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E2-E3

CONSUMER MOBILITY

Cellular Concepts & GSM Architecture


WELCOME

This is a presentation for the E2-E3 Consumer

Mobility Module for the Topic: Cellular Concepts &

GSM Architecture

Eligibility: Those who have got the Upgradation from

E2 to E3.

This presentation is last updated on 27-03-2011.

You can also visit the Digital library of BSNL to see

this topic.


Learning Objective

Define cell, cluster

Characteristics of cellular system

Understand ways in better communication

Various subsystems of GSM Network

GSM Architecture

GSM network structure

Functions of mobile station

Radio subsystem

Network and switching subsystems

Operation and maintenance center


A single antenna can transmit all the

GSM carriers in a city

GSM Antenna


Capacity of the system

Extract from Traffic Table - Erlang B Model

Number

of Access

Channels

GoS

0.5%

1.0%

2 %

5 1.13 1.36 1.66

10 3.96 4.46 5.08

20 11.1 12.0 13.2

33 21.5 22.9 24.6

50 36.0 37.9 40.3

56 41.2 43.3 45.9

99 80.0 84.1 87.0

100 80.9 85.0 88.0


Capacity of the system

TRUNKING EFFICIENCY GoS 2%

Sr.No

Number of

Access

Channels

Erlang

Capacity

Number of

users

served

1 5 1.66 16

2 10 5.08 50

3 20 13.2 132

4 50 40.3 403

5 100 88.0 880


A CLUSTER OF CELLS

4

5

6

7

2

3

11 2 3 4 5 6 7

GIVEN FREQ.

RESOURCE

CELLULAR MOBILE CONCEPTS


14

2

1

3

5

6

7 2

6

3

4

5

7DR

Frequency Reuse N=7

1 2 3 4 5 6 7Frequency Reuse Pattern “N”=7

Given Freq Resource




WHAT IS A CELL ?

A base station (transmitter) having a number of

RF channels is called a Cell. Or it is the area as

seen by a mobile station. OR Cells are base

stations transmitting over small geographic areas.

Each cell covers a limited number of mobile

subscribers within the cell boundaries ( Coverage

area)

Typical Cell Radius Approx = 15 Km (Start up), 1

KM (Mature)



WHAT IS A CLUSTER ?

A cluster is group of cells. Frequencies are nor

reused in a cluster.

The cluster is then repeated and the frequencies

are reused and system capacity is increased.

Typical cluster size is 4 or 7 cells.


Features of Digital Cellular Systems:

Small cells

Frequency reuse

Small, battery-powered handsets

Performance of handovers

Interference between cells


Cell clustering

There will be interference between the cells

using same frequency.

To reduce co-channel interference below the

acceptable level, the co-channel cells must be

set far apart.

Number of cells in a cluster is denoted by ‘K’,

also called as frequency reuse factor.

K=i²+j²+i*j where i and j are integers.


CO-CHL INTERFERENCE

R

Co Chl Interference is

a Function of “Q”

Lower Q Increased Co-Chl Interference Higher Q Reduced Co-Chl Interference

Q = D /R = 3K


Capacity/Performance Trade-offs

R

If K increases, then performance increases

If K increases, then call capacity decreases per

cell.

If K decreases, then performance decreases

If K decreases, then call capacity increases per

cell.



An Example

Freq

Reuse

Pattern

Voice

Chls/Cell

Calls/Cell/hour S/I dB

4 99 2610 14

7 56 1376 18.7

12 33 739 28.3

Assume Number of Voice Channels = 395,

Average Call Duration = 2 Minutes , GoS = 2%

Using Traffic Tables we get :


CELLULAR ENVIRONMENT

COUNTER MEASURES

Power Control

Channel Coding

Interleaving

Equalization

Slow Frequency Hopping

Antenna Diversity


GSM ARCHITECTURE


GSM

Various subsystems

Network Subsystem Includes the equipments and

functions related to end-to-end call.

Radio Subsystem Includes the equipments and

functions related to the management of the

connections on the radio path.

Operations and Maintenance subsystem Includes

the operation and maintenance of GSM equipment

for the radio and network interface.


Network Architecture

B

T

S

MSC VLR

HLR

PSTN

ISDN

Data

Networks

(

Air interface

OSS

B

T

S

B

T

SMSC VLR

BSCBSC

1 MSC=16 BSC

1 BSC=1024 TRU

A Interface

A-bis interface

BSC:BASE STATION CONTROLLER, BTS: BASE TRANSRECEIVER STATION, OSS: OPERATION AND SUPPORT

SUBSYSTEM.ss


GSM ARCHITECTURE

MSC

BSC

GMSC

AUC

BTS

BSC

BTS

BTS

BTS

BTS

HLR

PSTN

VLR

Voice/Data-Circuit

EIR

Netowrk

Management


GSM ARCHITECTURE

ME

SIM

BTS

BTS

BSC

BSC

MSC

HLR

EIR

VLR

AuC

Mobile Station Base Station Subsystem Network Subsystem

PSTN ISDN

PSPDN CSPDN

Um Abis

A

OMC


GSM Network Structure

GSM Service Area: Total area served by the combination of

all member countries where a mobile can be served.

PLMN Service Area: It is one N/W area.

MSC Service Area: There can many MSC/VLR in one PLMN

area . It is one Mobile Exch. Area.

GMSC: All I/C calls for PLMN N/W will be routed through

GMSC. Call connections between PLMNs , or to fixed N/Ws

must be routed to a GMSC. The GMSC contains the Inter

working functions to make these connections.


There are several LA in a MSC/VLR combination

A LA is a part of the MSC/VLR service area in

which a MS may move freely without updating

location information to the MSC/VLR exchange

that control the LA.

Within a LA a paging message is broadcast in

order to find the called mobile subs.

LA can be identified by system using the LAI.

LOCATION AREA


CELL

A cell is an identity served by one BTS.

It is area of Transmission/Reception between

MS and BTS.

The MS distinguishes between cells using the

BASE STATION IDENTIFICATION CODE(BSIC)

that the cell site broadcast over the air.


GSM PLMN Service Area

V

MSC

MSC

MSC

MSC

VLR

VLR

VLRI

II

IV

III

I

VLR


GSM MSC Service Area

MSC

VLRLA1

LA2

LA3

LA6LA4LA5


GSM Cells

MSC

VLR

LA1

LA2

LA3

LA6LA4

LA5

C

1

C

2

C3

C6C

5

C4

C=CELL


GSM

Relation between areas in GSM

Location AreaCell

Location AreaMSC Service AreaPLMN Service Area

GSM Service Area


GSM

LA Coding

MCC

LAI

LACMN

C3 digit 3 digit 2 Octets

MCC:Mobile Country Code,

MNC: Mobile N/W Code,

LAC: Location Area Code


FUNCTIONS OF MOBILE STATION

Voice and data transmission

Frequency and time synchronization.

Monitoring of power and signal quality of the Serving

& surrounding cells.

Provision of location updates even during inactive

state.

Equalization of multi path distortions.


Mobile Equipment

Portable, vehicle mounted, hand held

Mobile equipment is identified by unique IMEI(International Mobile equipment Identity)

Shall display at least last ten received, dialled and missed calls

Should Have Minimum talk time of 1hr 30 min. and standby time of 80 hrs

160 characters long SMS


SIM Card

Subscribers Identity module

Has Unique Subscriber’s ID:IMSI and ISDN

PIN( Personal Identification Number)

Key Ki ( Authentication key) , Kc (Cipheringkey) and A3, A5 and A8 algorithms

SIM has CPU, ROM, RAM and EPROM


Mobile Identification Numbers

IMEI

MSISDN

IMSI

TMSI

MSRN


MSISDN

Mobile Subscriber’s ISDN Number

The MSISDN is registered in the telephone directory and used by the calling party for dialing.

IT Is consists of

MCC-Mobile country Code

NDS-Network Destination Code--Give Routing Information to reach HLR

N(S)N--National Significant Number

CC NDC SN

1 to 3 digits Variable Variable

N(S)N


IMSI

International mobile subscriber’s Identity

The IMSI is an unique identity assigned to each authorized GSM User.

The IMSI is stored on the subscriber identity module (SIM), the HLR, VLR and AUC database.


IMSI

3 digits

MCC MNC MSIN

3 digits Not more than 9 digits

IMSI : Not more than 15 digits

MCC--Mobile Country Code, (ITU)

MNC—Mobile Network National Code, assigned by

Administration(DOT).

Mobile station Identification Number. It identifies the subs.

In a PLMN. First 3 digit identifies the Logical HLR-id of

Mobile subs.(Operator)


TMSI

Temporary Mobile subscriber’s Identity

It is MSC/VLR specific alias designed to maintain user confidentiality.

The VLR assigns a TMSI to each mobile subscribers entering the VLR area.

Assigned only after successful authentication.

TMSI changes on location update


MSRN

Mobile Station Roaming Number

MSRN--is a temporary identity which is assigned during the establishment of a I/C call to a roaming subs.

The MSRN is used in the HLR to set up a connection to the visited MSC/VLR.

It is assigned on demand by HLR on per call basis.

CC NDC SN

CC--Country Code, NDC--National Destination Code, SN-- Subs.

No.


RADIO /BASE STATION

SUB SYSTEM (RSS) /BSS

n BTS n BTS

BSC

BSC

BSC

MSC/VLR

RSS


Base Station System

It is set of BTS equipments and BSC as seen by MSC through A interface.

BSS may be composed of one or more cells(BTS).

The interface between BTS and BSC is A-bisinterface.

BSS is composed by BTS and BSC.

BTS serves one cell and controlled by BSC.

BTS handles 1 to 12 carriers and serves 8 to 90 subscribers simultaneously.


FUNCTION OF BTS -I

Transmit and receive signals from MS over air interface

Encodes, encrypts, multiplexes, modulates and feeds the RF signals to the antenna

Transcoding and rate adaption Functionality

Time and frequency synchronization signals transmission.

Transcoding to bring 13 Kbps speech to data rate of 16 Kbps and combining four of these signals to 64 Kbps. As these channels are to be mapped on PCM voice channels on A-bis interface.


FUNCTION OF BTS -II

Timing Advance is determined by BSC and BTS signals MS for proper Timing Advance.

Frequency hopping

Random access detection and sending it to BSC for channel assignment.

Uplink radio channel measurements corresponding to Downlink measurement made by MS.


TRANSCODER

This device takes 13 kbps speech adds additional synchronisation data to bring upto level of 16 kbps.

Four channels are combined in transcoder to provide 64 kbps channel within bss.

O/p data rate of transcoder is 64 kbps.

30 such 64 kbps channels are multiplexed on to 2.048 MBPS on A-bis CHANNEL. This carries 120 speech and control signals.


FUNCTIONS OF BSC-I

It is connected to BTS at one side and MSC on other side.

Radio resource management for cells under its control.

Asigns frequencies and timeslots to MS in its area.

Inter-cell handover (INTRA BSS)

Reallocation of frequencies to the BTSs in its area.

Power control of BTS and MS


FUNCTIONS OF BSC-II

Time delay measurement of the received signals from MS with respect to BTS clock.

Performs traffic concentration to reduce the number of lines from BSC to MSC.

Provides the time and frequency synchronization signal broadcasted by BTS.


Network and Switching Subsystem

(NSS)

MSC

(PSTN)

VLR

HLR AUC

EIR

D

C

SS7 Signaling

Traffic Path

F

(BSS)

A

E

Other

MSC


GSM

MSC ( MOBILE SWITCHING CENTRE)

Manages communication between GSMusers & users of other network

Call setup functions, basic switching are doneby MSC.

MSC takes into account the impact of RRsallocation and mobile nature of User inaddition to normal exchange functions.

MSC performs Switching function for allmobiles located in MSC service area.


MSC Functions - I

Paging, specifically call handling

Location updation

Handover management

Billing for all subscribers based in its area.

Handles different identities and numbers relatedto MS.

Identities are used in interface part of MS andMSC and Numbers in fixed part of network.

Transfer encryption parameters from VLR to BSSand MS for carrying out ciphering.


GSM MSC Functions - II

Echo canceller operation control

Signaling interface to databases like HLR,VLR.

Gateway to SMS between SMS centers andsubscribers

Handle interworking function while working asGMSC


VISITOR LOCATION REGISTER

(VLR)-I

It is collocated with MSC.

It constitute database that support the MSC instorage and retrieval of subscribers data

It controls mobiles roaming in its area.

VLR reduces the number of queries to HLR

One VLR may be incharge of one or more LA.

VLR is updated by HLR on entry of MS itsarea.

VLR assigns TMSI which keeps on changing.


Data Stored in VLR

The data is present in VLR as long as MS isregistered in area associated with it. Socalled as Dynamic or temporary database.

VLR Stores following data

IMSI & TMSI

MSISDN

MSRN.

Location Area

Supplementary service parameters

MS category

Authentication Key


Home Location Register(HLR)-I

It constitute database that permanently store datarelated to given set of subscribers.

It is a reference database for subscribers parameter.

It stores various identification numbers and address,authentication parameters, services subscribed andspecial routing information.

Current subscriber status and associated VLR.

Both VLR and HLR can be implemented in the

same equipment in an MSC.

One PLMN may contain one or several HLR.

It stores SMS gateway identification till SMS isdelivered to MS.


Home Location Register(HLR)-II

Permanent data in HLR

IMSI, MS-ISDN number.

Category of MS ( whether pay phone or not )

Roaming restriction ( allowed or not ).

Supplementary services like call forwarding


GSM

Home Location Register(HLR)-III

Temporary data in HLR

The data changes from call to call & isdynamic

MSRN

RAND /SRES and Kc

VLR address, MSC address.

Messages waiting data used for SMS


GSM

AUTHENTICATION CENTRE (AUC )-I

AUC is a separate entity but physicallyincluded in HLR

Protect communication through air interfaceagainst intruders .

Authentication (Ki) and ciphering (Kc) keysare stored in this data base.

Keys change randomly with each call

Keys are never transmitted to MS on air butOnly calculated response are sent.


AUTHENTICATION & ENCRIPTION

• AUC

Database

Generation

of Random

Number

RANDRAND

IMSI1

IMSI3

IMSI2

ki1

ki2

ki3

RANDSRESKc

Algorithm for

Ciphering

A8

Algorithm for

Authenticatio

n

A3

Kc

64 bits

SRES

32 bits

HLR


GSMEQUIPMENT IDENTITY REGISTER ( EIR )

This data base stores IMEI for all registeredmobile equipments .

IMEI is unique identity of Mobile Equipment.

Only one EIR per PLMN.

Three classes of Mobile Equipments stored inEIR are

White list : IMEI, assigned to valid ME.

Black list : IMEI reported stolen

Gray list: IMEI having problems like faultysoftware, wrong make of equipment etc.


It provide the Interfacing Capability to DataN/Ws.

IWF. A part of MSC, provides the subscriber withaccess to data rate and protocol conversionfacilities so that data can be transmittedbetween GSM Data Terminal Equipment ( DTE )and a land line DTE.

INTERWORKING FUNCTION


ECHO CANCELER(EC)

It is used on PSTN side of MSC for all voicecalls.

It is required to reduce the effect of GSM delay(Echo) when mobile is connected to PSTN circuit.

It cancels delay of the order of 70 ms.


The centralized operation of the variousunits in the system and functions needed tomaintain the subsystems.

Dynamic monitoring and controlling of thenetwork

Operations and Maintenance Centre(OMC)


Functions

-O&M data function

-Configuration management

-Fault report and alarm handling

-Performance supervision/management

-Storage of system software and data

Functions Of OMC


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