Overview on Network - the traditional network

The following slides are largely based on the book: S. Keshav, “An Engineering Approach to Computer Networking”, Addison Wesley. The Invention of the Telephone

„ When?When?

¾ MarchMarch 10,10, 1876.1876. „ WhoWho inventedinvented thethe telephone?telephone?

Alexander Graham Bell speaking into prototype telephone.

2 The First Voice Message

Mr. Watson, come here. I want to see you.

Speaking through the instrument to his assistant, Thomas A. Watson, in the next room, Bell said these famous first words.

3 : 1878

„ (1878)(1878) BellBell setset upup thethe firstfirst telephonetelephone exchangeexchange overover aa manualmanual switchboardswitchboard

A telephone operator manually connected calls with patch cables at a telephone switchboard.

4 From Analog to Digital (1960’s)

„ 19621962 TheThe firstfirst digitaldigital TT--carriercarrier systemsystem waswas introducedintroduced intointo commercialcommercial serviceservice byby AT&T.AT&T. „ 1960s1960s TelephoneTelephone networknetwork waswas thethe worldworld’’ss dominantdominant communicationcommunication network.network. „ CircuitCircuit SwitchingSwitching isis used:used: ¾ WhenWhen therethere isis aa call,call, aa pathpath fromfrom sourcesource toto destinationdestination isis setset up.up. ¾ TheThe bandwidthbandwidth ofof thethe pathpath isis reservedreserved forfor thethe callingcalling partiesparties forfor thethe wholewhole ofof thethe call.call.

5 Example:

„ For host A to send messages to B, the network must reserve one circuit on each of two links.

„ Each link can have more than 1 circuit. How?

6 Why Circuit Switching?

„ AA telephonetelephone callcall typicallytypically lastslasts forfor aa longlong timetime ¾ itit justifiesjustifies thethe costcost ofof settingsetting upup aa circuitcircuit beforebefore transmissiontransmission.. „ VoiceVoice signalsignal isis ofof constantconstant bitbit raterate (e.g.(e.g. 6464 kbpskbps forfor PCM).PCM). ¾ CircuitCircuit switchingswitching simplifiessimplifies thethe allocationallocation ofof .bandwidth.

7 Computer Networks

„ 1960s Computers became important. „ How to connect computers together? „ Is circuit switching appropriate? ¾ Data traffic are typically bursty. ¾ Typical Scenario: ) Sending a command to a remote computer ) A period of inactivity ) Sending another command ¾ Assigning a dedicated channel wastes bandwidth. „ was invented for this reason.

8 Learning objectives Know „ the simplified view of a typical telephone network „ how to route a call in telephonetelephone corecore networknetwork „ the essential issue/problem for routing a call „ features of telephone network routing „ the fundamental concepts for “transmission” „ why switching is needed instead of having a link for each pair of users „ the job of a signaling network „ how the state transition diagram can help a switch controller to decide what action to take according to the

incoming signal 9 Concepts

„ SingleSingle basicbasic service:service: twotwo--wayway voicevoice

¾ lowlow endend--toto--endend delaydelay ¾ guaranteeguarantee thatthat anan acceptedaccepted callcall willwill runrun toto completioncompletion „ EndpointsEndpoints connectedconnected byby aa circuitcircuit

¾ likelike anan electricalelectrical circuitcircuit ¾ signalssignals flowflow bothboth waysways ((fullfull duplexduplex)) ¾ associatedassociated withwith bandwidthbandwidth andand bufferbuffer resources resources 10 The big picture

„ Fully connected core

¾ simple routing ¾ telephone number is a hint about how to route a call ¾ hierarchically allocated telephone number space

11 The basic elements

1.1. RoutingRouting 2.2. SwitchingSwitching 3.3. TransmissionTransmission 4.4. SignalingSignaling

12 Two Key Network Functions

„ routing:routing: determinedetermine analogy:analogy: Traveling routeroute takentaken byby datadata fromfrom sourcesource toto „ routing:routing: processprocess ofof destinationdestination planningplanning triptrip fromfrom sourcesource toto destinationdestination ¾ routingrouting algorithmsalgorithms

„ switching:switching: movemove datadata „ switching:switching: processprocess ofof fromfrom switchswitch’’ss inputinput toto gettinggetting throughthrough singlesingle appropriateappropriate switchswitch’’ss interchangeinterchange outputoutput

13 1. Routing: Telephone

„ 3-level hierarchy, with a fully-connected core „ AT&T: 135 core switches with nearly 5 million circuits „ Local Exchange Carriers (LEC(LEC)) may connect to multiple cores 14 Routing algorithm

„ If endpoints are within same Central Office (CO), directly connect „ If call is between COs in same LEC, use one-hop (or the shortest) path between COs „ Otherwise send call to one of the cores „ Only major decision is at core/toll switch ¾ one-hop or two-hop path to the destination toll switch ¾ (why don’t we use paths with more than two hops?) „ Essence of problem/issue ¾ which two-hop path to use if one-hop direct path is full?

15 Features of telephone network routing

„ Stable load

¾ can predict network load throughout the day ¾ can choose optimal routes in advance „ Extremely reliable switches

¾ downtime is less than a few minutes per year ¾ can assume that a chosen route is available ¾ can’t do this in the „ Single organization controls entire core

¾ can collect global statistics and implement global changes „ Very highly connected network

„ Connections require resources (but all need the same)

16 2. Switching: motivation

„ Problem:Problem:

¾ eacheach useruser cancan potentiallypotentially callcall anyany otherother useruser ¾ cancan’’tt havehave directdirect lines!lines! (Why?)(Why?)

17 Why switching is needed instead of having a link for each pair of users?

A number of terminals connected with each other

To fully connect n terminals, how many links are needed?

18 Star Topology

Switch / Router

No. of links can be reduced

19 A 2-Tier Network

A larger network may be constructed recursively in the same way.

20 Switching

„ SwitchesSwitches establishestablish temporarytemporary circuitscircuits

„ SwitchingSwitching systemssystems comecome inin twotwo parts:parts: switchswitch andand switchswitch controllercontroller

21 Switching: what does a switch do?

„ TransfersTransfers datadata fromfrom anan inputinput toto anan appropriateappropriate outputoutput

„ SomeSome waysways toto switch:switch:

¾ spacespace divisiondivision

22 Switching

„ AnotherAnother wayway toto switchswitch

¾ timetime divisiondivision (time(time slotslot interchangeinterchange oror TSI)TSI)

„ ToTo buildbuild largerlarger switchesswitches wewe combinecombine spacespace andand timetime divisiondivision switchingswitching elementselements

23 3. Transmission

„ LinkLink characteristicscharacteristics

¾ information carrying capacity (bandwidth) ) information sent as symbols ) 1 symbol >= 1 bit ¾ propagation delay ) time for electromagnetic signal to reach other end ) light travels at 0.7c in fiber ~8 microseconds/mile ) NY to SF => 20 ms; NY to London => 27 ms

24 Transmission:

„ What is multiplexing?

¾ Enabling a number of lower bit rate connections to share a single higher bit rate . „ Frequency-Division Multiplexing (FDM)

¾ Applies to both analog and digital signals ¾ e.g. 4 kHz for each analog voice signal „ Time-Division Multiplexing (TDM)

¾ Applies only to digital signals ¾ e.g. digital voice using pulse-coded modulation (PCM) ) Sampling: 8 kHz, and Quantization: 8 bits per sample => Bit Rate: 64 kbps 25 Multiplexing: FDM and TDM Example: FDM 4 users

frequency

time TDM

frequency

time 26 Transmission: Multiplexing

„ MultiplexedMultiplexed trunkstrunks cancan bebe multiplexedmultiplexed furtherfurther

„ NeedNeed aa standard!standard! (why?)(why?)

„ US/JapanUS/Japan standardstandard isis calledcalled DigitalDigital SignalingSignaling hierarchyhierarchy (DS)(DS) Digital Signal Number of Number of voice Bandwidth Number previous level circuits circuits DS0 1 64 Kbps DS1 24 24 1.544Mbps DS2 4 96 6.312 Mbps DS3 7 672 44.736 Mbps

27 Transmission: Link technologies

„ Many in use today ¾ twisted pair ¾ coax cable ¾ terrestrial microwave ¾ satellite microwave ¾ ¾ ADSL (Asymmetric Digital Subscriber Lines ): the most chosen broadband option in the world (more than 60% of the broadband market) „ Increasing amount of bandwidth and cost per foot „ Popular ¾ fiber ¾ ADSL 28 Transmission: Analogue to Digital Conversion

„ ToTo representrepresent anan infiniteinfinite precisionprecision signalsignal originallyoriginally inin anan analogueanalogue formform byby aa finitefinite setset ofof numbersnumbers atat aa fixedfixed samplesample raterate

„ TwoTwo steps:steps:

¾ samplingsampling ¾ quantizationquantization

29 Transmission: Sampling

The sampling process leads to the PAM (pulse amplitude modulation) representation of the analogue signal 30 Transmission: Uniform Quantization

• Samples are quantized to the nearest quantization level • PAM + quantization ⇒ PCM (pulse code modulation) 31 Transmission: Non-uniform Quantization

„ SignalSignal compressioncompression ++ uniformuniform quantizationquantization ⇒⇒ nonnon--uniformuniform quantizationquantization

32 Transmission: Ways of Compression

„ μ law + μ x )1ln( = xxf )sgn()( μ + μ)1ln(

„ A law xA 1 = xxf )sgn()( 0 x ≤≤ A + A)ln(1 A + ln1 Ax 1 = xxf )sgn()( x ≤≤ 1 A + )ln(1 AA

33 Transmission: Voice Coding

„ ToTo representrepresent thethe digitizeddigitized voicevoice signalsignal atat aa reducedreduced bitbit raterate forfor narrowbandnarrowband transmissiontransmission andand digitaldigital storagestorage devicesdevices withwith limitedlimited capacitycapacity

34 Transmission: Codecs in ITU standards

„ G.711G.711

¾ approvedapproved inin 19651965 ¾ PCM,PCM, μμ lawlaw oror AA lawlaw ¾ 80008000 samplesample perper secondsecond ¾ eacheach samplesample isis encodedencoded asas anan octetoctet ¾ 6464 kb/skb/s

35 „ G.722G.722

¾ approvedapproved inin 19881988 ¾ providesprovides aa higherhigher qualityquality ofof digitaldigital encodingencoding ofof 77 kHzkHz ofof audioaudio spectrumspectrum ¾ supportsupport aa numbernumber ofof rates:rates: 48,48, 5656 oror 6464 kb/s,kb/s, usingusing SBSB--ADPCMADPCM ((subbandsubband -- adaptiveadaptive differentialdifferential PCM)PCM) ¾ goodgood forfor allall professionalprofessional conversationalconversational voicevoice applications,applications, butbut musicalmusical applicationsapplications areare notnot recommendedrecommended

36 „ G.726G.726 ¾ approvedapproved inin 19901990 ¾ ratesrates inin 16,16, 24,24, 3232 oror 4040 kb/s,kb/s, usingusing ADPCMADPCM (adaptive(adaptive differentialdifferential PCM)PCM) ¾ thethe qualityquality atat 3232 kb/skb/s isis takentaken asas aa referencereference forfor tolltoll qualityquality

„ G.728G.728 ¾ approvedapproved inin 19921992--9494 ¾ 1616 kb/s,kb/s, usingusing LDLD--CELPCELP (low(low delay,delay, codecode-- excitedexcited linearlinear prediction)prediction) ¾ qualityquality similarsimilar toto G.726G.726 37 „ G.723.1G.723.1

¾ approvedapproved inin 19951995 ¾ twotwo modesmodes ofof operationoperation ) 6.4 kb/s, using MP-MLQ (multipulse-maximum likelihood quantization) ) 5.3 kb/s, using ACELP (algebraic-code-excited linear prediction) ¾ hashas aa voicevoice activityactivity detection,detection, discontinuousdiscontinuous transmission,transmission, comfortcomfort noisenoise generationgeneration capabilitycapability ¾ 1.11.1 kb/skb/s duringduring silencesilence periodperiod

38 „ G.729G.729

¾ approvedapproved inin 19951995 ¾ 88 kb/s,kb/s, usingusing ABSABS CSCS--ACELPACELP (analysis(analysis byby sythesissythesis,, conjugateconjugate structurestructure –– ACELP)ACELP) ¾ therethere isis aa lowlow--complexitycomplexity versionversion G.729A,G.729A, whichwhich isis sometimessometimes usedused inin VoIPVoIP systemssystems

39 Transmission: Codecs from ETSI (Europe)

„ GSMGSM 06.1006.10 ¾ Approved in 1988 ¾ 13 kb/s, using RPE-LTP (regular pulse excitation – long term prediction) ¾ used in cellular mobile system

„ GSMGSM 06.6006.60 ¾ Approved in 1996 ¾ 12.2 kb/s, using ACELP

40 Transmission: Codec from IETF

„ iLBCiLBC (internet(internet LowLow BitrateBitrate Codec)Codec)

¾ 13.3313.33 kb/s,kb/s, LPCLPC andand blockblock basedbased codingcoding ofof thethe LPCLPC residualresidual signalsignal usingusing anan adaptiveadaptive codebookcodebook ¾ basicbasic qualityquality higherhigher thanthan G.729A,G.729A, highhigh robustnessrobustness toto packetpacket lossloss ¾ computationalcomputational complexitycomplexity inin thethe rangerange ofof G.729AG.729A ¾ royaltyroyalty freefree codeccodec ¾ http://http://www.ilbcfreeware.orgwww.ilbcfreeware.org// 41 4. Signaling

„ RecallRecall thatthat aa switchingswitching systemsystem hashas aa switchswitch andand aa switchswitch controllercontroller

„ SwitchSwitch controllercontroller isis inin thethe controlcontrol plane

¾ does not touch voice samples „ Manages the network

¾ call routing (collect dialstring and forward call) ¾ alarms (trigger the ring at receiver) ¾ billing ¾ directory lookup (for 800/888 calls)

42 Signaling network

„ SwitchSwitch controllerscontrollers areare specialspecial purposepurpose computerscomputers

„ LinkedLinked byby theirtheir ownown internalinternal computercomputer networknetwork

¾ Common Channel Interoffice Signaling (CCIS) network „ MessagesMessages onon CCISCCIS conformconform toto SignalingSignaling SystemSystem 77 (SS7)(SS7) spec.spec.

43 44 Signaling

„ OneOne ofof thethe mainmain jobsjobs ofof switchswitch controller:controller: keepkeep tracktrack ofof statestate ofof everyevery endpointendpoint andand taketake actionaction accordingaccording toto thethe incomingincoming signalsignal

„ KeyKey isis statestate transitiontransition diagramdiagram

45 46 QQ && AA

47