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AQA Computing A2

Computing A2, AQA specification

QuestionAnswer
Database A structured collection of data.
Database management system A software system that enables the definition, creation, and maintenance of a database and provides controlled access to this database.
Data model A method of describing the data, it's structure, the way it is interrelated and the constraints that apply to it for a given system or organisation.
Conceptual Model A representation of the data requirements of an organisation constructed in a way that is independent of any software used to construct the database.
Entity An object, Person, event, or thing of interest to an organisation and about which data is recorded.
Relation A set of attributes and tuples, modelling an entity (A table).
Attribute A property or characteristic of an entity.
Tuple A set of attribute values (A row in a table).
Primary Key An attribute that uniquely identifies a tuple.
Relational Database A collection of tables.
Composite Key A combination of attributes that uniquely identify a tuple.
Foreign Key An attribute in one table that is a primary key in another table.
Normalised entities A set of entities that contain no redundant data.
Normalisation A technique used to produce a set of normalised entities.
Data transmission Movement of data from one place to another.
Serial data transmission Single bits are sent one after another along a single wire.
Parallel data transmission Bits are sent down several wires simultaneously.
Baud rate The rate at which signals on a wire or line may change.
1 Baud One signal change per second.
Bit rate The number of bits transmitted per second.
Bandwidth For transmission medium; This is the range of signal frequencies it may transmit.
Asynchronous serial data transmission The arrival of data cannot be predicted by the receiver, so a start bit is used to signal the arrival of data and to synchronise the transmitter and receiver temporarily.
Communication Protocol A set of pre-agreed signals, codes, and rules to be used for data exchange between computers.
Handshaking protocol The sending and receiving devices exchange signals to establish that the receiving device is connected and ready to receive.
Baseband system A system that uses a single data channel in which the whole bandwidth of the transmission medium is dedicated to one data channel at a time.
Broadband system A multiple data channel system in which the bandwidth of the transmission medium carries several data streams at a time.
Local area network. (LAN) Linked computers are close in proximity.
Stand-Alone computer A computer that is not networked.
Topology In the context of networking: The shape, the layout, configuration or structure of the connections that connect devices to the network.
Wide area network. (WAN) A set of links that connect geographically remote computers and LANs.
Internet A collection of LANs and computers that are interconnected by a WAN.
Thin-Client Network A network where all processing takes place in a central server; The clients are dumb terminals with little or no processing power or local hard disk storage.
Peer-to-peer network A network that has no dedicated servers. All computers are equal, so they are called peers.
Client A computer that uses the services provided by a server.
Server A computer that provides shared resources to network users.
Server-based Network A network in which resources security and administration and other functions are provided by dedicated servers.
Web 2.0. Software that becomes a service accessed over the internet.
Web services Self-contained modular applications that can be described, published, located and invoked over a network.
Ajax A web technology that allows only the part of a webpage that needs updating to be fetched from the server.
Wireless network Any type of LAN in which the nodes (Computers or computing devices) are not connected by wires, but use radio waves to transmit data.
Bluetooth A wireless protocol for exchanging data over short distances from fixed and mobile devices.
Router A device that receives packets from one host (computer) or router and uses the destination IP address the packets contain to pass them to another host or router.
Gateway A device used to connect networks using different protocols so that information can be passed from one system to another.
Dynamic web page content Content that is generated when the web browser request is received.
Virus A small program attached to another program or data file. It replicated itself by attaching itself to other programs.
Worm A small program that exploits a network security weakness to replicate itself through computer networks.
Remote Login When someone connects to a computer via the internet.
Trojan A program that hides as desirable software, such as utility or a game, but attacks computers it infects.
Phishing When someone tries to get you to give them your personal information.
Encryption Using an algorithm and a key to convert the message data into a form that is not understandable without the key to decrypt the text.
Plain text Message data before it is encrypted.
Cipher text Message data after it is encrypted.
Decryption Using an algorithm and a key to convert encrypted message data into its plain text equivalent.
Cryptography the science of designing cipher systems.
Abstraction Representation that is arrived at by removing unnecessary details.
Algorithm A sequence of unambiguous instructions for solving a problem.
Computational complexity of a problem Measures how economical the algorithm is with time and space.
Time complexity of an algorithm Indicates how fast an algorithm runs.
Space complexity of an algorithm Indicates how much memory an algorithm needs.
Complexity of a problem Taken to be the worst-case complexity of the most efficient algorithm which solves the problem.
Basic operation The operation contributing the most to the total running time.
Order of growth Assesses by what factor execution time increases when the size of the input increases.
Asymptotic behaviour of f The behaviour of the function f(n) for very large values of n.
O(g) Called big O of g, represents the class of functions that grow no faster than g.
Order of complexity The order of complexity of a problem is its big O complexity.
Exponential time algorithm An algorithm whose execution time grows exponentially with input size.
Polynomial time algorithm An algorithm whose execution time grows as a polynomial of input size.
Linear time algorithm A polynomial time algorithm that executes in O(n) time.
State transition diagram A directed graph whose nodes represent the states. An edge leading from state s to state t is labelled with a symbolic code, e.g. a|b. The a part is called the transitions trigger, and denotes the input symbol. The b part denotes the output symbol.
Finite state machine (FSM) Consists of a set of symbols (Input symbol alphabet), and if it produces output, a set of output symbols, a finite set of states and a transition function that maps a state-symbol pair to a state, and possibly generates an output.
Transition function Maps (input symbol, current state) to (output symbol, next state).
Transition table Tabulates the mappings (input symbol, current state) to (output symbol, next state).
Deterministic finite state machine A FSM that has just one next state for each pair of state and input symbol.
Non-deterministic finite state machine A FSM that may have several possible next states for each pair of state and input symbol.
Halting state A state that has no outgoing transition.
Mealy machine A FSM that determines its outputs from the present state and from the inputs.
Moore machine A FSM that determines its outputs from the present state only.
Turing machine A FSM that controls one or more tapes, where at least one tape is of unbounded length.
Non-computable Describes an algorithmic problem that admits no algorithm.
Decision problem A yes/no Algorithmic problem.
Decidable Describes a decision problem that has a yes/no answer.
Undecidable describes a decision type algorithmic problem that is non-computable.
Tractable Describes a problem that has a reasonable (polynomial) time solution as the size of the input increases.
Intractable Describes a problem for which no reasonable (polynomial) time solution has yet been found.
Heuristic Describes an approach that uses know-how and experience to make informed guesses that assist in finding a polynomial time solution to an intractable problem.
Halting problem Is it possible in general to write a program that can tell, given any program and its inputs and without executing this program, whether the given program and its inputs will halt?
Regular language Any language that a FSM will accept
Regular expression A notation for defining all the valid strings of a formal language or a special text string for describing a search pattern.
Backus-Naur form (BNF) A notation for expressing the rules for constructing valid strings in a regular language.
Recursive definition _________ __________ Is one that is defined in terms of itself.
Object An instance of a class.
Instantiation An object is defined based on class.
Class definition. A pattern or template that can be used to create objects of that class.
Encapsulation Combining a record with the procedures and functions that manipulate it to form a new data type, a class.
Inheritance Defining a class and then using it to build a hierarchy of descendant classes with each descendant inheriting access to all its ancestors code and data.
Polymorphism Giving an action one name that is shared up and down a class hierarchy. Each class in the hierarchy implements the action in a way appropriate to itself.
Recursive routine A routine defined in terms of itself.
General case The solution in terms of itself for a value of n.
Base case A value that has a solution which does not involve any reference to the general case solution.
List A collection of elements with an inherent order.
Abstract data type (ADT). A data type whose properties are specified independently of any particular programming language.
Pointer A variable that contains an address. The pointer points to the memory location with that address.
Null pointer. A pointer that does not point to anything, usually represented by -1.
Pointer type A variable of pointer type stores an address of data value.
Heap The memory locations available to application programs for dynamic allocation.
Dynamic allocation Memory space that is only allocated when required at run time.
Memory leakage Successive calls to allocate memory space are made, but memory locations that are no longer required are not released. Eventually no memory is left in the heap.
Dynamic data structure the memory taken up by the data structure varies at run time.
Static data structure The memory required to store the data structure is declared before run time.
Stack A last-in first-out (LIFO) abstract data type.
Queue A first-in first-out (FIFO) abstract data type.
Circular queue When the array element with the largest possible index has been used, the next element to join the queue reuses the vacated location at the beginning of the array.
Linear queue Elements join the queue at one end, and leave the queue at the other end.
Priority queue Each element of a priority queue has an associated priority.
Graph A diagram consisting of circles, called vertices, joined by lines called edges, or arcs; each edge connects exactly two vertices.
Neighbours Two vertices are neighbours if they are connected by an edge.
Degree Degree of a vertex, the number of neighbours for that vertex.
Labelled or weighted graph A graph in which the edges are labelled or given a value called a weight.
Automation Turning an abstraction into a form that can be processed by a computer.
Directed graph (digraph) A diagram consisting of circles, called vertices, joined by directed lines, called edges.
Simple graph An undirected graph without multiple edges and in which each edge connects two different vertices.
Closed path or circuit A sequence of edges that starts and ends at the same vertex and such that any two successive edges in the sequence share a path.
Cycle A closed path in which all the edges are different
Tree A connected undirected graph with no cycles.
Rooted tree A tree in which one vertex has been designated the as the root and every edge is directed away from the root.
Linear search This search method starts at the beginning of the list and compares each element in turn with the required value until a match is found or the end of the list is reached.
Bubble sort During a pass through the list, neighbouring values are compared and swapped if they are not in the correct order. Several passes are made until one pass does not require any further swaps.
Real Number A number with a fractional part.
Scientific notation A real number represented by a sign, some significant digits, and a power of 10.
Significant digits. Those digits that carry meaning contributing to the accuracy of a number. This includes all digits except leading and trailing zeros where they serve merely as placeholders to indicate the scale of the number.
Floating-point notation A real number represented by a sign, some significant digits, and a power of 2.
Precision The maximum number of significant digits that can be represented.
Absolute error the difference between the actual number and the nearest representable value.
Relative error the absolute error divided by the actual number.
Underflow The value is too small to be represented using the available number of bits.
Overflow The value is too large to be represented using the available number of bits.
System program A program that manages the operation of a computer.
Operating system The most fundamental of all system programs.
Virtual machine The apparent machine that the operating system presents to the user, achieved by hiding the complexities of the hardware behind layers of operating system software.
Application programming interface A layer of software that allows application programs to call on the services of the operating system.
Interactive operating system an operating system in which the user and the computer are in direct two-way communication.
Real-Time operating system Inputs are processed in a timely manner so that the outputs can affect the source of the inputs.
Network operating system A layer of software is added to the operating system of a computer connected to a network. This layer intercepts commands that reference resources elsewhere on the network, then redirects the request to the remote resource unknown to the user.
Desktop operating system An operating system that allows a user to carry out a broad range of general-purpose tasks.
Client-server system A system in which some computers, the clients, request services provided by other computers the servers.
Server operating system An operating system optimised to provide one or more specialised services to networked clients.
Created by: arwfrmhvn