Data transmission and networking

Data Transmission and Networking

1.6.1 Networks

All the systems that have been mentioned so far have been individual computers,
sometimes with more than one user, but single processors. This means that the systems we
have discussed so far are not connected to other machines.
Imagine a classroom with 20 computers in it. Every time the lesson ends you would need to
store your files on secondary storage. It would be possible to store the files on a floppy disk
and take them away with you, but the likelihood is that the files will be stored on the hard
disk. This means that the next time you want to use those files you need to sit at the same
computer. It would be much more sensible to have a system that allowed access to the
same files through any of the 20 computers. To allow this the computers need to be
connected up to each other. When computers are connected together to share files they
make a network.
A network of 20 computers in a school classroom is obviously on a small scale, not because
20 is a small number, but because the communication is made easier because of the short
distances involved. If a business with head offices in London and factories in Leeds and
Manchester wanted to connect the computers on the three sites up there is an obvious
problem of distances to be overcome. Generally, networks over small distances are called
Local Area Networks (LAN) while those over great distances are Wide Area Networks (WAN).
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, so the classroom with 20 computers may only have 3
printers. Also those printers may be different types that could be used for different tasks.
This means that the type of printer used is dependent on the job that the user wants it to do
rather than the type of printer that happens to be connected to the computer on which the
work is being done.
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.
1.6.2 Hardware and Software

Network Basics.
In order to connect computers in a network there are a number of essential things
necessary
Each computer needs to be able to communicate with others. In order to do this there needs
to be a special piece of hardware attached to the processor, called a network card. It is
through this network card that the computer can communicate.
The computers need to be connected in some way. The standard method in schools is to use
a cable. The cable is coaxial, like a television aerial. The cable has a limitation in that the
signal gradually deteriorates as it is sent down the cable which means that the maximum
length of the cable is about 300 metres. This maximum depends on a number of things, not
least the quality of the cable used, but all cable is ultimately going to be limited in length.
With a number of computers attached to the network, communications are going to get
complicated. There is a need for something to control the signals being transmitted. This job
is carried out by a computer whose job is to control the network, it is known as a server or
a network server. One of the jobs of the server is to control access to the files held on a
hard disk, because of this it is sometimes known as the file server. Some networks have a
number of different servers controlling different parts of the system, for example, there
may be a printer server to control use of the printers. On many LANs these server functions
are all carried out by the single network server.
The communications around such a system are obviously quite difficult to control. It is
necessary to have a set of instructions that the network must follow, this is known as
network software or a network operating system.
A wide area network differs because the distances involved tend to be far greater than for a
LAN. Consequently it is not possible to connect all the computers by a cable. Actually the
cables already exist to do this, the telephone system. So to connect two computers over a
long distance, they can simply ring each other up! Unfortunately, the type of signals
produced by a computer are different to the type of signal that can be sent down a
telephone line. In order to send a computer signal down a telephone line it must be altered
first. This is done by another piece of hardware called a modem. A similar modem must be
present at the other end of the line in order to turn the signal back again.
1.6.3 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 effect that piece of hardware and none of the others. The
wiring up of the network can be very complex, though.
1.6.4 Different Types of Data Transmission

Serial and Parallel transmission of data
Data needs to be transmitted between devices in a computer system. The easy way to do
this is by using a single wire that links the two devices. Data is transmitted in the form of
bits down the wire, so an 8 bit byte, that stands for a single character, will be transmitted in
8 parts, one signal for each bit.

1

The diagram shows the data byte 01101101 being transmitted. As there is only one wire,
only one bit can be transmitted at a time. This is known as SERIAL transmission of data.
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.

1.6.5 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. This will be studied in more detail in the A2 part of the course.
1.6.6 Error checking and correcting

When data, of whatever type, is being transmitted from one part of a computer system to
another, it is transmitted as a series of binary digits. Any data that is transmitted is going to
be made up of a very large number of bits. Consequently, there are bound to be occasions
when the data is not transmitted correctly. Luckily there are only two possible mistakes that
can occur, either a 1 is received as a 0, or a 0 is received as a 1. Mistakes occur rarely, but
when they do occur they can be very serious, as the information is no longer correct. This
makes it very important that there should be methods for checking the data when it has
been transmitted.

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.
Notes:
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.
Earlier in this course it was said that a byte was the number of bits necessary to hold a
character code. Specifically, an ASCII character uses 8 bits in a byte, giving 256 different
characters. This is not true because one of the bits has to be reserved for a parity bit, the
bit that can change to make sure that the number of ones is always odd. This means that
there are 128 different characters possible.
The implication in all the above work is that odd parity is always used. Even parity can
equally 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
transferred between different parts of the CPU.

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.
1.6.7 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 reassembled
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.
1.6.8/1.6.9 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. There are a number of protocols that you may have heard of e.g. http,
tcp(ip) but don�t worry about them, you do not need to know specific examples of protocols,
but you should be able to discuss some of the rules that go to make up a particular protocol.
1.6.10 Layering of Interfaces

The interface between two devices is the connection between them through which the data
can be sent.
This connection between the devices can take the form of the physical connection, whether
wires are used or some other form of connecting them together. However, it also includes
many other things like how error correction is going to be carried out, how messages are
routed from one place to another, how the flow of data is going to be controlled, how the
data transfer can be synchronised so that data does not become mixed up, and many other
things that need to be agreed upon.
This is a very complicated idea, but, in essence, there are two ways of setting up this
interface. One way is to try to do the whole thing in one go, and the other way is to divide
the interface into sections and do one bit at a time. For instance, first set up the rules about
how the devices are to be connected physically, and later on worry about how to spot errors
that have been made in the data transfer. This is known as layering the interface because
the most basic layer has to be done first and then more layers may be put on top.
The main advantage of layering the interface is that changes may be made to one layer
without having to change them all. It also allows manufacturers to develop devices for
particular layers, knowing that they will be compatible with other devices at that level.
1.6.11 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 effect more than one stations, 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)
A. -In a WAN the computers on the network are remote from each other, while in a LAN the
computers are close to each other.
-The method of communication between nodes is likely to be different, with a LAN typically
being hard wired and a WAN being connected by using phone lines.
Notes: The use of the terms LAN and WAN are perfectly acceptable without reference to
what they stand for, as it has been stated in the question. Even if it hadn�t been stated in the
question, these sets of initials are stated in the syllabus so there is no problem in using
them without explanation. However, be careful when using acronyms like this as what might
be obvious to you may not have been heard of by the examiner, so a brief explanation on
first mention is generally a good idea.
Two mark points available, note that they are not given for saying that the WAN is remote
while the LAN is confined to a small area. This is really the same thing said the other way
around. Be very careful about saying the same thing twice and thinking you have both
marks where you have really only said one thing. Another example of this is when a
question asks for advantages and disadvantages of two things. The same thing given as an
advantage of one will not be given a second mark as the disadvantage of another.

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)
A.a)-Network cards in each of the computers
-Cable to carry the signals from one point to another on the network
-A network server to control access to peripheral devices and to files on the shared hard
drive.
b)-The distances between the computers are further than on a LAN�
-The cable used to connect up a LAN will only transfer signals over limited distances
-Other communication medium needed, e.g. modem and a telephone line.

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)
A. -Single ring of network cable shown clearly on diagram, may have spurs to single
machines
-Server as part of network
-Sensible peripheral devices shown
-Advantage: If cable is damaged the network can continue to operate.
Notes: If the question suggests drawing a diagram it is because that is the best way of
answering the question, use that method unless you are totally stuck. There are no marks
for doing so, but that will be the easiest method.
4. Explain the difference between
(i) simplex
(ii) half duplex
(iii) duplex
transmission of data, giving an example of the use of each. (6)
A. (i) �Simplex is the transmission of data in only one direction
-An example is the transmission of a television picture from the transmitter to the receiving
aerial, there is no signal sent back.
(ii) �Half duplex is a means of transmitting data in both directions, but only in one direction
at a time.
-An example of half duplex transmission is a walkie talkie where the handset needs to be
set to either send or receive.
(iii) �Duplex transmission of data is the ability to send messages over the same link, in
both directions, at the same time.
Notes: Nice, standard, question requiring a standard answer. As with so many of these
questions, if you try to make up your own examples you could be making a big mistake.

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)
A. -Colour picture requires more data because each pixel must be defined
-Page of text has less data because each character is defined by its own byte of data
-Bit rate is the speed at which a given amount of data can be transmitted which is more
important the more data that there is.
Notes: This answer is not quite right because it is possible to imagine a page of text written
in 2 point font which would use a large amount of data, while a colour picture of alternating
stripes of red and blue would not need a lot of data to describe it. Also this answer does not
take any account of compression algorithms that many students may know something
about. However, in principle this answer is true, and, considering there are only three
marks, any attempts to complicate the answer must be counter productive.

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)
A.a)-10101011
-Bytes should pass a parity check
-Other bytes are of even parity while this is odd
b) -If two bits in the same byte are in error the effect of each will cancel out the other.

7. Explain the difference between packet switching and circuit switching. (2)
A. -Packet switching sends message in equally sized pieces in any order, while in circuit
switching the message is sent in order meaning that it does not need to be reassembled at
destination.
-Packet switching involves the individual packets being sent onto the network before a
route is established, while circuit switching requires a route to be established before
transmission.
Notes: A difficult question. The immediate reaction is to say that circuit switching needs a
route to be established before transmission. Packet switching does not need a route to be
established. This is the same point from two perspectives, and hence is only worth one
mark. The second mark is much more difficult to find.

8. A computer is to use a printer to provide hardcopy output of jobs. In order for the data to
be transmitted and received properly a protocol must be set up between the two pieces of
hardware. State two parts of the protocol which would be essential in this example, giving
reasons why they are necessary. (4)
A. -Baud rate
-necessary because if the computer transmits at a different speed to that which the printer
can receive the message will become jumbled.
-Parity type
-because if the computer transmits using odd parity and the printer checks using even
parity correct bytes will never be accepted.
Notes: There are many possible answers to this type of question. You may know more than
the ones stated in the text of this chapter, be warned, stay with the ones in the text then
there is no chance of misunderstanding. Some students, on sight of the word protocol,
desperately want to write down TCP(IP) or http. Be very careful. It would be difficult to think
of a question, at this level, which could possibly have those as answers.

9. a) Explain the term �layered interface� when describing intercommunication between
devices. (2)
b) State a reason for arranging the interface between devices in a layered fashion. (1)
A.a)-Different rules have to be established to allow the communication to take place
-These rules are split up into types, for instance the physical connection between devices
and the agreement of error trapping routines will be handled separately
b) -The rules in one layer can be altered without having to alter those in other layers.

10. A school computer department has decided to invest in new hardware in the two rooms
that it uses for computer lessons in the school.
The teacher in charge has to decide between a network of computers or a collection of
stand alone machines. The teacher has asked your advice about which hardware to
implement.
a) State four advantages in implementing the network solution. (4)
b) State a disadvantage of using a network in this context and suggest a solution to the
problem. (2)
A.a)-Hardware can be shared. There is no need for a printer for each machine.
-Software can be shared. Only one copy of each piece of software is required.
-Pupils can access their files from any machine on the network, they do not have to use a
particular machine.
-Communication can be carried out between machines in the two rooms.
-Files stored on the disk drive only need to be updated once, so all amchines can access
the same information in common files.
b) -Files are less secure because of reliance on single drive
-regular back up copies of files should be taken.
-Privacy of files is more difficult to control because of common access to hardware
-Use of passwords to control access to files.