Please enable JavaScript.

Coggle requires JavaScript to display documents.

Computer Science GCSE - Coggle Diagram

- - - - concatenation joins strings together using the + operator
      - LEN(string) returns the length of the string
      - POSITION(string, character) returns the first position of a character in the string, starting from 0
      - SUBSTRING(x, y, string) extracts a substring from string starting at x and ending at y
  - - - constants remain the same throughout the program
      - variables can change throughout the program
    - - constant [NAME] = 30
      - var [name] = 30
  - - - Arithmetic Operators
        
        addition +
        
        subtraction -
        
        multiplication *
        
        division /
        
        integer division // or DIV
        
        modulus % or MOD
      - Data Type Operators
        
        pseudocode - STRING_TO_INT('1')
        
        python - str(1)
      - Comparison Operators
        
        equal to ==
        
        not equal to ≠ or <> or !=
        
        less than <
        
        greater than >
        
        less than or equal to ≤ or <=
        
        greater than or equal to ≥ or >=
      - Assignment Operator =
    - - BOOL - 1 byte (1 bit required)
      - CHAR - 1 byte
      - REAL - 4 or 8 bytes
      - STRING - 1 byte / character
      - INT - 2 or 4 bytes
- - - - they're normally centred around a single person and connect devices such as phones, watches and headphones
      - PANs often use Bluetooth, which has a strong signal but short range, making it good for connecting close devices
      - a personal area network connects devices over a very short range
      - PANs don't require any additional hardware, meaning you can also create a PAN on the move
    - - a WAN connects LANs in different geographical places
      - a wide area network
      - the Internet is actually one giant WAN
      - a WAN can be connected using telephone lines, satellite links or radio links
      - organizations hire infrastructure from telecommunications companies who own and manage the WAN, as it is much more expensive
    - - all the hardware is owned by the organisation that uses it
      - LANs can be wired (ethernet) or wireless (Wi-Fi)
      - covers a small geographical area on a single site
      - homes schools and businesses have LANs to connect devices, such as printers, smart TVs and PCs
    - - they can be vulnerable to hacking, and malware can easily spread between devices
      - if server dependent networks go down it can be very disruptive
      - they can be expensive to set up, as they can require lots of extra hardware
      - large networks are difficult to manage and may require a specialist
    - - you can mass install and update software on devices all at the same time
      - you can communicate cheaply (e.g. with email)
      - user accounts can be stored centrally, so users can log in from any device on the network
      - you can share hardware (e.g. printers)
      - you can share files easier, as well as collaborate and copy files between devices
    - - Cabling Examples
        
        CAT 5e & CAT 6
        
        cheaper than fibre optic cables and have a decent bandwidth
        
        commonly used in homes and offices to connect devices to a LAN
        
        contains pairs of twisted copper wires to reduce internal interference
        
        common types of ethernet cable
        
        coaxial
        
        tend to be very cheap, as they have a very low bandwidth
        
        surrounded by a plastic layer for insulation and a metallic mesh to provide shielding from outside interference
        
        contains a single copper wire
        
        fibre optic
        
        doesn't suffer interference
        
        can transmit over long distances at a high bandwidth without loss of signal quality
        
        high performance but expensive
        
        transmit data as light
      - routers transmit data between different networks, and are used to connect to the internet. most 'routers' are actually a router, switch and WAP
      - a network interface card (NIC) is a piece of hardware inside a device that allows it to connect to networks, that exist for both wired and wireless connections
      - switches are used to connect devices on a LAN
  - - - star topology networks can be wired or wireless
      - pros
        
        they have the best performance as data all goes to a central device, meaning all devices can transmit data at the same time
        
        there are fewer data collisions, resulting in less packet loss
        
        its simple to add new devices to the network, as each one uses a separate cable
        
        if a device fails or a cable is disconnected, the rest of the networks is unaffected
      - the central switch allows many devices to be connected simultaneously
      - cons
        
        if there is a problem with the switch the whole network is affected
        
        there is a maximum possible number of connections determined by the switch
        
        the switch itself is also an expensive piece of hardware
        
        if you need more you may have to buy another switch
        
        in a wired network, every device needs to be physically cabled to the switch, making it very expensive
      - all devices are connected to a central switch or server that controls the network
    - - two terminators are placed at each end of bus to stop data reflecting back
      - without the terminators, the network would suffer interference and be potentially unstable
      - bus topologies use a single 'backbone' cable to connect all devices (a bus)
      - pros
        
        the network is unaffected if a device fails
        
        it isn't dependent on a central switch to keep the whole network running
        
        relatively cheap to set up, as there is much less wiring needed, and the terminators and bus are much cheaper than a switch
      - cons
        
        data collisions are common, meaning data must be resent, slowing the network down
        
        to avoid collisions, devices must wait for the bus to be available before sending data, slowing it down
        
        the more devices you add, the more common collisions are, making it unsuitable for large networks
        
        if the bus is broken (mice etc.) the network is split
        
        a lack of terminators at both ends means lots of data is reflected, shutting down the entire network
  - - - its very easy to add new devices, no extra cabling is required
      - can be cheaper and more environmentally friendly as they don't require wires
      - convenience, as you can connect automatically, and move around
    - - they generally have a lower bandwidth and are less reliable than wired networks
      - distance from WAP, interference from other wireless networks, and physical obstructions can all reduce signal strength
      - WAPs are visible to all devices, making them less secure and allowing hackers to gain access
    - - wireless networks use radiowaves to transmit data
      - a wireless network requires a wireless access point (WAP)
      - generally these are referred to as Wi-Fi, however this is actually a specific family of WLAN protocols
      - this is essentially a switch that allows devices to connect wirelessly
      - local wireless networks are called WLANs (wireless LANs)
      - hotspots are locations where you can connect to a WAP
      - to connect wirelessly, devices need the necessary hardware, which most do
      - if a device doesn't have the necessary hardware, it can still connect using a dongle
      - dongles
        
        USB dongles can be plugged into a computer to allow it to access the internet
        
        HDMI dongles can use wireless networks to stream high quality video to a TV
  - - - FTP
        
        File Transfer Protocol
        
        used to access, edit and move files between devices on a network
      - IMAP
        
        Internet Message Access Protocol
        
        used to retrieve emails from a server
      - HTTPS
        
        HTTP Secure
        
        a more secure version, encrypts all sent and received information
      - SMTP
        
        Simple Mail Transfer Protocol
        
        used to send emails and transfer emails between servers
      - HTTP
        
        Hyper Text Transfer Protocol
        
        used by browsers to access websites and communicate with web servers
      - responsible for things such as file, email and data transfer
    - - TCP
        
        establishes a connection between the sending and receiving devices
        
        splits data into numbered packets, so it can be reordered easily upon arrival
        
        communicates with the receiving device to verify correct packet transfer, if not packets are resent
        
        Transmission Control Protocol
        
        the sending device gets confirmation when the transfer is complete
        
        this makes it better for a reliable connection, where uncorrupted packets is important, e.g. file downloading
        
        it also means that if some packets go missing, the whole file doesn't need to be sent again
      - UDP
        
        breaks data into packets without numbering them
        
        the device reads them in the order they arrive, without correctly ordering them
        
        only sends each packet once without checking if it arrives
        
        this saves time, but you won't know if packets have gone missing in transit
        
        User Datagram Protocol
        
        this makes it good for a fast efficient transmission, where a hiccup is better than a delay, e.g. video streaming
      - controls the packaging and unpacking of data
    - - each layer serves the layer above it, doing the hidden function required
      - the four layers of the TCP/ IP model:
        
        layer 4 - application
        
        providing networking services to applications, e.g. turning data into websites
        
        HTTP, FTP, SMTP
        
        layer 3 - transport
        
        setting up communication between devices
        
        TCP, UDP
        
        splitting data into packets and checking it is sent and delivered correctly
        
        layer 2 - internet
        
        adding IP addresses to packets, directing them between devices and handling traffic. used by routers
        
        IP
        
        layer 1 - link
        
        passing data over the physical network
        
        responsible for how data is sent as e-signals over cables, wireless using NICs, and interpreting signals with device drivers
        
        Wi-Fi, Ethernet
      - self contained - protocols in a layer don't need to know what's happening in other layers
      - a group of protocols with similar functions
      - advantages of layers
        
        breaks network communication into manageable pieces, helping devs focus on one area of the network individually
        
        layers are self-contained, so they can be changed without affecting other layers
        
        having a set of rules for each layer forces companies to make compatible, universal hardware and software
    - - IP
        
        responsible for directing data packets across the Internet or other IP networks, using packet switching
        
        packets are sent between a series of routers, each router reads the header IP address to decide where it goes next
        
        these addresses are added as a header to each packet
        
        direction of data changes depending on traffic, so packets can take different routes
        
        unique IP addresses are given to each device connected to an IP network
        
        if a router receives too many packets, it will prioritise some
        
        establishes connections between routers and handling network traffic
        
        packet switching is efficient because of the many routes a packet could take, so packets can move quickly, even in heavy traffic
    - - they cover how communication starts and ends, how the data is organised, and what the devices do if the data goes missing
      - data sent between networks is split into equal packets
      - a protocol is a set of rules that define how devices communicate and how data is transmitted across a network
      - each packet contains extra information, such as the destination and source addresses and a checksum (to find errors)
    - - Wi-Fi
        
        the frequencies are split into a small range
        
        the channels in 2.4 overlap, causing interference with nearby networks on a similar frequency
        
        2.4 has a greater range, but 5 is stronger over a short range
        
        it's important for data to be encrypted of a Wi-Fi network
        
        uses two radio frequencies - 2.4GHz and 5GHz
        
        there are security protocols such as Wi-Fi Protected access (WPA) for this
        
        units of data sent on this layer are called frames, not packets
        
        a family of protocols used in WLANs
      - Ethernet
        
        mostly the same as Wi-Fi, but is only for wired connections
- - - - breaking a complex problem down into smaller problems and solving each one individually
    - - picking out the important bits of information from the problem, ignoring the specific details that don't matter
    - - a logical way of getting from a problem to a solution
      - if the steps you take to solve a problem follows an algorithm then it can be reused for similar problems in the future
  - - - pseudocode should follow a similar structure and read like normal code
      - the idea is that it should clearly follow an algorithm without having to worry about the exact syntax
      - it should be quick to write and easily convertible
    - - Commands
        
        decision
        
        processes
        
        inputs/ outputs
        
        start/ end
        
        subroutine
      - flowcharts can show sequences, selections, iterations or a combination. see CGP for examples
  - - - Algorithm
        
        compare the search item to the current item, if higher, remove the lower half, vice versa
        
        repeat steps 1 - 3 with your new, smaller list, until you find your item or have searched every item
        
        check if this is the item you're looking for
        
        find the middle item in an ordered list
      - Advantages
        
        much more efficient
        
        much lower run time for longer, ordered lists
    - - Algorithm
        
        check if this is the item you're looking for
        
        look at the next item in the list
        
        look at the first item in a list
        
        repeat steps 2 - 3 until you've found the item or searched every item
      - Advantages
        
        much more simple
        
        doesn't have to be ordered
        
        just as effective as binary search on small, ordered lists
  - - - Advantages
        
        it's a simple algorithm that can be easily implemented
        
        it's an efficient way to check if a list is already in order
        
        it doesn't use very much memory as all the sorting is done to the original list
      - Disadvantages
        
        it is very inefficient
        
        due to this, it is very slow at sorting large lists
        
        the worst case scenario requires n (n - 1) / 2 comparisons
      - Algorithm
        
        move on to items 2 & 3 and repeat step 2
        
        repeat step 3 until you reach the end of the list
        
        if they are incorrectly ordered, swap them
        
        repeat steps 1 - 4 until there are no more swaps in a pass, after each pass, don't include 1 more item from the end
        
        look at the first two items in the list
    - - Advantages
        
        for short lists, it has a similar running time compared to bubble sort
        
        it has a very consistent running time, regardless of how pre-sorted the original list was
        
        for large lists, it is much more efficient and faster
      - Disadvantages
        
        even if the list is already sorted, it still has to go through the whole splitting and merging process
        
        it uses more memory, because it has to create additional lists
      - Algorithm
        
        Merge pairs of sub-lists, sorting the items correctly each time
        
        Repeat step 3 until all items are merged
        
        Keep repeating step 1 until all the lists only contain 1 item
        
        Split the list is half (into sub-lists)