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Networks (Network protocols (Layers (PROS (It breaks communication into…

- - - - Permanent addresses.
      - Used to connect printers on a LAN and for hosting websites on the Internet.
        
        Companies don't want the IP address of their website to change.
      - Can be very expensive.
    - - Assigned to the device by a network server.
        
        This means that a device may have a different IP address every time you log on to the network.
      - ISPs often use these as they are more cost effective and can be reused.
    - - 128-bit numbers are translated into eight hexadecimal numbers.
      - 32-bit numbers are converted into four denary numbers.
  - - - The control information is like the envelope of the letter- it includes the packet's destination address, the source address and the packet number.
    - - This may be part of an email, document, web page or streamed video.
    - - This is a form of validation used to check that the payload data hasn't been corrupted during transit.
      - The sending and receiving devices both calculate a checksum value by performing a function on the payload data.
      - If they match then the data has been received correctly.
    - - Used by routers to direct data packets on the Internet and other IP networks.
      - 1. The sending device splits the data into packets to be sent across the network.
        Each packet is given a packet number to show the order of data.
      - 2. Each router reads the packet header and decides which way to send the packet next, according to the IP rules.
      - 3. The way the data is sent changes depending on network traffic so the packets can take different routes.
        If a router receives too many packets at once it may prioritise some over others.
      - 4. As the packets take different routes, they can arrive in the wrong order.
        The receiving device used the packets numbers it reassembles them in the right order.
      - 5. Sometimes packets go missing in transit so the receiving device checks periodically that all packets have been received.
        If it hasn't been received within a certain time, it sends a timeout message back to the sending device.
      - 6. If all the data is received and the checksums match, a receipt confirmation is sent to the sending device.
  - - - Transmission Control Protocol (TCP)
        
        Sets the rules for how devices connect on the network.
        
        In charge of splitting the data into packets and reassembling the packets back into the original data once it reaches the receiving device.
        
        Responsible for checking that that data is correctly sent and delivered.
      - Internet Protocol (IP)
        
        Responsible for packet switching.
    - - HTTP (Hyper Test Transfer Protocol)
        
        Used by web browsers to access websites and communicate with web servers.
      - HTTPS (HTTP Secure)
        
        A more secure version of HTTP.
        
        Encrypts all data sent and received.
      - FTP (File Transfer Protocol)
        
        Used to access, edit and move files between devices on a network.
      - POP3 (Post Office Protocol- version 3)
        
        Used to retrieve emails from a server.
        
        The server holds the email until you download it, at which point it is deleted from the server.
      - IMAP (Internet Message Access Protocol)
        
        Used to retrieve emails from a server
        
        The server holds the email until you actually delete it, you only download a copy.
        
        Used by most web-based email clients.
      - SMTP (Simple Mail Transfer Protocol)
        
        Used to send emails.
        
        Used to transfer emails between servers.
  - - - E.g. when you send an email (on layer 4), this triggers actions in layer 3, which triggers actions in layer 2, all the way down to layer 1.
    - - Turning data into websites and other applications and vice versa.
        
        HTTP, FTP, SMTP.
    - - Controlling data flow.
        
        TCP.
    - - Making connections between networks, directing data packets and handling traffic.
        
        IP.
      - Used by routers.
    - - Passing data over the physical network.
        
        Ethernet.
      - Responsible for how bits are sent as electrical signals over cables, wireless and other hardware.
    - - It breaks communication into manageable pieces.
        
        This helps developers concentrate on only one area of the network without having to worry about the others.
      - As layers are self-contained, they can be changed without the other layers being affected.
      - Having set rules for each layer forces companies to make compatible, universal hardware and software, so different brands will have to work with each other and always work in essentially the same way.
- - - - Used to connect devices in a LAN.
      - Most common are CAT 5e and CAT 6.
      - Are 'twisted pair' cables, containing four pairs of copper wires twisted together to reduce internal interference.
    - - Made of a single copper wire surrounded by a plastic layer for insulation.
      - Have a metallic mesh to provide shielding from outside interference.
    - - Transmit data as light.
      - High performance cables.
      - Don't suffer interference.
      - Can travel very large distances without loss of signal quality.
  - - - A WAP is a essentially a switch that allows devices to connect wirelessly.
    - - Usually built in now but a USB dongle can also be used.
    - - Uses two radio frequency bands- 2.4 GHz and 5 GHz.
        
        These bands are split into numbered channels that each cover a small frequency range. These overlap.
      - WiFi performance is affected by interference between networks using adjacent channels
        
        To avoid problems, only certain channels that are spread apart tend to be used.
- - - - Fibre optic cables can give much better performance than copper cables.
      - Wireless performance depends on signal quality and so is affected by the range of the device, the amount of interference from other devices and physical obstructions like thick walls.
- - - - Web browsers are client programs which sens requests to web servers and the servers fulfil these.
    - - Easier to keep track of files.
      - Easier to perform back-ups.
      - Easier to install and update software.
      - Easier to manage network security.
      - Reliable.
      - Are always on
    - - Expensive to set up.
      - Needs IT specialists to maintain the network and server.
      - Sever dependence.
      - Sever may become overloaded if too many clients are accessing it at once.
  - - - Easy to maintain.
      - No server dependence.
    - - No centralised management.
      - Copying files between devices creates duplicate files.
      - Less reliable as data may be lost if one device fails.
      - Machines are prone to slow down when other devices access them.
- - - - If a device fails or a cable is disconnected the rest of the network is unaffected.
      - It's simple to add more nodes.
      - Better performance than other setups as data goes straight to the central device so all devices can transmit data at once and there are very few data collisions.
    - - Can be expensive as every device in wired networks will need a cable to connect to the central switch
      - If there is a problem with the switch then the whole network is affected.
  - - - No single point where the network can fail.
    - - Wired networks are very expensive due to the amount of wires needed, but wireless options are becoming more common.