CTEC1414 Lecture 20
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Transcript of CTEC1414 Lecture 20
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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