Answers · Communicating with waves

Communicating with waves — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. CK1 · Final check

Model answer. (a) A geostationary satellite is about 36 000 km up. A command sent up to it and down to the platform travels over 72 000 km, which takes about a quarter of a second: longer than 0.1 s, before the picture of the move has even started back. Low-orbit satellites are only a few hundred kilometres up, so the signal has a far shorter way to go and the delay is well under 0.1 s. (b) No. The satellite goes round once a day, but its orbit takes it over the North Pole and the South Pole. Seen from the roof, it moves north and south across the sky, and for part of each day it is below the horizon, so a fixed dish would lose it. A satellite stays in one place in the sky only if it goes round once a day above the equator: a geostationary satellite. (c) Wi-fi uses microwaves, which can penetrate some walls and floors, so the signal from the router reaches the gym. The cable from the town is an optical fibre carrying visible light or infrared. Glass is transparent to them, so they travel the 20 km, and they can carry high rates of data, enough for 300 films at once. Microwaves are sent through the air, not along a glass fibre.

A full answer includes

(a) I wrote that a geostationary satellite is about 36 000 km up.
(a) I wrote that a signal going up to it and down again takes longer than 0.1 s.
(a) I identified low-orbit satellites.
(a) I wrote that low-orbit satellites are only a few hundred kilometres up, so the delay is far shorter.
(b) I predicted that the dish could not point at it all the time.
(b) I wrote that the satellite would move north and south across the sky.
(b) I wrote that a satellite stays in one place in the sky only if it goes round above the equator.
(b) I did not predict that going round once a day is enough.
(c) I identified microwaves for the wi-fi.
(c) I wrote that microwaves can pass through some walls and floors, so they reach the basement.
(c) I identified visible light or infrared, in an optical fibre, for the cable.
(c) I wrote that glass is transparent to visible light and some infrared.
(c) I wrote that visible light and infrared can carry high rates of data, enough for 300 films at once.
(c) I did not choose microwaves for the cable.

Also accept

(a) geostationary is about 36 000 km up, so up and down takes longer than 0.1 s; use low-orbit satellites (b) no: it moves north and south across the sky; only above the equator does it stay put (c) wi-fi: microwaves, through walls and floors; cable: an optical fibre with visible light or infrared, through transparent glass, at high rates of data

Watch for

“(a) A geostationary link is quick enough, because microwaves travel at the speed of light.” — They do, but going up about 36 000 km and down again still takes about a quarter of a second: longer than 0.1 s.
“(b) Yes, because a satellite that goes round once a day always stays in one place in the sky.” — Only if it goes round above the equator. Over the poles, it moves north and south across the sky.
“(c) The cable carries microwaves, like the wi-fi.” — Microwaves are sent through the air. A cable for 300 films is an optical fibre: glass is transparent to visible light and infrared, which can carry high rates of data.

2. CK2 · Final check

Model answer. (a) The sensor’s voltage can take any value as a gust rises and falls, so it is analogue. Each pulse in the fibre is only on or off: the pulses have only two values, so they are digital. Row A. Light can carry either kind of signal, so “carried by light” is not the reason. (b) The microphone turns the heartbeat into a voltage that varies continuously: an analogue signal. To send it digitally, this voltage is measured thousands of times a second, and each measurement is sent as a code of 1s and 0s. At the doctor’s end the codes are turned back into a varying voltage, and a speaker turns it into the sound of the heartbeat. A sound can be sent as a digital signal as well as an analogue one. (c) Each pulse need only be read as on or off, so the pulses can be very short and sent close together: many millions of them each second. The pulses of the 30 calls take turns along the cable, so all 30 get through at once. This is the increased rate of transmission of data that digital signals give. The pulses do not travel faster.

A full answer includes

(a) I chose row A.
(a) I did not choose row C, which says the pulses are digital because they are carried by light.
(b) I wrote that the microphone’s voltage varies continuously, like the heartbeat: an analogue signal.
(b) I wrote that this voltage is measured thousands of times a second.
(b) I wrote that each measurement is sent as a code of 1s and 0s.
(b) I wrote that at the doctor’s end the codes are turned back into a varying voltage.
(b) I wrote that a speaker turns this voltage into the sound of the heartbeat.
(c) I wrote that each pulse need only be read as on or off, so the pulses can be very short and close together.
(c) I wrote that the pulses of the 30 calls take turns along the cable.
(c) I wrote that this is an increased rate of transmission of data.
(c) I did not write that the pulses travel faster than an analogue signal.

Also accept

(a) Row A (b) the varying voltage is measured, sent as codes and turned back into sound (c) short pulses close together, and the calls take turns: an increased rate of data

Watch for

“(a) Row B.” — The voltage can take any value as a gust rises and falls, so it is analogue; the pulses can only be on or off, so they are digital.
“(a) Row C.” — Light can carry either kind: a lamp dimmed smoothly is analogue. The pulses are digital because they have only two values.
“(a) Row D.” — The pulses carry readings of a smooth quantity, but each pulse is only on or off, so the signal is digital.
“(b) A digital signal cannot carry a real sound, only 1s and 0s.” — The 1s and 0s are codes for measurements of the sound, taken thousands of times a second. At the far end they are turned back into the sound.
“(c) The cable carries 30 calls because digital pulses travel 30 times faster.” — The pulses travel at the same speed. They are very short and close together, so far more data arrives each second.