1. COMPUTER NETWORKS

2. COMMUNICATION BETWEEN COMPUTERS
For a computer to communicate with each other (which may be a completely different system) an interface is needed.
Signals
Two types of signals are used for data transmission:
Digital and Analog
A digital signal is a stream of 0's and 1's. So this type is particularly appropriate for communication between computers because it is already in a format that can be processed.
Digital quantities have values which jump from one to the next without any ‘in between’ value, there is no state between ‘on’ and ‘off’. Eg. of digital data is – The no. of cars in a car park., traffic light sequences, digital watch etc.

3. An analog signal uses variations (modulations) in a signal to convey information. Analogue data varies over a whole range of values. Temp. is an eg., since it can be 10, 10.1, 10.01, C.
It is particularly useful for wave data like sound waves. Analog signals are what your normal phone line, the audio tones that travel along telephone cables and sound speakers use.

4. The need for conversion between analogue and digital data.
A computer cannot process analogue data. Analogue data must be converted into digital data by the interface. This is called digitising the data.
An interface is usually an interface card and it is inserted in one of the slots provided inside the computer. It could also be an external device plugged into one of the computer’s communication ports with its own power supply.

5. EXAMPLES OF INTERFACES
MODEM : (MODulator – DEModulator)
The purpose of a modem is to convert between the analogue signals used in telephone cables to the digital signals used by a computer.
It modulates a digital signal from the computer into an analog one to send data out over the phone line. Then for an incoming signal it demodulates the analog signal into a digital one
A modem works as an input and an output device because for incoming signals it converts the analogue signal into a digital signal and it works in the reverse way for outgoing signals.
A modem transmitting & receiving at a speed of 33,600 bps can communicate about one page of text every second.

6. SOUND CARD: It is needed to output for music or speech from programs, CD-ROMs & input analogue sound signals from a microphone. It can include a MIDI also.
NETWORK CARD: A network card lets computers communicate with other computers that are networked together. A cable is plugged into the card & each computer then has access to any shared programs, devices and files on other computers or a main server computer, as well as shared printing.
Digital Phone lines :
It is not necessary to use a modem when using a digital phone 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.

7. What is a network
A computer network is developed by linking computer systems together on a permanent basis, this enables them to share computing power or storage facilities. The network includes the individual computer systems, the connections and the hardware that allows communication to happen.
This can mean two computers cabled together on the same desk, in the same building or between computers in different parts of the country or thousands of computers across the world
The connections could be cable(metal or fibre-optic) or wireless (infra-red, microwave or radio) and may be between computers in one building or between computers in different parts of the world. They either link computers directly or more commonly link them through a hub or switch.

8. The hardware includes the network cards fitted inside the computers and the hubs or switches.  Often the computers in a network are linked to a server, this is a powerful computer which holds the software to run the network. It also holds the shared resources of the network like the users’ files, software packages and printer queues. It usually has a much higher specification than the other computers on the network, with a very large hard disk drive.
Typical hardware devices that may form part of a network are:
Personal computers used as terminals
One or more central processing units acting as dedicated file servers or
Print servers
Disk drives
Scanners
Printers
Point-of-sale terminals (In a retail setting)

9. DATA TRANSMISSION (Communication Links) :
In order for terminals to communicate, communication link is required. These links can be cables or radiowaves /infra-red / microwave signals, in which case data is sent between the terminals as a series of pulses of infra-red, micro or radiowaves.
Data is transmitted as signals with signals carried on a wave. The medium that carries the signal passes on its journey from one computer to another & this is called the ‘Transmission Medium’
Network Components:
A network is not just a number of computers connected by cables.
Parts found in a typical network are:
Network software - This may be part of the operating system, or it can be software designed specifically to manage a network.

10. Cables - Connecting cables are usually used to connect devices on a network, they could be metal or fibre optic cables. Although some networks make use of radio, infra-red or microwaves to provide the link.
When distance between sending & receiving computers is greater, cables are not practical to use & so wireless medium is used, here electromagnetic waves provide the medium – a carrier signal.
Connectors - Connectors are used to connect network cables to terminals or other devices.
Network cards - If a personal computer is to be used as a terminal in a network, a device called a network card must be built into it. The network card looks like a small circuit board and slots into one of the connectors on the main circuit board (called the mother board) inside the computer. Network cards have connectors on them for network cables.

11. DATA TRANSFER SPEEDS :(Meaasured in kbs)
It is the speed at which data is transferred between a server & a terminal.
If data transfer speed is slow, it means it takes ages to get connected to a website & secondly download time (Time to transfer a file from the server to the terminal) is more.
ACCESS RIGHTS :
These are a facility offered by most organisation’s network software. Like, for a particular user, some files can be viewed only & not altered, other files can be viewed & altered & some cannot be accessed at all.
SPEED OF ACCESS :
It depends on (a) Speed of Modem-How fast the pulses are sent & the arrival speed of pulses of the receiver’s Modem
(b) Type of cabling – Fibre optic cables are much faster than metal wire cables.
Together these factors determine the Bandwidth that limits the quantity of data that can be passed along the link in a certain time period.

12. TELECOMMUNICATIONS :
It is used to link up to many on-line databases. People use it to transmit their work from home to an office or to other workers in the electronic chain.
OPEN SYSTEMS INTERCONNECTION (OSI) :
Since each computer manufacturer has its own standards for encoding data, sending data down a communication line was a problem. So a standard called OSI is used for connection of PCs which enables communication between all the components.
PROTOCOLS :
Protocols in telecommunications ensure that each computer behaves predictably & provides information in an understandable way.
A Gateway translates the protocols between computers so that different computers are able to communicate with each other.

13. FILE COMPRESSION :
It is the process of condensing information(data), while sending it through telecommunication link. It is done by software before the data is sent or by the Modem, while the data is being sent. Smaller the file, cheaper will be the file transfer as transmission rate is reduced.
ELECTRONIC DATA INTERCHANGE (EDI) :
This system works as follows :
Companies receive invoices electronically from their suppliers asking for payment
These are checked by computer against the purchase ledger to make sure that they are correct.
The computer then sends details to the bank’s computer to make the payment electronically to the supplier

14. Benefits to the person sending Money :
No paper required to pay the accounts as everything is electronically done.
Relationship with the supplier is improved, since all invoices are paid promptly without reminders.
Future cash flow can be predicted with a greater degree of accuracy.
Bank charges are reduced.
Benefits to the person receiving Money :
No waiting for the cheques to be cleared, sure of receiving the funds.
Sales ledger can be updated immediately electronically
Since data is captured electronically no chances of any errors in data entry.

15. The advantages of networking:
Computers can communicate with each other and share data and files.
Computing power and/or storage facilities can be shared.
Hardware such as printers can be shared.
There is control over which programs and settings a user has access to

16. The disadvantages of networking:
Accessing anything across a network is slower than accessing your own computer
A virus can spread more easily. If a virus gets into one computer, it is likely to spread quickly across the network because they are linked.
As data is shared there is a greater need for security. Users of the network have to have user ids and passwords.
If the server fails, all the workstations are affected. Work stored on shared hard disk drives will not be accessible and it will not be possible to use network printers either.
The cost of installing the equipment is greater. Cabling can be expensive to buy and to install.
Damage to cables can isolate computers. Some sections of the network can become isolated and will not be able to communicate with the rest of the network.
Because networks can be complicated to maintain, a network manager may be needed to run the system.

17. Purpose & Use of User ID and Password
It is a number or name that is unique to a person using network.
The person who looks after the network uses this to allocate file space for each user’s work.
It is also used to permit users to access certain files.
A user ID tells the operating system that a certain person is using the terminal.
Password :
It is a string of characters (letters &/or numbers) that the user (or the person who looks after the network) can select.
It is used to authenticate the user to the system.Only if you type in the correct password, will you be allowed access.
The user ID will normally be shown on the screen, but any password is hidden. As each character of the password is typed, an asterisk is shown on screen.

18. Communications software:
These are specialist programs that allow your computer to communicate with another computer or device.
Usually extra hardware such as a network card or a modem is needed
Internet Browsers - for browsing the Internet and reading the Web pages. Examples: Microsoft Internet Explorer and Netscape Navigator
software - for writing, sending and receiving . Examples: Internet Mail or Outlook Express
Video conferencing software - This allows people in different locations to see and hear each other as if they were in the same room. It therefore requires hardware such as microphones, speakers and a video camera. The software controls the exchange of images and sound and the computers normally need a good communication links such as an ISDN line. Some software systems also allow the users to work on files as if they were all working at one computer.

19. FAX
A telecopying device that electronically transmits written or graphic material over telephone lines to produce "hard copy" at a remote location. A method of digitally copying a document then converting the copy to an analog signal to be transmitted to a remote location using standard phone lines. Because faxing isn’t exact, what is received at the other end is rarely a perfect copy, and usually just a "reasonable facsimile thereof" (thus the term facs—er, fax).
BULLETIN BOARD online information resources where individuals may post information that is of interest to others. Virtual communities often form around bulletin board systems (BBS). Although bulletin boards existed on systems that preceded the World Wide Web, most are now Web-based.

20. Types of networks:
Networks are divided into two types, a LAN (Local Area Network) or a WAN (Wide Area Network).
LANs - Local Area Networks
Definition - A LAN is a network that is limited to an area such as a building or school
Advantages of LANs:
Good quality printers can be shared so individual workstations do not need their own printer. When they print, the data is stored in a queue on the server. The data is then passed to the printer in turn.
All the users work can be stored in a central place (the dedicated file server) so a user can access their work through any computer on the network.

21. Software can be shared, software packages are stored on the server and downloaded to workstations as requested. Note that a licence still has to be bought for each copy of the software needed.
Data can be shared because database files stored in the server are available to users around the network; data from CD-ROMs can also be shared across the network.
Central back-up can take place automatically at regular intervals. A user will usually be able to retrieve work that has been deleted by mistake.
Local messages can be sent to people working at other computers on the network which can save time and paper.
It is possible to set up a local intranet such as that on the KLB school network. The web pages of information can be accessed only over the LAN. 
An intranet is free because it does not involve phone links.
There is control over users’ access rights to programs and data.

22. Disadvantages of LANs:
Printing can be slow. Where a lot of computers are served by only one or two printers, long print queues may develop.
A virus can spread more easily. If a virus gets into one computer, it is likely to spread quickly across the network because it will get into the central backing store.
As data is shared there is a greater need for security. Users of the network have to have user ids and passwords which they may let others know or make too easy to guess. User ids prevent unauthorised users from logging onto the network or accessing someone's files.
If the server fails, all the workstations are affected. Work stored on shared hard disk drives will not be accessible and it will not be possible to use network printers either.
The cost of installing the equipment is greater. Cabling can be expensive to buy and to install.
Damage to cables can isolate computers. Some sections of the network can become isolated and will not be able to communicate with the rest of the network.
Because networks can be complicated to maintain, a network manager may be needed to run the system.

23. WANs - Wide Area Networks
Definition - a Wide Area Network is not confined to one building. The computers and terminals forming part of the network can be spread around the world.
External communication links such as satellites, microwaves or telecommunication links will be used to connect the parts of a WAN. The connection must normally be paid for because the links are external.
LANs may be connected to WANs via a special gateway. The Internet is really a huge Wide Area Network and many LANs will be connected to the Internet in this way.
Advantages of WANs:
These are similar to those of LAN's except the scale of sharing etc. becomes far greater and can be world-wide.
Disadvantages of WANs:
Again these are similar to those of LAN's except that issues such as security become even more important as potential hackers could possibly be trying to break into a computer system from anywhere in the world

24. Network topology:
Network topology is the name given to the way in which the devices (called nodes) are connected in the network.
There are three common network topologies, called ring, line (bus) and star.
You will be expected to know about the ways a network can be constructed and you should be able to draw a simple line diagram to represent each one.

25. Ring topology:
In a ring topology, the nodes are connected in a circle with all the devices having equal importance.
Advantage :A Ring network provides the fastest form of local area networking because the data only flows in one direction.
Disadvantage : If the cable fails, all the workstations will be affected

26. Line (bus) topology:
Data is sent to all the nodes on the network at the same time and the devices are placed along a line like bus stops.
Advantage : Easy and inexpensive to install (least amount of cable required).
Disadvantage : If the main cable fails, all the computers will be affected.
- Performance of the network slows down with more users.

27. Star topology: Advantage : A cable failure does not affect other users
In this type of network, a central controller forms the main node and the subsidiary nodes are connected around it like a star.
+ Communication is fast because there is a direct path from the central controller to each terminal.  Performance is not as greatly affected by the number of other computers on the network are being used.
Advantage : A cable failure does not affect other users
Disadvantage : As the central machine controls the whole system, the whole system will be affected if it breaks down.
- Uses a lot of cables which is expensive
- Requires a ‘hub’ box at the file server to control all the cables

28. Types of servers:
Networks are often arranged in a hierarchical way. This means that one or more computers is more powerful than the rest and is known as a server. Servers look after all the printing, file maintenance and other peripherals and other, less powerful computers called clients are connected to the network.  This is known as a client-server relationship.
On some networks the jobs can be divided between two or more servers. The server(s) used to store both programs and data would be called a file server. It acts as a massive hard drive for the client terminals. The server used to manage all the printing on the network would be called a print server.
Where a computer is a dedicated file server, it cannot be used as a terminal and will be occupied all the time in managing the network. 
The clients may have no disk drives nor processing power of their own and they make use of the functions provided by the server.
+ Client computers can be very low specification and therefore cheap.
- The client computer can be totally dependant on the server.

29. Common Network Environments:
Common Network Environments are networks that are open for use by everyone and thus are known also as PUBLIC networks. The most well known common network environment is the INTERNET
Uses of Common Network Environments :
Searching for information.
Displaying information
Communicating with others via , discussion forums or chat rooms.

30. An intranet:
An intranet provides information on web pages but the information can only be accessed from the local network.
Advantages:
An Intranet is free because it does not involve phone links.
Information can be easily updated and shared inside a company or school.
The web pages can be viewed using a standard web page browser.
More of the information will be useful because it was created for particular users. 
Disadvantages:
Information cannot be shared outside the local network.

31. The Internet:
The Internet is a world-wide network of linked computers sharing a vast amount of information.
Advantages:
Millions of people contribute information.
Data can be rapidly added and updated to reach a huge audience.
It can be used to sell goods to a world-wide market.
Disadvantages:
There is a lot of useless information, much of which is out of data and it can be very difficult to find what you want.
Much of the information is misleading or has not been checked for accuracy.
It is not regulated and offensive and illegal material exists.
Issues such as security become even more important as potential hackers could possibly be trying to break into a computer system from anywhere in the world.

32. The risks to data:
It is important to take a backup of data because the data stored in a computer can often be far more valuable than the actual computer equipment. Losing such data could put a company out of business.
Examples of valuable data are: a company’s financial records; customers’ details; records of the stock held
Data can be damaged or destroyed in the following ways:
-Mistakes by users such as deleting files and not making a regular back-up
-Hackers gaining access to systems and changing or deleting data
-Computer fraud where data is changed to benefit individuals
-Theft of computer equipment
-Infection of systems and data by computer viruses.
-Deliberate and malicious damage by users of the system
-Disasters such as fire, floods, earthquakes etc. destroying equipment
-Breakdown of hardware (system crash ) particularly disk drives

33. Reducing the risks of data loss:
Making a back-up of data:
It is vital that a back-up of your data (either from the hard disk drive of a computer or from the hard disk of a server running a network) is regularly carried out. For eg.
A back-up of the whole KLB school system is carried out automatically every weekday night so the most that can be lost is one day’s work.
Special tape streamer units save the data onto magnetic tape cassettes capable of holding over 20 GB of data. This is enough capacity to allow all the data on the server’s hard drives to be backed up.
A number of tapes are used in rotation and previous back-up copy is taken away from the school premises each night in case of a disaster such as a fire. Tapes kept on-site should be stored in a fireproof safe.
Keeping hardware secure:
Protect the computer itself by using locks on doors and windows and using security bolts to fix the computer permanently to the desktop.
Floppy disks are easily physically damaged and must also be kept away from magnetic fields and dust.

34. Restricting who access computers:
Hacking involves breaking codes and passwords to gain unauthorised entry to computer systems. Once into a computer system a hacker can do an enormous amount of damage. Stand-alone computers are usually safe as there is no connection for the hackers to break into. Computers which form part of networks or those with external links, such as attached modems, are in danger from hackers.
Ways to restrict access:
User Ids (user identification) and passwords - each user must enter a unique username and a password to access either a computer or their own area on a server.
The password is a secret combination of letters and/or numbers to  prevent anyone else accessing the computer system with their username.
Usernames can be used to restrict access to certain parts of the system so they cannot delete or install programs/files or change the computer setup. Protecting passwords - to protect your password the system should: force you to change passwords regularly and use passwords with at least six characters (you should use passwords that have no meaning so they would
be hard to guess).

35. Blocking external access :
have an automatic lockout if a password is wrongly entered more than three times. This prevents access by password guessing software.
Blocking external access :
External hackers can be blocked by disconnecting modems from the telephone line when not being used. Systems with permanent phone connections need special software called a 'firewall' to prevent unauthorised access.
Having an anti-virus policy:
Computer viruses are programs which can be introduced into a computer via a floppy disk, attachment or the Internet.
A virus program contains instructions that make it attach itself to system files or programs and make copies of itself. It can therefore spread to other programs on your hard disk and onto floppy disks or itself to all the contacts in your address book.
Their effects can be devastating and cost a lot of money to fix. They can alter the host program, stop it working completely or cause a whole hard disk to become scrambled.

36. Anti-virus software is usually required to detect and destroy them but it is important to have regular updates to deal with new viruses. The anti-virus software scans the computer's disks looking for any viruses which it then removes and alerts the user.
Protecting floppy disks - The 'write-protection' hole in the corner of the floppy disk should be kept covered to prevent accidental wiping of the disk or the introduction of viruses.
To reduce the risk:
Never use a floppy disk given to you from an untrustworthy source or pass floppy disks around between your friends.
Do not start up a computer with a floppy disk left in the drive.
Set the computer BIOS in the boot sequence so it does not start with the floppy disk drive.
Keep the write-protection hole covered on floppy disks that are used to store original "clean" programs - so they can be reinstalled with confidence.
Never open an attachment that comes from someone unknown or is not clearly explained in the message.
Install a virus protection program.

37. Computer fraud: Software protection:
This is a criminal activity where computer operators use the computer to their own advantage.
Computer fraud can be difficult to detect because:
- It is very hard to track down and the people committing the crime are often very skilled .
-Offenders are often young, with no previous criminal records
-It is often not publicised, as news of the fraud may damage the image of the company .
Software protection:
Software companies try to prevent illegal copying of their disks using the following methods:
Copy protection - the disk (or CD-Rom) may be formatted in a special way so it cannot be copied.

38. Restricting the number of installations –
each installation is recorded on an installation disk and only a certain number are allowed.
Using a code number –
The software cannot be installed without a code number from the supplier.
Using the serial number of the computer –
The serial number is recorded onto the installation disk and it can only be installed on that computer in future.
Encryption –
Data can be scrambled up and cannot be read without the correct software

39. SUMMARY
Keeping data secure
Measures that can be taken to keep data safe include:
Making regular back-ups of files. (Back up copies should be stored safely in fireproof safes or in another building.)
Protecting yourself against viruses by running anti-virus software.
Using a system of passwords so that access to data is restricted.
Safe storage of important files stored on removable disks - eg locked away in a fireproof and waterproof safe.
Allowing only authorised staff into certain computer areas, eg by controlling entry to these areas by means of ID cards or magnetic swipe cards.
Always logging off or turning terminals off and if possible locking them.
Avoiding accidentally deletion of files by write-protecting disks.
Using data encryption techniques to code data so that it makes no apparent sense.

40. The Data Protection Act
It was developed to give protection and lay down rules about how data about people can be used. Usually this data is stored on a computer.
The Data Protection Act 1998 covers: - Information or data stored on a computer or an organised paper filing system about living people.
How the Act works: The basic way the Act works is:
By setting up rules that people have to follow
Having an Information Commissioner to enforce the rules.
It does not stop companies storing information about people. It just makes them follow rules.

41. Who's involved?
The Information Commissioner is the person (and her office) who has powers to enforce the Act.
A data controller is a person or company that collects and keeps data about people.
A data subject is someone who has data about them stored somewhere, outside their direct control. This makes us all data subjects as there can be few people in the UK who do not feature in computer records somewhere.
Registration with the Information Commissioner
Any organisation or person who needs to store personal information must apply to Register with the Information Commissioner.
A register of data controllers is kept detailing the data that will be stored so they have to say in advance what information will be stored and how