3. Exchanging Data

Question 1

What is Compression? How do Lossy and Lossless Compression differ?

Answer 1

Compression is a family of techniques which reduce the size of data. Compression is important for saving system resources (Hard Disk space), and increasing the speed of data transmission over Networks.

Lossy Compression will lose some quality in the original data when compressing, but usually creates a higher Compression ratio (i.e. smaller files). The lost data may be exhibited as artifacts in image files. Lossless Compression loses none of the quality of the original data, but has a lower Compression ratio.

Question 2

How does Run Length Encoding Compress data?

Answer 2

Run Length Encoding (RLE) is used to compress data which has consecutive repetitions of the same symbol. It represents the data as a series of symbols and the number of times they repeat. For example the data:

AAAAAAAABBBBBBBCCCAAAAAABBCCCC

would be encoded as:

A8B7C3A6B2C4

Data, such as text, which does not have many consecutive repetitions will not compress well with RLE.

Question 3

How does Dictionary Compression Compress data?

Answer 3

Dictionary Compression is used to compress data which has repeating words (like text). The words are stored in a Dictionary, with each word allocated a unique numeric index. The text is then replaced by a sequence of indexes into the Dictionary. This will compress the text if words repeat often.

Question 4

What is Encryption? What is the difference between Symmetric and Asymmetric Encryption?

Answer 4

Encryption is a family of techniques used to hide the content of a message from third parties. Symmetric Encryption uses the same Encryption Key both for encrypting and decrypting the message. Asymmetric Encryption uses a different Encryption Key for encrypting than for decrypting the message. Symmetric Encryption requires the sender to pass the key to the receiver, when it may be intercepted, this is called the Key Exchange Problem.

Question 5

How does the Caeser Cipher work?

Answer 5

A Caeser Cipher is a shift cipher which changes each character by moving a certain number of characters to the left or right. The number of places to move each character is the Encryption Key, and this is a type of Symmetric Encryption. For example, the text ‘Hello World’ shifted by 3 characters would give the ciphertext ‘Khoor Zruog’.

Question 6

How does Public Key Encryption work?

Answer 6

Public Key Encryption is an Asymmetric Encryption Technique commonly used on the Internet. Each device will have a Public Key which is freely available to all devices on the Network, and a Private Key which is kept secret. Both the Public Key and the Private Key are needed to encrypt and then decrypt the message. For example, if the Public Key is used to encrypt a message, then the Private Key must be used to decrypt it (and vice versa).

If device A is sending a message to device B, then it encrypts the message with device B’s Public Key, then transmits the encrypted message, and finally device B can decrypt it with its own Private Key.

Because the Private Key never needs to be shared, this solves the Key Exchange Problem.

Question 7

How are Checksums used?

Answer 7

Checksums are used for Error Detection.

A Checksum is a value (number) calculated using a mathematical function (a Hash Function) from the data to be transmitted. The Checksum is sent to the receiver along with the data. The receiver then applies the same mathematical function to the received data, and if the calculated Checksum matches the received Checksum then no error has occurred in transmission.

Question 8

What is a Flat File Database?

Answer 8

A Flat File Database consists of a single file which stores an entire dataset. A spreadsheet is an example of a Flat File Database.

Question 9

What is a Relational Database?

Answer 9

A Relational Database is a Database which stores information in Relations/Tables. Each Relation will consist of Records/Rows and Attributes/Columns.

Question 10

What is SQL?

Answer 10

SQL is Structured Query Language, a language for communicating with Relational Databases.

Question 11

What is a Primary Key?

Answer 11

A Primary Key is a unique identifier for each Record in a Relation.

Question 12

What is a Foreign Key?

Answer 12

A Foreign Key is an identifier for a Record in another Relation.

Question 13

What is a Composite Key?

Answer 13

A Composite Key is a Key (Primary or Foreign) which consists of multiple Attrributes.

Question 14

What is First Normal Form?

Answer 14

A Relation is in First Normal Form if it has no repeating Attribute or group of Attributes.

Question 15

What is Second Normal Form?

Answer 15

A Relation is in Second Normal Form if it is in First Normal Form and there are no Partial Key Dependencies.

A Partial Key Dependency is when an Attribute is dependent upon only part of a Composite Key.

Question 16

What is Third Normal Form?

Answer 16

A Relation is in Third Normal Form if it is in Second Normal Form and there are no Non-Key Dependencies.

A Non-Key Dependency is when an Attribute is dependent upon another Attribute which is not part of the Primary Key.

Question 17

What is Referential Integrity?

Answer 17

Referential Integrity refers to how Foreign Keys reference Records in other Tables. If the Record which is referenced does not exist then the Referential Integrity of the Database has been broken. This can happen when a Record is deleted from a Table. There are various methods for enforcing Referential Integrity including ‘Cascade on Delete’, where Records referencing a deleted Record are also deleted.

Question 18

What does ACID refer to?

Answer 18

Atomicity - all transactions must be fully processed, or not at all.

Consistency - no transaction can violate any of the validation constraints on the Database, for example rules enforcing Referential Integrity.

Isolation - there will be no difference in the resulting dataset between executing transactions in parallel (concurrently) or sequentially.

Durability - once a transaction is completed, the result of the transaction is permanent.

Question 19

Why do Databases implement Record Locks?

Answer 19

Record Locks prevent multiple users from changing the same Record in a Relation at the same time which can lead to inconsistent results or corrupt data.

Question 20

What is DeadLock?

Answer 20

DeadLock is when cyclical dependency appears between the Record Locks that users have created whilst interacting with a Database. For example user A may have locked Table 1 whilst waiting for user B to unlock Table 2, whilst user B is waiting for Table 1 to be unlocked to continue.

Question 21

What is a Network?

Answer 21

A Network consists of Hardware which allows computers and other devices to communicate with one another.

Question 22

What is the difference between a LAN and a WAN?

Answer 22

A LAN is a Local Area Network, a Network over a small geographical area.

A WAN is a Wide Area Network, a Network over a large geographical area.

Question 23

What is the Internet?

Answer 23

The Internet is the physical infrastructure which connects Networks together across the globe.

Question 24

What is Circuit Switching?

Answer 24

Circuit Switching is where communication is established as a direct end-to-end link between sender and receiver. An example of Circuit Switching is the public telephone system. This is an inefficient use of Network infrastructure, because there is often downtime when no data is being sent.

Question 25

What is Packet Switching?

Answer 25

Packet Switching is a method for sending data over Networks where messages are split into small Data Packets which are individually Routed across the Network to the destination.

Question 26

What might you find in a Data Packet?

Answer 26

A Data Packet will consist of a Header, Payload, and sometimes a Trailer. The Payload is the data itself.

The Header will contain:
Sender’s IP Address
Recipient’s IP Address
Protocol
Packet Number
Time To Live

The Trailer may contain a Checksum or other Error Detection information.

Question 27

What is a Protocol?

Answer 27

A Protocol is a set of rules which define how communication happens between two devices.

Question 28

What are the characteristics of a Bus Topology Network?

Answer 28

A Bus Topology Network consists of a shared backbone cable over which all connected devices communicate. Transmissions are broadcast on the cable and can be seen by all devices, which is bad for data security. Only a single communication can happen on the cable at any one time, which means Bus Topology Networks decline in performance as more devices are added. If the cable fails the Network stops functioning, a single point of failure. Because only a single device can broadcast at any one time, the CSMA/CD (Carrier Sense Multiple Access with Collision Detection) Protocol is used to manage the use of the shared cable.

Question 29

What are the characteristics of a Star Topology Network?

Answer 29

A Star Topology Network consists of a central Switch which is connected by cables to each of the devices in the Network. The Switch directs packets of data to the correct device. Each device has its own dedicated cable, and so no collisions can occur and only devices involved in a communication see the packets. Cable failure means that only a single device is removed from the Network. Data can be sent simultaneously between multiple devices. If the switch fails the Network will also fail.

Question 30

What are the characteristics of a Mesh Topology Network?

Answer 30

A Mesh Topology Network consists of nodes (devices) which have direct connections between each other. Packets are routed from node to node until they reach their destination if there is no direct connection between sender and receiver. Mesh Topology Networks are usually Wireless. Mesh Topology Networks have no central point of failure, and are easy to expand.

Question 31

What is WiFi? What Hardware does a WiFi Network require?

Answer 31

WiFi (Wireless Fidelity) is a technology for wireless Network communication.

Each device on a WiFi Network will require a Wireless Network Adapter. The Network also requires a Wireless Access Point (WAP) to coordinate the traffic in the Network.

Question 32

What is a URL?

Answer 32

A URL (Uniform Resource Locator) is an address for a Resource (e.g. Web Page, Image File etc.) on the Internet. An example of a URL is below:

https://www.google.com/search?q=hello+world

Question 33

What is a Domain Name?

Answer 33

A Domain Name is a human readable address which can be used instead of an IP Address. For example www.google.com is the Fully Qualified Domain Name for Google. The Top Level Domain is ‘com’, the website name is ‘google’, and the host is ‘www’.

Question 34

What is the Domain Name System?

Answer 34

The Domain Name System consists of Domain Name Servers which maintain the mapping between Domain Names and IP Addresses used for Routing.

Question 35

What is a MAC address?

Answer 35

A MAC address is a unique identifier for a Network device which is assigned by the manufacturer and cannot change.

Question 36

What is a Port Number?

Answer 36

A Port Number is a number appended to an IP Address which identifies the Application that the Data Packet is intended for. For example:

43.56.23.1:8080

identifies Port Number 8080 which is a Web Server (HTTP).

Question 37

How many bits are there in an IPV4 address?

Answer 37

An IPV4 address looks like this:

53.76.23.1

each of the individual numbers is in the range 0-255, which requires 8bits. So the whole IPV4 address is 32bits in size. There are 2^32 possible addresses using IPV4.

Question 38

How many bits are there in an IPV6 address?

Answer 38

An IPV6 address looks like this:

FE80:CD00:0000:0CDE:1257:0000:211E:729C

it consists of 8 groups of 4 digit Hexadecimal numbers. Each of these groups is 16bits in size, so the whole IPV6 address is 128bits in size. There are 2^128 possible addresses using IPV6.

Question 39

What order are the layers in the TCP/IP Stack?

Answer 39

Application
Transport
Network
Link

Question 40

What Protocols might you find at the Application Layer of the TCP/IP Stack? What happens at this layer?

Answer 40

The Application Layer is concerned with the purpose of the communication. Below are some Application Layer Protocols and what they are used for:

HTTP(S) - The Web Protocol for communicating with Web Servers
FTP(S) - File Transfer Protocol for communicating with a File Server.
POP - Post Office Protocol used to download Email from an Email Server.
SMTP - Simple Mail Transfer Protocol used to send Email between Email Servers.
IMAP - Internet Messaging Access Protocol used for managing Email on an Email Server

Question 41

What Protocols might you find at the Transport Layer of the TCP/IP Stack? What happens at this layer?

Answer 41

The Transport Layer is concerned with end-to-end transmission of data from sender to recipient. The message is split into Data Packets and the Port Number is added.

TCP - Transmission Control Protocol, slower more reliable connection which allows for resending missing Data Packets.
UDP - User Datagram Protocol, quicker less reliable connection with possible Data Packet loss.

Question 42

What Protocol might you find at the Network Layer of the TCP/IP Stack. What happens at this layer?

Answer 42

The Network Layer is concerned with routing Data Packets across Networks from sender to recipient. At this layer the Internet Protocol (IP) is used and IP Addresses are used for Routing.

Question 43

What happends at the Link Layer of the TCP/IP Stack?

Answer 43

At the Link Layer Data Packets are transmitted between devices on the same Network section. MAC Addresses are used to identify the sender and recipient.

Question 44

What does a Firewall do to improve Security?

Answer 44

A Firewall is either Hardware or Software which controls Network access to a device or Network. It performs the following functions:

Packet Filtering - the Firewall will prevent certain packets from passing through. This filtering will be applied using rules such as closing Ports or blacklisting certain IP Addresses.

Stateful Inspection - the Firewall examines the contents of Data Packets to identify whether they should pass through.

Question 45

What is a Proxy Server?

Answer 45

A Proxy Server is a device which hides the IP Address of its Clients from the devices they are communicating with. It does this by forwarding the Data Packets with its own IP Address as sender, and relaying Responses back to the Client.

Question 46

What is the difference between a Virus, a Worm and a Trojan?

Answer 46

All of these are Malware (Malicious Software).

A Virus is a self replicating file which hides itself inside of host files. Once the host file is executed the Virus will execute its code.

A Worm is a self replicating file which do not need to be directly executed in order to execute. They may take advantage of Network vulnerabilities to transmit themself to other devices.

A Trojan is Malware which pretends to be another kind of file in order to trick the user into executing the code.

Question 47

What is a Router, and what does it do?

Answer 47

A Router is a piece of Network Hardware which connects two Networks which use the same communication Protocols. A Router forwards a Data Packet into the connected Network identifying the quickest route for it to get to its destination (Routing). Data Packets will be Routed from Router to Router across the Internet (or inside a LAN/WAN) to get to their destination as quickly as possible. Routers takes into account Network Load (traffic) when identifying how to Route a Data Packet to its destination.

Question 48

What is a Gateway?

Answer 48

A Gateway is similar to a Router, but joins Networks which use different Protocols.

Question 49

What is HTML?

Answer 49

HTML is Hyper Text Markup Language, a language for describing Web documents. In a Markup Language the contents of a document are assocuiated with their role, for example: title, heading, or paragraph. HTML documents can contain links to other documents, which is why it is described as a web.

Question 50

What is CSS?

Answer 50

CSS is Cascading Style Sheets, a language for describing the visual elements of a document. CSS allows properties such as colour, font, and layout to be defined for elements in a HTML document.

Question 51

What is Javascript?

Answer 51

Javascript is a programming language used to define dynamic Client-side behaviour in HTML web documents. Client-side means that the Javascript code is executed on the Client Web Browser and not on the Web Server.

Question 52

What is the PageRank algorithm for? how does it work?

Answer 52

The PageRank algorithm is used by Google to provide search results from the World Wide Web. The documents in the web are nodes within a Directed Graph and the edges are the links between documents. The importance of a webpage is identified as a combination of the number of links to that page and the importance of the pages which are linking to it. This requires an iterative algorithm which converges on the ranking (PR(A)) of each document. The algorithm to calculate PR(A) is below:

PR(A) = (1-d) + d(PR(T1)/C(T1) + … + PR(Tn)/C(Tn))

The PR(Ti) values are the ranking of the documents which link to document A.
The C(Ti) values are the count of links from documents which link to document A.
d is a damping factor to prevent large changes in subsequent iterations.

Question 53

Describe how Client Server Network Applications work?

Answer 53

A Server consists of Software on a device that provides access to a resource. The resource may be files on a disk drive, a database, a printer amongst many other possibilities. In essence a Server provides a service to other computers called Clients. If a computer only runs Server Software then it is a Dedicated Server.

A Client will make a request to a Server which will process the request and create a response.

In Client-Server Network Applications, if the Server fails then the service it provides will stop functioning. Having all the data and Software related to a service on a single computer helps the management of that service.

Question 54

What is Client-Side Processing?

Answer 54

Client-Side Processing is the processing of data on a Client device in a Client-Server Network Appliction.

Question 55

What is Server-Side Processing?

Answer 55

Server-Side Processing is the processing of data on the Server in a Client-Server Network Application. Server-Side Processing if often used for sensitive information, for example payments processing.

Question 56

What distinguishes a Thin Client from a Thick Client?

Answer 56

A Thin Client does little or no processing, which is all done on the Server. A Thick Client participates by processing data locally.

Question 57

Describe how Peer-to-Peer Network Applications work?

Answer 57

In Peer-to-Peer Network Applications all the computers involved have an equal level of importance to the maintenance of the service. They may be acting in the role of a Server, a Client, or both at the same time. Resources are spread across all devices in the network. If any one computer is removed from the Network it shouldn’t affect the provision of the service to the other devices.

An example of Peer-to-Peer Networking is Bittorrent where files are distributed across multiple computers. Whilst a computer is downloading a file, it is also uploading the parts of the file it has already downloaded to other computers in the Network.