[PDF] SECTION 4: COMPUTER NETWORKS - Wellspring School

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MERCEDES PASSANO

INFORMATION TECHNOLOGY

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SECTION 4: COMPUTER NETWORKS

COMMUNICATING OVER LONG DISTANCES

Many computer users need to be able to connect their computer to other computers located at a remote site. For example : • Office workers want to communicate with computers at their workplaces from wherever they may be working in the world. • Home computer users want to be able to access the Internet . The Public Switched Telephone Network (PSTN) provides the most convenient communications channel for these users. Because the telephone network was designed before computers were in widespread use telephone lines were not designed to carry digital signals. They were designed to carry human voices in the form of analogue electrical waveforms. An analogue signal can take any value in a given range, e.g. -5 to +5 volts. The binary digital signals that a computer generates (0s and 1s) which are often represented as 0 volts (off) for a 0 and 5 volts (on) for a 1 can not be transmitted directly down a telephone line. A special device known as a Modem must be used to enable a computer to communicate over a telephone line. The modem converts the binary digital data that the computer wants to transmit into an analogue signal for transmission down the telephone line. Another modem must be used at the other end of the telephone line to convert the analogue signal back into a digital one that the receiving computer can understand. This situation is shown in figure 2. A Modem (MODulator - DEModulator) is a device used to allow a computer to communicate over a telephone line. Modulation is the process of converting digital electrical signals generated by a computer into analogue electrical signals which can be transmitted over a telephone line.

Demodulation is the process of converting received analogue signals back into digital signals

which a computer can understand.

Transmission Speed

The speed at which data can be transmitted over a telephone line has increased steadily. The very latest modems can operate at speeds up to 56.6KBps. Such speeds are only possible over very short distances. These modems are most useful for connecting a home computer to the Internet. This is because the telephone number that you have to dial to connect to the Internet via an Internet Service Provider is usually a local one. Transmission speeds over a telephone line are much slower than those over a Local Area Network (LAN ). The actual speed at which a particular communication takes place between two computers will depend upon : ▪ The maximum speed of the transmitting computer"s modem. ▪ The maximum speed of the receiving computer"s modem.

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2 ▪ The quality of the telephone line. A poor quality line will result in errors which may mean data has to be resent.

Compression

Data can be transferred between two computers more quickly if it is compressed (reduced in size) before it is transmitted. Some modems can compress data down to one quarter of its original size before it is sent. This will effectively quadruple the speed at which data can be transmitted. The data must be decompressed by the modem connected to the receiving computer. Both modems include the appropriate compression software for compression to be used during data transmission.

Data Transmission

Telephone lines will only allow the transmission of data at very slow speeds (e.g. 3Kb per second).

To transmit a picture the size of an A4 page can take several minutes. If data that is to be

transmitted can be compressed before it is sent then there can be considerable advantages :

Error Correction and Detection

When data is sent over a telephone line it often encounters noise. Noise is electrical interference which corrupts the data that is received. The data that is received does not match the data that was transmitted. Sources of noise include crossed-lines and electrical pulses. It is very important that the modem receiving data can detect whether or not the data has been corrupted. If it has been corrupted the modem will attempt to correct it.

Validation Checks such as:

• Parity Checks or • Cyclic Redundancy Codes can be used to detect and correct errors.

Standards

One modem must be able to transmit and receive data to and from modems of different makes in

possibly different countries using different telephone systems. Therefore a variety of standards

specifying transmission speeds, compression and error correction and detection methods have been established. Such standards which describe how information is communicated are known as

Protocols.

COMPUTER NETWORKS

A computer network is a group of computers which are connected together to allow communication between them. A network is a system that sends and receives data and messages enabling two or more computers to communicate with each other. Networks typically allow computers to share files, share printers and send messages to each other.

Types of Network

There are two distinct types of network :

1. Local Area Network (LAN) : A network which is located entirely on one site such as an office

or school.

2. Wide Area Network (WAN) : A network which connects together computers on a number of

sites in different areas.

Examples of LANs Examples of WANs

1. A typical school computer network. 1. The Internet.

2. A network in one office block. 2. A network of bank cash dispensers.

3. The network of ticket machines in lottery shops.

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Connecting to a Network

Computers which are connected to a network are called stations. Stations on a LAN are normally connected to the network using a network interface and some cable. The cable might be made of wire or be fibre-optic (made of glass fibres). Connections to other computers on a WAN are normally made using a modem (MODulator- DEModulator) and a telephone line. A modem converts digital signals (1s and 0s) sent out by the transmitting computer into analogue signals (noises) which can be sent down the telephone line. It also converts analogue signals received down the telephone line back into digital signals so that they can be understood by the computer that data is being sent to. Computer networks can also use radio links or satellites as well as cables to connect computers together.

Why use a Network ?

The main reason for using a network is to allow sharing of resources between many computers and thus save money. • Expensive resources such as printers and disk drives can be shared between many computers. • A user can use any software from any station and can load and edit files that they have created on any station. • You can use a network to send electronic mail or to access huge databases such as the World Wide Web or a list of all the products a company sells.

Networks - Advantages

▪ Files can be stored on a central computer (the file server) allowing data to be shared

throughout an organisation.

▪ Files can be backed up more easily when they are all on a central fileserver rather than when

they are scattered across a number of independent workstations.

▪ Networks also allow security to be established, ensuring that the network users may only have

access to certain files and applications. ▪ Software and resources can be centrally managed.

▪ Network versions of software often allow for their speedy installation on workstations from the

file server. ▪ Expensive devices such as laser printers or scanners can be shared. ▪ Users can access their files from any workstation.

Some Disadvantages of Networks

• The cabling, interface cards, file servers etc. that are required to set up a network are

expensive. • If one computer, cable or interface breaks the whole network may stop operating and you may not be able to use any of the computers until the fault is repaired. In practice this rarely happens. • The files stored on computer networks can be broken into more easily that files stored on floppy disk and locked away.

Networks - Intranet

Recently it was realised that Internet technology could be harnessed within a company or

organisation, allowing network users to use web browsers to access company information.

The "Intranet" was thus born.

The advantages are obvious: technical information, manuals, etc. can be placed on an internal web server to be accessed by employees within the company. It is probable that within the next

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few years most organisations will have Intranets in place of, or in addition to, the more traditional

networks.

Networks - Security

Most companies and organisations have computer files that are critical to their business. The files may be technical specifications of products, customer accounts etc. Military organisations and industries will have files that are important to the defence of the country. This makes it vitally important to ensure that the files are secure.

Networks store their files centrally on a file server. This makes it much easier to maintain security

than with a large number of stand-alone computers with the files saved on their own hard disks. Security is maintained by ensuring that users have login names and passwords and are restricted to

files and directories for which they have clearance. One important aspect of security is the regular

backing-up of important files. This can easily be done when files are centrally stored.

INTERNET

: The Internet has become so ingrained in today"s world that people forget it has really only come about in the last few years. The Internet started as an American defence project in the 1970s to try and make the military computer network infrastructure resistant to a Russian nuclear attack. The Internet as we know it today did not come into existence until the early 1990s. Today the Internet has many thousands of servers and many millions of users throughout the world and it has revolutionised communication and information retrieval.

Information is kept on special servers called "Web Servers" which contain pages of information

written in a language called HyperText Mark up Language (HTML), that allows them to be viewed by any computer with special "Web Browser" software. Any company or individual can set up a Web Server to distribute material ranging from atomic physics to the latest ski conditions in the Alps. The main purposes of the Internet are to allow people to search for and obtain information on any number of subjects via the World Wide Web and to send and receive electronic mail. The number of uses to which the Internet is being put is continuously expanding. For example, companies are now using the Internet to advertise and sell products, and video conferencing is becoming available.

LOGIN, USER ID and PASSWORD:

Login Name:

A login name is the account name used to access a computer system. Also called

user ID or user name, it is a way people identify themselves to their online service or Internet access

provider. Your user name, also called user ID or account name, is the same as your login name. This is the name by which you and your electronic mailbox are identified online. Not a secret (contrast with Password).

Password

: A password is a code or word used to gain access to restricted data on a computer network or code used to gain access (login) to a locked system. While passwords provide security against unauthorized users, the security system can only confirm that the password is legitimate, not whether the user is authorized to use the password. Good passwords contain letters and non-letters and are not simple combinations such as virtue7. A good password might be: 5%df(29) That"s why it is important to safeguard passwords: ▪ Never disclose your password. ▪ Devise a password that consists of letters, numbers, and symbols. ▪ Change your password frequently.

MERCEDES PASSANO

INFORMATION TECHNOLOGY

5 METHODS OF COMMUNICATION: FAX - E-MAIL - BULLETIN BOARDS - TELE/VIDEO CONFERENCING.

Introduction:

Communicating is transferring information from one person or location to another. There are many different methods that can be used to communicate information. The best method to use on a particular occasion will depend upon factors such as:

1. What type of information you are sending, e.g. a package, a document or a video.

2. How quickly the information must arrive at its destination.

3. What equipment you have to help you send the information and what equipment the

person receiving the information has.

4. How much you are prepared to pay to send the information.

5. Whether the information is private and being sent to just one person or public and being

sent to many people.

Private Communications Systems :

▪ Post ▪ Telephone ▪ Cellular Telephone ▪ Fax ▪ Email ▪ Video Conferencing

Broadcast Communications Systems

▪ Teletext ▪ Digital TV Communications systems can be classified as being either private or broadcast. Private systems are used for private communications between a small number of people. Broadcast systems allow one person to send the same information to lots of people at the same time. Some of the most common communication systems are listed below. FAX: A fax machine can be used to send copies of documents from one location to another. A fax machine consists of a scanner, printer and modem all built into one device. The document that is to be sent is inserted into a fax machine. It scans the pages and sends them via the telephone network to another fax machine. The receiving fax machine then prints a copy of the document. A fax machine can be connected to the telephone network just like a telephone. A fax machine is

identified by a number which you must dial to send a document to it. Few people have fax

machines at home but most businesses have a fax machine. Charges for sending faxes are set in the same way as for making telephone calls.

Advantages:

▪ Documents can be sent very quickly to another fax machine anywhere in the world. Faxing is much quicker then using the post. ▪ There is a permanent record of any communication by fax.

Disadvantages:

▪ The quality of the printed copy of the document is not usually very good. ▪ Fewer people have access to a fax machine than to the post or a telephone. ▪ Parcels can not be sent by fax.

ELECTRONIC MAIL (EMAIL

) is a system which allows users of a computer network to send messages to each other. Users can usually : ▪ Send a message to another user, or group of users (whose names are on a mailing list). ▪ Keep their messages in a mailbox. ▪ Read, print and delete messages from their mailbox. Each user has to have his or her own address that other users need to know to send messages to him or her. An example address is : mark@mwchemicals.co.uk An email system can transmit other types of information as well as simple text. Pictures, music,

videos, in fact almost any sort of information that you can think of can also be sent. This is

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6 achieved by attaching files created in another package such as a word processor or drawing program to the end of an email message. These files are known as attachments. An electronic mail system can be private and only operate within a particular company on the company"s own WAN. However most email is now sent via the Internet so that any Internet user can send and receive email. The cost of sending an email is very low. It is much cheaper to communicate by email than by

post, fax or telephone. This is especially true if the person you are communicating with lives a long

way away.

Advantages:

▪ Any sort of information (e.g. text, video) can be sent by email.

▪ Information sent by email arrives very quickly, typically within a few minutes of being sent.

▪ Emails can be sent very cheaply around the world. ▪ A received email document is identical to the sent document. Unlike fax transmissions there is no decrease in quality. ▪ There is a permanent record of any communication by email.

Disadvantages:

▪ Not many people have access to email. In the UK about one quarter of the population can send and receive email. ▪ Parcels can not be sent by email. ▪ It can be difficult to check who has sent you an email.

Communication Methods : Video Conferencing:

Video conferencing uses the telephone network to transmit video images from one location to another. These video images can be transmitted and received by computers with cameras attached or by specialised video conferencing hardware. Video images can not be sent down a single telephone line. Usually many lines are required at the same time to send a video picture, so using video conferencing is expensive. The quality of the transmitted pictures is not usually very good. Video conferences can take place between just two people. Frequently however many people, all at different locations, take part in a video conference at the same time. Large companies often use video conferencing systems. It can be much cheaper to hold a video conference than to

bring a lot of people together to hold a normal conference. This is especially likely to be true if the

people involved in the conference work a long way away from each other.

Advantages:

▪ Video conferences can bring a large number of people together to hold a conference without them having to travel. This can save money. ▪ Live video pictures can be sent during a video conference.

Disadvantages:

▪ The quality of transmitted video images is not usually very good. ▪ It is much more expensive to hold a video conference than to talk on the telephone. ▪ You need specialised equipment to hold a video conference.

BULLETIN BOARD SYSTEM

An electronic message centre. Most bulletin boards serve specific interest groups. They allow you to dial in with a modem, review messages left by others, and leave your own message if you want. Bulletin boards are a particularly good place to find free or inexpensive software products. In the United States alone, there are tens of thousands of BBSs. A computerized meeting and announcement system that allows people to carry on discussions, upload and download files, and make announcements without the people being connected to the computer at the same time. In the early 1990"s there were many thousands (millions?) of BBS"s

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7 around the world, most are very small, running on a single IBM clone PC with 1 or 2 phone lines. Today almost there are disappear, replaced by the WWW. ADVANTAGES: More fast and more cheap than Internet.

A bulletin board system (BBS) is a computer or an application dedicated to the sharing or

exchange of messages or other files on a network. Originally an electronic version of the type of bulletin board found on the wall in many kitchens and work places, the BBS was used to post simple messages between users. The BBS became the primary kind of online community through the 1980s and early 1990s, before the World Wide Web arrived. A BBS may be accessible from a dial-up modem, Telnet, or the Internet. Because it originated before the graphical user interface (GUI) became prevalent, the BBS interface was text-based. Although recent Web-based versions have a graphical, interactive user interface, the text-only interface preferred by BBS purists can often be accessed by Telnet. According to Justin Scott, co-

founder of Sceiron Interactive and a former sysop, a Web-based BBS is essentially a Web site that is

powered by BBS software rather than a Web server.

Most BBSes are devoted to a particular subject, although some are more general in nature.

Among special interests represented on BBSes are dentistry, law, guns, multi-player games, Druidic

practices, and information for the disabled. A significant number of BBS sites offer "adult-oriented"

chat and images that can be downloaded. The BBS is often free, although some charge a membership or use fee. Many BBSes have Web sites, and many Internet access providers have bulletin board systems from which new Internet users can download the necessary software to get connected. The BBS has its own culture and jargon. For example, a sysop is the person who runs the site. Online chat became widely popular through the BBS and many chat acronyms originated there. The first BBS, called the Computerized Bulletin Board System (CBBS), was created in 1978 by Ward Christensen and Randy Suess. Although ARPANET was in operation at that time, it was restricted to institutions funded by the U.S. Department of Defense. When CBBS went online, it became the first non-military computer-based community, other than timesharing systems (which allotted portions

of mainframe processing time to a group of computers). An article by Christensen and Suess

published in Byte magazine described CBBS and outlined the technology they had used to develop it, sparking the creation of many tens of thousands of BBSes all over the world.

Despite the vastly greater reach of the Internet, the BBS is still fairly common in parts of the world

where the Internet is less established and is still valued by many with Internet access for its ability to

foster a sense of community.

ENCRYPTION:

Encryption is the process of inputting data that is to be transmitted into some form of secret code to prevent unauthorised access.

Encryption provides a high level of security for sensitive data, such as credit card numbers. It uses

mathematical algorithms to scramble up the data, which can only be opened by having the

correct "key" to the encryption process. Both sender and user must have a copy of the "key" to encode and decode the message. One of the most popular encryption systems used on the net is called "pretty good privacy". Encrypted or coded data is referred to as cipher text and unencrypted data is called plain text.

AUTHENTICATION:

is a term used in computer security. This process verifies the identity of a user and his eligibility to access the computer. For example, when using the schools computer you have to enter your user ID and password to use the system. The computer checks to see if you have permission by a process of authentication.