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5.1.4 - Communication Devices



Revision Points:
Candidates are expected to be able to:

Information System links/channels:

The hardware in an information system has to be connected in some way so data can be transferred between components.  These links or channels can be wires (i.e. metal), electromagnetic waves (i.e. radio waves, infra-red waves, micro-waves) or fibre-optic cables (i.e. glass).

All information systems can be broken down into stages with links/channels between them. The most common links/channels are between the input, processing and the output devices although many systems also have links/channels to storage devices.

An example of the stages in an information system - the photocopier:

Stage What happens
One A sheet of paper with information on it is scanned and digitised into data which is passed to the CPU (input)
Two The data is processed by the CPU (resized, rotated etc.)
Three The data is passed from the CPU to the laser printing part of the photocopier (output)
Four A page of information is ejected from the photocopier

The need for conversion between analogue and digital data:

Digital data is easier and faster to communicate between computers because it is already in a format that can be processed. 

Analogue data data cannot be processed by a computer so it must be converted into digital data by an interface. This is called digitising the data.

Many sensors produce analogue signals (usually a changing voltage) and must be connected to an interface which is then connected to the computer.  The interface therefore converts the analogue signal into digital data which can be understood by a computer.

The interface may also protect the computer from high voltages and if it is protected it can be placed in locations that could damage a computer.


An external modemModem (MODulator - DEModulator):

  • A modem is an example of an interface.
  • The purpose of a modem is to convert between the analogue signals used in telephone cables and the digital signals used by a computer.
  • A computer can only process digital data and phone lines can only transmit analogue data so there is a need to convert between the two using a modem if a computer needs to access the Internet, email, video-conferencing or fax communications.
  • A modem works as an input and an output device because for outgoing signals it converts the digital signal  into an analogue signal (modulation) and for incoming signals it works in the reverse way (demodulation).

Digital to analogue to digital conversion

  • A modem transmitting and receiving at a speed of 33,600 bps (bits per second) can communicate about one page of text every second (4200 bytes or characters a second).

A diagram showing a modem converting a digital signal into an analogue sound signal and back for transmission along phone lines


Digital phone lines:

It is not necessary to use a modem when using a digital telephone line such as an ISDN (Integrated Services Digital Network) line to connect a computer (or a LAN) to the internet because the signal is already a digital one which a computer can process.


Comparing digital and analogue Data:

You need to be able to explain the difference between digital and analogue data.  To digitise means to convert a data from an analogue form into a digital form.

Analogue data:

  • Analogue data can be any value within a defined range. For example, an analogue sound level can have any loudness between zero and the maximum recorded and any frequencies between the highest and lowest recorded. A traditional watch face is an analogue output and a vinyl record is an example of an analogue storage device.

Disadvantages of analogue data:

  • Because the electrical signal can be any value, it is easily easily distorted by other electrical signals (interference).
  • Analogue signals cannot be processed by a computer so they have to be converted into digital form (digitised).

Digital data:

  • Digital data is made up of the two binary numbers, 0 and 1. Text, music, images, video etc all have to be converted into digital data (digitised) before they can be processed and stored by a computer.
  • Digital data is transferred as a stream of on/off or high/low pulses and can have many different forms such as electrical, light, radio, infrared, microwave for example).
  • These pulses have only two states representing the two binary numbers. In an electrical digital signal for example, the voltage pulses are either on, representing a 1, or off , representing a 0. .
The diagram shows the digital data:
10001110
10100111
1011010
as a series of pulses (a digital signal)

Advantages of digital data:

  • Because digital signals are either on or off they are not easily distorted and error checking can easily be built into the signal so digital data can be transferred reliably.
  • Digital signals are in a form that can be processed by a computer without them having to be converted.

Digital data can be transferred by anything that can transmit and receive two different states (high/low or on/off for example). For example, a TV remote control uses pulses of infrared light to transmit a digital signal to the television.


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