Senior 3 practice book · Unit 9 of 10

Telecommunication Channels

50 questions that complete the Senior 3 quiz for this unit: 28 core and 22 challenge. Easy and hard questions are mixed. Try each question, then tap “Show answer”.

Common misconceptions

Optical fibres carry electricity, like copper wires.Optical fibres carry pulses of light, kept inside the glass core by total internal reflection. That is why lightning and electric motors do not disturb them.
Radio waves are a kind of sound wave.Radio waves are electromagnetic waves travelling at 3 × 10⁸ m/s. Sound is changed into an electrical signal and carried by the radio wave, then changed back into sound by the receiver.
Digital signals never suffer from noise.Noise affects every signal. A digital signal can be cleaned up because only two levels must be recognised, but very heavy noise can still turn a 0 into a 1 and cause errors.
A mobile phone call travels the whole way by radio waves.Radio waves carry the call only between the phone and the nearest base station; from there it usually travels through optical fibre or microwave links to the other base station.

What this unit covers

Topics marked new are not tested much in the quiz, so this book gives them extra questions.

Parts of a communication system: source, transmitter, channel, receiver, destination
Guided (wired) channels: twisted pair, coaxial cable, optical fibre
Unguided (wireless) channels: radio, microwave, infrared and satellite links
Analogue and digital signals; binary code
Noise, attenuation, repeaters and regeneration of signals
Advantages and disadvantages of digital compared with analogue communication
Electromagnetic waves used in telecommunication and the wave equation c = fλ
Carrier waves and modulation: AM and FM
Transducers and conversion: microphone, loudspeaker, ADC, DAC and sampling
Bits, bytes, data rate and bandwidth
Propagation of radio waves: ground waves, sky waves (ionosphere) and line-of-sight space waves
Mobile phone networks: cells, base stations, handover and frequency reuse

Go to the questions

Questions (1–50)

Easier and harder questions are mixed, just like in a real exam. The 22 harder ones are marked ★ Challenge.

1True or false
All electromagnetic waves, including radio waves and light, travel at the same speed in a vacuum.
Show answer
Answer: True
They all travel at about 3.0 × 10⁸ m/s; they differ in frequency and wavelength.
Common mistakeLearners often think higher-frequency waves travel faster; the speed is the same, only λ and f change.
2True or false · ★ Challenge
Bluetooth and Wi-Fi are short-range wireless channels that use radio waves.
Show answer
Answer: True
Both use radio waves of about 2.4 GHz (Wi-Fi also uses 5 GHz) at low power, so they reach only tens of metres.
Common mistakeSome learners think Wi-Fi travels through the phone cable; it is a wireless radio link between the device and the router.
3True or false
Most noise enters a communication system in the channel, between the transmitter and the receiver.
Show answer
Answer: True
Along the cable or through the air, the signal picks up interference from motors, lightning and other signals, and it becomes weaker.
Common mistakeLearners sometimes think noise is produced only by the receiver; the long channel is where most of it is picked up.
4True or false · ★ Challenge
A phone call between two people in the same town never uses any cables at all.
Show answer
Answer: False
Only the links between each phone and its nearest base station are radio; base stations are joined to the network's switching centres by optical fibre or microwave links.
Common mistakeLearners think 'mobile' means completely wireless; most of the journey is usually through cables.
5True or false
Copies of a digital music file are identical to the original, however many times the file is copied.
Show answer
Answer: True
Copying digital data copies exact 0s and 1s, so no noise is added; analogue copies (such as cassette tapes) get worse each time.
Common mistakeLearners may think every copy loses some quality; this is true for analogue copies but not for digital ones.
6True or false · ★ Challenge
Taking more samples per second when digitising a sound gives a closer copy of the original, but produces more data to send.
Show answer
Answer: True
More samples follow the wave's shape more closely, but each sample adds bits, so the bit rate and file size rise.
Common mistakeLearners may think more samples is 'free'; better quality always costs more data and bandwidth.
7True or false
Morse code, made of short and long pulses with gaps, is an early example of a digital way of sending messages.
Show answer
Answer: True
The message is sent as separate pulses with only a few fixed forms, not as a smoothly varying signal.
Common mistakeLearners often think digital means 'computers only'; any code made of a few fixed symbols is digital.
8Fill in the blank
One byte is made up of ______ bits.
Show answer
Answer: 8
A byte is a group of 8 bits; file sizes are usually given in bytes and link speeds in bits per second.
Common mistakeLearners sometimes treat MB and Mbit as equal; a megabyte is 8 times larger than a megabit.
9Multiple choice · ★ Challenge
FM radio usually sounds clearer than AM radio. What is the main reason?
1. AFM waves travel faster, so the noise has much less time to act on them
2. BFM stations always use much more powerful transmitters
3. CFM signals are digital, so they cannot pick up any noise
4. DNoise mostly changes amplitude, which an FM receiver ignores
Show answer
Answer: D. Noise mostly changes amplitude, which an FM receiver ignores
Electrical noise adds spikes to the wave's amplitude. In FM the information is in the frequency, so the receiver can cut off the amplitude changes and the noise with them.
Common mistakeChoosing 'FM signals are digital' is wrong: ordinary FM broadcasting is analogue; its advantage comes from coding the sound in the frequency.
10True or false
Using thicker copper cable or placing repeaters closer together both help to deal with the weakening of a signal along a line.
Show answer
Answer: True
Thicker cable has less resistance, so less energy is lost; closer repeaters restore the signal before it becomes too weak.
Common mistakeLearners sometimes think only the transmitter power matters; the channel and the spacing of repeaters matter too.
11Multiple choice · ★ Challenge
A slightly scratched music CD still plays perfectly, but a scratched vinyl record crackles. What is the main reason?
1. AThe laser in the player removes every scratch
2. BCDs store the music as a smooth wavy groove
3. CVinyl records are digital, so noise adds up
4. DDigital data includes error-correcting codes
Show answer
Answer: D. Digital data includes error-correcting codes
Extra check bits stored with the data let the player detect and repair small numbers of wrong bits, so the music is restored exactly.
Common mistakeChoosing 'CDs store a smooth wavy groove' describes the analogue vinyl record; a CD stores pits representing 0s and 1s.
12Fill in the blank
Radio waves that travel along the surface of the Earth, following its curve, are called ______ waves.
Show answer
Answer: ground (surface)
Ground waves are used by long-wave and medium-wave stations to reach a few hundred kilometres.
Common mistakeSome learners write 'sky waves'; sky waves go up and are reflected by the ionosphere.
13Fill in the blank · ★ Challenge
The binary number 1010 is equal to ______ in decimal.
Show answer
Answer: 10
1010 = 1 × 8 + 0 × 4 + 1 × 2 + 0 × 1 = 10.
Common mistakeSome learners read 1010 as one thousand and ten; in binary each place is worth twice the place to its right.
14Fill in the blank
Light stays inside the core of an optical fibre because the cladding has a ______ refractive index than the core.
Show answer
Answer: lower (smaller)
Total internal reflection needs light to travel from the denser (higher n) core towards the less dense (lower n) cladding.
Common mistakeSome learners write 'higher'; with a higher index in the cladding the light would simply refract out of the core.
15Multiple choice · ★ Challenge
Why does a mobile network use many low-power base stations instead of one very powerful transmitter for a whole city?
1. ALow-power stations make the radio waves travel much faster to each of the phones
2. BOne powerful transmitter could not send any digital signals at all
3. CThe same frequencies can be reused in distant cells, so more people can call
4. DMany small stations stop phones from needing batteries
Show answer
Answer: C. The same frequencies can be reused in distant cells, so more people can call
Each cell can reuse the frequencies of cells that are not next to it, multiplying the number of calls; phones near a mast also need less power, saving battery.
Common mistakeChoosing 'radio waves travel faster' is wrong: the speed is always the speed of light; the advantage is capacity.
16Fill in the blank
If two radio stations in the same area broadcast on the same carrier frequency, listeners hear a mixture because of ______.
Show answer
Answer: interference
The two signals overlap at the receiver; that is why each station is given its own frequency by the regulator.
Common mistakeSome learners write 'noise'; the problem here is interference between two wanted signals, not random noise.
17Multiple choice · ★ Challenge
Medium-wave AM radio can often be heard in a valley behind a hill, but FM radio and TV signals from the same mast cannot. Why?
1. AFM waves travel more slowly through the air
2. BAM transmitters always use far more power than FM transmitters
3. CFM waves are absorbed by the ionosphere
4. DLonger waves follow the ground and bend round hills
Show answer
Answer: D. Longer waves follow the ground and bend round hills
Medium waves (hundreds of metres long) travel as ground waves and diffract round obstacles. FM and TV use VHF/UHF waves only a few metres long, which travel in straight lines and are blocked by hills.
Common mistakeChoosing 'FM waves travel more slowly' is wrong: all radio waves travel at the speed of light.
18Multiple choice
In a telephone, the microphone changes:
1. AAn electrical signal into sound energy
2. BLight pulses into an electrical signal
3. CA digital signal into an analogue signal
4. DSound energy into an electrical signal
Show answer
Answer: D. Sound energy into an electrical signal
The microphone is the input transducer; the loudspeaker (earpiece) at the other end changes the electrical signal back into sound.
Common mistakeChoosing 'electrical signal into sound' describes the loudspeaker, the transducer at the receiving end.
19Multiple choice
In radio broadcasting, 'modulation' means:
1. ATurning the loudness of the speaker up and down
2. BChanging a digital signal into an analogue signal at the receiver
3. CChanging a high-frequency carrier wave to match the signal
4. DAmplifying a weak signal at a repeater station
Show answer
Answer: C. Changing a high-frequency carrier wave to match the signal
The audio signal is used to vary the amplitude or frequency of a carrier wave of much higher frequency, which is then transmitted.
Common mistakeChoosing 'turning the loudness up and down' confuses the volume control with modulation of the carrier wave.
20Multiple choice · ★ Challenge
A school's internet link carries 20 megabits per second. If 40 learners use it at the same moment and share it equally, what data rate does each learner get?
1. A2.0 Mbit/s
2. B800 Mbit/s
3. C0.50 Mbit/s
4. D0.050 Mbit/s
Show answer
Answer: C. 0.50 Mbit/s
Each learner gets 20 ÷ 40 = 0.50 megabits per second.
Common mistakeChoosing 2.0 Mbit/s divides the wrong way (40 ÷ 20); the shared total must be divided by the number of users.
21Multiple choice
The word 'attenuation' in telecommunication means:
1. AThe signal being changed into binary code
2. BThe signal being reflected back to the sender
3. CThe signal getting weaker as it travels along the channel
4. DThe signal travelling faster in the cable
Show answer
Answer: C. The signal getting weaker as it travels along the channel
Energy is lost along the channel (resistance in copper, absorption in glass, spreading out in air), so the signal becomes weaker with distance.
Common mistakeChoosing 'changed into binary code' confuses attenuation with digitising; attenuation is a loss of strength.
22Short answer · ★ Challenge
Two hilltop towns 20 km apart must be linked. Give one advantage of a microwave link and one advantage of an optical fibre link for this job.
Show answer
Model answer: Microwave link: no trench or cable is needed, so it is quicker and cheaper to set up across valleys, and only towers at each end need maintenance. Optical fibre: much higher bandwidth (more data), not affected by heavy rain or electrical interference, and harder to tap, so more secure.
Common mistakeLearners often say microwaves are 'faster'; both microwaves and light travel at about the speed of light, so speed is not the difference.
23Fill in the blank
A device that changes energy from one form into another, such as a loudspeaker changing an electrical signal into sound, is called a ______.
Show answer
Answer: transducer
Microphones, loudspeakers, LEDs and photodiodes in communication systems are all transducers.
Common mistakeSome learners write 'transformer'; a transformer changes voltage, while a transducer changes the form of energy.
24Multiple choice · ★ Challenge
For a phone call, a voice signal is sampled 8000 times per second and each sample is stored as an 8-bit number. What bit rate is needed?
1. A1000 bits per second
2. B64 000 bits per second
3. C8008 bits per second
4. D512 000 bits per second
Show answer
Answer: B. 64 000 bits per second
Bit rate = samples per second × bits per sample = 8000 × 8 = 64 000 bit/s (64 kbit/s).
Common mistakeChoosing 1000 bits per second divides instead of multiplying; every one of the 8000 samples needs 8 bits.
25Multiple choice
Which of these displays shows information in DIGITAL form?
1. AA clock whose second hand sweeps smoothly
2. BA fuel gauge with a moving needle
3. CA clock that shows the time as 14:05 in figures
4. DA mercury thermometer in a beaker
Show answer
Answer: C. A clock that shows the time as 14:05 in figures
Digital information is shown in separate steps (figures that jump from one value to the next); the others vary smoothly, which is analogue.
Common mistakeChoosing the sweeping second hand confuses an electronic clock with a digital one; a smooth movement is analogue.
26Multiple choice
A TV remote control sends its commands to the television using:
1. AInfrared radiation, along a line of sight
2. BSound waves that are too high for people to hear
3. CVisible light reflected from the ceiling
4. DLong-wave radio reflected from the sky
Show answer
Answer: A. Infrared radiation, along a line of sight
The remote's LED gives out infrared pulses; that is why it works poorly if something blocks the path to the TV.
Common mistakeChoosing 'sound waves' is a common guess because nothing is seen, but infrared is an invisible electromagnetic wave, not sound.
27Short answer · ★ Challenge
TV and FM transmitter masts in Rwanda are tall and placed on hilltops. Explain why.
Show answer
Model answer: TV and FM signals travel in straight lines (line of sight) and are blocked by hills and by the curve of the Earth. A tall mast on a hilltop can 'see' further, so the signal reaches a much larger area before the hills and horizon block it.
Common mistakeLearners often say the height makes the signal 'stronger'; the main point is the clear straight path to more receivers.
28Multiple choice
In television broadcasting, the main job of the receiver (the TV set) is to:
1. ATurn the pictures and sound from the studio into a radio signal
2. BChange the received signal back into pictures and sound
3. CCarry the signal from the station to the house
4. DBoost the signal so that it reaches other houses
Show answer
Answer: B. Change the received signal back into pictures and sound
The receiver picks up the signal from the channel and decodes it into the original information for the viewer.
Common mistakeChoosing 'turn the pictures into a radio signal' describes the transmitter at the TV station, not the receiver.
29Multiple choice · ★ Challenge
Which is the main drawback of using a geostationary satellite link for a live two-way conversation?
1. AThe signal cannot cross the sea or the mountains
2. BThe satellite can only carry analogue signals
3. CThe link stops working every night when it gets dark outside
4. DThere is a noticeable delay before each reply is heard
Show answer
Answer: D. There is a noticeable delay before each reply is heard
Each one-way trip up and down is about 72 000 km, taking about 0.24 s, so a question and its reply take about half a second, which makes conversation awkward.
Common mistakeChoosing 'stops working at night' confuses the satellite's radio link with solar power; satellites carry signals day and night.
30Multiple choice
Which list puts these waves used in communication in order of INCREASING frequency?
1. ARadio, microwave, infrared, visible light
2. BVisible light, infrared, microwave, radio
3. CMicrowave, radio, visible light, infrared
4. DInfrared, radio, microwave, visible light
Show answer
Answer: A. Radio, microwave, infrared, visible light
Radio waves have the longest wavelength and lowest frequency; then come microwaves, infrared and visible light.
Common mistakeChoosing 'visible light, infrared, microwave, radio' gives increasing WAVELENGTH, which is decreasing frequency.
31Short answer · ★ Challenge
Explain what is meant by the bandwidth of a channel, and why optical fibre is said to have a very high bandwidth.
Show answer
Model answer: Bandwidth is the range of frequencies a channel can carry, which decides how much data it can carry each second. Light has an extremely high frequency (about 10¹⁴ Hz), so a fibre can carry a huge range of frequencies and therefore millions of calls or very high data rates at the same time.
Common mistakeLearners often think bandwidth is the thickness of the cable; it is about the range of frequencies (and so the data rate), not physical size.
32Multiple choice
In AM (amplitude modulation), which property of the carrier wave changes with the sound signal?
1. AIts frequency
2. BIts amplitude
3. CIts speed
4. DIts wavelength only
Show answer
Answer: B. Its amplitude
AM varies the height (amplitude) of the carrier; FM varies its frequency instead.
Common mistakeChoosing 'frequency' describes FM; the letters AM stand for amplitude modulation.
33True or false
Microwaves pass straight through the ionosphere, which is why they are used for links to satellites.
Show answer
Answer: True
Their high frequency means the ionosphere does not reflect them, so they reach satellites in space.
Common mistakeSome learners think all radio waves bounce off the ionosphere; only lower frequencies (such as short wave) do.
34Short answer · ★ Challenge
Explain why even a digital signal needs repeaters along a long cable, and what goes wrong if they are placed too far apart.
Show answer
Model answer: The signal weakens and picks up noise as it travels. A repeater must decide whether each pulse is a 0 or a 1 and send out a clean, strong copy. If the repeaters are too far apart, the signal becomes so weak and noisy that the repeater cannot tell the levels apart, so some bits are read wrongly and errors appear in the message.
Common mistakeA common belief is that digital signals can travel any distance; they can be regenerated only while the 0s and 1s are still recognisable.
35Multiple choice
Short-wave radio from another continent can be heard in Rwanda at night mainly because the waves are:
1. ACarried round the Earth by undersea cables
2. BReflected back to Earth by the ionosphere
3. CBent around the Earth by mountains
4. DSent through the ground beneath the oceans
Show answer
Answer: B. Reflected back to Earth by the ionosphere
These sky waves bounce between the ionosphere (charged layers of the upper atmosphere) and the ground, travelling far beyond the horizon.
Common mistakeChoosing 'bent around by mountains' is wrong; mountains block waves, while the ionosphere reflects them.
36Short answer · ★ Challenge
Cells in central Kigali are small (a few hundred metres across), while cells in rural areas can be many kilometres across. Suggest why.
Show answer
Model answer: Each base station can handle only a limited number of calls at once. In the city there are many users close together, so small cells are needed to share the load and reuse frequencies. In rural areas there are few users, so one base station can cover a large area cheaply.
Common mistakeLearners often say rural cells are bigger because the land is flatter; the main reason is the number of users each cell must serve.
37Multiple choice
A mobile network divides a region into small areas, each served by its own base station (mast). Each small area is called a:
1. ACell
2. BNode
3. CGrid
4. DCircuit
Show answer
Answer: A. Cell
That is why mobile phones are also called cell phones: each cell has a base station that talks to the phones inside it.
Common mistakeChoosing 'grid' confuses the phone network with the electricity grid.
38Multiple choice
What is the smallest number of bits needed to give 64 different codes?
1. A6
2. B8
3. C32
4. D64
Show answer
Answer: A. 6
n bits give 2ⁿ codes, and 2⁶ = 64.
Common mistakeChoosing 32 halves 64; the number of codes doubles with each extra bit, so you count how many doublings give 64.
39Multiple choice · ★ Challenge
A radio station transmits waves of wavelength 250 m. What is their frequency? (c = 3.0 × 10⁸ m/s)
1. A75 GHz
2. B1.2 MHz
3. C0.83 MHz
4. D1.2 kHz
Show answer
Answer: B. 1.2 MHz
f = c ÷ λ = 3.0 × 10⁸ ÷ 250 = 1.2 × 10⁶ Hz = 1.2 MHz.
Common mistakeChoosing 75 GHz multiplies c by λ; the wave equation c = fλ gives f = c ÷ λ.
40Multiple choice
Why are the two wires of a telephone or network cable twisted around each other?
1. ATo make the cable stronger so that it does not break when pulled
2. BTo make the signal travel faster along the cable
3. CTo reduce interference from outside and between pairs
4. DTo let the wires carry light as well as current
Show answer
Answer: C. To reduce interference from outside and between pairs
Twisting means any interference affects both wires almost equally, so it largely cancels out at the receiver.
Common mistakeChoosing 'stronger cable' is a natural guess, but the twist is there for the electrical signal, not for strength.
41Short answer · ★ Challenge
With a weak signal, an old analogue TV picture became 'snowy' but still watchable, while a digital TV picture is either perfect or freezes and breaks into blocks. Explain the difference.
Show answer
Model answer: In analogue TV the noise adds directly to the picture signal, so the picture gets gradually worse as the signal weakens. In digital TV the receiver only has to recognise 0s and 1s, so it gives a perfect picture until the signal is so weak that many bits are lost; then error correction fails and the picture freezes or shows blocks.
Common mistakeLearners often conclude that digital TV is 'worse' at weak signals; it is better until a threshold, then fails suddenly instead of gradually.
42Short answer
Name the three main layers of an optical fibre cable, from the centre outwards, and state the job of the cladding.
Show answer
Model answer: Core (thin glass that carries the light), cladding (glass of lower refractive index around the core) and the outer protective jacket (buffer/sheath). The cladding's lower refractive index makes light hitting the core–cladding boundary at large angles undergo total internal reflection, so it stays in the core; it also protects the core surface.
Common mistakeLearners often say the cladding is just a cover; its lower refractive index is what makes total internal reflection possible.
43Short answer · ★ Challenge
Speech has frequencies of only a few kilohertz. Give two reasons why radio stations do not transmit speech directly as low-frequency radio waves but use a high-frequency carrier wave.
Show answer
Model answer: A 1 kHz radio wave would have a wavelength of 3.0 × 10⁸ ÷ 1000 = 300 km, needing an enormous aerial. Also, all stations would use the same audio frequencies and interfere with each other; giving each station its own carrier frequency lets many stations broadcast at once and the listener tune to one.
Common mistakeLearners sometimes say 'sound cannot travel through air'; sound does travel, but not far, and as a radio wave low frequencies are impractical.
44Short answer
A passenger is on a phone call in a bus travelling from Kigali to Muhanga. Explain what happens to the call as the bus moves from one cell to the next.
Show answer
Model answer: As the signal from the first base station becomes weaker and the signal from the next one becomes stronger, the network transfers the call to the new base station (a handover). This happens automatically in a fraction of a second, so the call continues without being cut.
Common mistakeLearners often think the phone stays connected to the first mast all the way; each mast covers only its own cell.
45Multiple choice · ★ Challenge
Which is a real DISADVANTAGE of optical fibre compared with copper cable?
1. AIt suffers much more electrical interference from nearby motors
2. BJoining two fibres needs special tools and skilled workers
3. CIt carries far less data every second than copper
4. DIts signal fades much faster with distance than copper
Show answer
Answer: B. Joining two fibres needs special tools and skilled workers
Fibre ends must be cut and fused very precisely, so repairs and connections are slower and more costly; fibre also cannot carry electrical power to equipment.
Common mistakeChoosing 'carries less data' is the opposite of the truth: fibre has a much higher bandwidth than copper.
46Multiple choice
Which is a disadvantage of digital transmission compared with analogue transmission?
1. AIt needs extra converters and more bandwidth
2. BIt cannot be stored on computers or phones
3. CIt is always spoilt by small amounts of noise
4. DIt cannot carry speech, music or pictures
Show answer
Answer: A. It needs extra converters and more bandwidth
Sound and pictures must be converted (ADC and DAC), and sending many bits per second usually needs a wider range of frequencies than the original analogue signal.
Common mistakeChoosing 'spoilt by small amounts of noise' describes analogue signals; small noise does not change digital levels.
47Short answer
Describe the jobs of the analogue-to-digital converter (ADC) and the digital-to-analogue converter (DAC) in a mobile phone call.
Show answer
Model answer: In the caller's phone, the ADC measures (samples) the analogue signal from the microphone many times a second and turns each value into a binary number for transmission. In the listener's phone, the DAC turns the received binary numbers back into a smoothly varying analogue signal that drives the loudspeaker.
Common mistakeLearners often put the DAC in the sending phone; conversion to digital happens before sending, and back to analogue after receiving.
48Short answer · ★ Challenge
Draw or describe a block diagram for sending a voice note from one phone to another, and give the job of each block.
Show answer
Model answer: Speaker's voice (source) → microphone (changes sound into an electrical signal) → transmitter in the phone (digitises, codes and sends it as radio waves) → channel (radio waves to the mast, then fibre or microwave links) → receiver in the other phone (picks up and decodes the signal) → loudspeaker (changes it back into sound) → listener (destination).
Common mistakeLearners often leave out the transducers at each end; the microphone and the loudspeaker are needed to change sound to an electrical signal and back.
49Short answer
Write the decimal number 13 as a 4-bit binary number, and explain why binary is suitable for electronic communication.
Show answer
Model answer: 13 = 8 + 4 + 1 = 1101. Binary uses only two states (0 and 1), which can be shown by off/on, low/high voltage or no light/light; these two states are easy to tell apart even when there is some noise.
Common mistakeLearners sometimes write 1011 by reading the place values in the wrong order; the leftmost bit is worth 8, then 4, 2 and 1.
50Multiple choice · ★ Challenge
How does the woven metal shield (braid) of a coaxial cable improve the signal it carries?
1. AIt blocks electrical interference from outside reaching the centre
2. BIt carries a second copy of the signal in case the first one fails
3. CIt reflects light back into the central copper conductor
4. DIt raises the voltage of the signal as it travels along
Show answer
Answer: A. It blocks electrical interference from outside reaching the centre
The braid surrounds the central conductor and is earthed, so electric and magnetic interference from outside is stopped before it reaches the signal.
Common mistakeChoosing 'reflects light back' mixes up coaxial cable with optical fibre, which uses total internal reflection.