Download 13. Digital Communications

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Introducing communications
What types of communications technology can you think of?
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How is information sent?
Telecommunications involves sending,
receiving and storing information, which
can be sound, images or computer data.
In all communications technology,
information is sent (transmitted) as signals.
There are two types of signals – analogue
and digital. These can be transmitted by
wires, radio waves or optical fibres.
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What is an analogue signal?
Most of the signals sensed by humans are analogue signals.
Everyday examples include sound, light and temperature.
An analogue signal
can be converted
into an electrical
signal. This graph
shows how the
voltage of an
analogue signal
varies with time.
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voltage
An analogue signal may have any value within a
continuous range.
time
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What is an analogue signal?
Analogue signals can be sent directly down a copper wire
or turned into radio waves. Originally, all telephone and
television signals travelled as analogue signals.
The earpiece/speaker turns
analogue electrical signals
back into sound waves.
The microphone in a telephone
mouthpieces turns sound waves
into analogue electrical signals.
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What is a digital signal?
Modern communication systems carry information from
analogue signals as digital signals.
Unlike analogue, a
digital signal does not
vary smoothly, and so
is described as being
a discontinuous
signal.
current
A digital signal only contains two values: ‘0’ (off) and
‘1’ (on). These are used to encode analogue information.
time
Digital signals can be sent as electrical signals, radio waves
or as pulses of light through optical fibres.
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Converting analogue to digital
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What are the advantages of digital signals?
Most forms of communication,
such as television and radio,
now involve digital signals.
This is because digital signals
have two main advantages over
traditional analogue signals.
 No loss of signal quality. Analogue signals lose quality
during transmission but digital signals are received
unchanged, so there is no loss of information.
 More information. Digital signals can carry more
information than analogue signals. For example, digital
signals can carry more television and radio stations and
‘interactive’ services.
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Why do analogue signals lose quality?
All types of signals lose strength as they travel. This is
called attenuation and means that signals often have to
be amplified so they can be used.
Signals can also pick up noise (interference).
original
attenuated
amplified
Amplifying an analogue signal also amplifies the noise.
It is difficult to distinguish the noise from the signal, because
an analogue signal can have any value. This means that an
analogue signal is of lower quality by the time it is received.
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Why don’t digital signals lose quality?
Digital signals also lose strength and need to be amplified.
original
attenuated
amplified
threshold
During transmission, a digital signal
can be regenerated by comparing the
amplified signal to a threshold value.
It is easy to distinguish the noise from a
digital signal and so the signal does not
lose quality by the time it is received.
regenerated
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Analogue or digital?
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Communications – matching activity
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What is analogue and digital storage?
The terms ‘analogue’ and ‘digital’ do not just apply to the
different types of signals that can be used to send information.
They also refer to the different methods used for storing
and retrieving information.
Vinyl records and video
tapes are examples of
analogue storage.
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Most modern methods of
storing data are digital, such
as CDs, DVDs and hard drives.
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Opinions on analogue and digital
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What are optical fibres?
Optical fibres are thin strands of solid
glass, about the width of a human hair.
Light can travel along an optical fibre,
even if it is bent. The light is reflected
internally and detected at the other end.
Light transmitted through an optical fibre
is reflected internally about 6,000 times
per metre.
In telecommunications, information is encoded as digital
signals in the form of pulses of visible or infrared light and
transmitted along optical fibres at almost the speed of light.
A single optical fibre can carry the digital signals for about
12,000 phone calls, all at the same time!
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What’s in a cable of optical fibres?
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How do messages travel along optical fibres?
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Sending messages
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What are the advantages of optical fibres?
Optical fibres are increasingly
being used for communications
instead of traditional copper wires.
Optical fibres have a number
advantages over copper wires:
 They are cheaper.
 They are thinner and lighter.
 Signals can travel further.
 Not affected by electromagnetic signals so no interference.
 Several digital signals can be carried by one fibre.
 Signals do not weaken as much while they travel.
 More secure – they are almost impossible to bug.
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Why don’t messages get mixed up?
Multiplexing is the method that allows lots of messages to
be sent at the same time without getting muddled up.
There are two main types of multiplexing:
 Wavelength division
Each message is sent using
a different wavelength.
The receiver tunes into the
wavelength of each message.
 Time division
Each message is divided
into chunks for transmission.
The messages are put back
together at the other end.
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optical fibre
messages on
different wavelengths
sections of messages
sent at different times
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Optical fibres – true or false?
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Optical fibres – the downsides?
Optical fibres have changed the way people work and play,
but is this necessarily a good thing?
Should we be able to get films
and music online for free?
Can working faster and dealing
with more information increase
stress at work?
Does having books and articles
available on computers make it
easier for students to cheat?
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Using radio waves in communications
Different types of radio waves are used to transmit signals for
radio, TV, mobile phones, Wi-Fi and satellite communications.
In a transmitter, sound
waves are turned into
oscillating current, which
is then turned into radio
waves in an aerial.
aerial
transmitter
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In a receiver, radio waves
are picked up by another
aerial, turned back into
oscillating current, and then
back into sound waves.
radio
waves
receiver
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Why do some radio waves travel so far?
Low frequency (up to 3 MHz) radio
waves travel along the Earth’s
surface and can travel hundreds of
miles. These ground waves are
used for local and national radio.
ionosphere
Higher frequency radio waves
(3–30 MHz) are reflected off the
ionosphere high in the
atmosphere. These sky waves
are used for international and
amateur radio.
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Microwaves and satellite communications
Microwaves are very high frequency radio waves
(over 30 MHz).
Unlike other radio waves,
they are not reflected by the
ionosphere but pass straight
through and into space.
ionosphere
This means that they can be
picked up by satellites and
transmitted over very large
distances right round the
Earth.
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What’s the frequency, Kevin?
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What is a wireless network?
A wireless network allows computers, mobile phones and
other devices to access the internet and communicate with
each other without wires.
Wireless technology,
sometimes called Wi-Fi,
uses high-frequency radio
waves (around 2.4 GHz) to
transmit data.
A Wi-Fi device can connect to the internet via an access
point. The region covered by an access point is a ‘hotspot’.
Although Wi-Fi is convenient, it can be less secure than a
wired network and has a limited range, usually around 100 m.
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Why is my laptop wireless enabled?
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Glossary
 amplification – Increasing the strength of a signal.
 analogue – A signal with continuously varying intensity.
 attenuation – The loss of strength of a signal during


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transmission.
digital – A signal with only two states; ‘on’ and ‘off’.
encoding – The process of turning an analogue signal into
a digital signal.
multiplexing – Sending multiple digital signals at the same
time.
optical fibres – Long, thin strands of glass or plastic used
to carry digital signals as pulses of infrared or light.
telecommunications – Sending, receiving and storing
data via wires, radio waves or optical fibres.
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Anagrams
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Multiple-choice quiz
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