An Introduction To

Course Outline

History of CellularCreation of GSMCellular Networks

Components of GSMCommonly used GSM abbreviations

Commonly faced GSM problems

Situation of Global Cellular Mobile Radio before Digital

• Different Standards World-wide• Up to 1992 exclusively Analog Radio

Technology• Analog Cellular still exists in most

countries• Based on Different Standards

Different Standards World wide

• AMPS - Advanced Mobile Phone Service Mainly North and South America, Asia

• TACS - Total Access Communications Mainly UK, Middle East, European Countries - Ireland, Spain, Italy and Austria

• NMT - Nordic Mobile Telephone System Dominates the European Market

Different Standards World wide

The world is covered with a quilt of Analog Cellular Radio Systems, but ....– they operate on different Frequency Bands – they use different Signalling Protocols.

As a result it is not possible to use a Singular Cellular Phone Throughout

Commercial Aspects of Cellular

Cellular Networks Investment is Driven By Two Goals Coverage and Capacity

Coverage determines the geographical area where a system is available

Capacity determines the max number of subscribers supported by the network

The Capacity Syndrome

• Restricted Spectrum presented limitations to operators

• Operators asked for more spectrum • New Techniques were sought to enhance

capacity without sacrificing quality

The Capacity Syndrome

Regulators, Operators and the Industry worked hard to achieve the utmost Network Capacity by using the latest techniques:– Narrowband Radios – Microcells– Sectored Cells– Time - multiplexing digital radio techniques

The Capacity Syndrome

• Spectrum remained (and still remains) a precious resource

• Which technology was to provide to best spectral efficiency

Digital Radio is not, and will not be, the expected or even the predicted perfect

overall solution to all capacity problems

Aspects of Quality

Analog has the effect of directly passing physical influences, disturbances in radio transmission

Digital Radio hides these effects

10

GSM

Analog

Quality

Noise and Interference

Performance Comparison

Aspects of Quality

Under critical and more difficult reception conditions, digital radios generally perform significantly better than analog radios ...... but the transition from good quality speech to weird noises is extremely pronounced

Political and Economic Aspects

• The Regulators demanded a system that would:– have the capability and capacity to serve very

large subscriber bases– provide excellent quality of speech and data

communications– effect the service with minimal cost to the

subscriber

Political and Economic Aspects

The combination of all these facts and factors, parameters and restrictions, demands and compromises, led to the development and introduction of Digital Cellular Mobile Standards

Political and Economic AspectsThere were three different approaches adopted in different regions of the world for the task of modernising cellular services:– The US Way

• Develop a system, see if it works, standardize– The European Way

• get everyone round a table and develop the standard– The Rest of the World’s Way

• watch the US and Europe and chose the best

GSM - A Digital Cellular Mobile Radio System

Different requirements and the dedication to meet them led to the development of the GSM standard

GSM – Groupe Speciale Mobile Global System for Mobile Communications

Development and Introduction of the GSM Standard

• Spectrally Efficient• Excellent Speech Quality• High degree of Security• New Services and Features• International Roaming• Reduced Power Consumption

Development and Introduction of the GSM Standard

• GSM is an evolving standard• Standardisation incomplete before launch• Phased approach

– GSM Phase 1– GSM Phase 2– GSM Phase 2+– 3 G Phase

GSM Phases

• Networks in operation since 1992 using Phase 1 Technical Specifications

• Phase 2 and 2+ features are now available for Operators to incorporate into their networks

• 3G will see the continuous introduction of additional services and features to GSM long into the future

Cellular Concept

• The area over which a single radio can support subscribers (its coverage area) is called a cell or sector

• The Operator’s service area is sub-divided into many cells or sectors

• The size of these cells is determined by the transmitter power

Cellular Concept

• The Frequency Spectrum allocated to the Operator is shared within a group of cells called a cluster

• Adjacent clusters of cells can “re-use” the frequencies provided the resulting interference is below a certain threshold

• The frequency re-use allows a small set of frequencies used to cover a huge area

Cellular Structure

• Early implementations of cellular radio systems were designed according to regular cluster patterns of hexagonal cells

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567 123

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567

123

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567 123

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567

E.G. A Seven Cell Repeat Pattern

Omni-directional Cells

• Initially Cells were illuminated from the centre by base sites that radiated equally in all directions

• OMNI sites are still used where subscriber densities are low

Sector Cells

• Sector Cells have multiple Cells Radiating from one Radio Site

Sector Cells

• The use of Sector cells grew from the need to support greater subscriber densities

• Sector Cells:– improve frequency re-use

(i.e. less cells per cluster)– reduce the required number of sites to cover a

given area (particularly attractive in urban environments)

High Capacity Cellular Structures

• When subscriber densities get very high or there are localised traffic hot-spots, Operators have the option to install microcellular architectures

• Microcells are very limited in size• Generally microcellular antennas are below

roof level and use buildings to limit the extent of coverage

Hybrid Coverage Strategies

In practice a mixture of cell sizes and often types of cell are used to effect a real world coverage plan

System Architecture

MSCPSTN

BSCBSC

BTS BTS

Mobile Stations

• End User Equipment– Car Mounted– Transportable– Handsets– Car kit for Handset

Subscriber Identity Module

• SIM Card holds all the information to identify and validate the mobile subscriber

• The SIM concept allows a subscriber to use many different mobile stations while being treated by the network as a single number

• The SIM uses Smart Card technology

CONTENTS OF SIM :

- SIM SERIAL NUMBER- IMSI ( INTERNATIONAL MOBILE SUBSCRIBERS IDENTIFICATION)- TMSI (TEMPORARY MOBILE SUBSCRIBERS IDENTITY )- SUBSCRIBERS AUTHENTICATION KEY (Ki)- MSISDN NUMBER ( MOBILE STATION INTEGRATED SERVICE DIGITAL NETWORK )- PIN ( PERSONAL IDENTIFICATION NUMBER )- THIS IS THE SECURITY LOCK FOR YOUR SIM- PUK ( PERSONAL UNBLOCKING KEY )

Base Transceiver Station

• The BTS communicates with the mobile station over a radio channel

• The BTS can have several transceiver units each capable of holding upto eight simultaneous voice communications BTS

Base Station Controller

• Base Station Controllers look after the management of a group of BTSs

• The main function of the BSC is to ensure the successful hand-over of calls between BTSs

Mobile Switching Centre

• The MSC interfaces the mobile Network to the Public Switched Network

• It is responsible for the management of calls to and from mobile subscribers, including:– call routing– call set-up– supervision

• The MSC also generates billing tickets

Switching• Types of Switching

– Circuit Switching– Message Switching– Packet Switching

• Exchange - Technology Evolution– Cross Bar– Strowger– Stored Program Controlled - PCM/TDM

Switching• Types of Exchanges

– Terminal– Tandem– Trunk / TAX– Integrated Local cum Transit ( ILT )

• Exchange Plan– Primary– Secondary– Tertiary

• Numbering Scheme

Operation & Maintenance Centre

• The OMC provides remote monitoring and control of network elements

• Alarms are sounded at the OMC when the network detects a malfunctioning Element

• The OMC collects network performance statistics

• It is the OMC that actually provisions subscribers onto the network

Switching Elements

HLR VLR

MSCEIRAUC

Home Location Register

• The HLR stores all the permanent data associated with all subscribers registered within its PLMN. This includes:– IMSI– MSISDN– Current controlling VLR – Call Restrictions – Supplementary Services allowed

Visitor Location Register

• The VLR stores all the non permanent data associated with subscribers within the service area of the MSC. This includes:– TMSI– Current location area – Mobile Station Roaming Number

Authentication Centre

• The authentication centre is responsible for validating the mobile subscriber prior to allowing them access to network services

• It is also responsible for ensuring that the encryption of speech across the air interface is initialized properly

Equipment Identity Register

• The EIR stores the unique identities of all mobile stations (IMEI)

• At the discretion of the operator the IMEI is checked against 3 database before each call:– White List– Grey List– Black List

Registration of the Subscriber

• Subscribers must register with a network whenever they turn their phones on

• During registration the network: – checks the validity of the subscriber (AuC)– updates the dynamic information the network

holds on the subscriber (VLR)

A registered subscriber will have one entry in an HLR and one in a VLR

Location Updating

• The service area is divided into location areas

• The VLR stores which location area the subscriber is/was last known to be in

• The network (VLR) does not know which cell the subscriber is in!!!

• Whenever a subscriber crosses a location area boundary it must inform the network

Call Establishment From Mobile

Mobile Station requests a channel from the network If channel is available connection to MSC is established MSC collects subscriber information from the HLR to see

if subscriber is allowed the requested service The MSC checks with AuC that the subscriber is valid Also MSC may check the mobile station’s IMEI a) If call is to PSTN, call is routed to the nearest point of

interconnect b) If call is to another AirTel mobile subscriber MSC sets up call to the called subscriber

Call Establishment To Mobile PSTN sees a 98100XXXXX number and delivers the call to

the AirTel Network (to the MSC) MSC collects subscriber information from the HLR and

checks with AuC that the subscriber is valid MSC interrogates the VLR to determine which location area

the subscriber was last known in All BTSs in the location area page the subscriber The Mobile Station monitors the paging channel and

responds to its own pages Mobile Station identifies itself MSC optionally checks IMEI and connects the call

Hand Over

• Hand Over (Handoff) of the call between BTSs allows subscribers to move freely around the service area whilst maintaining an established call

• GSM is able to use sophisticated hand over algorithms based on mobile assisted hand over (MAHO) techniques

Mobile Assisted Hand Over

• During a call the mobile monitors the signals from all the neighbouring BTSs

• When a different BTS is better able to support the call the mobile station requests a hand over to the new BTS

• The network evaluates the request and may then set up a new channel for the call on the new BTS

Other Interfaces to the Switch

VMS

MSCSMSC CCBS

IN

Voice Mail System

• The VMS allows callers to leave messsages for subscribers who are busy or unavailable

• The VMS greatly improves call completion rates and is therefore a significant revenue earner for the operator

• A VMS platform can also be used to provide audio text and bulletin board facilities

Short Messaging Service Centre

• The SMSC allows alpha-numeric paging messages to be delivered to a mobile station

• Notification of voice mail messages • Mobile to mobile short messaging

Intelligent Network

• Intelligent Network platforms enable the Operator to offer Value Added Services in addition to those defined in the GSM Standards, for example:– Single number dialling– Call hunting– Pre Paid

Switching - Commonly Used Abbreviations

• PSTN - Public Switched Telephone Network• PABX - Public Automatic Business Exchange• PLMN - Public Land Mobile Network• TAX - Trunk Automatic Exchange• CUG - Closed User Group• DID - Direct Inward Dialling

GSM - Commonly Used Abbreviations

• GSM - Group Speciale Mobile• MSC - Main Switching Center• HLR - Home Location Register• VLR - Visitor Location Register• EIR - Equipment Identity Register• AuC - Authentication Center• BTS - Base Transceiver Station• BSC - Base Station Controller• TRX – Trans Receiver

GSM - Commonly Used Terms• Cell / Sector• Traffic Capacity - Erlang• MOU - Minutes of Usage• Frequency Spectrum - Bandwidth• Traffic Channels• Signalling Channels• BCCH - Broadcast Control Channel• SDCCH-Standalone Dedicated ControlChannel• Paging• Location Update• Hand Over• Drop Call

GSM - Commonly Used Terms• Uplink / Downlink • Interference • Clipping• Attach / Detach• Micro Cell / Macro Cell• Repeater / Booster• Tower Mounted Amplifier - TMA• GSM / Microwave Antenna• Diversity• BER - Bit Error Rate• Full Rate / Half Rate / Enhanced Full Rate• Grade of Service

GSM - Commonly Used Terms• Timing Advance • Signal Strength• Voice Quality / Speech Quality (Rx Qual)• Roaming• IN - Intelligent Network• LOS - Line of Sight• IMSI - International Mobile Subscriber Identity• IMEI - International Mobile Equipment Identity• MSISDN - Mobile Station Integrated Services

Digital Number

GSM - Commonly Used Terms• DTX - Discontinuous Transmission• TRX - Transmitter / Receiver• OMC - Operations Maintenance Center• NMC - Network Management Center• SIM - Subscriber Identity Module• SMS - Short Message Service• dB - Decibel & dBm• SFH - Synthesised Frequency Hopping• FLP - Fractional Load Plan

GSM - Commonly monitored Parameters• Parameters monitored:

– Drop Call Rate • Signalling Phase - Drop 1 in %• Traffic Channel - Drop 3 in %

– Congestion in • Signalling - in Seconds• Traffic Channel - in Seconds• PSTN in attempts/calls

– Handover Success Rate in %– Handover Failure Rate in %– Answer to Seizure Ratio in % – Total Traffic/Peak Traffic in Erlangs / Minutes– Call Success Rate in %– Blocked Calls due to Congestion in Traffic &

Signalling Channles

Commonly Used Terms

• Dual Band / Tri Band • WLL - Wireless Local Loop• CDMA - Code Division Multiple Access • Limited Mobility• HSCSD - High Speed Circuit Switched

Data,Ph 2, 57.6 KBPS • GPRS - General Packet Radio Service,Ph 2+• UMTS - Universal Mobile Telecommunications

Service Vehicle - 144 kbit/s; Pedestrian 384 kbit/s; In-building 2 Mbit/s.

Commonly Used Terms• EDGE - Enhanced Datarate for GSM

Evolution, Data rates up to 384 KBPS, 3G Evolution

• CAMEL - Customised Applications for Mobile Networks Enhanced Logic– GSM Feature for including IN – Pre-paid roaming services between networks– Fraud control

• Bluetooth -To replace wires for connection between PCs,Printers,Palm Tops,Laptops,Mobiles. 720 KBPS, distance of 10 Meters

Common observed Problems in GSM• Network Busy• No Connection• After dialling ‘Airtel’ comes Back• Number Busy• All Lines in this route are busy-Announcement• One Way Voice• Call lands into Voice Mail• Call Drops suddenly• Call Echo• Call Clipping

Customer Care and Billing SystemCredit

ControlCustomerCare

PaymentProcedure

AutoPayments

StandardLetters

FraudManagement

DealerCommission Reports

LeadsTracking

SalesLedger

Customer Management

System

CreditVetting

Rating

BillProduction

CustomerAdministration

CCBS Interfaces

Credit BureauStock Control

S.M.S.

Banks

POSSystem

CollectionAgencies

GeneralLedgerM.S.C.

H.L.R.

Intelligent Network

Credit Card

DocumentManagement

Customer Management

System