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 + 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