Mobile Cellular Networks.ppt

7/29/2019 Mobile Cellular Networks.ppt

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Mobile Cellular Networks

Evolution

1st generation, 1980s

analogue

voice

2nd generation 1990s

digital

Voice, fax data

95% coverage of UK by 1991

3rd generation - within 10 years digital

anywhere, anytime, anything

Most significant development in

telecommunications in recent years


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Cellular principle

Proposed as a solution to the bandwidth

problem

Restrict the radio range of Base Station

(transmitter)

Can now reuse BS frequency in other parts of

the network

Taking this one step further

tessellate network coverage area with cell

reuse pattern (cluster)

Each cell in cluster operates on a different

frequency

Cluster sizes of 4,7,9 etc are common

Result - increase in capacity of network in

terms of max number of simultaneous callsthe network can support


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Cellular principle

Cells are hexagonal shape

Base station located in middle

Radius of cell is governed by power of Base

Station

Increasing the power increases geographical

size of cell

Smaller sizes automatically increase the

network capacity

but can also increase interference


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A

B

C

D

E

F

GF

A

B

C

D

E

G

A

B

C

D

E

F

G

Figure 1Cellular structure and frequency spectrum allocation plan


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Cellular principle

Trend is to have sophisticated cell structures

essentially overlay large cells on smaller

cells

Common cell sizes

Pico cells

floor of a building

a few metres

Micro cells Street

10 -400 metres

Base station mounted below roof level

Street canyons

Macro cells 5 kms

special masts erected for Base station

Pico cells give large capacity for a small area

Macro cells give small capacity for a large area


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Global System for Mobility (GSM)

Small amount of radio spectrum allocated for

cellular networks

For GSM

890-915MHz uplink (Mobile station to

Base station)

935-960Mhz downlink

Each call requires a dedicated full duplex

channel (circuit switched)

Typically a network provider is allocated a

subset of these for operation

Note Mobile station must operate across all

frequencies


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plink 890-915 MHz

ownlink 935-960 MHz

1 2 3 123 124

100kHz

200kHz

100kHz

124 X 200 kHz carriers + 2 X 100kHz guard bands at each end

Figure 3

Structure of the Frequency Sprectrum for GSM


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Network Components

Mobile Station

Mobile Equipment (e.g. phone)

antenna

Subscriber Identity Module (SIM)

Smart card

SIM must be inserted into ME before

ME will work

Essentially personalises ME Contains subscription information

Other information - subscribers short

dialing codes

Can make emergency codes

Future is multi-media mobile stations


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Network Components

Base Station System

Base Transceiver Station

Antenna

Interfaces to MS

Able to transmit /receive signals on

many channels simultaneously

Base Station Controller

Controls a number of Base TransceiverStations

Essentially a concentrator (multiplexer)

Multiplexes Base Transceiver

Stations onto high speed link

Undertakes some radio managementtasks

passes Location Area Code to Base

Transceiver Station for

broadcasting to MSs

Also translates 13kbps speech from

radio channels to 64Kbps PCM for

transmission on fixed network


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Network Components

Mobile Switching Centre (MSC)

ISDN switch enhanced to operate in mobile

network

In addition to switching

manages calls for all MSs within its

domain

Billing

Handover

Authentication


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Network Components

Intelligence (databases) in Network

Equipment Identity Register (EIR)

Stores information on lost or stolen

MSs

Each MS has a unique International

Mobile Equipment Identifier (IMEI)

Network can refuse access id IMEI is

stored on EIR.

Authentication Centre (AuC)

provides access security for network


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Network Components


Home Location Register (HLR)

One logical HLR in network

Contains an entry for every subscriber

Stores fairly static information about

subscriber

services subscribed to

But also location information to allowmobility

Location Area Code where MS is

currently operating


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Network Components


Visitor Location Register (VLR)

One VLR for every Location Area in

network

Typically an Mobile Switching Centre

covers a location area

In this instance VLR is integrated

with MSC

VLR contains information on every

subscriber (visitor) currently operating

in the domain of VLR

Entries are added when visitors enter

VLR domain

Entries are deleted when visitors leaveVLR domain

HLR and VLR jointly facilitate mobility


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MSC

EIR VLR HLR

AUC

NMC/

OMC GMSC

BSC

MS

BTS BTS

BTS

MS

BSC

MS

BTS BTS

MS

MS

D

C

E

To

PSTN/ISDN

A

FB

Figure 2

GSM network architecture

Um

Abi s

N.B. The letters shown on the links arehe network interfaces defined in GSM


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Radio Interface

Traffic channels

Full duplex, circuit switched

Control channels for signalling

Broadcast

e.g. Broadcast Control Channel

(BCCH)

Gives Location Area Code

Private e.g. Fast Associated Control Channel

(FACCH)

Used during call establishment, for

handover etc.

Other Interfaces - very like N-ISDN


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Mobility

Terminal Mobility (1st generation systems)

Keeps track of MS

MS / subscriber more or less the same entity One-to-one relationship between subscriber and

MS

Give your MS to someone else, they will

receive your calls

Not unlike the fixed network


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Mobility

Personal mobility in GSM (2nd generation

systems)

More flexible than terminal mobility Subscriber can receive calls on any MS

provided their SIM card is inserted

One-to-many relationship

Separated MS from subscriber


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Mobility

Full personal mobility (3rd generation

systems)

MS can be used by many subscribers Subscriber can be registered to receive calls on

any MS

Many-to-many relationship


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Mobility Management (Network Mobility)

Needed to deliver Incoming calls

Various Approaches

Network doesnt keep track of

subscribers moves

To deliver an incoming call needto broadcast to every cell in the

network

Implications of extending this to

an international level


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Mobility Management

Another approach (used by 2nd generation

systems)

Network keeps track of subscribers moves HLR and VLR used for this purpose

Two operations involved

Update (location update)

Find (finding subscriber to deliver an

incoming call)


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if subscriber highly mobile

(frequent updates)

if subscriber receives many calls

(frequent finds)

location update approach is bestelse subscriber receives few calls

..............

else (subscriber is stationary)

if subscriber receives many calls

..............

else subscriber receives few calls

..............


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Mobility management

Update - when a subscriber moves to a

new location area

i.e. comes under the domain of a new VLR MS detects it has roamed into a new location

area

MS requests a location update from new MSC

New MSC enters subscribers details in

associated (new) VLR New VLR forwards location update to HLR

HLR is updated with new VLR address

HLR requests old VLR to delete subscribers

entry


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MS NewMSC

NewVLR

HLROldVLR

Location Update

Request Update Location

Area Update LocationCancel Location

Cancel Location

AckInsert Sunscriber

Data

Insert SunscriberData Ack

Update Location

AckUpdate Location

Area Ack

Location Update

Accept

Location Update OperationFigure 5


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Mobility management

Find (mobile subscriber)

HLR is used to find the subscribers current

location (VLR)

HLR requests a temporary roaming number

from VLR

VLR returns roaming number to HLR

HLR returns roaming number to call source

Call may now be routed (to VLR) VLR meanwhile will be alerting MS that a call

is expected


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PSTN/ISDN

GMSC

HLR

MSC

VLR

BS

1

2

3

4

6 7

8

9

10

5

MSC/VLR whereGMSC/HLR on MS'shome network MS is currently located

Figure 6

Sequence of Operations for an Incoming CAll

MS


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GMSC

HLR

MS C

VLR

BS

1

2

3

4

6 7

8

9

10

5

MSC/VLR where calledGMSC/HLR on calledMS's home network MS is cur rently located

MS

GMSC MS C

HLR/VLR

BS

MSC w here c allingGMSC in network wherecalling MS is currently located MS is currently located

MS

PSTN/ISDN

12

3

4

5

* * HLR if calling MS is a subscriber VLR if calling MS is a visitor


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Mobility Management

Roaming Agreements


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UKNET SwissNET

UKNET SubscriberUKNET Subscriber

Incoming call from UKNET subscriber to remote UKNET subscriber

HLR

VLR


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UKNET SwissNET

UKNET Subscriber

Incoming call from SwissNET subscriber to UKNET subscriber resident in Switzerland

HLR

VLR

SwissNET Subscriber

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Mobile Cellular Networks.ppt