Wireless Networks
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Cellular and land line telephone systems are
responsible for providing coverage throughout
a particular territory, called a coverage region
or market
The interconnection of many such systems
defines a wireless network over a country or
continent
Types of Networks
Fixed Networks(PSTN) Wireless Network
•Highly static
•High bandwidth possible
(fiber Optic or coaxial)
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•Highly dynamic
•Low RF bandwidth
provided for each user
Interconnection of networks
PSTN
Mobile
Switching
Center (MSC)
Voice and
Data Links
Radio
Link
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BS1
BS2
Functions of PSTN
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Highly integrated communications network that
connects almost all of of world’s population
Each country is responsible for the regulation of
PSTN within it’s borders. Over time,
government to private transition takes place.
PSTN has a network of fixed telephone
exchanges
Over time, transition has occurred from manual
to automatic exchanges
History of PSTN network
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Controlled by long distance companies like
AT&T, Sprint etc.
1984 – Supreme Court issues Modified Final
Judgement (MFJ)
Break up of AT&T into 7 major Bell operating
systems (BOCs) each with its own service
region.
PSTN Exchange system
City
City
Local
Exchange
Carrier
(LEC)
City
Local Access
and Transport
Area (LATA)
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Local
Exchange
Carrier (LEC)
Inter
Exchange
Carriers
(IXC)
Local PSTN telephone network
IXCA
PBX
Other CO.s
Tandem
Switch
Home
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Central
Office (CO)
IXCC
IXCB
Wireless Networks
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Wireless Networks are extremely complex,
unlike static local, fixed telephone networks.
Wireless networks requires air interface
between base stations and randomly spaced
subscribers
Complex propagation media
MSC must eventually provide connection to
PSTN and other MSCs
Schematic of Wireless Network
BS
BS
MSC-1
(Home MSC)
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HLR AUC
VLR
SS7
PSTN
MSC-2
(Visitor MSC)
HLR AUC
VLR
Wireless network comparison
First, second and third generations
 Modulation techniques (Analog, Digital, Spread
Spectrum)
 Switching (channel) techniques (Circuit
switching, packet switching)
 Signaling (control) techniques (in-band, CCS)
 Practical systems (USDC, GSM, CDMA)
 Network capabilities
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First Generation Networks
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Analog Technology -FM modulation
AMPS (Advanced Mobile Phone Services) – in
U.S.
Mobile Terminals
Base Stations
MSCs
PSTN is separate network from the signalling
network
Functions of MSC
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Provides overall System control for each region
Maintains mobile related information and
handoff control
Performs all call processing; billing; fraud
detection within the market
Wireless component linkages
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Base station to Mobile user : Analog speech,
low rate data
Data transmission between BS – user
RVC
RCC
FVC
FCC
Base station - MSC: 9600 B/S Data Link
MSC - PSTN Landline Trunked lines and
Tandem Switch
IS-41 Network Protocol
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Network protocol standard to allow automatic
registration of roamers – inter-operator roaming
Allow MSCs of different service providers to
pass information about subscribers to other
MSCs on demand
HLR – Home Location Register – real time user
list
VLR – Visitor Location Register
AUC – Authentication Center
Roaming process
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Mobile periodically keys up and transmits its
identity information which allows MSC to
constantly update it’s customer list.
The registration command is sent in over
control channels at 5-10 minute intervals MIN – Mobile Identification Number (Telephone
No.)
ESN – Electronic Serial Number
Registration
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By comparing MIN of roaming subscriber with
HLR database, the MSC identifies roamers
MSC sends registration request over landline
signaling network to subscriber’s home MSC
Home MSC validates roamer’s MIN and ESN
and returns a customer profile to visited MSC
Home and visitor MSC update their HLR and
VLR
The roamer is then registered in the visited
MSC.
SS7 (Signaling System 7) Process
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Switch controlled services for users
Call return, call formatting, repeat dialing, call
block, call tracing, caller ID
800 Series - Toll free access to calling party –
paid by service subscriber
Alternated Billing Service and line information
database (ADB/LIDB)
Enables calling party to bill a call to a
personal number (third party number, calling
card or collect call) from any number
Functioning Of SS7
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MSC controls the switching and billing functions
and interacts with PSTN to transfer between
global grid and its cluster of base stations
MSC uses the SS7 signaling network for
location validation and call delivery to its users
which are roaming and relies on several
information databases
Switching in Networks
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First generation Circuit Switching
MSC dedicates a voice channel connection
between base station and PSTN for duration of
cell phone call
As calls are initiated and completed, different
radio circuits and dedicated PSTN voice circuits
are switched in and out to handle traffic
Properties of circuit switching
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Wireless data networks are not well supported
by circuit switching, due to their short, bursty
transmissions => often, time required to
establish a circuit exceeds the duration of data
transmission.
Circuit switching is best suited for dedicated
voice-only traffic, or for instances where data is
continuously sent over long periods of time.
Second Generation Wireless Networks
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Employ digital modulation and have advanced
call processing capabilities.
Examples:
o Global system for Mobile (GSM)
o DECT (Digital European Cordless
Telephone)
o Cordless Telephone (CT2) -British system
o JDC – Japanese Digital Cellular system
Packet Switching network
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Second generation switching network
Packet Switching (or virtual switching)
implements connectionless services for large
number of data users, who remain virtually
connected
Packet switching breaks each message into
smaller units for transmission and recovery
When a message is broken into packets,
control information is added to each packet to
provide source/ destination information and
identification.
Packet Structure
Header
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User Data
Trailer
Header – contains source address, destination
address, packet sequence number, and other
routing and billing information
Trailer - contains cyclic redundancy checksum
which is used for error detection at receiver
Packet Switching properties
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Packet switching is also called Packet Radio
(PR) when used by a wireless link
Provides excellent channel efficiency for data
transmission, since the channel is utilized only
when sending or receiving bursts of information
X25 is widely used packet radio protocol –
developed by CCITT (ITU-T) International
Telecommunication Union.
Second Generation
Wireless Networks…
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New network architectures that reduce
burden of MSC - BSC (base station
controller) inserted between several base
stations and MSC
All systems are digital voice coding and
employ digital modulation
Systems employ common channel
signaling for simultaneous voice and
control information
Advances in Second Generation Networks
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While 1st generation systems were primarily
designed for voice, second generation networks
provide paging, facsimile and high –data rate
network access
Handoff is mobile-controlled-MAHO(Mobile
assisted Handoff)
The mobile units in this generation perform
functions like received power reporting,
adjacent base station scanning, data encoding
Common Channel Signaling (CCS)
Second generation signaling network
 Digital Technique that provides simultaneous
transmission of user data, signaling data and
other related traffic through a network.
[Mobile <> BS <> MSC <>MSC ]
 Uses out of band signaling channels which
separate the network data from the user (voice
or data) on the same channel or using TDM.
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Advantages of CCS
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Advantage – high speed signaling (50kbps –
Mbps) is not limited by low speed voice data
(20bps –20kbps)
Substantial increase in the number of users
CCS network structure
STPs
SMS
SS7
SS7
MSC
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SEPs
STPs
CCS network components
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CCS network architecture is composed of
geographically distributed central switching
offices.
Service Management system (SMS)
Switching end points (SEPs)
Signaling transfer points(STPs)
Database service management system(DBAS)
Third Generation Wireless Networks
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Aim is to provide a single set of standards that
can meet a wide range of wireless applications,
and provide universal access around the globe
=> voice, data and video
Integrated Services Digital Network (ISDN)
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Parallel worldwide network for CCS signaling
traffic that can be used to either route traffic on
PSTN or provide new services between
network nodes and end users
ISDN has 2 types of channels:
o Information bearing channels called Bearer
channels (B channels) –64kbps
o Out of band signaling channels, called data
channels (D channels)
Broadband ISDN (B-ISDN)
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End user applications require greater bandwidth
for computer systems and video imaging
Based on asynchronous transfer mode (ATM)
technology which allows packet switching – 100
Gbps
Future Wireless Networks
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Based on B-ISDN to provide access to
information networks such as Internet and
other public and private databases
PCS - Personal Communication System
PCN - Personal Communication Network
International Mobile Telecommunication (IMT
–2000)
Universal Mobile Telecommunication System
(UMTS)- Europe