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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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
Network Components
Intelligence (databases) in Network
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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Mobile Cellular Networks
Network Components
Intelligence (databases) in Network
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
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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Mobile Cellular Networks
Mobility Management
Roaming Agreements
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Mobile Cellular Networks
UKNET SwissNET
UKNET SubscriberUKNET Subscriber
Incoming call from UKNET subscriber to remote UKNET subscriber
HLR
VLR
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Mobile Cellular Networks
UKNET SwissNET
UKNET Subscriber
Incoming call from SwissNET subscriber to UKNET subscriber resident in Switzerland
HLR
VLR
SwissNET Subscriber
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