adfa gsm bijoy

8/13/2019 Adfa Gsm Bijoy

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1

Presentation Objectives

Introduction to GSM

Learning about GSM Network Architecture

Network components

Radio Interface

Cell structure

Frequency re-use and tri-sector antenna

Learning about GSM Operational Principles

Call delivery

Location Updating

Speech and Data transmission

Understanding of GSM Localization

Recognizing the Services provided by GSM

Understanding of GSM Security Issues

Conclusions



2

Introduction to GSM

The most popular 2G cellular standard developed to cater

voice services and data delivery using digital

modulation.

Development background:

Developed by Grou pe Spéci ale Mob il e (founded

1982) which was an initiative of CEPT (Conference

of European Post and Telecommunication).

In 1989, ETSI (European Telecommunications

Standards Institute) took the control of GSM and

defined new acronym “Global System for Mobile

Communications.”

Commercial use has been started since 1991.

Features of GSM Standards:

Supports international roaming and

handheld terminals.

Good subjective speech quality and

wide-range of new services.

Low cost (?).

Compatible with other systems, say

ISDN and PSTN.

In March 2005, Around 1.3 billions subscribers in more than 135 countries(more than 70% world market) used GSM. Among them 43% users are

from Europe and 40% users are from Asia specific region.



3

Network Components: GSM Architecture

Base TransceiverSystem (BTS)

Base StationController (BSC)

Base Station (BS)

VLR HLR

EIR AuC

Mobile Switching Centre

(MSC)

PSTN

Other

GSM

Mobile

Terminal (MT)

Wireless Connection

P2P Wireless or optic fiber connection

There are three main components of GSMnetworks:

Mobile Terminal (MT)

Mobile equipment for transmitting and receiving signals.

Subscriber Identity Module (SIM) for storing necessarypermanent and temporary data.

Base Station (BS)

Base Transceiver System (BTS) for

transmitting and receiving signals; and

manipulating signals such as encoding/decoding,encrypting, multiplexing and modulating.

Base station Controller (BSC) for assigning and managing resources, controlling handoff and power level, etc.

Mobile Switching Centre (MSC)

Manage communications, mobility, and billinginformation.

Home Location Register (HLR): central master database for users under the MSC.

Visitor location Register (VLR): Local database for the users currently under the domain of MSC.

Authentication Center (AuC): Authenticate mobile terminals and encrypt user data.

Equipment Identity Register (EIR): register MTs and locked stolen or malfunctioning MTs.



4

Radio Interface: GSM Architecture

Most GSM Networks operate at 900 MHz and/or 1800MHz,

850 MHz and/or 1900MHz in Parts of Americas (including USA and Canada).

900 MHz Frequency Band

Uplink

Downlink

TDMA Frame:

Each frame contains 8 time slots.

Channel data rate: 270.833 kbps

Frame duration: 4.615 mS

There are two types of logicalchannels:

Traffic channels

Signaling Channels – for broadcasting,common control, and dedicated control.124 carriers

890MHz 915MHz 935MHz 960MHz

Downlink Uplink

124 carriers

Each Carrier has 200KHz Frequency band

TDMA divides each RF channel (i.e., carrier)to 8 voice Channel

t1 t2 t3 t4 t5 t6 t7 t8

Time-slots

Time slot duration 577 µs



5

Cell Structure: GSM Architecture

Cell is a area covered by one BS.

GSM Network Areas

Public Land Mobile Network(one operator’s network)

MSC/VLR Service Area(Covered by one MSC)

Location Area

(one MSC covers severallocation areas)

Cell(Covered by one BS)

Cell Representation

Depending on the sizes, there are four types of cells:

Macro-cell: 5-35 km radius. For rural or suburban areas. Micro-cell: 1-5 km radius. Used in town or urban areas.

Pico-cell: radius <100 meters. For a campus area.

Umbrella-cell: Fill-up the gaps between two cells or covershadow areas of a cell.

Macro-cell Micro-cellUrban

Pico-cell

Suburban

In buildingcommunications

Cell radius varies depending on antenna height, antenna gain, and condition of propagation.

GSM supports the longest distance up to 35 km,

because, Timing limitations: Longer distance takes more propagation

time. So, utilization decreases with the increase is guard timebetween two time-slots.

Interference: More noises are added and signal is attenuatedwith the increase in distance, requiring more power level.

Limiting User: Longer cell radius reduces number of users.Ideal cells

R

Fictitious cells

R

R = Radius of cell



6

Frequency re-use: GSM Architecture

For the purpose of filtering a given frequency used by a cell, a distance should bemaintained between two cells using same frequency.

For reusing frequency, cells are divided into clusters (each having K cells) so thata frequency band is used by only one cell of a cluster.

Tri-Sector Antenna

Used for 1 cell:

Cell SplittingUsed for 3 cells

Impacts of reducing cell-radius or

splitting cells: Increase cellular systems capacity.

Decrease transmission power in BS andMT

The Cell Structure for K = 7

1

2

3

4

5

6

7

1

2

3

4

5

6

7

1

2

3

4

5

6

7

4.5873

3Distance,

R R

R K D

D

R = Radius of the cell.



7

Call delivery: GSM Operational Principles

MT BS1. Call Request

10. Reply

MSC

2 . C al l R e q u e s t

9 . R e p l y

HLR

VLR

3. LocationRequest

MSC

HLR

VLR

4. RouteRequest

4. RouteRequest5. Route

Request

7. Forwardinformation

8. Send Requestto called MSC

TLDN: Temporary localdirectory number.

1-2. Send call request from calling MT to MSC

via BS.3. MSC determines the address of HLR of

the called MT and sends locationrequest message.

Call delivery Steps:

4-5. HLR determines the serving VLR of thecalled MT and send route request message. VLR then send the message

to the MSC serving the MT.6. MSC allocates a TLDN to the MT

and reply to HLR with TLDN.

7. HLR forward information to theMSC of the calling MT.

8. Calling MSC requests a call

set up to the called MSCthrough SS7 networks.



8

Location Updating: GSM Operational Principles

MT BS1. Location Update

MSC

2 .L o c a t i on

U p d a t e

HLR

VLR

Location Updating Steps:

VLR

If VLR Changes.

3. Registrationquery

old

4 .L o c a t i on

R e gi s t r a t i on

5 . R e g i s t r a t i o n

A c

k n o w l e d g e m e n t

6. RegistrationCancellation

7. CancellationAcknowledgement

1-2: When the MT enters a new LA, Location updatemessage is sent to MSC via BS.

3: MSC launches a registration query to VLR andVLR updates the records.

4: If new LA belongs to a different VLR, the new VLR

determines the address of HLR of the MT frommobile identification number (MIN).

sends location registration message to HLR.

5: HLR authenticates the MT and records informationof new VLR of the MT.

6-7: HLR sends a registration cancellation message toold VLR. Old VLR then remove records andreturns a cancellation acknowledgementmessage to the HLR.



9

Speech and Data Transmission: GSM Operation

Input SpeechSpeechencoder

Dataencryption

DataModulator

10101 10101

Speech

decoder

GSM to

PSTN

PSTN to

GSM

Speech

encoder

Data

demodulator

Data

decryption

Speech

Decoder

10101 10101Output Speech

Add-on module in standard GSM handset

Add-on module in standard GSM handset

Base station subsystemBase station subsystem64-bit PCMwaveform

GSM uses Gaussian-Filtered Minimum shift Keying (GMSK) Modulation.



10

GSM Localization

GSM localization is the use of GSM

mobile phones to determine the locationof the user. Location of an MT can be determined using Time

of Arrival (TOA), Time difference of Arrival

(TDOA), and Angle of Arrival (AOA) measures.

Location measurement from TOA

R1

R2

R3 BS1

BS2

BS3

1. TOAs t1, t2, t3are measuredfrom signalstrength.

2. measure base station distances as R1= ct1, R2= ct2,and R3=ct3, where c is light speed.

3. Circles are formed with radius R1, R2, and R3.

4. Intersection point is the position of MT.

Location measurement from TDOA

R1

R2

R3

1. measure |R2-R1| = c(t2-t1) and |R3-R1| = c(t3-t1)and plot them.

2. Two hyperbolas will be found. Position of MT isthe crossing point of hyperbolas at R1 distancefrom BS1.

BS1

BS2

BS3

Location measurement from AOA

θ 1 θ 2

θ 1 θ 2 1. Angles can be measured

by MT and sent to therespective BS.

2. Crossing point of tangentbrown at BSs is theposition of the MT.

However, GPS is the most popular way ofgeolocating an MT precisely within 50 meters.



11

GSM Services

GSM network Provides three types of services:

Tele Services: Normal voice calls.

Voice conversation

Emergency services

Bearer Services: data services.

Short message services (SMS), Videotex, teletex, and advanced messagehandling services.

Supplementary Services:

call forwarding and call barring,

Calling/connected line identification presentation and restriction

Malicious call identification

Multi-party services (i.e., tele-conferencing), etc.



12

GSM Security IssuesGSM provides security in three areas:

Data and signaling confidentiality

Authentication of a user

Confidentiality of a user

Data and signaling confidentiality:transmit Cipher-text

A5/1, A5/2, and A5/3 ciphering algorithmsare used.

A8 algorithm is used to generate key forciphering algorithms.

User confidentiality is maintain by protecting SIM using a PIN and PUK (PIN unlock)code. If invalid PIN code is entered for a given time, PUK code should be collectedfrom the operator. Using invalid PUK several times damage the SIM permanently.

Authentication Procedure: when a connection is attempted.

Network MT SIM1. Identity (initial message) TMSI

2. Authentication Request (RAND) 3. Run GSM (A3) Algorithm (RAND)

4. Response (SRES)5. Authentication Response (SRES)

Network compares the SRES with its own SRES. If Authentication fails networkmay choose to repeat with International Mobile Subscriber Identity (IMSI).

TMSI: Temporary mobile subscriber identity SRES: Signed Response. SIM: Subscriber Identity Module.



13

Conclusions

GSM extended circuit-switching through introducing TDMA and Digital

Technology. GSM introduces multimedia services for the first time.

Digital services of GSM networks enable tremendous growth of mobilecommunications.

People are now expecting wireless worldwide connectivity of their laptops and powerfulhandheld devices.

GSM provides moderate level of security.

Safe for general personal use, but not secure for top organizations like Government spyagencies.

Capacity is the main limitation of GSM networks. GSM cannot support thetoday’s demand of wireless multimedia services.

2.5G General Packet Radio Service (GPRS) and 3G CDMA2000 are now being used in someareas to get more capacity.

Indeed !

GSM makes the world a global village !

adfa gsm bijoy

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