2.

1 The structure of a data packet

The amount of data that is stored in a file can be very large. If you tried to
transmit this data from one device to another, all at once, this just wouldn’t be
possible or practical. The wires or radio waves used would simply not be able
to accommodate sending such large amounts of data at a single time.
Therefore, for the data in a file to be transmitted, it is broken down into very
small units called packets.
Each packet of data contains three different sections:
Packet header
Payload
Chapter 2 Trailer
The packet header contains a lot of important information about the data
Data transmission enclosed in the packet and its destination. The information it includes is the:
• destination address
• packet number
• originators address.
IN THIS CHAPTER YOU WILL: The destination address is normally an IP address. It is the IP address of the
device where the data is being sent. Without this address, the hardware
• learn how data is broken down into packets to be transmitted, transmitting the data would not know where to send the data.
including the contents of each packet
Each data packet in the file is given a packet number. The packets of data may
• understand how data is transmitted using a range of different not have all been sent in the correct order, this will depend on the type of
transmission methods transmission used; you will learn more about this in Sections 2.2 and 2.3. This
• be able to choose a data transmission method for a given scenario means that when the destination device has received all the data packets, it
can use the packet number to put them back into the correct order to recreate
• learn about the universal serial bus (USB) interface and how it is used the file.
to transmit data
The originators address is also normally an IP address. It is the IP address of
• learn how errors can occur when transmitting data the device from which the data has been originally sent. So, if you send data to
• be able to describe how a range of different error detection methods another device, the originator’s address is the address of your device. This
work address isn’t crucial to the transmission process, but it does help to trace
where data has been sent from in situations such as illegal activity, or to simply
• understand why it is useful to encrypt data when it is transmitted
request the original device to resend the data if an error is detected.
• be able to describe how data is encrypted using both symmetric and
The payload of the data packet is the actual data from the file that you are
asymmetric encryption.
sending. The data is broken up into many small units to be sent as the payload
in each packet.
The trailer section of the packet is sometimes known as the footer. This
GETTING STARTED contains two main pieces of information. The first is the marker to indicate it is
the end of the packet and also the data for any error detection systems that are
Your data is transmitted to other devices, sometimes all over the world, on being used. You will learn more about this in Section 2.2.
a regular basis. Have you thought about how it gets there? Draw a
diagram to represent how you think data travels from your device to your Questions
friend’s device when you send them a message. It doesn’t matter if you
don’t think you know how this is done, just draw what you think happens 1 Can you name the three main sections of a data packet?
and you can see at the end of the chapter how close you were with your 2 What information does the packet header contain?
diagram.
3 Which part of the data packet is the actual data from the file?
4 Why does data need to be broken down into packets to be transmitted?
DATA TRANSMISSION IN THE FINANCIAL WORLD
ACTIVITY 2.1
A bank has to transmit data all around the world for thousands, possibly
millions, of people on a daily basis. The accuracy of this data is Ask your teacher if you are allowed to use a packet tracer. Your teacher
imperative. Imagine what would happen if one person’s hard earned may want to complete this activity together as a class. Use the packet
money went into another person’s bank account! When data is tracer to send data to another device and look at the data that the packet
transmitted, there are certain things that can cause it to suffer errors. tracer software provides. What kind of information does it give you? What
Interference is a big issue in data transmission that can cause errors in route did your packets take to get to the destination device?
the data, therefore banks have to make sure that they have many error
detection methods in place to make sure that data is accurate, even after
transmission. As the accuracy of the data is so crucial for banking
transactions, banks have to combine several types of error detection, to 2.2 Packet switching
make sure that the data has been thoroughly and completely analysed for You have learnt that data is broken down into packets to be sent from one
errors. device to another. The process of transmitting these packets is called packet
switching. In this process, each packet of data is sent individually from one
device to another. There could be several pathways across a network that the
packets could be transmitted. Each data packet could be sent along a different
pathway. A router is the device that controls which pathway will be used to
transmit each packet. You will learn more about how a router does this later in
the chapter. Look at the diagram of a network in Figure 2.2:

Figure 2.1: Financial transactions can be done online

The importance of checking for errors in data transmission can costs

banks a lot of money. It takes very large and powerful systems to check
the millions of transactions that are made every day. However, it would
cost them even more in compensation money, if they messed up the data
of their customers, and possibly their money, so they put a lot of time, Figure 2.2: A network
money and effort into the process.
Discussion questions The packets of data start at device A, they could be transmitted down any of
1 How would you feel about having a bank account with a bank that did the pathways between device A and device B. Each packet can be sent using
not have any error detection systems in place? What problems do a different pathway. When a packet reaches a router, the router decides which
you think this could cause? pathway to send the packet along next. This will continue until all the packets
have arrived at device B. It is likely that the packets will arrive out of order.
2 Do you think it is right that the bank is responsible for checking errors Once all packets have arrived, they are reordered to recreate the file.
in the transmission of data? Much of the hardware, that will be used
to transmit the data from one device to another, will not be owned by
them. So, why do you think the bank should be made responsible for
checking for errors if the errors may occur using hardware that isn’t
theirs?
 ACTIVITY 2.2 There are several advantages and disadvantages of parallel data transmission,
see Table 2.2:
Five packets of data need to be sent from device A to device B.
The pathway the first packet takes is: Advantages Disadvantages

• Router 1 As data is sent multiple bits at the As data is sent multiple bits at the
same time, the transmission of data is same time, bits do not arrive in order
• Router 2. quicker. and need to be reordered after
The pathway the second packet takes is: transmission. This increases the risk
of the data being skewed.
• Router 1
Many computers and devices use As data is sent along multiple wires,
• Router 4 parallel data transmission to transmit there is more chance of interference.
• Router 5. data internally. Therefore, there is no This means there is more chance of
requirement to convert this to serial error in the data.
The pathway the third packet takes is:
data transmission to transmit the data
• Router 1 across a network.
• Router 3 Multiple wires are needed for a
• Router 2. parallel transmission cable, therefore,
it is more expensive to manufacture
The pathway the fourth packet takes is: and also more expensive to buy.
• Router 1 Table 2.2: Advantages/disadvantages of parallel data transmission
• Router 4
• Router 3 Parallel transmission has an increased chance of data being skewed and an
increased chance of interference. This makes it more suitable for the
• Router 5. transmission of data over shorter distances. This is often limited to
The pathway the fifth packet takes is: approximately 5 metres.
• Router 1 Question
• Router 3
5 A marketing company prints leaflets to deliver to people’s houses. The
• Router 5. leaflets are designed on a computer and sent to a large printer that is 3
Draw a copy of the diagram of a network. Use coloured pens to draw the metres away. Large numbers of leaflets need to be printed in a short
pathway that each of the packets of data takes. period of time, to makes sure that they are ready for delivery. Would you
use a serial or a parallel data transmission cable to connect the computer
to the printer? Why would you choose this type of cable?
Self Assessment
Simplex, half-duplex and full-duplex transmission involve the direction in
What did you do to check if your diagram is correct? Have you tried which the data is transmitted.
tracing each route to see if it goes through the correct routers?
Simplex data transmission
2.3 Methods of data transmission In simplex data transmission, data is transmitted from one device to another in
one direction only. Figure 2.5 shows an example:
You might think that there is only one method that can be used to transmit data
from one device to another, but there are actually several. You need to
understand how data is transmitted using each of these methods. These
include:
• serial
• parallel
Figure 2.5: Simplex data transmission
• simplex
• half-duplex You would only use this type of data transmission when there is no need to
• full-duplex. send data both ways between the devices. One example is connecting a
keyboard to a computer. Simplex data transmission can be used to do this as
Serial and parallel data transmission involve the number of wires that are
data only needs to be sent from the keyboard to the computer, so that the
used to transmit the data, and how many bits of data are sent at a time.
computer screen can display the keys that are pressed. There is no reason
Serial data transmission why the computer would need to send data to the keyboard, so a simplex data
transmission connection is used.
In serial data transmission, data is transmitted using a single wire. Each bit of
data is transmitted one at a time along the single wire. Figure 2.3 shows an
example:

Figure 2.3: Serial data transmission

There are several advantages and disadvantages of serial data transmission,

see Table 2.1:
Advantages Disadvantages Figure 2.6: Simplex data transmission is used between a computer and a
keyboard
As data is sent one bit at a time, it As data is sent one bit at a time, the
should arrive in order of sequence. transmission of data is slower.
This means there is less chance of Half-duplex data transmission
the data being skewed. In half-duplex data transmission, data can be transmitted in both directions
As data is sent along a single wire, Additional data may need to be sent between the devices, but only one direction at a time. Figure 2.7 shows an
there is less chance of interference. to indicate to the receiving device example:
This means there is less chance of when the data transmission has
error in the data. started and stopped. These are called
a start bit and a stop bit.
Only one wire is needed for a serial
transmission cable, therefore, it is
cheaper to manufacture and also
cheaper to buy.
Table 2.1: Advantages/disadvantages of serial data transmission

Serial transmission has less chance of data being skewed and less chance of
interference. This makes it more suitable for the transmission of data over long
distances.

Parallel data transmission

In parallel data transmission, data is transmitted using multiple wires. Multiple
bits of data are transmitted along each wire at the same time. Figure 2.4 shows
an example: Figure 2.7: Half-duplex data transmission

Figure 2.4: Parallel data transmission

You would use this type of data transmission when you need to send data in COMPUTER SCIENCE IN CONTEXT
both directions between the devices, but there is no requirement for this to be
at the same time. It can also be used when the communication between two Many devices are connected using a USB cable. This is because the USB
devices needs to have a high level of performance. This is when two half- interface is an industry standard, which means that manufacturers don’t
duplex connections can be set up between devices to allow data to be need to worry about compatibility issues when connecting devices. One of
transmitted in both directions at the same time, but using two different the most recent versions of the USB interface to be released is USB 4.
channels of communication. Can you work out how this would be set up?

ACTIVITY 2.6

esearch the data transfer speed of USB 4; is this different to USB 1, 2 and
3? What other differences can you find between the different versions of
USB?

Figure 2.8: Wi-Fi uses halfduplex data transmission

There are several advantages and disadvantages of the USB interface, see
Table 2.3:
ACTIVITY 2.3
Advantages Disadvantages
Use the internet to research how half-duplex data transmission is used to It is a simple interface. The USB cable The length of a USB cable is limited,
create a Wi-Fi connection. Why is half-duplex, rather than full-duplex data to device can only fit into the USB one normally to 5 metres.
transmission used in Wi-Fi connections? way. Therefore, it means less errors in
connecting devices are likely to be
made.
Full-duplex data transmission The speed of a USB connection is The transmission speed is relatively
relatively high, so data can be high for a USB connection, but it isn’t
In full-duplex data transmission, data can be transmitted in both directions
transferred quickly. as high as other types of connection,
between the devices, at the same time. Figure 2.9 shows an example:
such as ethernet.
The USB interface is a universal
standard for connecting devices,
therefore a USB port is included in
many different devices.
When a USB device to cable is
inserted into a USB port, it is
Figure 2.9: Full-duplex data transmission
automatically detected. The first
connection will normally involve the
You would use this type of data transmission when it is essential for data to be download of drivers to operate the
both sent and received by each device, at the same time. One example is a hardware that has been connected.
telephone conversation. The person speaking into Device A can speak at the Each connection after this the driver
same time as the person speaking into Device B. It may be a little bit of a should not need to be downloaded
chaotic conversation if you both try and speak at the same time all the time again.
though! However, during most conversations, people do sometimes speak at
the same time, so full-duplex data transmission is needed. A USB connection can also be used
to power a device, so it does not need
When a connection is created between two different devices it will either be a another power source. It can also use
serial or parallel data connection. It will also be a simplex, half-duplex or full- this to charge a device, such as a
duplex connection. This means that a connection can be, for example, a serial mobile phone.
simplex connection, or a parallel half-duplex connection, or possibly a serial
full-duplex connection. You need to understand when each of the different
Table 2.3: Advantages/disadvantages of the USB interface
methods of transmission are most suitable.

Questions Questions
6 Which method of data transmission sends data along a single wire? 12 What does USB stand for?
7 Which method of data transmission sends data multiple bits at a time? 13 What type of data transmission method does a USB interface use?
8 If a data transmission connection sends data one bit at a time, in both 14 What are two advantages of using the USB interface?
directions, but not at the same time, what kind of data transmission
15 Can you name five devices that are connected using a USB interface?
methods are used?
16 What is the length a USB cable limited to? Can you think why it is limited
9 What are two advantages of serial data transmission?
to this length?
10 What are two disadvantages of parallel data transmission?
11 A business manager transmits data about its customers to a central file 2.5 Detecting errors in data transmission
server. The file server is 100 metres away from the business manager’s
office. They need to be able to send and receive customer data to and The transmission of data from one device to another is often not a perfect
from the server, at the same time. The accuracy of the customer’s data is process. In the process of transmitting the data, interference can occur. This
imperative. Which methods of data transmission should be used to create can cause data to be lost, data to be gained and data to change. This would
the connection between the business manager’s computer and the file not be helpful and could also lead to many issues. If a password wasn’t
server? Why would you choose those methods? transmitted correctly, then that person may not be able to log into their account.
If a person’s address wasn’t transmitted correctly, they may not get email that
needs to be sent to them. If a person’s details are not transmitted correctly to a
ACTIVITY 2.4 company, they may not get a product or service that they order. The accuracy
of data is often vital, therefore there needs to be a procedure in place to detect
Write a data transmission scenario like the one given in Question 11. any errors in data so that actions can be taken to correct this.
Think about which data transmission methods could be used for your
scenario and why they would be the most suitable.

Peer Assessment
Give the scenario to a friend and ask them which data transmission
methods they would use. Are they the same methods that you thought
should be used? If you have chosen different methods, discuss and come
to an agreement about which methods should be used. If you chose the
same methods, see if you had the same reasons for choosing those
methods.

Figure 2.10: Procedures need to be in place to handle errors in data

2.4 The universal serial bus (USB) interface transmission
The concept of USB can be a little confusing, as it is used to describe several
aspects. You may have heard of a USB connection, a USB cable, or maybe There are several error detection methods that can be used, these include:
you call a storage device ‘a USB’. You need to understand the difference • parity check
between several of these aspects. A USB interface includes items such as a
USB port, a USB cable, a USB connection and a USB device. • checksum

A USB device, such as a USB memory stick, is a device that uses the USB • echo check.
interface. It plugs into a USB port on a computer and it transmits data using a Each of these methods is designed to check for errors after the data has been
USB connection. A USB cable can be used to connect devices, such as a transmitted from one device to another.
mouse, to a computer.
A USB connection uses serial data transmission to transmit data between the Parity check
devices. However, it a special type of serial data transmission connection that A parity check can use an odd or even check method. Each byte of data has
is used that is designed to transmit data at high speeds. 7 bits and 1 extra bit that is called a parity bit (see Figure 2.11). The parity bit is
normally the first or last bit of data in the byte.
ACTIVITY 2.5

One of the issues with serial data transmission is the transmission speed
is slower than parallel. Research how a USB connection is able to
transmit data at a faster rate.

Figure 2.11: The parity bit is the first bit of data in this byte
Before transmission begins, the parity check is set to be either odd or even
parity. The number of 1s in the 7 bits of data is totalled. In the example in In a negative acknowledgement method:
Figure 2.11, the result of that would be 2. If an odd parity check is used, then a • The sending device transmits the first data packet.
1 is added as a parity bit. This is because all the 1s in the byte now add up to
3, which is an odd number. If an even parity check is used a parity bit of 0 • The receiving device receives the data packet and checks it for errors.
would have been added instead. This is because all the 1s in the byte would • If the receiving device detects no errors, no further action is taken.
then add up to 2, which is an even number. When the parity bit has been
added to each byte, the data can be transmitted. After transmission, the • If the receiving device does detect errors, it will send a negative
receiving device will check each byte of data for errors. If an odd parity check acknowledgement back to the sender.
has been used and the device finds a byte that has an even number of 1s, • If the sender receives a negative acknowledgement, it knows this means
then it knows that an error has occurred with this byte of data. The error is the data was received incorrectly, so it can resend the data packet.
detected!
• A timeout is set by the sending device when it sends the data. This is just
Questions so that the sending device knows that if it doesn’t receive a negative
acknowledgement back within that set time period, it doesn’t need to be
17 Which of these bytes would have been transmitted incorrectly if an even still be waiting for it and can send the next data packet.
parity check has been used?
a 10111011 ACTIVITY 2.10
b 01110111
Draw a diagram to represent how an ARQ system operates using
c 10101000 negative acknowledgement.
18 Which of these bytes would have been transmitted incorrectly if an odd
parity check has been used?
a 00110001 REFLECTION
b 10110101 After completing activities 2.8, 2.9 and 2.10: How have you made sure
c 10001000 that your diagrams for an echo check and both ARQ methods are correct?
19 Can you think of an instance in which an error could occur in the data,
but this would not be detected by a parity check?
Check digit
Checksum
You have learnt that errors, such as data loss, data gain and data being
A checksum uses a calculated value to check for errors. A value is calculated changed can occur when data is transmitted. You now know that these errors
from the data that will be transmitted, before transmission takes place. A normally occur due to interference. It is also possible for errors to occur when
method such as modulus 11 could be used to calculate the value. data entry is performed. This could be manual data entry, for example, a
human typing in a value, or it could be automatic data entry, for example, a
ACTIVITY 2.7 barcode scanner scanning a barcode to obtain the data stored in the barcode.

Use the internet to find out how the modulus 11 method calculates a value • A check digit value is previously calculated from the data that will be
from the data. You do not need to be able to remember how to do this entered at some point, for example, a barcode number or an ISBN
method of calculation for the exam, but it will help you to know how a number. This number is stored with the data.
checksum is calculated.
• When the data is entered, for example, the barcode is scanned, the check
digit is recalculated from the data entered.
Once the checksum value has been calculated it is added to the data to be • If the previously calculated check digit and the stored check digit match,
transmitted with it. After transmission, the receiving device uses the same the data entered is correct.
method to calculate a value from the received data. If the values match, then
the device knows that no error has occurred during transmission. If the values • If the previously calculated check digit and the stored check digit do not
do not match, the device knows that an error has occurred during match, the data entered is incorrect.
transmission. The error is detected! Although the process is similar, you should not confuse the operation of a
check digit and a checksum. Make sure that you take the time to understand
Questions the difference between the two.
20 What is the checksum value calculated from?
21 What happens to the checksum value when it has been calculated before ACTIVITY 2.11
transmission?
With a partner, discuss and write down the differences between a check
22 Why would checksum values that do not match show that an error has digit and a checksum.
occurred?

Echo check
COMPUTER SCIENCE IN CONTEXT
An echo check involves a simple comparison of the data sent to the data
received. The sending device transmits the data to the receiving device. The Companies will normally combine several error checking methods to
receiving device then transmits the data it receives back to the sending device. make sure that the data they transmit and receive is error free. This
The sending device compares the data it sent to the data it has received back means that the data can be checked in several ways to make sure that
from the receiving device to see if they match. If they do, then no error has any error that could occur is being checked. This is because the data that
occurred. If they don’t match, then the sending device knows the data was many companies use is vital to their business, they want to do everything
received with error. The error is detected! they can to make sure that it does not contain any errors caused by
transmitting the data.
ACTIVITY 2.8

Draw a diagram to represent how an echo check detects errors in data.

Question
23 If the data does not match that which is sent back by the receiving
device, will this always mean that the receiving device did not receive the
data correctly? How do you know that?

Automatic repeat request (ARQ)

When an error has been detected after the data is transmitted, it is likely that
the data will need to be retransmitted. Either the sending device will need to
ask the receiving device if it received the data correctly, or the receiving device
will need to tell the sending device it did or did not receive the data correctly.
This will allow the data to be retransmitted if necessary.
A method called automatic repeat request (ARQ) can be used to do this.
There are two main ways that an ARQ can operate and each method uses
either a positive or negative acknowledgement and a timeout.
In a positive acknowledgement method:
• The sending device transmits the first data packet.
• The receiving device receives the data and checks it for errors.
• Once the receiving device knows it has received the data error free, it
sends a positive acknowledgement back to the sending device.
• When the sending device receives this positive acknowledgement, it
knows the receiving device has received the data packet error free and it
sends the next data packet.
• If the sending device does not receive a positive acknowledgement within
a set timeframe, a timeout occurs.
• When a timeout occurs, the sending device will resend the data packet. It
will keep doing this when a timeout occurs, until it receives a positive
acknowledgement, or sometimes a limit (such as 20 times) is set and
when this limit is reached it will stop resending the data.

ACTIVITY 2.9

Draw a diagram to represent how an ARQ system operates using positive

acknowledgement.
 SUMMARY
2.6 Encryption
You have learnt a lot about how data is transmitted in this chapter. You have Data is broken down into packets to be transmitted from one device to
also learnt why it is important for data to be received correctly and how errors another.
can be detected to make sure this happens. You also need to understand that Each packet contains three parts: the packet header, the payload and the
the data that is transmitted often needs to be protected during transmission. trailer.
This is because data is valuable, especially when it contains our personal
details and information that is personal to us. A hacker may try and steal the The packet header includes the destination address, the packet number
data during transmission so that they can use it for their own gain. One method and the originator’s address.
of protection that can be used when transmitting data is encryption. The payload is the data the sender wants to transmit.
Data before encryption takes place it is called plain text. An encryption The trailer contains data such as the error detection method to be used.
algorithm, called an encryption key, is used to scramble the data and make it
A process called packet switching can be used to send the data from one
meaningless. This meaningless data is called cipher text. The cipher text can
device to another across a network.
then be transmitted from one device to another. The receiving device uses the
key to decrypt the cipher text and return it to its plain text form. This means the Data is transmitted using serial or parallel transmission.
data will have meaning again if read. Data is also transmitted using simplex, half-duplex or full-duplex
One important thing to note about encryption is that is does not stop a hacker transmission.
stealing the data that is transmitted. It just means that the data that is stolen An interface called USB can be used to transmit data. This is often used
will be meaningless, as it will be encrypted and therefore scrambled. They will to connect hardware such as a keyboard to a computer.
not understand any of the data in the file.
Errors can occur when transmitting data due to interference.
There are two main methods of encryption that can be used symmetric and
Methods are required to detect any errors in transmission.
asymmetric.
A parity check is an error detection method that uses a parity bit to detect
The process for symmetric encryption is:
errors. An odd or even parity check method can be used.
• Plain text is encrypted into cipher text using an encryption key.
A checksum is an error detection method that uses a calculated value to
• The cipher text and the encryption key are sent separately to the receiving detect errors.
device. An echo check compares data that is sent and received to see if they
• The same key is then used to decrypt the cipher text back into its plain text match, to detect errors.
form. An ARQ can be used to monitor whether data is received correctly after
This is the simplest method of encryption and is still used for data that is not of transmission.
a high level of importance. It is sometimes used by people just wanting to A check digit is an error detection method that is used for data entry.
encrypt the data that they store on their own computer or external storage
device, as it isn’t being transmitted across a network. However, people began Data can be encrypted to keep it secure during transmission.
to worry that it could be too easy for a hacker to intercept both the cipher data Encryption can be performed using a symmetric or asymmetric method.
and the encryption key if it is sent across a network, or the internet, so a more
Symmetric encryption uses the same key to encrypt and decrypt the
secure method was developed called asymmetric encryption.
data.
The process for asymmetric encryption is:
Asymmetric encryption uses different key, a public and a private key, to
• Plain text is encrypted into cipher text using a public key. This is also a encrypt and decrypt the data.
type of encryption algorithm.
• The cipher text is transmitted to the receiving device.
• The cipher text cannot be decrypted using the public key, it is decrypted
using a private key.
A person who uses an asymmetric method of encryption has a public
encryption key. This is called a public key because it is made public for the
encryption process. Making something public means that anyone can see and
use it. Any device that wants to send you encrypted data may obtain your
public key and encrypt the data using your public key. You also have a private
key. Any encrypted data that your device receives that has been encrypted
with your public key can be decrypted with your private key. They act as a pair.
Your private key is always kept private though, your device does not make it
available to any other unless authorisation is given to do so.
The process can work in reverse as well (a private key is used to decrypt and a
public key is used to decrypt), and this is what can occur in processes such as
digital signatures. These are used in security protocols such as SSL (secure
sockets layer). You will learn more about these in Chapter 5, about applications
of the internet.

COMPUTER SCIENCE IN CONTEXT

A bank uses asymmetric encryption for all the banking transactions that
are completed on a daily basis. As you can imagine, the data that is
transmitted for these banking transactions is very sensitive and personal
data, so the bank has to make sure that they use a very secure method of
encryption for this process.

Questions
24 What is the name given to an encryption algorithm?
25 What is the name given to data that has been encrypted?
26 Does encryption stop data being stolen by a hacker? Why or why not?
27 Why is asymmetric encryption a more secure method of encryption than
symmetric?
EXAM-STYLE QUESTIONS SELF-EVALUATION CHECKLIST

After studying this chapter, think about how confident you are with the
1 Identify two items of data that would be included in the different topics. This will help you to see any gaps in your knowledge and
header of a packet of data. help you to learn more effectively.
[2]
2 Explain why and how packets of data are reordered after You might find it helpful to rate how confident you are for each of these
packet switching has been used to transmit data across a statements when you are revising. You should revisit any topics that you
network. rated ‘Needs more work’ or ‘Getting there’.
[2]
I can... See Needs Getting Confident
3 Describe how data is transmitted using serial half-duplex data
topic more there to move
transmission. [4] work on
4 Give two benefits of using the USB interface to connect describe the structure of a packet 2.1
hardware devices to a computer. [2] of data.
describe the process of packet 2.2
5 A company has a central file server that is located 500 m away from
switching.
the main office where employees work at their computers.
describe how data is transmitted 2.3
Employees send and retrieve files to and from the file server on a
using serial and parallel data
daily basis, often at the same time.
transmission.
Identify the type of data transmission that would be most
describe how data is transmitted 2.3
suitable for the given context. Explain why this would be the
using simplex, half-duplex and full-
most suitable method of transmission. [6]
duplex data transmission.
6 Four statements are given about error detection methods. choose a suitable data 2.3
Tick (✓) to show which statement applies to which error transmission for a given context
detection method. Some statements may apply to more than and explain why that is the best
one error detection method. [4] method of data transmission.
describe how data is transmitted 2.4
Statement Checksum Echo check Parity check using the USB interface.
This method checks for explain how errors can occur 2.5
errors in data after it has during data transmission.
been transmitted.
describe how a parity check uses 2.5
This method can use an a parity bit to detects errors.
odd or even process.
describe how a checksum value 2.5
This method sends data can be used to detect errors.
back from the receiver to
the sender to compare the describe how an echo check 2.5
data to check for errors. compares data to detects errors.

This method uses a value describe how an ARQ can monitor 2.5
that is calculated from the if data is received correctly.
data to check for errors. describe how a check digit can be 2.5
used to detect errors involving
7 The given paragraph describes how data is encrypted using data entry.
symmetric encryption. explain why data may need to be 2.6
Complete the missing terms in the paragraph using the given list. Not encrypted.
all terms need to be used. describe how data is encrypted 2.6
• a different using symmetric encryption.
• cipher text describe how data is encrypted 2.6
using asymmetric encryption.
• decrypted
• encrypted
• key
• lock
• meaningless
• plain text
• the same
• understandable
Before encryption the data is known as …………………………
. This data is encrypted using an algorithm that is known as a
………………………… . After the data has been encrypted it is
known as ………………………… . This type of data is
scrambled and ………………………… if it is stolen. When the
data is received it is ………………………… using
………………………… encryption algorithm. [6]