01 What is Mobile Computer

8/9/2019 01 What is Mobile Computer

1/31

Mobile Computing

Networks

Introduction


2/31

What is mobile computer?

A computerwhich youcantake with you

allaround.

Youcandoallthethings which can be

done with adesktopcomputer.

Youshould beabletousesame

software, which youuseonadesktopcomputer.


3/31

Mobile computer - How?

Onepossibility isto haveastandalone

computercapableof

storinglargeamountofsoftwareanddatafiles,

processingpowertosupporttherequiredapplications.

Modernday laptopcomputeraresomethinglikethis.


4/31

Mobile computer - How? Contd..

Wheneveryouarestatic,connectto

internetthrough anaccesspointand you

candothefiletransfer,telnet, webbrowsingetc..

Whileonthemove,connectivity is

desiredforusingsoftware which requirecooperationofatleasttwomachines.


5/31

Option forconnectivity

Aneasy option

Usecellularmobilephonenetworktoconnecttosome

Internet Service Providerand hencetointernet. What youneed - cellularphone/correspondingmodem.

Notagoodoptionforcampus widemobile

network.

dependentonthemobiletelephonenetworkoperator cheaperwireless LANoptionsavailable


6/31

Otherextremeofmobile

computer

Mobilecomputingdevice

actsasaterminal

have wirelessconnectivity tothenetwork Whatevercommandorapplication yourunis

executedonaremoteserver.

Mobilecomputingdeviceactsasremoteterminal.


7/31

Advantages/ disadvantagesof

thisextreme

Very small,compactandlight weight

mobilecomputer

Lesspowerconsumptionandlarge

battery life - thismay not betrue,ifradio

communicationrequiresmorepower

thantopowerupallnecessary devicesforastandalonecomputeronaverage

basis.


8/31

Advantages/ disadvantagesof

thisextreme contd..

Higherreliability ofdata - theftof

standalonecomputermeanseverything

islost. But hereallinformationisstoredonsomestaticserver.

Butdataprivacy whilecommunicatingtoserversviathenetwork hasto beensured.

Theperformance highly dependentupon

networkand wirelesschannelconditions.


9/31

Issuesinmobile computing

networks

ActualMobilecomputer- somewherein

betweenthetwoextremes.

Issuesdueto

natureofmedium Mobility

Portability


10/31

Wirelessmedium

Disconnection Commoninradioenvironments - dueto

noise. duetomovingintodarkareas Blocking by serverse.g.,duetotoomany

requeststoafileserver.

Standalonemobilecomputercantoleratethiskindofproblem better.

Portableterminals willnotfunction


11/31

Wirelessmedium contd..

Asynchronousoperations - roundtriplatencyandshortdisconnectionscan betakencareof.(X11 window system operates this way.)

Synchronousremoteprocedurecalls - Aftereachrequest waitforresponse. Asynchronousremoteprocedurecalls - Afterseveral

requests have beensentthenacknowledgementisaskedfor.

Pre-fetching/lazy write backdecouplescommunicationfromdatausage/generation.

Decouplingallowstheprogramtoprogressevenduringdisconnection.


12/31

Coda - distributedfilesystem,developed

fornotebookcomputers with less

frequentdisconnections on boardcache usersprofileisusedtokeep bestselectionof

filesinthecache

Wholefilesarecachedinsteadoffixed blockofdata.


13/31

Afterdisconnection,thecacheisautomatically synchronized with file

server. Filemodificationareallowedevenduring

disconnection. Probability ofhavingmorethantwomodifiedversionsofafile

islessthan 1% even whenmorethanoneuserisgiven writeaccesstosamefile.


14/31

Low Bandwidth

typical bandwidth

Wireless Infrared - 1 Mbps Radio - 2 Mbps cellulartelephony - 9-14 Kbps

Wired

Ordinary Ethernet - 10 Mbps FDDI - 100 MbpsATM - 155 Mbps


15/31

Network design to copupwith

lowbandwidth

Morenumberofcellsforagivenuserarea

usesmallerwirelesscells.The bandwidth is shared byall the mobile computers within a cell.

Useofoverlappingcellsoperatingindifferentpartofspectrum

Advantagesoffirsttechnique

Simplertechnique

Reducespowerrequirements transceiverscoveringlessareasachieves higher

bandwidth.


16/31

Software techniques to combat

lowbandwidth

There is no way bandwidth can be increased, it canonly beefficiently utilized.

compressionatthetwoendsofwirelesslink.

Logging - since bulkusagemoreefficientthanmultiplesmallusage.

Loggingalong with compression - still better- Largerblock compress better.

Thedatacommunication - bursty.Thetechniquesfor

disconnectioncanalsoimprovetheperformanceinlowbandwidth environment

Duringa burstdemandtemporarily exceedsthecapacity -burstislikeshortdisconnection.


17/31

Prefetching/ Lazy write backcanusevalleystoreducedemandatpeaks - leading toeffective usage of bandwidth.


18/31

Schedulingcommunicationintelligently

Whendemandexceedsthecapacity,givepriority totheprocesses waitingforaresponse.

The jobslike writing backupstotheserveretc.shouldbedoneinleftoverbandwidth.

Coming Emailscan benotifiedtouseronceit hasfullycometomobilecomputer.Itcan betransferredaslow

priority trafficoverthe wirelesslink. These techniques dont increase bandwidth

but improves users satisfaction


19/31

Bandwidthvariability

Withinacellnumberofmobilecomputerscanvary.Thus bandwidth varies with time.

Themobilecomputerwhenpossiblecan be

connectedto wirednetwork (plugged).Bandwidth variation betweenpluggedandwirelessaccessislarge.

A mobilecomputercanassume high bandwidth connectionandoperatesonly when

plugged. Low bandwidth anddonttakeadvantageofhigh

bandwidth whenplugged.


20/31

Bandwidthvariability contd..

Variable bandwidth andadapttocurrently availableresources

Variable bandwidth meanslargerdelay

variation

Subnetshould bepreferably connection

oriented (e.g.,virtualcircuits)

forconnectionlesssubnetdueto bandwidth variability,delay variations will belargeleadingtolarge

unnecessary retransmissionsattransportlevel.


21/31

Heterogeneousnetworks

Switching betweeninterfaces Infrared LANs

WirelessRadio LANs Cellularnetworks Switching betweenvariousaccess

protocols

Mechanisminsoftware & hardwareneededfortheabove


22/31

Mobility

AddressMigration

Current wirednetworks - notdesignedfordynamically changingaddresses.

Each hostnameis boundtoanetworkaddress (inTCP/IP worldDNS serverdoes

this binding)

Movingtonew locationmeansacquiringnewnetworkaddress.The bindingneedto bemodified.


23/31

Mobility contd

The bindinginfo cannot becached with longexpirationtimeinthe

network.

Theaddressofmobile hostischangingmuch fasterduetomobility.

Tocommunicate with mobile host - messageshould besenttoitsmostrecentaddress.

Forallactiveconnections,mobile hostshouldkeeponinformingaboutchangeinaddresstoallthe hostsatotherends.


24/31

Findingmobilehostaddresses

Fourmechanismsforfindingmobile host

addresses

selective broadcast centralservices home bases Forwardingpointers


25/31

Selectivebroadcast

Broadcast - amessageissenttoallcells

requestingthemobile hosttorespond

with itsnew networkaddress. Expensivemethodforlargenetworks

Ifavailableapriory knowledgeofgroup

ofcells where hostislikely to bethere -doselective broadcasttothesecells.


26/31

Central services

currentaddressofeach mobile hostkeptinlogically centralizeddatabase

Every mobile host whenchangesaddress - anupdatemessageissentanddatabaseisupdated

Databaseislogically centralized, butcan

bephysically distributedusingreplicationandcachingforbetterefficiency.


27/31

Homebases

limitingcaseofdistributingcentralservice.

Only asingleserver(home base)knowsthe

currentaddressofmobile host.(Each mobilehost may have separate or common homebase

for keeping track of its network layer address)

Mechanismfortrackingthemobile hostis

sameasincentralserver.


28/31

Homebases contd

Problems

Ifhome baseisdown, how totrackthemobilehost. Backup home bases - keepthereplicaof

databasein home base.

Mobile hostcanchange home baseitself.This happensonlongduration basis.The

mobile host havetonotify allofitsservers

regardingthischange.


29/31

THANK YOU


30/31

Forwardingpointers

Wheneveramobile hostchangesaddress,copy ofnew addressleftatoldlocation

Each message - sentalongthechainusingthesepointers

Toavoidlongchains henceinefficientrouting,pointersupdatedatsource hostsand homebaseperiodically.

Agentsarerequired which receivesandforwardsthemessage.


31/31

Portability

Minimizepowerconsumption - largerbattery life,smallerbattery size - lesser

weight Smalleruserinterface - handwriting,

voicerecognition,pens,virtualrealityscreens.

Smallstoragecapacity - Compressedfilesystems

Documents

01 What is Mobile Computer