1.3.3

• Network


  Two or more computers connected together with the ability to transmit data between each other
• Types of networks


  • Local area network (LAN)
  • Wide area network (WAN)
• Local area network (LAN)


  • Spread over a small geographical area or a single site
• Wide area network (WAN)


  • Spread over a large geographical area
• Network topologies


  • Physical topology
  • Logical topology
• Physical topology


  The physical layout of the wires and components which form the network
• Logical topology


  The layout which shows how data flows
• Bus topology


  • All terminals (devices) are connected to a backbone cable, the ends of which are plugged into a terminator
• Star topology


  • Uses a central node, often a switch, to direct data through the network
  • MAC (Media Access Control) addresses are used to identify each device on the network

  
• Mesh topology


  • Every node is connected to every other node
  • Most commonly found with wireless technology such as Wi-Fi
• Protocol


  A set of rules defining how two computers communicate with each other
• Commonly used protocols


  • HTTP (Hypertext Transfer Protocol)
  • HTTPS (Secure)
  • TCP/IP (Transmission Control Protocol / Internet Protocol)
  • POP3 (Post Office Protocol)
  • IMAP (Internet Message Access Protocol)
  • FTP (File Transfer Protocol)

  
• Internet


  A network of networks which allows computers on opposite sides of the globe to communicate with each other
• TCP/IP stack


  • Application layer
  • Transport layer
  • Network layer
  • Link layer

  
• Application layer


  Specifies what protocol needs to be used in order to relate the application that's being sent

  
• Transport layer


  • Uses TCP to establish an end-to-end connection between the source and recipient computer
  • Splits data up into packets and labels these packets
  • Requests retransmissions of any lost packets

  
• Network layer


  • Adds the source and destination IP addresses
  • Routers operate on the network layer and use the IP addresses to forward the packets
  • The sockets are then used to specify which device the packets must be sent to and the application being used on that device

  
• Link layer


  The connection between the network devices, it adds the MAC address identifying the Network Interface Cards of the source and destination computers

  
• LAN


  A network spread over a small geographical area
• WAN


  A network typically spread over a large geographical area, requiring extra telecommunication hardware
• Domain name system (DNS)


  The system used to name and organise internet resources
• TLD


  Top Level Domain
• 2LD


  2nd Level Domain
• Packets


  Segments of data containing information such as sender and recipient IP addresses, protocol being used, and order of the packets

  
• Domain


  Separated from the larger domain by a full stop. For example, 'leeds.gov.uk'.
• Domain name


  • www.pmt.education
• Domain names


  Much easier to remember than IP addresses, which is why they are used to link to servers across the world
• Domain name system server (DNS server)

  Translates domain names into IP addresses when we wish to access a website
• Data Packets


  Segments of data
• Information contained in data packets

  • Header
  • Payload
  • Trailer
• Header


  Contains sender and recipient IP addresses, protocol being used, order of the packets, and Time To Live / Hop Limit
• Payload


  Raw data to be transmitted
• Trailer


  Contains checksum or cyclic redundancy check to detect errors during transmission
• Circuit switching


  A method of communication where a direct link is created between two devices, maintained for the entire conversation
• Packet switching


  A method of communication in which data is communicated using packets across a network, with packets sent across the most efficient route

  
• Advantages of packet switching


  • Multiple methods to ensure data arrives intact eg. checksums and cyclic redundancy checks
  • Multiple routes can be used between devices, so if one path breaks, another can be used
  • Packets can be transferred over very large networks to allow communication globally

  
• Disadvantages of packet switching

  • Time is spent deconstructing and reconstructing the data packets
  • Must wait for all packets to arrive before data can be received
• Advantages of circuit switching

  • Data arrives in a logical order which results in a quicker reconstruction of the data
  • Enables two users to hold a call without delay in speech
• Disadvantages of circuit switching

  • Bandwidth is wasted during periods of time in which no data is being sent
  • Devices must transfer and receive data at the same rate
  • Using switches means electrical interference may be produced which can corrupt or destroy data
  • Ties up sections of the network which cannot be used by others data until transmission has been completed
• Firewall


  A device designed to prevent unauthorised access to a network, consisting of two network interface cards (NICs) between the user and the Internet