Unit 2 Data Transmission & Encryption

Question 1

What is the trailer in a data packet?

Answer 1

The trailer has data that indicates the end of the packet and a
checksum for the destination to check it transmitted correctly

Question 2

What is the payload in a data packet?

Answer 2

The payload is the contents that need to be sent

Question 3

What is the header in a data packet?

Answer 3

The header contains the destination address, originating
address (source address) and the packet number so that the
correct order is restored once it arrives

Question 4

What is packet switching?

Answer 4

As each packet may take a different route through
the network, they may arrive out of order

The destination computer or server will need to reorder the
packets into the correct order

The packet number is used to do this

Question 5

What is serial transmission?

Answer 5

Bits are sent one signal at a time over a single wire

One bit is sent at a time

Once the bit has been received, the next bit is sent

Question 6

What is parallel transmission?

Answer 6

In parallel transmission a number of wires are used
so that many bits may be sent at the same time

Question 7

What is wrong with parallel transmission?

Answer 7

1. Crosstalk may occur when a signal on one line
   creates a disturbance in another line ( The longer a line, the more likely it is for a crosstalk to occur.
2. Skew
   Each wire in a parallel cable has slightly
   different properties. Signals therefore arrive at slightly different times

The receiver must wait until all
lines are received until the next
set of bits may be sent

Transmission needs to be
over very short distances
to reduce the problem

Question 8

What are some advantages of serial transmission?

Answer 8

Serial cables only require one wire for the data itself

This means that far fewer wires are needed in the cable,
making it cheaper to make and smaller

This also allows for longer cables without the risks of greater
crosstalk and skew

Question 9

What are the features of the USB

Answer 9

Universal serial bus (USB) is a serial cable that is
commonly used to connect external
devices to computers, laptops,
tablets and smartphones

The USB cable may be plugged in or
removed whilst the device is turned on

To connect older cables, such as serial mice
or keyboards and parallel printers, the
computer first needed to be turned off

Question 10

What are some upsides of the USB?

Answer 10

USB 1 connections operate between
1.5 and 12 Mbps

Parallel cables operate at a much
slower 110 kbps

The later standard of USB 3 operates
up to 20 Gbps

Older parallel and serial cables
normally required small screws to
connect the cables

All sorts of devices could be connected
by simply installing the necessary drivers

Question 11

What are some drawbacks of the USB?

Answer 11

USB connections should only be
5 meters in length before a
USB hub is used

The data transfer speeds are
not fast enough to use in
internal devices such as CPU
and hard drive

The transfer of data with DDR4 RAM is
up to 200 Gbps

This is around 10 times faster than USB 3

Question 12

What is simplex transmission?

Answer 12

In simplex transmission, data travels in one
direction only

Question 13

What is Half-duplex transmission?

Answer 13

Data is able to travel in both directions along a single
connection, but it cannot travel at the same time

Question 14

What is Full duplex transmission?

Answer 14

In full duplex transmission, traffic is able to move in
both directions simultaneously using two cables

Question 15

What are transmission errors?

Answer 15

When data is transmitted, errors may
be introduced due to:

Electrical interference

Power surges

Synchronisation issues

Wear and tear on the cable or connectors

These errors may cause:

Data to be lost

Data to be gained

Data to be changed

Question 16

What is error checking?

Answer 16

Computers need methods to check for data
transmission errors caused by interference

These methods include the use of:

Parity bits

Check digits

Check sums

Echo checks and

Automatic Repeat reQuests (ARQ)

Question 17

What are parity bits?

Answer 17

The parity checking protocol determines whether bits in a transmission have been corrupted
Every byte transmitted has one of its bits allocated as a parity bit
The sender and receiver must agree before transmission whether they are using odd or even parity
If odd parity is used then there must be an odd number of 1’s in the byte, including the parity bit
If even parity is used then there must be an even number of 1’s in the byte, including the parity bit

Question 18

What are check digits?

Answer 18

A check digit is an additional digit at the end of a set
of other numbers designed to check for mistakes in
input or transmission

Printed books have a unique barcode with an ISBN
(International Standard Book Number)

Other products contain bar codes using formats such as UPC
(Universal Product Code)

Both codes make use of check digits to make sure
they have been scanned or entered correctly

Question 19

What are checksums?

Answer 19

When transmitting data, in particular files, it is
important to be able to check that the file received
has no errors introduced when it was transmitted

The file is run through an algorithm that produces a checksum
which it also sends

When the file arrives, the receiving computer generates the
checksum and compares it with the checksum that was sent

If the checksums don’t match, the file may have been
corrupted or altered in transmission

Question 20

What is an echo check?

Answer 20

An echo check acts in the same way as an echo
from a wall

The data is transmitted to the receiver

The same data is then echoed back to the transmitter

The transmitter compares the data with the original. If it isn’t
the same, the data is transmitted again

Question 21

What is an Automatic Repeat query(ARQ) and how is it used?

Answer 21

Automatic repeat requests
(ARQs) work as follows:

The sender first sends a block of data

The receiving computer then sends
back an acknowledgement

When the sender receives the
acknowledgement it sends the next
block of data

If the acknowledgement is not
received after a period of time, a
timeout occurs and the data is resent

This process continues until the
acknowledgement is received

Question 22

Why is encryption needed in data transmission?

Answer 22

Large amounts of personal data are now stored and
transmitted on computers

Data is also stored on computers and servers on the Internet
which may be hacked

If a hacker gains access to the data, they won’t be able to
understand it if it is encrypted

Question 23

What is encryption?

Answer 23

The purpose of encryption is to encode a file or data
into something that cannot be read or understood

Question 24

What is symmetric encryption?

Answer 24

In symmetric encryption, a key is first shared
between the sender and receiver

This key encrypts the data before it is transmitted

The receiver uses the key to decrypt the data

Question 25

What is asymmetric encryption?

Answer 25

In asymmetric encryption, two keys are created by
the receiver

The public key is accessible by anyone and used to
encrypt data

The private key is kept secret by the receiver and used to
decrypt the data

The technique is also known as public-key cryptography

This means that anyone can
encrypt the data, but only the
receiver can decrypt it

There is no need to share a secret key
as happens with symmetric encryption

Question 26

What is the shared key?

Answer 26

the secret key that is used to encrypt and decrypt
the data

Question 27

What is the private key?

Answer 27

the key used to decrypt data

Question 28

What is plaintext?

Answer 28

text that hasn’t been encrypted

Question 29

What is the public key?

Answer 29

a key that anyone may see and use to
encrypt data before it is sent to someone