellular networks used by mobile devices such as tablets, first
deployed in Finland in December 1991.As of 2014, it has become the
global standard for mobile communications – with over 90% market
share, operating in over 193 countries and territories.
GSM uses a variation
of time division multiple access (TDMA) and is the most widely used
of the three digital wireless telephony technologies (TDMA, GSM, and
CDMA). GSM digitizes and compresses data, then sends it down a
channel with two other streams of user data, each in its own time
slot. It operates at either the 900 MHz or 1800 MHz frequency band.

Features
of GSM Module:

Improved spectrum efficiency

International roaming

Compatibility with integrated
services digital network (ISDN)

Support for new services.

SIM phonebook management

Fixed dialing number (FDN)

Real time clock with alarm
management

High-quality speech

Uses encryption to
make phone calls more secure

Short message service (SMS)

Block Diagram of GSM:

The GSM network architecture consists of three major subsystems:

Mobile Station (MS)

Base Station Subsystem (BSS)

Network and Switching Subsystem
(NSS)

The wireless link interface between the MS and the Base
Transceiver Station (BTS), which is a part of BSS. Many BTSs are
controlled by a Base Station Controller (BSC). BSC is connected to
the Mobile Switching Center (MSC), which is a part of NSS. Figure
shows the key functional elements in the GSM network 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.
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.
IWF-

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.
Serivces offered by 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.