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Aim
: To implement a case study on GSM.

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Requirements
: MS Word, Internet.

Theory
: Introduction to GSM
GSM is a
mobile communication modem; it is stands for global system for mobile
communication (GSM). The idea of GSM was developed at Bell
Laboratories in 1970.  It is widely used mobile communication
system in the world. GSM is an open and digital cellular technology
used for transmitting mobile voice and data services operates at the
850MHz, 900MHz, 1800MHz and 1900MHz frequency bands.

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)

The security
strategies standardized for the GSM system make it the most secure
telecommunications standard currently accessible. Although the
confidentiality of a call and secrecy of the GSM subscriber is just
ensured on the radio channel, this is a major step in achieving
end-to- end security.

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.

The
generic GSM network architecture is composed of three subsystems as
the Radio Subsystem (RSS), the network and Switching Subsystem (NSS)
and the Operation Subsystem (OSS). The subscriber carries the Mobile
Station, which is part or RSS.

 
The
RSS basically consists of radio specific equipment such as mobile
station (MS), Base Station Subsystem (BSS) to control the radio link.
The connection between RSS and NSS is established with A interface
based on circuit switched PCM·30 system with 2.048 Mbit/s date rate.
The chief components of ass are BSS, cellular layout and base station
controller (BSC).

Radio
Substation (RSS)
The
RSS consists of the components that are necessary in order to
allocate the radio resources to the individual subscribers. It
principally consists of the mobile terminals (mobile phone or mobile
station, MS) and the base station subsystem (BSS).
Base
Station Subsystem (BSS)
The
Mobile Station and the Base Station Subsystem communicate across the
Um interface, also known as the air interface or radio link. A BSS is
controlled by a BSC.A BSS maintains radio connections to an MS,
coding/decoding of voice and data rate adaptation to/from the
wireless network part. There may be many BSS in a GSM network and
each BSS contains several MS. Base transceiver station (BTS) and a
base station controller (BSC) along with the cellular layout. These
communicate across a standardized Abis interface, allowing operation
between components made by different suppliers.

Mobile
Station (MS)
MS
is basically mobile equipment (the handset) which comprises all user
equipment and software needed for mobile communication and a smart
card called the Subscriber Identity Module (SIM). The MS, which
contain a SIM, card in the form of a very small chip bside the
equipment.
 
The
Subscriber Identity Module (SIM) contains all subscriber information
necessary for identifying GSM subscriber. Broadly, it holds a
subscriber’s International Mobile Subscriber Identity (IMSI),
authentication key and algorithm. SIM is independent of the device or
handset In which it is being used because an MS can be identified via
International Mobile Equipment Identity (IMEI). As soon as the SIM is
inserted into handset, it becomes immediately programmed for use.
Therefore, it can be inserted into any handset. If you have forgotten
to carry your handset but are carrying your SIM card, it can be
inserted in any borrowed phone for use. Without SIM a handset can
access only emergency services. Advances in memory and processing
capacity has enabled SIM cards to be programmed to display custom
menus for personalized services and therefore, makes it different
from conventional cellular phones, Typically, mobile stations have
transmitted power from 2 W to 1W depending upon the cell size, If
cell size is smaller, the transmitted power will be less.

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.
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.

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.
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.

Conclusion
: Thus we have studied a case study on GSM architecture.

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