Chapter 1.6 Data Transmission and Networking

Chapter 1.6 Data Transmission and Networking.

1.6 (a)        Networks

A network is an interconnection of two or more computers. Generally, networks over small distances are called Local Area Networks (LAN) while those over great distances are Wide Area Networks (WAN). A Metropolitan Area Network (MAN) is one within a district or country.

Whether the network is a WAN or a LAN it will have the advantage of offering the users the chance to communicate with one another, to share information centrally, to share copies of software and to allow multiple access to files of data. In a LAN there is the added benefit of being able to share hardware such as printers and scanners.

To summarise:

·         Computers can be linked together to form networks

·         If the distances are short the network is called a LAN, if longer the network is a WAN

·         Networks allow computers to communicate

·         Networks allow the sharing of both hardware and software.

1.6 (b)        Hardware and Software

Components of a network

·         Network card. It is through this network card that the computer can communicate.

·         Cables

·         Servers

·         Network operating system.

Note: A modem is used to convert digital signal into analog at sending side and vice versa at receiving side. It is used to communicate over a WAN.

1.6 (c)        Network Topologies

There are a number of ways that the machines on a network can be connected together. The shape of the network is known as its topology. We are interested in three different network topologies.

1. Bus network

In a bus network the hardware items all use the same, central, communication line, known as a bus. The advantage is that the installation of the system is comparatively easy because there is only one communication line. However, the disadvantage is that a single break in the cable will stop all communications in the network.

2. Ring network

 

In a ring network the two ends of the bus are joined up. This creates a loop so that data can be sent in both directions along the cable. If there is a break in the cable, the network will continue to operate because the messages can go in the other direction.

3. Star network

 

In a star network the individual pieces of hardware are connected to some central point, usually the network server. This means that the failure of a single piece of hardware or the breaking of a connector can only affect that piece of hardware and none of the others. The wiring up of the network can be very complex, though.

1.6 (d)        Different Types of Data Transmission

Serial and Parallel transmission of data

In serial transmission only one bit of data can be transmitted at a time. Serial transmission has the advantages of being simple and reliable because the next bit 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 more than one wire, then more bits can be sent at once. A sensible number of wires would be 8, because then a whole byte can be sent at once instead of having to send one bit at a time.

This type of data transfer is called PARALLEL transmission. Parallel transmission of data is obviously faster than serial because all the bits are travelling at the same time, but because of the fine tolerances in the transmission, it is less reliable as the bits can become muddled up.

Modes of Transmission.

If data is to be transmitted between devices, there are three different modes of transmission possible.

a) Simplex mode. If data can only travel in one direction then it is known as a SIMPLEX transmission. A good example of a simplex transmission of data is teletext information which is passed to a television receiver, but there is no way to send data in the other direction.

 

b) Duplex mode. When data can pass in both directions at the same time it is known as DUPLEX transmission of data. An example of duplex transmission would be a telephone conversation as both users can speak and be heard at the same time.

 

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

 

 

1.6 (e)        Bit Rates

Information needs to be sent to devices in a computer system from other devices. For example, a picture stored in the memory of the computer needs to be sent down a telephone line to another computer. Remember that the more pixels that there are and the more colours that can be represented, the better the picture will be. However there is a limit to the amount of information that can be transmitted in a given time down the phone line, so a decision needs to be made. One choice is to have as much detail as possible and accept that it will take a long time to transmit it. The other is to limit the resolution of the picture, meaning that there is less information to send so that the message will be sent more quickly.

The number of bits that can be sent in one second is known as the BIT RATE. The units used to measure the bit rate are BAUD. 1 baud = 1 bit per second.

Note that text can be sent much more quickly than other forms of information because it needs far fewer bits (1 byte per character) than other types of data. When data other than text is being transmitted, e.g. on the internet, it is important to limit the amount of data that needs to be sent or the time it takes to download the data can be unreasonably long. The data can be limited by such simple things as reducing the size of pictures so that they only take up a small part of the screen, or that they are restricted to a few colours. Speeding up the transmission of the information by reducing the amount of data that is sent is known as compression

1.6 (f)         Error checking and correcting

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

b) Parity. All data is transmitted as bits (0s and 1s). The Number of 1s in a byte must always be either an odd number or an even number. If two devices that are communicating data decide that there will always be an odd number of 1s, then if a byte is received that has an even number of 1s, an error must have occurred. E.g. the byte 01011000 has 3 ones in it. 3 is an odd number, so it fits the rule that it must have an odd number of ones. When it is sent there is an error in transmission so that the first bit is received as a one. So, the byte received is 11011000. This has 4 ones in it, which is an even number, so there must be an error. The receiving device would ask for it to be sent again.

Note:

·         If two mistakes are made in the same byte they will cancel each other out and the faulty data will be accepted. This problem can be overcome, and in the same way, a clever way of correcting error mistakes can be implemented. This method is not part of this course.

c) Check Sum. Data will normally be sent from one place to another as a block of bytes rather than as individual bytes. The computer can add numbers together without any trouble, so another checking procedure is to add all the bytes together that are being sent in the block of data. The carry, out of the byte, is not taken into account, so the answer is an 8 bit number, just like the bytes. This answer is calculated before the data is sent, and then calculated again when it is received, and if there are no errors in the transmission, the two answers will match.

1.6 (g)        Switching

When a message is being sent from one machine to another, particularly over a wide area network, the message may have to pass through other machines first. This may be forced on the system because there is no direct route from one machine to another.

 

In the network shown, it would be easy to send a message from A to D or from A to B because A is directly connected to both of them. However, sending a message from A to C is much more difficult because there is no direct route. There are two ways that the message can be sent

a) Packet switching. The message is split into a number of equal sized packets. Each packet has a label saying where it is meant to be going and what number packet it is. These packets are sent along communication lines towards the destination. Each time a packet reaches a node on the network the node decides which direction to send it on. So, one packet in the message from A reaches node D. The obvious route to take is the one directly to C, but it is already in use for another message, so D decides to send it to E instead. The next packet arrives at D and, this time, the line to C is free, so the packet is sent direct to C. When the message has all arrived at C it has to be reassembled in the correct order.

b) Circuit switching. Before the message is sent, the network reserves a route from A to C. The message can then be sent directly from A to C and will not need to be reordered when it gets there.

Packet switching allows optimum use of the connections around the network because as many routes are in use at one time as possible, whereas circuit switching means that the whole message is kept together so it does not need to be reassembled at the destination.

1.6 (h) and (i)      Protocols

When data is being transferred from one place to another in a computer system there must be rules set up as to how the transfer is going to be done. Typical rules would be

·         A rule about the wire connecting the two parts of the system. Errors would occur if one device had a serial connection and the other was expecting a parallel connection

·         If one device sent data at a particular bit rate and the other device read what it received at a different rate, the message would never be received correctly.

·         If one device used even parity and the other device used odd then no correctly sent byte of information would ever be accepted.

The set of rules that needs to be set up to allow the transfer of data to be carried out is known as a protocol.

1.6 (j)      Advantages and Disadvantages of Networking

Advantages.

·         The main advantage of linking computers in a network is that they can share data. If two computers each have their own copy of a database then, unless no changes can be made to it, one computer will always have a more up to date version than the other because a change made on one machine will not register on the copy of the database on the other machine. If the two machines are networked there only needs to be one copy of the database, therefore any change made to the database will be used by both machines.

·         The machines can be used to communicate, a good example being the use of e-mail over the internet.

·         On a LAN the sharing of hardware and software between machines can reduce the cost of a large amount of computer equipment being needed in a small area, although, the saving on software is not necessarily what might be thought because the license for network software is far more than for a single machine.

·         Also on a LAN, access to files does not depend upon the physical location of the user, but on who the user is. For example, in a school classroom, if a pupil was working at a particular machine at the end of one lesson they do not have to use the same machine the next lesson, they may even be in another room.

Disadvantages.

·         The fact that the network allows access to user files from more than one machine means that keeping files secure is far more difficult. Methods like user ID and password systems have to be employed to ensure that the correct users are given access to the correct files.

·         If there is a fault on the network it can affect more than one station, and if the server is faulty the whole network will fail.

Example Questions

1.      Explain the difference between a wide area network (WAN) and a local area network (LAN).                                                                                    (2)

2.      a) State three pieces of hardware that are needed to create a LAN from a set of stand alone computers.                                                                      (3)

         b) Explain why the communication over a WAN differs from that across a LAN and state how the hardware necessary for communication would differ from that used in part (a).                                                                                        (3)

3.      By drawing a diagram, or otherwise, describe a ring network and state an advantage that a ring network has over a bus network.                                            (3)

4.      Explain the difference between

         (i) simplex

         (ii) half duplex

         (iii) duplex

         transmission of data, giving an example of the use of each.                 (6)

5.      Explain why the bit rate is more important when sending a colour picture from one device to another, than it is when sending a page of text.                 (3)

6.      The following bytes of data are received by one device after being transmitted from another.

                                    01001101

                                    10001000

                                    10101011

                                    00011011

         An automatic checking technique is used to check that the data has been transmitted without error.

    a)  State which byte has been received incorrectly, explaining how you arrived at your answer.                                                                                         (3)

    b)  Explain why it is possible that a byte of data could still be incorrect despite passing the test that you used in part (a).                                                   (1)

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