Cambridge IGCSE Computer Science

HTTP - hypertext transfer protocol

a set of rules which must be followed when transferring files across the internet

URL - uniform resource locator

used to find/identify a webpage on the internet 1. webserver name 2. file name 3. protocol

web browser

software that allows a user to display a webpage on their screen - they interpret html code and show the result

actuator

used in many control operations involving sensors and ADC/DACs. Electromagnetic device eg. motor that outputs result of sensor operation

loudspeaker

output device that produces sound using voltages at different levels which vibrate a cone in speaker housing

LED

Used for backlighting a monitor/tv - they are thin, produce a white light, and do not require much power

CCFL

Old technology used for backlighting - took time to warm up, and produced yellow-ish lighting + required a lot of power

volatile

memory that loses its contents when the power is off eg. main memory

non-volatile

memory that doesn't lose its contents when the power is off e.g. hard disk

off-line storage

storage that can be physically moved away from a device and stored somewhere else e.g. CD, USB stick, DVD

SRAM

type of RAM that has 'flip flops' to hold each bit of memory. Does not need constant refreshing.

magnetic media

secondary storage with mechanical parts which move over the surface to read and write data magnetically e.g. a hard disk, tape or floppy disk

program

our way of telling a computer what to do, how to do it, and when to do it

assembly language

code that is specific to a particular hardware, but needs to be translated into machine code before it can run

machine code

the binary code that is specific to a machine and does not need translating in order to run

translator

program that changes higher level language into machine code

compiler

this translates HLL into machine code. Once it has done this, the program can be used multiple times without translating it again

UPS

uninterruptable power supply

cracking

editing source code with malicious intent

firewall

this examines traffic between a user's computer and the internet by checking what is/isn't allowed and blocking traffic that does not meet certain criteria

proxy server

this filters web traffic, keeps the IP address secret, and, using a cache, speeds up access to websites that have been visited before

TLS - Transport Layer Security

this is a form of secure data transmission made up of 2 layers: the record protocol, and the handshaking protocol. It is very secure and fairly new, and can make use of session caching, which improves performance. It involves many complicated encryption algorithms. prevents third party hacking into communication between web server and the user's computer.

session caching

this is used in TLS to avoid using too much computer time; TLS is able to attempt to resume an existing session, which increases speed.

Denial of Service (DoS) attack

attempting to prevent a user from accessing part of a network (usually the server); this is usually temporary, but may be very damaging, or a big security breach. overloads web traffic by sending thousands of requests.

free software

software that is free to run, copy, change or adapt, and share with friends, family or colleagues.

CSS

Cascading Style Sheet. Controls presentation of page. Enable multiple pages or elements on a page to be changed in one go.

Gigabyte (GB)

1024 megabytes. 2 to the power of 30 bytes.

HTML

HyperText Mark-up Language. Used to write and develop webpages to present text. Uses tags.

Terabyte (TB)

1024 gigabytes. 2 to the power of 40 bytes.

Lossless image compression

Find groups of repeating data. Records the data once and the number of times it was repeated.

Lossless text compression

Finds patterns in original text. Encodes each pattern in a dictionary

PNG

Lossless picture file format that uses alpha channel to keep transparent backgrounds.

MIDI

A set of instructions for digital instruments to play synthesised sounds. Musical Instrument Digital Interface.

.WAV

uncompressed music files

.FLAC

lossless compressed music files

Parity bit

Set to 1 or 0. Sent with a byte of data to make the total number of 1s odd or even for each byte.

1KB (kilobyte)

1,024 bytes (2¹⁰ bits)

1MB (megabyte)

(2²⁰ bits)

1GB (gigabyte)

(2³⁰ bits)

1TB (terabyte)

(2⁴⁰ bits)

1PB (petabyte)

(2⁵⁰ bits)

A

10

B

11

C

12

D

13

E

14

F

15

Simplex

Data sent in 1 direction only

Asynchronous

Agreed bit pattern with control bits

Parity Check

Uses parity bits. Odd Parity / Even Parity agreed between sender and receiver. If it is not the agreed number, an error has occurred.

ARQ (Automatic Repeat Request)

Uses acknowledgement and timeout. If acknowledgement isn't sent back to sender before the timeout occurs, message automatically resent.

Checksum

Additional value sent at the end of a data block. When block is about to be transmitted, additional value calculated and then transmitted. Value recalculated at the end to make sure they match. If they don't match, an error has occurred.

Echo Check

Data sent to and from another device twice. Sender compares 2 sets of data, if the sets of data are different, an error has occurred.

Wardriving

Using wireless internet connections illegally through a portable device

Secure Sockets Layer (SSL)

When a user logs onto a website, it encrypts the data. Only the user's computer + web server can make sense of what's being said. A green padlock in the status bar.

Record Protocol

Can be used with or without encryption

Handshake Protocol

Permits the website and user to authenticate and make use of encryption algorithms which only the user and website know.

Encryption

Makes data meaningless unless recipient has the decryption key

Asymmetric Encryption

Private and public key are both needed. Encryption keys generated using a hashing algorithm.

Authentication

Used to verify data is coming from a trusted source.

MAC address

a number that uniquely identifies a device on the Internet. Often refers to network card. Rarely changed.

ISP (internet service provider)

company charging monthly fee that provides internet access

IP (internet protocol) address

assigned by ISP, unique to each internet session

MAC address

number which uniquely identifies a device - 1st half = manufacturer, 2nd = device itself - made up of 6 groups of hex digits

HTML Presentation

how the document looks/sounds

HTTPS

a SECURE set of rules which must be followed when transferring files across the internet

OLED

new technology used for tv/monitor screens, using a cathode and anode. Flexible, no backlighting required.

DLP

light projector using mirrors to produce 1024 different grey shades. A bright white light source passed through colour filter allowing 16million colours to be created.

ROM

memory that is hard coded at time of manufacture. Stores start-up programs, bios and BOOTSTRAP

secondary memory/storage

long-term, non volatile storage media e.g. hard disk

virtual memory

part of the hard disk that is programmed to work as an extension of main memory

DRAM

type of RAM where each chip consists of transistors and capacitors. Not too expensive to manufacture, has high storage capacity, and doesn't consume too much power. Needs refreshing every 15s

SRAM

type of RAM that has 'flip flops' to hold each bit of memory. Does not need constant refreshing.

bootstrap

the first program loaded from ROM by BIOS; loads operating system from secondary storage

optical media

secondary storage that is read using lasers e.g. CD, DVD

solid state/flash memory

secondary storage with no moving parts; used in memory sticks, cameras and phones

FAT - file allocation table

a map of where each file is and which sectors on the magnetic disk are free

high level language

portable language designed with the programmer in mind - it is easy to understand and debug, but it needs to be translated in order for a computer to understand it

low level language

language relating to specific architecture and hardware of a particular device - can refer to assembly lang or machine code

interpreter

this translates HLL into machine code one line at a time.

assembler

this translates assembly language into machine code. Once it has done this, the machine code can be used multiple time without translating it again.

wardriving

stealing someone's WiFi

spyware/key logging software

hardware or software that can be used to monitor key presses and gain information.

SSL

this is used on web browsers to allow data to be sent/received securely. It encrypts data so that only the sender and receiver can understand the information being transmitted

symmetric encryption

this uses a secret key, applied to the message, file that could be a combination of characters. The same key is needed by both sender and receiver to encrypt and decrypt the message

asymmetric encryption

this is a type of encryption using both a public and a private key. The public key can be seen by anyone, and used to encrypt messages to the receiver. The private key is used by the receiver to decrypt the message sent to them.

plain text

text/data before it has gone through encryption algorithm

cypher text

output from encryption algorithm

authentication

used to verify that data comes from a reliable source; used with encryption to strengthen internet security

computer ethics

this considers 3 factors; intellectual property rights, privacy issues, and a computer's effect on society. It is a set of principles that regulates the use of a computer

code of ethics

this is a set of rules set up by the ACM and IEEE to ensure that computers are used correctly, and to avoid plagiarism.

freeware

software that a user can download for free from the internet, but they are not allowed to edit the source code

shareware

software that users are allowed to try out, free of charge, for a certain period of time. It often lacks certain features that can be downloaded in the full, paid version.

ASCII

American Standard Code for Information Interchange is a character set. Each letter on the keyboard has a unique value that can be represented in hexadecimal or decimal.

Bit

Binary Digit. Either 1 or 0, On or Off.

Byte

Smallest unit of memory. A collection of 8 bits.

Check digit

A final digit calculated from all the other digits. Used to identify errors in mis-typing or mis-scanning a bar code.

Cookie

A file/packet of information sent by a web server to a web browser. Not a program. Store user preferences and creates anonymous user profile, and used in advertising. Resides on client machine, not server.

Full-Duplex Data Transmission

Data can be sent in both directions at the same time.

Half-Duplex Data Transmission

Data can be sent in both directions but only one at a time. e.g. walkie talkies