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.

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The Invention of the Telephone

When?When?

March 10, 1876.March 10, 1876.

Who invented the telephone?Who invented the telephone?

Alexander Graham Bell speaking into prototype telephone.

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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.

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Telephone Exchange: 1878

(1878) Bell set up (1878) Bell set up the first telephone the first telephone exchange over a exchange over a manual switchboardmanual switchboard

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

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From Analog to Digital (1960’s)

1962 The first 1962 The first digital Tdigital T--carrier systemcarrier systemwas introduced into commercial was introduced into commercial service by AT&T.service by AT&T.

1960s Telephone network was the world1960s Telephone network was the world’’ssdominantdominant communication network.communication network.

Circuit Switching is used:Circuit Switching is used:

When there is a call, When there is a call, a path from source to a path from source to destination is set up.destination is set up.The bandwidth of the path is The bandwidth of the path is reservedreserved for for the calling parties for the whole of the call.the calling parties for the whole of the call.

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Example: Circuit Switching

For host For host AA to to send messages send messages to to BB, the , the network must network must reserve one reserve one circuitcircuit on each on each of two links. of two links.

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

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Why Circuit Switching?

A telephone call typically A telephone call typically lasts for a long lasts for a long timetime

it justifies the cost of setting up a circuit it justifies the cost of setting up a circuit before transmissionbefore transmission..

Voice signal is of Voice signal is of constant bit rateconstant bit rate (e.g. (e.g. 64 kbps for PCM). 64 kbps for PCM).

Circuit switching simplifies the allocation of Circuit switching simplifies the allocation of bandwidth.bandwidth.

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Computer Networks

1960s Computers became important.1960s Computers became important.

How to connect computers together?How to connect computers together?

Is circuit switching appropriate?Is circuit switching appropriate?

Data traffic are typically Data traffic are typically burstybursty..Typical Scenario:Typical Scenario:

Sending a command to a remote computerSending a command to a remote computerA period of inactivityA period of inactivitySending another commandSending another command

Assigning a dedicated channel wastes bandwidthAssigning a dedicated channel wastes bandwidth..Packet switchingPacket switching was invented for this reason.was invented for this reason.

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Learning objectivesKnowKnow

the simplified view of a typical telephone networkthe simplified view of a typical telephone network

how to route a call in telephone core networkhow to route a call in telephone core network

the essential issue/problem for routing a callthe essential issue/problem for routing a call

features of telephone network routingfeatures of telephone network routing

the fundamental concepts for the fundamental concepts for ““transmissiontransmission””why switching is needed instead of having a link for why switching is needed instead of having a link for each pair of users each pair of users

the job of a signaling network the job of a signaling network

how the state transition diagram can help a switch how the state transition diagram can help a switch controller to decide what action to take according to the controller to decide what action to take according to the incoming signalincoming signal

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Concepts

Single basic service: twoSingle basic service: two--way voiceway voice

low endlow end--toto--end delayend delay

guarantee that an accepted call will run to guarantee that an accepted call will run to completioncompletion

Endpoints connected by a Endpoints connected by a circuitcircuit

like an electrical circuitlike an electrical circuit

signals flow both ways (signals flow both ways (full duplexfull duplex))

associated with bandwidth and buffer associated with bandwidth and buffer resourcesresources

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The big picture

Fully connected coreFully connected core

simple routingsimple routing

telephone number is a hint about how to route a calltelephone number is a hint about how to route a call

hierarchically allocated telephone number spacehierarchically allocated telephone number space

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The basic elements

1. Routing1. Routing

2. Switching2. Switching

3. Transmission3. Transmission

4. Signaling4. Signaling

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Two Key Network Functions

routing:routing: determine determine route taken by data route taken by data from source to from source to destination destination

routing algorithmsrouting algorithms

switching:switching: move data move data from switchfrom switch’’s input to s input to appropriate switchappropriate switch’’s s outputoutput

analogy:analogy: Traveling

routing:routing: process of process of planning trip from planning trip from source to destinationsource to destination

switching:switching: process of process of getting through single getting through single interchangeinterchange

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1. Routing: Telephone network topology

33--level hierarchy, with a fullylevel hierarchy, with a fully--connected coreconnected core

AT&T: 135 core switches with nearly 5 million circuitsAT&T: 135 core switches with nearly 5 million circuits

Local Exchange Carriers (LEC) may connect to multiple Local Exchange Carriers (LEC) may connect to multiple corescores

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Routing algorithm

If endpoints are within same Central Office (CO), If endpoints are within same Central Office (CO), directly connectdirectly connect

If call is between COs in same LEC, use oneIf call is between COs in same LEC, use one--hop (or hop (or the shortest) path between COsthe shortest) path between COs

Otherwise send call to one of the coresOtherwise send call to one of the cores

Only major decision is at core/toll switchOnly major decision is at core/toll switchoneone--hop or twohop or two--hop path to the destination toll switchhop path to the destination toll switch(why don(why don’’t we use paths with more than two hops?)t we use paths with more than two hops?)

Essence of problem/issueEssence of problem/issuewhich twowhich two--hop path to use if onehop path to use if one--hop direct path is full?hop direct path is full?

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Features of telephone network routing

Stable loadStable load

can predict network load throughout the daycan predict network load throughout the day

can choose optimal routes in advancecan choose optimal routes in advance

Extremely reliable switchesExtremely reliable switches

downtime is less than a few minutes per yeardowntime is less than a few minutes per year

can assume that a chosen route is availablecan assume that a chosen route is available

cancan’’t do this in the Internett do this in the Internet

Single organization controls entire coreSingle organization controls entire core

can collect global statistics and implement global changescan collect global statistics and implement global changes

Very highly connected networkVery highly connected network

Connections require resources (but all need the same)Connections require resources (but all need the same)

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2. Switching: motivation

Problem:Problem:

each user can potentially call any other each user can potentially call any other useruser

cancan’’t have direct lines! (Why?)t have direct lines! (Why?)

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

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

A number of terminals connected with each other

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Star Topology

Switch / Router

No. of links can be reduced

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A 2-Tier Network

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

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Switching

Switches establish temporary Switches establish temporary circuitscircuits

Switching systems come in two parts: switch Switching systems come in two parts: switch and switch controllerand switch controller

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Switching: what does a switch do?

Transfers data from an input to an Transfers data from an input to an appropriate outputappropriate output

Some ways to switch:Some ways to switch:

space divisionspace division

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Switching

Another way to switchAnother way to switch

time division (time slot interchange time division (time slot interchange or or TSI)TSI)

To build larger switches we combine space To build larger switches we combine space and time division switching elementsand time division switching elements

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3. Transmission

Link characteristicsLink characteristics

information carrying capacity (bandwidth)information carrying capacity (bandwidth)information sent as information sent as symbolssymbols

1 symbol >= 1 bit1 symbol >= 1 bit

propagation delaypropagation delaytime for electromagnetic signal to reach other time for electromagnetic signal to reach other endend

light travels at 0.7c in fiber ~8 microseconds/milelight travels at 0.7c in fiber ~8 microseconds/mile

NY to SF => 20 ms; NY to London => 27 msNY to SF => 20 ms; NY to London => 27 ms

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Transmission: Multiplexing

What is multiplexing?What is multiplexing?

Enabling a number of lower bit rate connections to Enabling a number of lower bit rate connections to share a single higher bit rate transmission line. share a single higher bit rate transmission line.

FrequencyFrequency--Division Multiplexing (FDM)Division Multiplexing (FDM)

Applies to both analog and digital signalsApplies to both analog and digital signals

e.g. 4 kHz for each analog voice signale.g. 4 kHz for each analog voice signal

TimeTime--Division Multiplexing (TDM)Division Multiplexing (TDM)

Applies only to digital signalsApplies only to digital signals

e.g. digital voice using pulsee.g. digital voice using pulse--coded modulation coded modulation (PCM)(PCM)

Sampling: 8 kHz, and Quantization: 8 bits per Sampling: 8 kHz, and Quantization: 8 bits per sample => Bit Rate: 64 kbpssample => Bit Rate: 64 kbps

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Multiplexing: FDM and TDM

FDM

frequency

timeTDM

frequency

time

4 usersExample:

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Transmission: Multiplexing

Multiplexed trunks can be multiplexed furtherMultiplexed trunks can be multiplexed further

Need a standard! (why?)Need a standard! (why?)

US/Japan standard is called US/Japan standard is called Digital SignalingDigital Signalinghierarchy (DS)hierarchy (DS)

Digital Signal Number

Number of previous level circuits

Number of voice circuits

Bandwidth

DS0 1 64 KbpsDS1 24 24 1.544MbpsDS2 4 96 6.312 MbpsDS3 7 672 44.736 Mbps

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Transmission: Link technologiesMany in use todayMany in use today

twisted pairtwisted paircoax cablecoax cableterrestrial microwaveterrestrial microwavesatellite microwavesatellite microwaveoptical fiberoptical fiberADSL (Asymmetric Digital Subscriber Lines ): the ADSL (Asymmetric Digital Subscriber Lines ): the most chosen broadband option in the world (more most chosen broadband option in the world (more than 60% of the broadband market) than 60% of the broadband market)

Increasing amount of bandwidth and cost per footIncreasing amount of bandwidth and cost per foot

PopularPopular

fiberfiberADSLADSL

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Transmission: Analogue to Digital Conversion

To represent an infinite precision signal To represent an infinite precision signal originally in an analogue form by a finite originally in an analogue form by a finite set of numbers at a fixed sample rateset of numbers at a fixed sample rate

Two steps:Two steps:

sampling sampling

quantizationquantization

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Transmission: Sampling

The sampling process leads to the PAM (pulse amplitude modulation) representation of the analogue signal

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Transmission: Uniform Quantization

• Samples are quantized to the nearest quantization level

• PAM + quantization ⇒ PCM (pulse code modulation)

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Transmission: Non-uniform Quantization

Signal compression + uniform quantization Signal compression + uniform quantization ⇒⇒nonnon--uniform quantizationuniform quantization

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Transmission: Ways of Compression

μμ lawlaw

A lawA law

Ax

AxA

xxfA

10 )ln(1

)sgn()( ≤≤+

=

11 )ln(1

ln1)sgn()( ≤≤

+

+= x

AAAx

xxfA

)1ln()1ln(

)sgn()(μμ

μ +

+=

xxxf

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Transmission: Voice Coding

To represent the digitized voice signal To represent the digitized voice signal at a reduced bit rate for at a reduced bit rate for narrowband narrowband transmissiontransmission and and digital storage devices digital storage devices with limited capacitywith limited capacity

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Transmission: Codecs in ITU standards

G.711G.711

approved in 1965approved in 1965

PCM, PCM, μμ law or A lawlaw or A law

8000 sample per second8000 sample per second

each sample is encoded as an octeteach sample is encoded as an octet

64 kb/s64 kb/s

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G.722G.722

approved in 1988approved in 1988

provides a higher quality of digital encoding provides a higher quality of digital encoding of 7 kHz of audio spectrumof 7 kHz of audio spectrum

support a number of rates: 48, 56 or 64 support a number of rates: 48, 56 or 64 kb/s, using SBkb/s, using SB--ADPCM (ADPCM (subbandsubband --adaptive differential PCM) adaptive differential PCM)

good for all professional conversational good for all professional conversational voice applications, but musical applications voice applications, but musical applications are not recommendedare not recommended

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G.726G.726

approved in 1990approved in 1990rates in 16, 24, 32 or 40 kb/s, using rates in 16, 24, 32 or 40 kb/s, using ADPCM (adaptive differential PCM)ADPCM (adaptive differential PCM)the quality at 32 kb/s is taken as a the quality at 32 kb/s is taken as a reference for toll qualityreference for toll quality

G.728G.728

approved in 1992approved in 1992--949416 kb/s, using LD16 kb/s, using LD--CELP (low delay, codeCELP (low delay, code--excited linear prediction)excited linear prediction)quality similar to G.726quality similar to G.726

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G.723.1G.723.1

approved in 1995approved in 1995

two modes of operationtwo modes of operation6.4 kb/s, using MP6.4 kb/s, using MP--MLQ (MLQ (multipulsemultipulse--maximum maximum likelihood quantization)likelihood quantization)

5.3 kb/s, using ACELP (algebraic5.3 kb/s, using ACELP (algebraic--codecode--excited excited linear prediction)linear prediction)

has a voice activity detection, has a voice activity detection, discontinuous transmission, comfort noise discontinuous transmission, comfort noise generation capabilitygeneration capability

1.1 kb/s during silence period1.1 kb/s during silence period

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G.729G.729

approved in 1995approved in 1995

8 kb/s, using ABS CS8 kb/s, using ABS CS--ACELP (analysis by ACELP (analysis by sythesissythesis, conjugate structure , conjugate structure –– ACELP)ACELP)

there is a lowthere is a low--complexity version G.729A, complexity version G.729A, which is sometimes used in VoIP systemswhich is sometimes used in VoIP systems

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Transmission: Codecs from ETSI (Europe)

GSM 06.10GSM 06.10Approved in 1988Approved in 198813 kb/s, using RPE13 kb/s, using RPE--LTP (regular pulse excitation LTP (regular pulse excitation ––long term prediction)long term prediction)used in cellular mobile systemused in cellular mobile system

GSM 06.60GSM 06.60Approved in 1996Approved in 199612.2 kb/s, using ACELP12.2 kb/s, using ACELP

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Transmission: Codec from IETF

iLBCiLBC (internet Low (internet Low BitrateBitrate Codec)Codec)

13.33 kb/s, LPC and block based coding of 13.33 kb/s, LPC and block based coding of the LPC residual signal using an adaptive the LPC residual signal using an adaptive codebookcodebook

basic quality higher than G.729A, basic quality higher than G.729A, high high robustness to packet lossrobustness to packet loss

computational complexity in the range of computational complexity in the range of G.729A G.729A

royalty free codecroyalty free codec

http://http://www.ilbcfreeware.orgwww.ilbcfreeware.org//

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4. Signaling

Recall that a switching system has a switch Recall that a switching system has a switch and a switch controllerand a switch controller

Switch controller is in the Switch controller is in the controlcontrol plane

does not touch voice samples

Manages the network

call routing (collect call routing (collect dialstringdialstring and forward call)and forward call)

alarms (trigger the ring at receiver)alarms (trigger the ring at receiver)

billingbilling

directory lookup (for 800/888 calls)directory lookup (for 800/888 calls)

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Signaling network

Switch controllers are special purpose Switch controllers are special purpose computerscomputers

Linked by their own internal computer Linked by their own internal computer networknetwork

Common Channel Interoffice Signaling (CCIS) Common Channel Interoffice Signaling (CCIS) networknetwork

Messages on CCIS conform to Messages on CCIS conform to Signaling Signaling System 7 (SS7) System 7 (SS7) spec.spec.

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Signaling

One of the main jobs of switch controller: One of the main jobs of switch controller: keep track of keep track of statestate of every endpoint and take of every endpoint and take action according to the incoming signalaction according to the incoming signal

Key is Key is state transition diagramstate transition diagram

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Q & AQ & A