- Communication Devices
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.
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:
||A sheet of paper with information on it is scanned and
digitised into data which is passed to
the CPU (input)
||The data is processed by the CPU
(resized, rotated etc.)
||The data is passed from the CPU to the laser printing part of the
||A page of information is ejected from the photocopier
The need for conversion between analogue and digital
Digital data is easier and faster to communicate between computers because it is already
in a format that can be processed.
data data cannot be processed by a computer so it must be converted into
by an interface. This is called digitising
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.
(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
or fax communications.
- A modem works as an input and
an output device because for outgoing signals it converts the
into an analogue
signal (modulation) and for incoming signals it works in the reverse way
- 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).
Digital phone lines:
It is not necessary to use a modem when using a digital telephone
line such as an ISDN
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 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 is made up of the two binary
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
- 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
- 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:
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.