Ch 20B - Communicating Information

8/17/2019 Ch 20B - Communicating Information

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Topic 30:Topic 30:

Communicating InformationCommunicating Information30.1 Principles of Modulation30.1 Principles of Modulation

30.2 Sidebands and bandwidth30.2 Sidebands and bandwidth

30.3 Transmission of information by digital means30.3 Transmission of information by digital means

30.4 ifferent channels of communication30.4 ifferent channels of communication

30.! The mobile"phone networ#30.! The mobile"phone networ#



30.4: Channels of30.4: Channels of

CommunicationCommunication $ire"pairs$ire"pairs

%oa&ial cables%oa&ial cables 'adio wa(es'adio wa(es Microwa(esMicrowa(es )ptic fibres)ptic fibres



Channels of CommunicationChannels of Communication

%hannels of communication%hannels of communication are the differentmeans*media of transferring signals.

%oa&ial %ables

$ire"pairs

Microwa(e +in# )ptic ,ibres

'adio +in#



Wire-PairsWire-Pairs

%onsists of a pair of insulated copper wires -sed mainly for (ery short distances with low freuencies.

/&amples +in#ing telephones to the nearest e&change+in#ing telephones to the nearest e&change +in#ing door bell in a house to the switch outside+in#ing door bell in a house to the switch outside

igh attenuation of the signal energy is lost as heat in the resistance of the wires as radiation since the wires act as aerials

/asily pic# up e&ternal interference that degrades theoriginal signal

Morse %ode Transmitter



Wire-PairsWire-Pairs

f se(eral wire"pairs are arranged ne&t to one anotherthey will pic# up each other5s signals.

This effect is #nown as cross-talk or cross-linking andgi(es (ery poor security as it is easy to 6tap5 a telephonecon(ersation.

The bandwidth of a pair of wires is only about !00 #7. %onseuently as a means of carrying a large amount of

information it is e&tremely limited.

8 cable of wire"pairs



Coaxial CableCoaxial Cable




/ssentially a pair of wires arranged so that onewire is shrouded by the other

The signal is transmitted down the innerconductor and the outer conductor acts as thereturn wire and also shields the inner one frome&ternal interference.

The outer conductor is usually connected toearth.




More e&pensi(e than wire"pairs +ess attenuation of the signal. This means that for long

distance communication repeater amplifiers can bearranged further apart.

+ess prone to e&ternal interference though not immune toit so they do offer slightly greater security.

The bandwidth of coa&ial cable is about !0 M7. t is

capable of carrying much more information than a wire"pair.

T9 cable



Raio !in"Raio !in"



Raio !in" #$%Raio !in" #$%

8erials are reuired for radio lin#. /nergy is radiated from an aerial in the form of

electromagnetic wa(es. These wa(es tra(el outwards from the aerial with the speed

of light. /lectromagnetic wa(es in the freuency range 30 #7 to 3

:7 are generally referred as radio wa(es.



Raio !in" #&%Raio !in" #&%

The first radio wa(es used for communication were of (ery lowfreuencies and (ery long wa(elengths. The radio wa(es wereswitched on and off so that communication was by morse code.

+ater use of higher freuencies and the de(elopment of amplitudemodulation ;8M8M< enabled (oice communication.

,urther de(elopment including ,M,M broadcasts and the use ofdifferent carrier freuencies enabled higher"uality communication and also more radio stations to operate in the same area.



Raio !in" #3%Raio !in" #3%

The choice of aerial for broadcasting determines whether the radio wa(esare emitted

in all directions ;for broadcasting to a whole area< or in one direction only ;for point"to"point communication<.

Similarly for the recei(ing of radio signals the choice of aerial isdetermined by whether the signal is to be recei(ed from one direction orall directions.

8erials with dish reflectors enable the radio wa(es to be transmitted asparallel beams.




Some ata on ,reuencies and 'anges of 'adio $a(es

Surface wa(es $a(es that tra(el along

the contour of the /arth by diffraction

S#y wa(es $a(es that are reflected bythe ionosphere

Space wa(es $a(es that aretransmitted in a straight line ;line"of"

sight +)S< from transmitter to recei(er



Raio !in" #'%Raio !in" #'%

The three types of transmittingwa(es complement each other.

S#ip 7one is an area where the

wa(es are not recei(ed.



(icro)a*e !in"(icro)a*e !in"



(icro)a*e !in" #$%(icro)a*e !in" #$%

Microwa(es are radio wa(es in the S, wa(eband from 3 :7 to 30

:7. $ith such high freuencies it has (ery short wa(elengths of only afew centimetres.

The wa(elength of the radio wa(es determines the length of theaerial.

,or mobile phones the aerial must be for the sa#e of con(enienceshort and hence microwa(es are suitable for its use.

8s the freuency of the carrier wa(e increases the bandwidth alsoincreases.

The ,reuency =ands used for 'adio %ommunication



(icro)a*e !in" #&%(icro)a*e !in" #&%

The bandwidth of a microwa(e lin# is of the order of :7. Thislarge bandwidth means that the microwa(e beam has a largecapacity for transmitting information.

Microwa(es are generally used for point"to"pointcommunication where signals are transmitted directly fromtransmitter to recei(er ;space wa(es<.

,or terrestrial use the range of the transmissions is limited toline"of"sight and relay stations are used.

,or (ery long distance transmission microwa(es are

transmitted beyond the atmosphere and sent bac# to /arth bycommunication satellites.



(icro)a*e !in" #3%(icro)a*e !in" #3%

The transmitting element is placed at the focus of a parabolicreflector.

The parabolic reflector reflects and focuses the wa(e poweron to a recei(ing element.

n this way the wa(e power is radiated in a parallel beam.

>ote The reflecting parabolic dish is not the aerial. The aerial is found at the focus of the reflecting dish.




The intensity of the wa(es will always be reduced;attenuated< as the distance from the transmitterincreases.

They need to be strengthen * amplified at regulardistance.



+ptic ,ibres+ptic ,ibres




)ptic fibres)ptic fibres are strands of optically pureglassglass as thin as a human hair that carrydigital information o(er long distances




%+%? M8:/



What are +ptic ,ibresWhat are +ptic ,ibres

)ptic fibres are solid glass.)ptic fibres are solid glass.

%+%? M8:/



o) oes +ptic ,ibre Wor"/o) oes +ptic ,ibre Wor"/

%+%? M8:/

)ptic fibres transmit optical pulses using the principle of)ptic fibres transmit optical pulses using the principle oftotal internal reflection.total internal reflection.



tructure of an +ptic ,ibretructure of an +ptic ,ibre

A single optical fibre, will have the following parts: Core - Thin glass center of the fibre where the light travels Cladding – An outer optical material (glass of lesser density)

surrounding the core uffer coating - !lastic coating that protects the fibre fromdamage and moisture

"undreds or thousands of these optical fibres are arranged inbundles in optical cables# The bundles are protected by the

cable$s outer covering, called a %ac&et#



Transmitting +ptical PulsesTransmitting +ptical Pulses

)ptic fibres carry digital information in the form of pulses of light or infra"red radiation.

These pulses are pro(ided by lasers and the light produced has

(ery high freuencies of the order of 10@ M7. n theory a bit or indi(idual light wa(e could last for only 10"14 s.

This would allow hundreds of thousands of indi(idual informationtelephone calls for e&ample to share the same optic fibre.

owe(er present technology does not allow control at such highfreuencies. The duration of a bit is go(erned by how fast the laserpro(iding light to the fibre can be switched on and off. This is at

present of the order of :7 but is increasing as technologyde(elops.

)ne single opticfibre cantransmitter asmuch data as abundle of

coa&ial wires



Transmitting +ptical PulsesTransmitting +ptical Pulses

!ulses of light or infra-red radiation travel along the fibre!ulses of light or infra-red radiation travel along the fibreas a result ofas a result of total internal reflectiontotal internal reflection##

As there isAs there is no loss of energyno loss of energy in total internal reflection, thein total internal reflection, thelight impulses can travel great distances#light impulses can travel great distances#

"owever, some of the light signal"owever, some of the light signal do degradedo degrade within thewithin thefibre, mostly due tofibre, mostly due to impurities in the glassimpurities in the glass##



1*antages of +ptic ,ibres 1*antages of +ptic ,ibres

'arge bandwidth, giving rise to large transmission capacity

uch lower cost than metal wires

iameter and weight of cable is much less than metal cable,hence easier handling and storage

uch less signal attenuation, so far fewer regeneratoramplifiers are re*uired, reducing the cost of installation

o not pic& up electromagnetic interference, so very highsecurity and negligible cross-tal&

Can be laid alongside e+isting routes such as electric

railway lines and power lines#



+ptic ,ibre: (a2 &00 $&+ptic ,ibre: (a2 &00 $&

(a) )ptic fibre transmission has in some instances replacedtransmission using co"a&ial cables and wire pairs.

)ptic fibres ha(e negligible cross"tal# and are less noisythan co"a&ial cables.

/&plain what is meant by(i) cross"tal# A2B(ii) noise. A2B

(b) 8n optic fibre has a signal attenuation of 0.20 d= #mC1.

The input signal to the optic fibre has a power of 2D m$.The recei(er at the output of the fibre has a noise power ofD.! E$.

%alculate the ma&imum uninterrupted length of optic fibre

gi(en that the signal"to"noise ratio at the recei(er must notbe less than 30 d=. A!B



olution: (a2 &00 $&olution: (a2 &00 $&



Channels: 5o* &00 6Channels: 5o* &00 6

ifferent freuencies and wa(elengths are used indifferent channels of communication.

Suggest why

(a) infra"red radiation rather than (isible light isusually used with optic fibres A2B

(b) the base stations in mobile phone networ#soperate on -, A2B

(c) for satellite communication freuencies of theorder of :7 are used with the uplin# ha(ing adifferent freuency to the downlin#. A2B



olution: 5o* &00 6olution: 5o* &00 6

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Ch 20B - Communicating Information