Paper 2 Theory

Question 1

Describe Serial Transmission

Answer 1

Bits are sent one at a time down a single wire

Question 2

Describe Parallel Transmission

Answer 2

Several bits are sent simultaneously down several wire

Question 3

Give the advantages of serial transmission over parallel transmission

Answer 3

Cost -> Only one wire is needed
No Crosstalk -> Interference between parallel wires can cause the corruption of bits, meaning data means to be transmitted
Distance -> Serial transmission is more reliable over long distances, as bits cannot come out of sync
Frequency -> Due to a lack of interference, signal frequencies can be higher, leading to a higher rate of data transfer

Question 4

Explain Synchronous Transmission

Answer 4

Sender and receivers clocks are synchronised for the whole transmission, with the sender controlling the speed of transmission. Once they are in sync, the data may be transferred without the need for further information exchange

Question 5

Explain asynchronous transmission

Answer 5

Sender and receivers clocks are not in permanent sync, but are instead only in sync for the duration of the transmission, with the data being sent with a start and stop bit

Question 6

What is the purpose of the start bit in asynchronous transmission?

Answer 6

Sent at the beginning of transmission, causing the receiver to synchronise it’s clock with the senders

Question 7

What is the purpose of the stop bit in asynchronous transmission?

Answer 7

Indicates that the data has arrived, giving the receiver time to process the information, and allowing the receiver to identify when the next start bit arrives

Question 8

What is the Baud Rate?

Answer 8

The rate at which the signal changes, with each different signal coding for a bit pattern

Question 9

What is the Bit Rate?

Answer 9

The speed at which data is transmitted, in bits per second
May be higher than the baud rate if multiple bits are encoding in each signal

Question 10

Bit rate = ?

Answer 10

Baud Rate * Number of bits per signal

Question 11

What is the Bandwidth?

Answer 11

The range of frequencies that the transmission medium can carry
The larger the bandwidth, the higher the bit rate, as more data can be sent across it in a given time

Question 12

What is Latency?

Answer 12

The dealy between a packet being sent and the same packet arriving at its’ destination

Question 13

What is a parity bit?

Answer 13

A bit that is added to a group of bits, used for error checking (To check if any of the bits have been flipped)

Question 14

Explain Odd Parity

Answer 14

Parity bit is set to a 1 or 0 to make the total number of 1’s in the bit pattern an odd number

Question 15

Explain Even Parity

Answer 15

Parity bit is set to a 1 or 0 to make the total number of 1’ in the bit pattern an even number

Question 16

What is a protocol, and why are they used?

Answer 16

A set of rules relating to communication between devices
Allows equipment from different suppliers to be connected on the same network, and communicate with each other

Question 17

What is the TCP/IP stack?

Answer 17

Transmission Control Protocol/Internet Protocol stack
A set of networking protocols for sending and receiving data packets across the internet

Question 18

What are the four layers of the TCP/IP stack?

Answer 18

Application layer, Transport Layer, Network Layer and Link Layer

Question 19

What is the function of the application layer

Answer 19

Uses protocols, relating to the application, to transmit data across the network

Question 20

What is the function of the Transport Layer?

Answer 20

Establishes a connection between the sender and the receiver
If data is too large to be sent in one go, breaks it up into segments (Packets made up of only the data)
Adds source and destination port numbers (Identify which application the data relates to)

Question 21

What is the function of the Network Layer?

Answer 21

Adds source and destination IP addresses, used to identify the two devices

Question 22

What is the function of the Link Layer?

Answer 22

Adds source + destination Mac addresses, which are fixed on each device

Question 23

What is encryption and what is a key?

Answer 23

The process of scrambling data, using a key, turning plain text into cipher text
A cryptographic algorithm used to encrypt or decrypt data

Question 24

Explain the difference between symmetric and asymmetric encryption

Answer 24

Symmetric - The same key is used to encrypt and decrypt the data
Asymmetric - Two different but related keys are used, one to encrypt and the other to decrypt

Question 25

What are sockets and what is their role in the TCP/IP stack?

Answer 25

A socket is the combination of the IP address and port number
It specifies which device the packet must be sent to, and the application being used on that device

Question 26

What is a MAC address, and what are their role?

Answer 26

The Media Access Address is a unique number that is burned into the Network Interface Card of each machine
They identify specific devices, meaning that once the packet finds the correct network, the MAC address can be used to find the correct machine

Question 27

What is an IP address?

Answer 27

A Internet Protocol address that is assigned to a network device, identifying it on the Internet
It indicates where a packet of data is to be sent or where it is sent from, allowing routers to direct the packet accordingly

Question 28

What is packet switching?

Answer 28

A method of communicating data across a network
Data is split into manageable chunks, and is then sent across the Internet

Question 29

What is usually contained in the header of a data packet, and what is the purpose of each?

Answer 29

Checksum - Used for error checking
Source IP address - Included if a response is needed, and/or to request packets be resent
Destination IP address - The packets final destination
Hop Limit - A limit to how many routers a packet may visit before it self-destructs
Sequence Number - Packets will arrive in the wrong order, so this is used to reassemble the data

Question 30

How is routing achieved across the Internet?

Answer 30

Packets are split up and each sent along the least congested route to the recipient

Question 31

What is the difference between a router and a gateway, and what is their purpose?

Answer 31

Router - Used to connect at least two networks using the same protocols
Gateway - Used to connect networks that use different protocols, and will translate between them
To move a data packet towards its’ final destination

Question 32

How does a firewall work?

Answer 32

Packet filtering - Packets are rejected/accepted based on things like their source IP address or what ports they are trying to access
Stateful inspection - Firewall maintains information about current connections, and only allows packets relevant to these connections
Proxy Server - Intercepts all packets entering and leaving the network, hiding the true IP address of the source from the recipient

Question 33

Explain how Public Key Private Key encryption works

Answer 33

First, the sender uses the receivers public key, made available to others, to encrypt the data they want to send
The public key cannot be used to decrypt the data
The data is then sent across the network to the receiver
The receiver then uses their private key, known only to them, to decrypt the data

Question 34

Explain how a digital signature works

Answer 34

First, a value is calculated from the unencrypted message.
The sender then encodes this value using their own private key, and attaches it to the message (Encrypted with the recipients public key)
When the receiver receives the message, they can recalculate the value
The receiver then decrypts the senders value, and if the two values are the same, the message must be correct, and is genuinely from the sender

Question 35

Explain the purpose of Domain Names, DNS and how it works

Answer 35

Domain Names are easier for humans to remember than IP addresses, and are ordered in a hierarchy of smaller domains
The Domain Name System catalogs all domain names and their corresponding IP addresses
When a web page is requested, the browser requests the corresponding IP address from the local DNS server
If it has it, it returns the IP address
If not, the DNS will send the request to a larger DNS database to try and find the IP address
Once the IP address is found, the user senders a request for the web page to that location

Question 36

What is the difference between a worm and a virus?

Answer 36

A virus needs it’s host program to be run to begin self replicating, whereas a worm does not