COMMUNICATIONS
Data representation
numbers/letters
Graphics and pixels
Transmission speeds
Modems
NETWORKS and SECURITY
Hub
Internet
WWW World wide web
Lost or invalid files
Operator error
Power problems
Hardware failure
Viruses
Computer crime
Back up
Encryption
Transmitting Data
Data is transmitted to another computer in several ways.
directly by cable, by the telephone network through modems, or sometimes satellites,
or microwave. The transmission can be syncronous (set timing) or asyncronous (in bursts)
in packets, blocks of data, or as single signals. Over long distances it will need tobe
re-amplified several times. The risk always is that some data will be lost or modified during
the transmission process. So a lot of effort has gone into providing checking of data.
Parity checking sums the data and adds an extra bit to see that the sum is either odd or even.
In a network the data is addressed to a another point in the network. So the addressing system
has to deal with wrong or missing addresses. When sending data it must be remembered that the
transmission system can always fail so a back-up procedure is essential to avoid loss of data.
Data is built up of combinations of zeros 0 and ones 1. All data in a computer is represented
this way, whether it is words, mathematics, pictures or sounds. This is binary representation.
An ASCII code is commonly used to represent a character set of numbers, letters and special characters
where each number or letter has a unique 8 bit code. A bit like morse code. The resulting code
or binary pattern is recognised by the screen or printer etc. and re-interpreted back into its
original number or letter for display.
To display pictures(graphics) the screen is divided into a matrix table (commonly 800 x 600) and
each block or point is known as a pixel. It has a colour that it displays at that point.
By displaying many pixels over the screen an image or picture appears.
Recent times have seen transmission speeds go from 1200/2400baud to 33.4 or 56.6kb. This means
we can send large volumes of text very quickly, or graphic images at an acceptable rate, but not
quite fast enough for full animation. Compression techniques are being developed to improve speeds
but the big break-through will come when optical fibres and microwaves replace the wires. Then
speeds tens or hundreds of times faster will be possible. Full screen animations will be available.
Increased speed is only of benefit if the reliability is maintained.
Modems are used when a signal has to travel over the phone system or for a long distance. Their
function is to take a binary signal and convert it to an analogue signal (or visa versa).
so they MOdulate and DEModulate.
Networked Computers
Computer networks describe the connection of two or more computers to enable the transfer of data.
The connections are referred to as channels, and a channel has a bandwidth or transfer volume.
The networks can be STAR with a central computer and others connected into the centre.
Or a ring where each is connected to the one before and the one after so a ring is formed.
A BUS where each joins a central core channel, or a Tree where as you go further it spreads,
or a combination of the above.
Networks have the benefits of being able to share data, and peripherals, thus saving costs.
The hub refers to the master computer in a network from which other computers radiate.
It is a good way to co-ordinate and control things but the Hub computer must be reliable.
for if it failed the whole network would go down as well.
The internet is a computer network that is open to all the world, to share information, send email,
and to set up newsgroups and chat sessions. It is common to all users and uses TCP/IP to address
other sites.
The Web ebables users to search the net for information by using the URL name and key-word searches
to retrieve the data you want. The web caused the boom in the internet as it brought simple
searching methods to all, and made it extremely easy to access information.
Files are lost, data is lost as a result of the following reasons. Virus code can delete or corrupt the
data stored on a disk, the disk itself may be faulty, the program may have faulty code, the user may
accidently delete a file, the power or other environmental problems may damage the disk surface,
or the file may be hacked by an unauthorised user. All the above can have steps taken to procect the data
and the ultimate is to have adequate back-ups.
Whenever people are involved mistakes get made. So the people involved with the computer should
have adequate training, not be stressed, have a user friendly interface, and this will reduce errors.
However there will always be errors so checking and double -checking is needed for some data.
Power surges can burn out computers, cause transmission faults and leave processing half done.
All these are serious problems so power surge protection is essential, Uninterruptable power supplies
battery back-up power, and fuses all help minimise the problem.
Computers break down, due to faulty manufacture, no maintenance, bad care and environment and
due to abuse. The effets can be minimised with regular checks, maintenance and back-up of data.
These are pieces of code that are attached to program routines and when the program is run they
cause faults or failures to occur. They are spread whenever a file is copied or transferred
between computers and the virus code is secretly attached to the file.
The defence is to run VIRUS check programs on all new files, and dont trust any file.
People try to hack into other computers to steal data, programs, or to cause a problems for the
computer owner or business. It is illegal. They may try to gain advantage by using the information
to make money, or fiddling financial records to their benefit. It is hard to detect so the
defence is to use passwords to stop un-authorised access, and control the use of data within a
network.
Files do get lost. No matter how we try they somehow find a way to destroy themselves.
So we take a security copy (back-up). That is the only sure way to avoid loss of data.
This is the coding of the data into a secret code. It must be de-coded at the other end.
This means if anyone taps into the transmission along the way it will be a jumble of letters
and won't be understood. So VISA card numbers etc. will be safe. But when used it will take
time to code and de-code.