Overview on Telephone Network - the traditional telecommunication 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 Telephone Exchange: 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: Circuit Switching 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.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. Packet switching 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 network topology 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 Internet 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: Multiplexing What is multiplexing? ¾ Enabling a number of lower bit rate connections to share a single higher bit rate transmission line. 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 optical fiber 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 ln(+ 1μ x ) (f ) x= sgn( x ) μ ln(+ 1μ ) A law A x 1 (f ) x= sgn( x ) ≤ x 0 ≤ A 1+ ln(A ) A 1+ lnAx 1 (f ) x= sgn( x ) ≤ x ≤1 A 1+ ln(AA ) 33 Transmission: Voice Coding ToTo representrepresent thethe digitizeddigitized voicevoice signalsignal atat aa reducedreduced bitbit raterate forfor narrowbandnarrowband transmissiontransmission andand