COMPLETE 1.3 OCR A LEVEL COMPUTER SCIENCE

Why is compression necessary?

To reduce the storage space of files on disk - to speed up the sharing and transmission of data - to reduce buffering.

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Describe Lossy compression.

Reduces the size of files by removing all non-essential information.

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Describe Lossless compression.

Records patterns in data and creates a set of instruction on how to use the data. The file can then be reassembled with exact accuracy. No data is lost. This is a larger file than using Lossy compression, but still significantly smaller than original

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Describe Run Length Encoding.

Rather than recording every pixel in a sequence, it records its value and number of times it repeats.

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Describe dictionary-based compression.

Creates a table with each word being saved as a separate entry in a dictionary. These are then translated to binary accordingly, and may be translated back once received by the end user.

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Define encryption.

Transformation of data from one form to another to prevent an unauthorised third party from being able to understand it.

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How does the Caesar cipher work?

By shifting the letters of the alphabet along by a given number of characters; this parameter being the key.

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How does the Vernam cipher work?

It uses a one time pad, which must be equal or longer in characters than the plain text, must be truly random and of one time use only.

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Describe symmetric encryption.

The same key is used to encrypt and decrypt data. Requires both parties to have copy of the key and this couldn’t be transmitted over the internet or an eavesdropper monitoring the message may see it.

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Describe asymmetric encryption.

Uses two separate keys - public key and a private key. One key encrypts the data which can be publically distributed and a different key to decrypt it which is kept private.

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Describe hashing.

It provides a mapping between an arbitrary length input and a usually fixed length or smaller output. Used to generate digital signatures and certificates.

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Define relational database

A collection of tables in which relationships are modelled by shared attributes.

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What is the purpose of relational databases?

To avoid data duplication, inconsistency, redundancy and improve data integrity and security.

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Describe flat file database.

A single file data structure that is easy to maintain but of limited use due to dada inconsistency, duplication and redundancy.

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Define primary key

A unique identifier used to define every record in a table.

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Define foreign key

An attribute which creates a join between two tables. It is common in both tables and serves as a primary key in one of the two.

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Describe secondary key

Allows tables to be sorted and searched quickly and differently from the PK based on different attributes.

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Describe entity relationship modelling

Using diagrams to show relationships between tables, e.g. one-to-one tables

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Describe indexing

Creating an index of primary keys which is automatically maintained by the database software, giving the position of each record according to its primary key.

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Define normalisation

A process used to resolve many-to-many relationships to design data tables optimally. Minimises repetition to reduce data redundancy.

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What makes a table 1st normal form?

When there are no repeating attributes or groups of attributes. First normal form separates out multiple items of data in a row.

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When is a table in 2nd normal form?

When it is in its 1st normal form and contains no partial dependencies. 2nd normal form removes data items occurring in multiple rows into a new table linked by repeated fields.

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When is a table in 3rd normal form?

When it is in its 2nd normal form and contains no key-dependencies. All of those are removed to their own linked tables so every field is definable by key.

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Why is normalisation important?

Removes data redundancy - allows easier maintenance and modification of the database - allows faster sorting and searching of the database - makes sure no elements can be deleted accidentally.

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Describe referential integrity.

Keeping the database in a consistent state. Makes sure no component is deleted if it is used in links to existing data in another table.

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What is SQL?

A declarative database language that allows the creation, interrogation and alteration of a database

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Define transactions

Changes to the state of the database based on a set of rules called ACID.

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What does ACID stand for?

Atomicity, Consistency, Isolation, Durability.

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Describe Atomicity

A transaction must be processed in its entirety or not at all, never partially.

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Describe Consistency

No transaction can violate any of the defined validation rules for maintaining the integrity of the database.

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Describe Isolation

Concurrent execution of transactions leads to the same results as if transactions were processed one after the other.

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Describe Durability

Once a transaction has been committed, it will remain so no matter what.

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Describe record locking

A technique used to prevent simultaneous access to objects in a databases in order to prevent updates being lost or inconsistencies in the data arising.

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Describe data redundancy.

Repetition of data that leads to inconsistencies but might allow data recovery if part of the database is lost.

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What is the physical structure of the internet?

Backbone cables are connected by trans-continental leased lines across the sea beds. The national ISPs are connected to the backbone and distribute the internet connection to smaller providers, who in turn provide access to individuals

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What is the uniform resource locator (URL)?

The full address of an internet resource. Specifies the location of the resource, resource name and the file type.

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Describe internet registrars.

Hold records of all existing website names and the details of domains available for purchase.

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Describe internet registries.

Five global organisations with worldwide databases that hold records of all the domain names currently issued to individuals and companies, and their details.

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Define protocol.

A set of rules governing network communication between devices.

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Describe domain name system (DNS)

Domain name identifies the area that an internet resource resides in. These are structured into a hierarchy of top level domains to 3rd level domains. Each domain has one or more equivalent IP addresses.

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Define WAN

wide area network. Relies on third party carriers/connections. Typically spread over a large geographical area.

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Define LAN

A number of computing devices on a single site connected together by cables. Covers small geographical area.

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What are the topologies used by LAN?

Bus, Star, Mesh topologies

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Describe Bus topology

All computers are connected to a single cable. The ends of that cable are plugged into a terminator.

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Describe star topology

Consists of a central node which acts as a router to transmit messages. A switch keeps a record of the unique MAC address of each device on the network.

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Describe mesh network topology

Each node is connected to every other node, by transmitting data across any intermediate nodes. Only one node requires a connection to the internet and others share that connection.

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Describe circuit switching

Creates a direct link between two devices for the duration of the communication.

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Describe packet switching

A method of communicating packets of data across a network on which other similar communications are happening simultaneously.

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Describe data packets

Data to be transmitted is broken down into parts called packets. Each packet contains a header and a payload containing the body of data being sent.

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Define the TCP/IP protocol stack

A set of networking protocols that work together as four connected layers, passing incoming and outgoing data packets up and down the layers during network communication.

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

Application layer - Transport layer - Network layer - Link layer

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Describe the function of the application layer

Collects, packages and delivers data to and from users.

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Describe the function of the transport layer

Uses TCP to establish an end-to-end connection with the recipient computer.

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Describe the function of the network layer

Adds the source and destination IP addresses. Used in transmission of packets.

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Describe the function of the link layer

Acts as the physical connection between network nodes and adds the MAC addresses.

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Describe the file transfer protocol (FTP)

A method used to transfer data across the network. Works as a high level protocol in the application layer using appropriate software.

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Define firewall

A security checkpoint designed to prevent unauthorised access between two networks.

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What is the purpose of packet filtering?

Controls network access according to network administrator rules and sources by examining the source and destination IP addresses in packet headers

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Describe the role of proxy servers.

Intercepts all packets moving in and out the network, hiding the true network addresses of the source from the recipient, enabling privacy and anonymous surfing. Often used to filter requests providing administrative control over the content.

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What are the 3 most common threats to computers and what is their purpose?

Worms, Trojans, Viruses. They are designed to cause inconvenience, loss or damage to programs, data or computer systems.

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What are the differences between worms, trojans and viruses?

Viruses and worms can self-replicate. Viruses require a host, whereas the worms do not. Trojans cannot self-replicate.

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What affects the pagerank of a site?

– The number of sites that link to their site – The PageRank of the linking sites – The number of outward links from the site

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Describe client-server network

one or more computers (called clients) are connected to a powerful central computer (server)

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What are the advantages of client-server network?

Better security - Backups are done centrally - Data and resources can be shared.

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What are the disadvantages of client - server network?

Expensive to install and manage - Professional IT staff is required to maintain the servers and run the network.

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Describe peer - peer network

There is no central server. Individual computers are connected to each other so that they can share the files.

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What are the advantages of peer- peer network?

Cheap to set up - Enables users to share resources - easy to maintain

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What are the disadvantages of peer-peer network?

It supports piracy as it is impossible to trace the files which are being illegally downloaded.

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How does server-side processing work?

Search requests are sent to the server where they are applied to a database using SQL. The database search results are then sent back to the client browser.

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How does client-side processing work?

Data is processed on the client computer. It reduces the load on the server and the amount of web traffic.

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COMPLETE 1.3 OCR A LEVEL COMPUTER SCIENCE

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