Wireless Wide Area Network-Cellular Telephone and Satellite Networks

Cellular telephony is designed to provide communications between two moving units, called mobile stations (MSs), or between one mobile unit and one stationary unit, often called a land unit.

1. Wireless WANs:
Cellular Telephone
and Satellite Networks
Dr R JegadeesanProf &CSE
JyothishmathiInstitute of technology and
Science, karimnagar

2. Cellular Telephony
➢Cellulartelephonyisdesignedtoprovide
communicationsbetweentwomovingunits,calledmobile
stations(MSs),orbetweenonemobileunitandone
stationaryunit,oftencalledalandunit.
➢InordertoprovidesuchtypeofcommunicationService
provider,musthavetheability
➢tolocateandtrackacaller
➢assignachanneltothecall
➢transferthechannelfrombasestationtobasestationas
thecallermovesoutofrange

3. ◼Tomakethistrackingpossible,eachcellular
serviceareaisdividedintosmallregions
calledcells.
◼Eachcellcontainsanantennaandis
controlledbyasolarorACpowerednetwork
station,calledthebasestation(BS).
◼Eachbasestation,inturn,iscontrolledbya
switchingoffice,calledaMobileSwitching
Center(MSC).

4. ◼The MSC coordinates communication between
all the base stationsand the telephone central
office
◼Itisacomputerizedcenterthatisresponsible
for
➢ConnectingCalls
➢RecordingCallInformation
➢Billing

6. ➢Cellsizeisnotfixedandcanbeincreasedor
decreaseddependingonthepopulationofthe
area.
➢Thetypicalradiusofacellis1to12miles.
➢Thetransmissionpowerofeachcelliskeptlow
topreventitssignalfrominterferingwiththose
ofothercells.
➢High-densityareasrequiremore,geographically
smallercellstomeettrafficdemandsthando
low-densityareas

7. Frequency-Reuse Principle
◼Ingeneral,neighbouringcellscannotusethe
samesetoffrequenciesforcommunication
becauseitmaycreateinterferenceforthe
userslocatednearthecellboundaries.
◼However, the set of frequencies available is
limited, and frequencies need to be reused.

8. ◼AfrequencyreusepatternisconfigurationofN
cells,Nbeingthereusefactor,inwhicheach
cellusesauniquesetoffrequencies.
◼Whenthepatternisrepeated,thefrequencies
canbereused.Thereareseveraldifferent
patterns

9. Figure 16.2 Frequency reuse patterns

10. ◼Thenumbersinthecellsdefinethepattern.
Thecellswiththesamenumberinapattern
canusethesamesetoffrequencies.
◼Wecallthesecellsthereusingcells.
◼AsFigureshows,inapatternwithreuse
factor4,onlyonecellseparatesthecells
usingthesamesetoffrequencies.
◼Inthepatternwithreusefactor7,twocells
separatethereusingcells

11. Transmission
◼To place a call from a mobile station, the caller enters
a code of 7 or 10 digits (a phone number) and presses
the send button.
◼Themobilestationthenscanstheband,tofindasetup
channelwithastrongsignal,andsendsthedata
(phonenumber)totheclosestbasestationusingthat
channel.
◼The base station relays the data to the MSC.
◼The MSC sends the data on to the telephone central
office. If the called party is available, a connection
is made and the result is relayed back to the MSC.

12. ◼Atthispoint,theMSCassignsanunusedvoice
channeltothecall,andaconnectionis
established.
◼Themobilestationautomaticallyadjustsits
tuningtothenewchannel,andcommunication
canbegin.

13. Receiving
◼Whenamobilephoneiscalled,thetelephone
centralofficesendsthenumbertotheMSC.
◼TheMSCsearchesforthelocationofthe
mobilestationbysendingquerysignalsto
eachcellinaprocesscalledpaging.
◼Oncethemobilestationisfound,theMSC
transmitsaringingsignal
◼whenthemobilestationanswers,assignsa
voicechanneltothecall,allowingvoice
communicationtobegin

14. Handoff
◼Itmayhappenthat,duringaconversation,
themobilestationmovesfromonecellto
another.
◼Whenitdoes,thesignalmaybecomeweak.
◼Tosolvethisproblem,theMSCmonitorsthe
signaleveryfewseconds.
◼Ifthestrengthofthesignalbecomesweak,
theMSCseeksanewcellthatcanbetter
accommodatethecommunication.

15. ◼TheMSCthenchangesthechannelcarrying
thecall(handsthesignalofffromtheold
channeltoanewone).
◼Handoffareoftwotypes
◼HardHandoff
◼SoftHandoff

16. Hard Handoff
◼Inahardhandoff,amobilestationonly
communicateswithonebasestation.
◼WhentheMSmovesfromonecelltoanother,
communicationmustfirstbebrokenwiththe
previousbasestationbeforecommunication
canbeestablishedwiththenewbasestation.

17. Soft Handoff
◼Inthiscase,amobilestationcancommunicate
withtwobasestationsatthesametime.
◼Thismeansthat,duringhandoff,amobile
stationmaycontinuewiththenewbasestation
beforebreakingofffromtheoldone

18. Roaming
◼Roamingmeansthatausercanhaveaccess
tocommunicationorcanbereachedwhere
thereiscoverage.
◼Aserviceproviderusuallyhaslimited
coverage.
◼Neighbouringserviceproviderscanprovide
extendedcoveragethrougharoaming
contract.

19. ◼SATELLITE NETWORKS

20. Asatellitenetworkisacombinationofnodes,someof
whicharesatellites,thatprovidescommunicationfrom
onepointontheEarthtoanother.
Anodeinthenetworkcanbe
➢Satellite
➢EarthStation
➢End-userTerminalOrTelephone.

21. ◼Anaturalsatellite,suchastheMoon,canbe
usedasarelayingnodeinthenetwork,butthe
useofartificialsatellitesispreferredbecause
wecaninstallelectronicequipmentonthe
satellitetoregeneratethesignalthathaslost
itsenergyduringtravel.
◼Another restriction on using natural satellites
is their distances from the Earth, which create
a long delay in communication

22. ◼Satellitenetworksarelikecellularnetworksin
thattheydividetheplanetintocells.
◼Satellitescanprovidetransmissioncapability
toandfromanylocationonEarth,nomatter
howremote.
◼This advantagemakes high-quality
communicationavailabletoundevelopedpartsof
theworldwithoutrequiringahugeinvestmentin
ground-basedinfrastructure.

23. Orbits
◼Anartificialsatelliteneedstohavean
orbitthepathinwhichittravelsaround
theEarth
◼The orbit can be
➢Equatorial Orbits
➢Inclined Orbits
➢PolarOrbits

24. ◼TheEquatorialorbitisacircularorbitwhich
liesexactlyintheplaneofequator.
◼TheInclinedorbitsaretheorbitsthatmake
angleswithequatorandpolarplanesofearth
◼Polarorbitsaretheorbitsthataredirectly
perpendiculartotheequatorialorbits.
◼Theperiodofasatellitei.e.thetimerequired
forasatellitetomakeacompletetriparound
theEarth,isdeterminedbyKepler'slaw.

25. Three categories of satellites
◼Basedonthelocationoftheorbit,satellites
canbedividedintothreecategories
➢Geostationary Earth orbit (GEO),
➢Low-Earth-orbit (LEO),
➢Middle-Earth-orbit (MEO).

27. Geostationary Earth Orbit(GEO)
◼Toensureconstantcommunication,the
satellitemustmoveatthesamespeedasthe
earthsothatitseemstoremainfixedabovea
certainspot.
◼Suchsatellitesarecalledgeostationary.
◼Thegeosatellitesareatanaltitudeof
36000kmandabove

28. ◼Butonegeostationarysatellitecannotcover
thewholeEarth.
◼Onesatelliteinorbithasline-of-sightcontact
withavastnumberofstations,butthe
curvatureoftheEarthstillkeepsmuchofthe
planetoutofsight.
◼Ittakesaminimumofthreesatellites
equidistantfromeachotherinGeostationary
Earthorbit(GEO)toprovidefullglobal
transmission.

29. Satellites in geostationary orbit

30. Low-Earth-orbit (LEO)
◼Low-Earth-orbit(LEO)satelliteshavepolar
orbits
◼Thealtitudeisbetween500and2000km,
witharotationperiodof90to120min.
◼Thesatellitehasaspeedof20,000to25,000
km/h.
◼AnLEOsystemusuallyhasacellulartypeof
access,similartothecellulartelephone
system.
◼Thefootprintnormallyhasadiameterof
8000km

31. ◼An LEOsystem is made of a constellation of
satellitesthat work together as a network;
each satellite acts as a switch.
◼Satellites that are close to each other are
connected through inter satellite links (ISLs).
◼A mobile system communicates with the
satellite through a user mobile link (UML).
◼A satellite can also communicate with an
Earth station (gateway) through a gateway
link (GWL).

32. Figure 16.19 LEO satellite system

33. Middle-Earth-Orbit(MEO)
◼Medium-Earth-orbit(MEO)satellitesare
positionedbetweenthetwoVanAllen
radiationbelts.
◼A satellite at this orbit takes approximately 6-
8 hours to circle the Earth.

34. ◼Itissometimescalledintermediatecircular
orbit(ICO),istheregionofspacearoundtheEarth
abovelowEarthorbit(altitudeof2,000kilometres)
andbelowgeostationaryorbit(altitudeof35,786
kilometers).
◼OneexampleofaMEOsatellitesystemistheGlobal
PositioningSystem(GPS).
◼The GPS system consists of 24 satellites and is used
for land, sea, and air navigation to provide time and
locations for vehicles and ships.
◼GPS uses 24 satellites in six orbits, as shown in
Figure

36. Thank you