OCR GCSE Computer Science [Custom for my weaknesses]

CUSTOM DONT USE THIS ONE

What is the purpose of the CPU?

to fetch, decode and execute an instruction

What is the purpose of a computer?

An electronic device that takes an input, processes data and delivers an output.

Fixed vs Stored-program

Fixed does not change, i.e. basically embedded, while stored-program does change.

3 (main) CPU performance factors

Clock speed, Cache size, Cores (number of cores)

Cache

Fast access to frequently used instructions and data

Primary storage

Instructions and data needed while CPU is running

Secondary storage

Long term storage of data

Characteristics of RAM?

• OS, Programs and all data currently being used

• Temporary storage

• Volatile

• Physically close to CPU

• Orders of magnitudes faster than the fastest SSDs

• Read and Write

Characteristics of ROM?

• Holds the bootstrap

• Small piece of read-only memory

• On the motherboard

• Non-volatile

• Permanent software called firmware [UEFI used today]

What are the very first instructions for the computer called?

Bootstrap

BIOS full form

Basic Input Output System

How does the computer wake up all components

POST (Power On Self Test)

What is the name for the permanent software programmed into the ROM?

Firmware

What is it called when a SSD/HDD is overworked by moving information between the RAM and itself?

Disk Thrashing

Optical Storage Types

CD-R, CD-RW, DVD-R, DVD-RW, Blu-Ray

Optical storage advantages/disadvatanges

Advantages

• Easy to mass produce

• Thin lightweight and portable

Disadvantages

• Low capacity

• Slow to access data

Magnetic storage types

HDDs, Tape and Floppy Disks

Magnetic storage pros/cons

Advantages

• Extremely affordable

• High storage capacity

• Faster than optical

Disadvantages

• Moving parts, so not robust, will fail eventually and is noisy

• Defragmentation needed to be done regularly to prevent slowdown

Solid state storage types

SSDs, USB Drives, Flash Memory like SD cards

Solid State storage pros/cons

Advantages

• Extremely fast

• No need to defragment

• No noise

• Low power

• Very reliable (no moving parts)

Disadvantages

• Expensive

• Limited read-write cycles

SIX IMPORTANT PARTS FOR SUITABLE STORAGE

• CAPACITY

• SPEED

• COST

• PORTABILITY

• DURABILITY

• RELIABILITY

Why two states

• Easy to manufacture

• Therefore cheaper

• and more reliable

How is data stored in optical storage?

Lands and pits (0s and 1s)

How is data stored in flash memory?

By trapping electrons (1) or not (0)

1 Terabyte

~10^12 bytes

1 Petabyte

~10^15 bytes

Data capacity formula

Total storage / File size

Text file size formula

Bits per character * Number of character

Calculating image file size (bitmap)

colour depth x height x width

Calculating sound file size

bit depth x sample rate x duration

Sample rate

Number of samples taken every second

Bit depth

Number of bits needed to store a sample

Colour depth

Number of bits needed to store a single colour

Binary → Denary

Add together all the values

Is 128 at the left or at the right? [L R]

LEFT

Addition rules

0 + 0 = 0
0 + 1 = 1
1 + 1 = 0 (carry 1)
1 + 1 + 1 = 1 (carry 1)

Overflow error

When the computer tries to store a bigger number than it can handle, resulting in data loss or accuracy loss.

Denary → Hexadecimal

Denary → Binary → Split in 2 nibbles → Hexadecimals

Binary shift right [L R]

Divides by 2

Binary shift left [L R]

Multiplies by 2

Define character set (2 marks)

A defined list of characters (1) recognised by the computer (2)

ASCII bit length

7 bits old, 8 bits extended/new

UNICODE

24 bits

Why use unicode?

So we can include symbols and other languages

Metadata

Additional data stored with the (image) file (e.g. define the width, height, colour depth and colour palette)

Compression

Reducing the number of bits in a file

Lossy compression PROS/CONS

PROS

• Greatly reduces file size

• Suitable for images, sound and video

CONS

• Data loss (unrecoverable)

• Reduces quality

• Cannot be used for text or .exe files

Lossless compression

PROS

• No data loss, differently encoded

• Can be used on any file format

• Can be reverted to original

CONS

• Less effective than lossy compression

Networking pros

PROS

• Users can share files

• Share peripherals

• Access files from other computers

• Servers can control security, software updates and backup

• Communcation

Networking cons

CONS

• Increased security risks of malware

• If central server fails, downs the entire network

• Performance loss if lots of data on network

LAN (Local Area Network)

• Small geographical area

• All hardware owned by organisation

• UTP cable, fibre optic or wifi

WAN (Wide Area Network)

• Large geographical area

• Connects LANs

• Infrastructure owned by telecommunication companies

• Connected with telephone lines, fibre optic cables or satellite

• E.g. the Internet

5 factors affecting network speed

• Bandwidth

• Number of users

• Transmission media

• Error rate

• Latency

Bandwidth

The amount of data that can be sent/received at any given time, measured in bit rate

Transmission medias

Wired (UTP (copper), Fibre optic) Wireless

Error rate

Percentage of packets lost in networking

Factors in error rate

• Signal degredation (long cables/distances)

• Grade of material for cables

• Environmental factors

Client-server model

• A server controls access and security to one shared file store

• Server manages internet, printing jobs, emails, etc.

• Client makes request to server

Peer-to-peer model

• A peer is a computer on a network

• Serve their own files to each other

• Peer is responsible for own security and backup

• Peers need to be on and able to communicate

Pros/Cons of p2p model

PROS

• Easy to maintain

• No dependency on single component

• Cheap to setup

CONS

• Less secure network

• Users need to manage own security and backup

• Difficult to maintain well ordered file storage

Pros/Cons of client-server

PROS

• Central server managing files, security and backup

• Central server manages internet, emails, etc.

• Easy file storage, security and backup.

CONS

• Specialist staff may be required for maintenance

• Expensive cost

LAN Hardware

• NIC

• WAP

• Router

• Cables

Network Interface Controller (NIC)

Connects a device to a wired/wireless connection with a specific mac address. Uses a protocol to ensure successful communcation.

WAP

Allows wireless-enabled devices to access the network.

Switch

Uses NIC addresses to route traffic between devices

Router

Sends data between networks

Router’s need?

Needed to connect a LAN to a WAN

What does a router use to route traffic?

IP address(es)

UTP copper cable full form

Unshielded Twisted Pair (copper cable)

UTP vs Fibre optic

UTP is cheaper, Fibre optic has higher bandwidth and suffers from less interference

What does a DNS (Domain Name System) consist of?

Multiple Domain Name Servers

DNS resolver servers purpose

To translate a web address to an IP address

Where are websites’ web addresses stored and fetched from

A collection of DNS servers

What is web hosting?

When providers provide a place to host (make your website public) your website

What are the benefits of hosting?

• 24/7 access

• Greater security

• Multiple users

Cloud storage

Servers that store data and programs remotely

Star network

• Most popular wired network type

• Central switch

• All devices connect to the switch

• Broken cable only affects one device

• Intelligent switch only routes to desired device

Mesh network

• Devices connected to other devices

• If a break occurs, other connections available

• More costly than star network

Full mesh

• Every device connected to every other device

• Very expensive

Partial mesh

• Multiple connections

• Compromise solution

Ethernet

• network standard

• includes a number of protocols

What does a network frame consist of?

• Preamble of bits to sync

• Start frame deliminator for start of frame

• Source and destination MAC address

• Actual data

• CRC - Cyclic Redundancy Check

Wi-Fi advantages

• Free movement

• Convient

• Easy setup

• Can support large number of users

Problems with Wireless connectivity

• Slower speed than wired

• Obstructed signals

• Less secure than wired

Bluetooth

• Another wireless standard

• ideal for connecting personal devices

• very short range

• low power consumption

Why do wireless modes of data transmission use encryption

Since they broadcast data, it must be encrypted to be secure against interceptors.

How is the data scrambled?

Scrambling the data using a master key created from the SSID and password

Encryption protocols:

WEP, WPA, WPA2

MAC addresses

• used to route frames on a LAN

• Unique to every NIC

IP addresses

• used to route packets on a WAN as the destination

• IPv4 and IPv6

IPv4

• 4 partitions, e.g. 192.168.0.1

• 0-255

• Running out of IPv4 addresses

IPv6

• 128 bits

• 8 groups separated by colons

• Uses hexadecimal 0000-ffff

What are standards?

• set specification for Hardware and Software

• makes it possible for products and services to be compatible with each other

Why do we need standards?

Without standards we would not be able to

• successfully

• communicate or interact

Give 4 standards

• ASCII/UNICODE

• IEEE (cables)

• HTML

• PDF, TXT, etc.

• PNG, MP3, WAV, etc.

TCP

• Transmission Control Protocol

• Error free transmission between two routers

IP

• Internet Protocol

• Routes packets across a WAN

TCP/IP protocol stack

• protocol stack used to connect a device to the internet

HTTP(S)

• Hypertext Transfer Protocol (Secure)

• A client-server method of requesting/delivering HTML web pages

• HTTPS uses encryption for sensitive data

FTP

• File Transfer Protocol