Networks 5 Mobile Communications Dr John Cowell phones off (please) 1.

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CTEC1414Lecture 20

Networks 5Mobile Communications

Dr John Cowell

phones off (please)

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Overview Developmental stages

Analogue High-power transmitter based networks Low-power transmitter based networks (cellular)

Digital GSM & WAP 3G

Overall architecture Cellular telephony Hands-off protocol

Common applications SMS, network broadcast, voice mail, etc

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Basic PrinciplesThere are some mobile transmitters, i.e. phonesThere are some stationary bases called mobile

base stationsThey communicate through radio frequencies,

typically in the band 800-900MhzThe radio frequencies are divided into channels,

typically 30KHz eachA conversation uses 2 channels, i.e. duplex

communication (send & receive)Concurrent calls must be on different pairs of

channels

The early days

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Analogue mobile networks1GHigh-power transmitters

Very much like terrestrial TV

One transmitter covers 100s of sq. miles

Small number of transmitters

Each transmitter has a dedicated number of channels

Limited number of calls ((900-800)*1000/30)/21600And that’s over an area of

100s of sq. miles

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Cellular networksA collection of small,

low-power transmitters carpet an area

The signal is still analogue and each transmitter works in the same frequency band

Butthe number of calls is

again 1600, but over a cell, i.e. much smaller area!

therefore, the number of calls increases over the whole area of a country

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Cellular networksHowever

frequencies from close cells can interfere between each other

a notion of cluster is introduced

no two adjacent cells use the same frequency band

this reduces the number of calls in a cell by a factor of 7 on a hexagonal network,

but it allows reuse of frequencies

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Capacity of cellular networksWithin a cluster, a

frequency can’t be re-used

Over the whole range of 800-900Mhz, there are 1600 calls

This range is split between 7 cells in a cluster

Therefore each cell can handle 200 calls

Multiply by the number of cells in the whole country

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Capacity of cellular networksThe number of calls a network can handle

depends on the number of cells availableIf increased capacity is required in a particular

area (say in Leicester city centre), the number of cells will have to be increased

This means that the size of each cell will get smaller

Hence a cell will have to be split create a cluster within a cell

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Capacity of cellular networks

Digital Mobile Telephony

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Digital Mobile TelephonyWhen the signals are encoded digitally, we have

digital mobile telephonyThe benefits are several

time sharing can be used i.e. one channel is used by 3-6 users the capacity is increased 3-6 times

quality of transmission can be improved by increasing noise resistance

security can be improved by scrambling the parts in the timeshared slots

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Components of Digital Mobile TelephonySIM

Subscriber Identification Module smart card address book details of transmitted/received calls network specific encoding

Handsetsearch for closest network base stationradio transmission/reception for calls, messages, etcuser interface to network services

Cell Systemcommunicate with handsettransparent re-connection to a new cell whilst user is in motioncall routingall other applications

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Digital mobile telephonyTwo types of digital standards

TDMA (Time Division Multiple Access) used in GSM (the most popular standard) the channel is shared in time between 3-6 users capacity increases up to 15 times by using on-the-fly

compression of the quiet pauses in a conversationCDMA (Code Division Multiple Access)

each call has an identifier (code) this allows many users to use one and the same

frequency there is no interference between different cells the cluster has just one cell in it

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Digital mobile telephonyGSM

(Global Standard for Mobile telecom) uses TDMA Used by over 3 billion people over 212 countries Easier roaming. Standard emergency number (112) 2G (SMS) Short Message Service. – Text messaging 2.5G (WAP)

3G Wideband CDMA (WCDMA)144KbpsVideoInternet access

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Code Division Multiple AccessAllows everybody in a cell to use the same

frequencySeparates calls by encoding each one uniquely

analogy: international cocktail party with many people talking in

different languages if you hear a voice speaking in your language, your brain

filters the background noise and locks onto to that one person

however, every other conversation is background noise!CDMA base station controls the mobile phone’s

power output which saves battery lifeadjusts signal strength according to distance from base

station

GSM: overall architecture

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Top level viewThe GSM

network is divided into three major systems: the switching

system (SS)the base

station system (BSS)

the operation and support system (OSS)

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The Switching System The switching system (SS) is responsible for

performing call processing and subscriber-related functions

The SS includes the following functional units: home location register (HLR)

database used to store and manage subscriptionsmobile services switching centre (MSC)

telephone switch visitor location register (VLR)

database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers

authentication centre (AUC) authentication and encryption of users

equipment identity register (EIR) database with the identity of mobile equipment that prevents

calls from stolen, unauthorized, or defective mobile phones each handset has a unique IMEI number

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The Base Station System (BSS) All radio-related

functions are performed in the BSSconsists of base

station controllers (BSC) and the base stations (BS) BSC - controls functions

and physical links between the mobiles and BS

BS - handles the radio interface to the mobile station antenna

Tower pictures from http://www.telstra.com.au/newsroom/photob.htm#towers

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The Operation and Support System The OSS is the functional entity from which the

network operator monitors and controls the systemcost-effective support for centralized, regional, and

local operational and maintenance activities that are required for a GSM network call routing, bill charging, administration, etc

provides a network overview and support the maintenance activities of different operation and maintenance organizations eg turn off a cell for maintenance

Hands-off Protocol

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Problem StatementAll is well until a mobile moves away from a base

stationthe signal between the mobile and the base station

weakensMoving away from one base station results (in

most cases) in getting closer to another• Different base stations use different frequencies– a call must either be dropped

or transferred to a different frequency• but dropping a call is

unacceptable!

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How is it doneThe (home) base continuously monitors the

strength of a mobile’s signalThe adjacent bases also monitor the signalIf it becomes weak, the base requests (to the

switching system) a frequency transferThe switching system finds out (from the

neighbouring bases) which cell the mobile is closest to

The call is transferred to a new pair of frequencies without the user noticing

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ApplicationsSMS

Short Message Service is a globally accepted Allows 160 characters and transfer between mobile, e-mail, voicemail

and paging Uses a subsystem called Short Message Service Centre (SMSC) as a

part of the Switching System SMSC is a “store-and-forward” service; sometimes with “advice of

delivery”Network broadcast

sends messages to all mobiles in a cell used for traffic, emergencies and weather updates

Voicemail “store-and-retrieve” service, part of the Switching System

Fax, e-mail, notification, etc

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Push-To-TalkIn the USA text messaging is not widely used

alternative voice system – PtTUses half-duplex communication.

Real-time direct one-to-one and one-to-many voice communication

Active call group - ‘always on’ connection – permanently listening. Usually between people on the same network.

Extra button on handsetpush to talk (release to listen)

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Push-To-TalkBased on half-duplex Voice over IP (VoIP)

technology over the 2nd generation GSM/GPRS network

Uses cellular access and radio resources more efficiently than circuit-switched cellular servicesnetwork resources reserved only for duration of

talk spurts instead of for an entire call session

Read http://en.wikipedia.org/wiki/Push_to_talk

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And Finally… A payphone on

Lake Victoria in Uganda using GSM Technology and Solar Power

From http://www.payphone-

project.com/payphones/photos/africa/

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Summary Developmental stages

Analogue High-power transmitter based networks Low-power transmitter based networks (cellular)

Digital GSM & WAP 3G

Overall architecture Cellular telephony Hands-off protocol

Common applications SMS, network broadcast, voice mail, etc