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
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 calls
the network can support
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
Mobile Cellular Networks
A
F
A
B
F
B
G
E
C
G
D
C
E
D
A
F
B
G
E
C
D
Figure 1
Cellular structure and frequency spectrum allocation plan
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
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
Mobile Cellular Networks
Uplink
Downlink
890-915 MHz
935-960 MHz
1
2
3
123
100 200
kHz kHz
124 X 200 kHz carriers + 2 X 100kHz guard bands at each end
Figure 3
Structure of the Frequency Sprectrum for GSM
124
100
kHz
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 - subscriber’s short
dialing codes
– Can make emergency codes
• Future is multi-media mobile stations
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 Transceiver
Stations
– Essentially a concentrator (multiplexer)
• Multiplexes Base Transceiver
Stations onto high speed link
– Undertakes some radio management
tasks
• 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
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
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
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 allow
mobility
• Location Area Code where MS is
currently operating
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 leave
VLR domain
• HLR and VLR jointly facilitate mobility
Mobile Cellular Networks
AUC
D
EIR
VLR
F
B
HLR
C
To
PSTN/ISDN
E
NMC/
OMC
GMSC
MSC
A
BSC
BSC
A bis
BTS
BTS
BTS
BTS
BTS
Um
MS
MS
N.B. The letters shown on the links are
the network interfaces defined in GSM
Figure 2
GSM network architecture
MS
MS
MS
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
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
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
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
Mobile Cellular Networks
• Mobility Management (Network Mobility)
– Needed to deliver Incoming calls
• Various Approaches
– Network doesn’t keep track of
subscribers’ moves
– To deliver an incoming call need
to broadcast to every cell in the
network
– Implications of extending this to
an international level
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)
Mobile Cellular Networks
if subscriber highly mobile
(frequent updates)
if subscriber receives many calls
(frequent finds)
location update approach is best
else subscriber receives few calls
..............
else (subscriber is stationary)
if subscriber receives many calls
..............
else subscriber receives few calls
..............
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
Mobile Cellular Networks
MS
New
MSC
New
VLR
Old
VLR
HLR
Location Update
Request
Update Location
Area
Update Location
Cancel Location
Cancel Location
Insert Sunscriber
Data
Insert Sunscriber
Data Ack
Update Location
Ack
Update Location
Area Ack
Location Update
Accept
Figure 5
Location Update Operation
Ack
Mobile Cellular Networks
• Mobility management
• Find (mobile subscriber)
– HLR is used to find the subscriber’s 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
Mobile Cellular Networks
PSTN/ISDN
1
8
6
7
GMSC
MSC
9
BS
2
5
3
HLR
VLR
4
GMSC/HLR on MS's
home network
MSC/VLR where
MS is currently located
Figure 6
Sequence of Operations for an Incoming CAll
10
MS
Mobile Cellular Networks
GMSC in network where
calling MS is currently located
MSC where calling
MS is currently located
4
GMSC
2
MSC
1
BS
MS
3
5
HLR/VLR *
* HLR if calling MS is a subscriber
VLR if calling MS is a visitor
PSTN/ISDN
1
8
6
7
GMSC
MSC
2
5
3
HLR
VLR
4
GMSC/HLR on called
MS's home network
MSC/VLR where called
MS is currently located
9
BS
10
MS
Mobile Cellular Networks
• Mobility Management
• Roaming Agreements
Mobile Cellular Networks
UKNET Subscriber
UKNET Subscriber
VLR
HLR
UKNET
SwissNET
Incoming call from UKNET subscriber to remote UKNET subscriber
Mobile Cellular Networks
UKNET Subscriber
VLR
HLR
UKNET
SwissNET Subscriber
SwissNET
Incoming call from SwissNET subscriber to UKNET subscriber resident in Switzerland