ELEC3030 Computer Networks

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Oct 23, 2013 (4 years and 20 days ago)

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ELEC3030(EL336)ComputerNetworks
SChen
ELEC3030ComputerNetworks
ProfessorShengChen:
Building86
,
Room1021
E-mail:sqc@ecs.soton.ac.uk
Downloadlectureslidesfrom:http://www.ecs.soton.ac.uk/∼sqc/EL336/
orgetthemfromCourseOffice(
ECSStudentServices
)
ReadingText:A.S.Tanenbaum,ComputerNetworks,4thedition,PrenticeHall,
2003.
2ndReadingText:L.ChappellandE.Tittel,GuidetoTCP/IP,2ndedition,
Thomson,2004.
Iwillcoverhalfoftheunit,including
•Introduction•Physicallayer
•Datalinklayer•Networklayer
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ELEC3030(EL336)ComputerNetworks
SChen
Overview
•Twomostimportantaspectsofcomputernetworksare
hardware
and
software
•Hardware:ascommunicationisaprimaryconcerninanetwork,wearedealing
withbothcomputersand
communicationtechnologies
•Intermsofscale,historicallycomputernetworksareclassifiedas
–Localareanetworks(LAN):withinroom,buildingorcampus,andsizefrom
10muptoafewkm
–Metropolitanareanetworks(MAN):sizeintenskmandmaycoveracity
–Wideareanetworks(WAN):withinacountryorevenwholecontinent,and
sizefrom10kmto100km
–Internetworks:dealwithhowtoconnectdifferentkindsofnetworks→resulting
theInternetwhichreallycoversthewholePlanet
•Software:whatactuallymakescomputernetworksissoftware.With“software”we
arenottalkingcomputercodes,butdesignthinking,
methodology
and
framework
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ELEC3030(EL336)ComputerNetworks
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Overview(continue)
•Itisusefultoviewcomputernetworks
software
asconsistingof
–Protocols:describehowtwocommunicatingpartiesexchangeinformation
–Services:describewhatanetworkofferstopartiesthatwanttocommunicate
–Interfaces:definehowtheservicesofferedbyanetworkcanbeaccessed
•Referencemodelisreferredtoasthewayoforganisingnetworksoftwares
•Differentpeopleandorganisationswouldlikeyoutoorganiseyournetworkintheir
referencemodels
,andsomeexamples:
–OSI7-layerreferencemodel(OSI:opensystemsinterconnection)
–TCP/IPmodel(TCP/IP:transmissioncontrolprotocol/Internetprotocol)
–Hybrid5-layermodelweadopt:acompromisebetweenOSIandTCP/IP
•Standardisationiscriticalincomputernetworks,andmajorplayersin
standardisations
include:ITU(internationaltelecommunicationunion),ISO
(internationalstandardsorganisation)andIEEE
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ELEC3030(EL336)ComputerNetworks
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TheHardware
•Transmissiontechnology:therearetwobasicmethods –Broadcast:transmissionisbroadcastedtoandreceivedbyall,asinbroadcast
networkssuchaspacketradio,satelliteandLANs
–Point-to-point:transmissiongoesfromsendertoreceiverpossiblyviasome
intermediateswitchingnodes,asinswitchednetworkssuchasWANsandInternet
•Arewirelessnetworks
WIFI
broadcastorpoint-to-pointlinkbased?
•LANs:usebroadcasttransmissiontechnology
TwotypicaltopologiesforLANsarebusand
tokenring
Cable Computer
(b)
(a)
Computer
•MANs:arebiggerversionsofLANs
Twoexamplesarethedistributedqueue,dual
busonrightandairinterfaceforfixedbroadband
wirelessaccesssystem(IEEE802.16)
WMAX
1
2
3
N
Bus A
Direction of flow on bus B
Head end
Direction of flow on bus A
Bus B
Computer
. . .
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ELEC3030(EL336)ComputerNetworks
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TheHardware(continue)
•LANsandMANsdonothaveany
switching
nodes:thewire(orwireless)doesall
thework→Thismakesthemextremelyefficientbuthardertogetbigger
•WANs:hostsareconnectedtoasubnet,which
containsrouters(switchingnodes)andtrunks
Routersgenerallyadoptastore-and-forward
(packet-switched)principle
SubnetRouter
Host
LAN
•Internetwork:manynetworksexistwithverydifferenthardwareandsoftware,and
interconnectingdifferentnetworksiscalledinternetwork –ConnectacollectionofdifferentLANswithinadepartment
–ConnectdifferentLANsthroughaWAN,withWANactingasasubnet
–ConnectdifferentWANstoeachotherbymeansofgateways→theInternet
•Subnet:collectionofroutersandtrunks;Network:combinationofasubnetand
itshosts;Internetwork:distinctnetworksinterconnectedtogether
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ELEC3030(EL336)ComputerNetworks
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LayeredStructureandProtocol
•Acompletecommunicationtaskacrossnetworkisverycomplicated→mostnetworkarchitectures
orsoftwaresareorganisedasaseriesof
layers
–Eachlayerperformsarelatedsubsetofthefunctionsrequired
tocommunicatewithanother
system
–Itreliesonthenextlowerlayertoperformmoreprimitivefunctions
andtoconcealthedetailsof
howthosefunctionsareactually
implemented
connection
entity
protocol
entity
N-1
N
interface
interface
service access
point
layer
•Logically,peerlayersacrossthenetworktalktoeachotherusing
protocols
•Thecommunicationisactuallycarriedoutusingsome
services
providedbylowerlayer
•Adjacentlayersinthesamemachinecommunicateviaan
interface
•Interfaceisrealisedinoneormoreserviceaccesspoints,whichfunctioninthemannerofports
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ELEC3030(EL336)ComputerNetworks
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Services
•Therearetwodifferenttypesofservices:
connection-oriented
and
connectionless
–Connection-oriented:theuserfirstestablishesaconnection,thenusestheconnectionto
communicate,finallyreleasestheconnection(thinkthisasusingtelephone)
–Connectionless:eachdataunithasthecompletedestinationaddressandisroutedthroughthe
networktothedestinationindependently(thinkthisaspostingaletter)
•Eachserviceofferscertain
quality
:e.g.whetherordereddeliveryandhowreliable
•Aserviceisspecifiedbyasetof
primitives
(operations)availabletoa
usertoaccesstheservice
–Fourclassesofserviceprimitivesare:
request
entitywantstheservicetodosomework
indication
entityistobeinformedaboutanevent
response
entitywantstoresponsetoanevent
confirm
responsetoearlierrequesthascomeback
•Considerasimplestconnection-orientedservicewith8primitives
1.CONNECT.request–requestaconnectiontobeestablished(dialaphonenumber)
2.CONNECT.indication–signalthecalledparty(phonerings)
3.CONNECT.response–usedbythecalleetoaccept/rejectcalls(pickupthephone)
4.CONNECT.confirm–tellthecallerwhetherthecallwasaccepted(callerhearsringingstop)
5.DATA.request–requestthatdatabesent(saysomething)
6.DATA.indication–signalthearrivalofdata(calleehearsyou)
7.DISCONNECT.request–requestthataconnectionbereleased(callerhangsup)
8.DISCONNECT.indication–signalthereleaseoftheconnection(calleehearsbusytone)
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ELEC3030(EL336)ComputerNetworks
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OSI7-layerReferenceModel
•Recallthatsoftwaremakes
computernetworksandlayered
structureisabasicprinciplein
organisingcomputernetwork
software
•Indesign,onewouldliketo
havea
framework
fordividing
networksoftwareintoseveral
layers,definingwhateachlayer
does,andfinallyproviding
implementationspecifications
→Thiskindofframeworkis
called
referencemodel
•AwellknownexampleistheOSI
7-layerreferencemodelspecified
byISO
Layer
Presentation
Application
Session
Transport
Network
Data link
Physical
7 6 5 4 3 2 1
Interface Interface
Host A
Name of unit
exchanged
APDU PPDU SPDU TPDU Packet Frame Bit
Presentation
Application
Session
Transport
Network
Data link
Physical
Host B
Network
Network
Data link
Data link
Physical
Physical
RouterRouter
Internal subnet protocol
Application protocol
Presentation protocol
Transport protocol
Session protocol
Communication subnet boundary
Network layer host-router protocol Data link layer host-router protocol Physical layer host-router protocol
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ELEC3030(EL336)ComputerNetworks
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OSI7-layerModel(continue)
1.Physicallayer:isconcernedwiththetransmissionofrawbits,anddealswithmechanical,electrical
andproceduralinterfaces,andphysicaltransmissionmedium
2.Datalinklayer:describeshowasharedcommunicationmediumcanbeaccessed,andhowto
makeanunreliablenoisylinkreliable
3.Networklayer:isconcernedwithcontrollingtheoperationofsubnet,forexample,howroutingis
done
4.Transportlayer:providestheactualnetworkinterfacetoapplications,jobslikemakingnetwork
connections,multiplexing,flowcontrol.Itisatrueend-to-endlayer,fromsourcetodestination
5.Sessionlayer:tellshowtosetup“long-lasting”communications(sessions).Thisisthedumbest
andill-defined
6.Presentationlayer:describeseverythingthatisneededtoexchangedatainaplatform-independent
way.Anexampleisdataencoding
7.Applicationlayer:containsthestuffthatusercansee,suchase-mail,filetransfer,remotelogin,
web’sexchangeprotocols
Notethatthe
user-network
interaction
occursatthebottomthree-layerlevels:the“net”isessentially
unconcernedwithhigherlayers
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ELEC3030(EL336)ComputerNetworks
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TCP/IPReferenceModel
•ThisiswhereInternetstarted:usedtobeawildcowboy’sworldbutnowisbetterstandardised
1.Applicationlayer:doessimilarthings
asOSIapplicationlayer
2.Transportlayer:doessimilarthings
asOSItransportlayer
Twoend-to-endprotocolsaredefined:
TCP–transmissioncontrolprotocol
(forreliableconnection-oriented)and
UDP–userdatagramprotocol(for
unreliableconnectionless)
3.Internetlayer:similarinfunctionality
toOSInetworklayer
TCP/IPOSI
Application Presentation Session Transport
Network Data link
Physical
7 6 5 4 3 2 1
Application Transport
Internet Host-to-network
Not present in the model
4.Host-to-networklayer:anythingbelowtheinternetlayer,notverywelldefined
•Tworeferencemodelsrepresenttwodifferentviewsoftheworld:telecommunicationcampand
computercamp.OSIcampviewstheworld(i.e.thenetwork)asrigid,welldefinedandorganised,
TCP/IPcamphistoricallyviewstheworldashostileandchaotic
Newstandards
arenowoftendefinedwith
best
of
both
referencemodels
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ELEC3030(EL336)ComputerNetworks
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ComparisonofOSIandTCP/IPModels
•Threeimportantconcepts,
services
,
interfaces
and
protocols
,arewelldefinedin
OSImodel,butnotinoriginalTCP/IPmodel

Transport
layerdoesthehardestjob,dealingwithend-to-end“connection”

Datalink
layerisalsoveryimportant:Anend-to-end“connection”consistsof
many“links”,andeachpossiblynoisylinkneedtobemadereliable
•Ontheotherhand,noonereallyknowspreciselywhat
session
layerdoes
•Itmayalsobearguedthataseparate
presentation
layerisnotstrictlynecessary
•Basedoncomparison,wewilladopt
thehybrid
5-layerreferencemodel
:
Thisisagood
framework
for
discussionofcomputernetworks
5
Applicationlayer
4
Transportlayer
3
Networklayer
2
Datalinklayer
1
Physicallayer
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ELEC3030(EL336)ComputerNetworks
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Summary
•Computernetworkhardwareoverview
Twocommunicationtechnologies:broadcastandpoint-to-pointtransmissions
Networksintermsofscale:LAN,MAN,WAN,internetwork
•Computernetworksoftwareoverview:
Layeredstructure,definitionsofprotocol,serviceandinterface
Connection-orientedandconnectionlessservices,servicequality,serviceprimitives
•Referencemodels:OSI7-layer,TCP/IPandhybrid5-layer
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