Networking -by Chris

= Data Transmission and Networking =

work in progress

Introduction
This self study material is a handout meant for students of AS and A2 level studying for computing.  It covers the necessary notes on Data transmission and Networking to prepare the students for an advanced  level examination for Cambridge for that topic

The materials  -describe the characteristics of a Local Area Network (LAN)  -discuss the hardware and software requirements for LANs and WANs to function -describe for each type of network topology their relative strengths and weaknesses -describe the characteristics and uses of Serial and Parallel transmission of data -describe the characteristics of Simplex, Duplex and Half Duplex data transmission  -explain bit rate/baud rate -explain the advantages and disadvantages of networking

(1) The advantages of Networks over standalone computers
Think of a classroom with 10 stand alone computers.  Reflect on the inconveniences of having these computers not connected together?

Every time the lesson ends pupils would need to store their files on a computer’s hard disk. It would be possible to store the files on a floppy disk or pen-drive and take them away with you, but most of the time the files will be stored on the hard disk. A pupil who wants to use those files  again later would have to sit at the same computer. <BR> Pupils would not be able to share their files, data and information.<BR> They would not be able to connect to each other.<BR>

Linking the computers together would change the situation and bring more advantages:<BR> The system would allow access to the same files through any of the 10 computers, which means <BR> that the student could sit anywhere and use any computer. The computers then, need to be <BR> connected up to each other. When computers are connected together, they make a network. A network of 10 computers in the school classroom would allow the pupils to share data, <BR> information, files and communicate among themselves in a more collaborative way.<BR>

This communication is made easier because of the short distances involved.<BR> But if a business situated in one country and having its  offices in remote countries <BR> wanted to connect the computers together, they would have to overcome the problem of distances.<BR> Fortunately it is easy to connect computers even though they are far apart.

Networks connected over small distances are called Local Area Networks (LAN) <BR> while those connected over great distances are Wide Area Networks (WAN).<BR>

Whether the network is a WAN or a LAN it will have many advantages such as offering <BR> the pupils the chance<BR> to communicate with one another, <BR> to share data and information, <BR> to share copies of software and to allow multiple access to files of data.<BR> to update software at only one place <BR>

In a LAN there is, furthermore, the added benefit of being able to share hardware,<BR> so the classroom with 10 computers may only have 3 printers of different types that could <BR> be used for different tasks.<BR>

(2) Hardware and Software
What is needed for networking<BR> To connect computers in a network we need the following <BR> This network card gives each computer its identity and allow them to communicate.<BR> It has a limitation in that the signal gradually deteriorates as it is sent down the cable which<BR> means that the maximum length of the cable is about 300 metres. This maximum depends on a number <BR> of things, not least the quality of the cable used, but all cable is ultimately going to be limited in length.<BR> This is done by a computer which acts as a server or a network server and whose job is to<BR> control the network. Another job of the server is to control access to the files and software<BR> held on the hard disk. The server can also act as a printer server and control the effective use<BR> of the printers. <BR> A wide area network differs because the distances involved are greater than for a LAN.<BR> The telephone system is used to connect the computers over a long distance. Here modems are  used <BR> at the other ends to alter the signals used by the phone and the computer so that good communications take place.<BR>
 * a special piece of hardware attached to the processor, called a network interface card(NIC).<BR>
 * To connect the computers we use a cable. The cable is coaxial, like a television aerial. <BR>
 * The more and more computers are attached to the network, communications have to be controlled.<BR>
 * To allow good communications around such a system we need a network software or a network operating system.<BR>

(3) Network Topologies
Network topologies are the different ways the machines can be connected on a network.<BR> The shape of the network is known as its topology. Here we consider three different network topologies.<BR>

1. Bus network
The Bus network have all the hardware items on the  same, central, communication line,<BR> known as a bus. The advantage is that this setup is easy because there is only one communication line.<BR> But the disadvantage is that if the cable breaks at one place it will stop all communications in the network.<BR>

2. Ring network
In a ring network the two ends of the bus are just joined up. This forms a loop where data can travel <BR> in both directions along the cable. Here if there is a break in the cable, the network will still <BR> continue to operate as messages can go in the other direction.<BR>

3. Star network
In a star network all the hardware are connected to the network server. <BR> This means that if a piece of hardware breaks only that  hardware will be affected.<BR> But if the network server breaks the whole system is down.<BR>

4.1 Serial and Parallel transmission of data.
There are two ways that data can be transmitted between devices in a computer system.<BR> When a single wire that links the two devices is used, the data can be  transmitted <BR> in the form of bits down the wire, so that one byte(8 bits), that stands for a single character,<BR> will be transmitted as 8 signals .<BR> As there is only one wire, only one bit can be transmitted at a time. This is known as SERIAL<BR> transmission of data.<BR> Serial transmission has the advantages of being simple and reliable because the next bit <BR> is not transmitted until the current one has arrived at its destination. However, because only one bit can be transmitted at a time, the speed of transmission is slow. If the devices are connected by 8 wires, then more bits can be sent at once.<BR> Therefore a whole byte can be sent at once instead of having to send one bit at a time.<BR> This type of data transfer is called Parallel transmission. Parallel transmission of data<BR> is faster than serial because all the bits are travelling at the same time<BR>

4.2 Different modes of transmission.
There are three different modes of transmission .<BR>

a) Simplex mode.
If data travels in one direction then it is known as a SIMPLEX transmission. A good example <BR> is teletext information which is passed to a television receiver in one direction and there is <BR> no way to send data in the other direction.

b) Duplex mode.
Where data can pass in both directions at the same time this is known as DUPLEX transmission of data.<BR> An example is a telephone conversation where both users can speak and be heard at the same time.<BR>

c) Half duplex mode.
Where data can pass in both directions, but only in one direction at a time, the transmission is called <BR> HALF DUPLEX. An example is a CB radio system in which each handset can either be set to receive mode <BR> or send mode.<BR>

(5) Bit Rates
Information are usually sent to devices in a computer system from other devices. If a picture needs <BR> to be sent down a telephone line to another computer it will take more time than when a piece of <BR> text is sent. This is because the picture is made of many pixels and takes a longer time to transmit <BR> than the text which is composed of fewer bits (1 byte per character).<BR> The number of bits that can be sent in one second is known as the BIT RATE. It is measured in BAUD.<BR> 1 baud = 1 bit per second.<BR> When pictures are being transmitted, e.g. on the internet, it is important to limit the amount<BR> of data that are sent as the time taken to download the data can be long. Reducing the size of pictures<BR> so that they take up a small part of the screen, or that they are restricted to a few colours will ,<BR> obviously, speed up their transmission and is known as compression. <BR>

(6) Checking and correcting errors
When data is transmitted from one part of a computer system to another, it is transmitted as <BR> a series of binary digits. There are two possible mistakes that can occur, either a 0 is received<BR> as a 1, or a 1 is received as a 0. These mistakes can occur when there are noise along the transmission.<BR> There are three methods for checking the data when they are transmitted.<BR>

(a) Echoing Back.
The is the sending of data  back again and checking them. If the data that is sent back is the <BR> same as the data that was sent then the original data must have reached its destination correctly,<BR> if not then it needs to be sent again. This is known as ECHOING BACK. Echoing back suffers from<BR> having to send data twice, thus taking longer than necessary, and needing to be a duplex,<BR> or half duplex, system to allow data transfer in both directions.<BR>

(b) Parity.
All data is transmitted as bits (0s and 1s). The Number of 1s in a byte must always be either an <BR> odd number or an even number. If two devices that are communicating data decide that there will <BR> always be an odd number of 1s, then if a byte is received that has an even number of 1s, an error <BR> must have occurred. The receiving device would ask for it to be sent again.<BR> Notes: •The implication in all the above work is that odd parity is always used. Even parity can equally <BR> well be used, whichever has been agreed between the two devices. •Parity is not only used during data transfer between devices, but also when data is <BR> transferred between different parts of the CPU.<BR>

(c) Check Sum.
Here Data is sent from one place to another as a block of bytes rather than as individual bytes.<BR> The computer can add numbers together without any trouble, so another checking procedure is to <BR> add all the bytes together that are being sent in the block of data. This answer is calculated<BR> before the data is sent, and then calculated again when it is received, and if there are no <BR> errors in the transmission, the two answers will match.

Advantages.
•The main advantage of connecting computers in a network is that they can share data. <BR> When machines are networked there only needs to be one copy of the database. If any change <BR> is made to the database it will be available by other machines.<BR> •The machines can be used to communicate. The pupils can collaborate on one project as each student<BR> does a bit of the work or e-mail can be used as part of an intranet or over the internet.<BR> •On a LAN the sharing of hardware and software between machines can reduce the cost of a large amount<BR> of computer equipment and software.<BR> •Also on a LAN, access to files does not depend upon the user working on the same machine every time.<BR> Access through password can allow him to retrieve his file from anywhere on the LAN<BR>

Disadvantages.
•Security of files may be a problem. But Methods like user ID and password systems have to be<BR> employed to ensure that the correct users are given access to the correct files and folders. <BR> Besides there are a log which record any work done on the network <BR> •If there is a problem on the network server  it can effect more than one station,<BR> and the network will fail.<BR>