1.3.3 Networks

• A network is where two or more computers connected together with the ability to transmit data between each other
• There are two main types of networks: local area networks (lan) and wide area networks (wan)
• LAN = name given to a network which is spread over a small geographical / single site, for example: a school
• WAN =name given to a network which is spread over a large geographical area. Large corporations with offices in multiple locations will often have a WAN allowing them to communicate between different sites
• Protocol is a set of rules defining how two computers communicate with each other
• Protocols = standard so that all devices have a designated method of communicating with each other, regardless of manufacturer.
• ● HTTP (Hypertext Transfer Protocol) – Used for web page rendering.
• TCP/IP (Transmission Control Protocol / Internet Protocol) - a networking protocol used in the routing of packets through networks
• POP3 (Post Office Protocol) and IMAP (Internet Message Access Protocol) – Mailing protocols, used for email access.
• FTP (File Transfer Protocol) – Used for the transmission of files over networks.
• The Internet is a network of networks which allows computers on opposite sides of the globe to communicate with each other
• Continents = connected to each other using large international backbone cables. Many of these pass underwater, linking continents to one another.
• TCP/IP stands for Transmission Control Protocol / Internet Protocol. A stack of networking protocols that work together passing packets during communication.
• The application layer is at the top of the stack. It defines what protocol needs to be used in order to relate the applications that being sent
• The transport layer uses TCP to establish an end - end connection between the source and the recipients computer, the data is then split into packets and labels. These packets have their packet number, the total number of packets and the port number used for communication.
• If any packets ever get lost, the transport layer requests retransmissions of these lost packets
• The network layer adds the sources and destinations IP address. Routers operating on the network layer use IP addresses to send packets to destination, which is a router. These sockets specify which device the packets must be sent to.
• The link layer is the connection between network devices, it adds MAC address thus identifying the network interface cards of the source and destination. For devices on the same network, the destination MAC address is the address of the recipients computer. Otherwise it will go to the router.
• This is a stack. The layers are looked at from bottom to top, once the destination has been reached, the MAC address is removed by the link layer, then the IP addresses are removed by the network layer, then the transport layers remove the port number and reassemble the packets. The application layer presents the data to the recipient in the form they requested.

  
• Summarise the layers

  1. Application layer decides what protocol to use
  2. Transport layer separates the packets and labels them, including the port number so that the recipients computer can handle the data correctly.
  3. Network layer adds the source and destination IP, routers use this information to forward the packets, socket specifies where the packet must be sent to
  4. Link layer creates a physical connection between the network nodes, it adds the MAC addresses of the source and destination computers so the packets can be sent properly
• WAN usually requires extra telecommunication hardware. Infrastructure used in WANS is owned by 3rd parties.
• DNS (domain name system) used to name and organise internet resources.
• TLD = Top level Domain and 2LD = 2nd level domain
• Domain names are easier to remember than IP addresses, which is why they're used to link servers across the world. The role of a dns = to translate these domain names into IP addresses, when we wish to access a website.
• Packets = segments of data
• Packets contain information in the form of:

  1. Headers, which contain: the sender and recipients IP addresses, the protocol, the order of the packets and the time to live/hop limit
  2. Payload
  3. Trailer
• • Header:

  1. The sender and the recipients IP addresses: act like a postcode, allowing the packet to be delivered to the correct destination and enabling the recipients device to trace where the packet came from.
  2. Protocol being used: Allows the recipients computer to understand how to interpret the packet
  3. Order of packet: Packets are reconstructed in the appropriate order as specified by the header, when reaching the recipients device.
  4. Time to live: Tells the packet when to expire so it doesn't travel forever.
• Payload

  Raw data to be transmitted.
• Trailer:

  Checksum / Cyclic redundancy check, The trailer contains a code used to detect whether any errors have occured during transmission.
• Two techniques to exchange data: circuit switching and packet switching
• Packet switching = method of communication in which data is communicated using packets across a network. Packets are sent across the most efficient route, which can vary for each packet.
• Three advantages of packet switching:

  1. Multiple methods are done to ensure the data arrives intact, e.g.: use of checksums
  2. Multiple routes can be used between devices, so that if one path breaks, another can be used.
  3. Packets can be transferred over large network allowing global communication.
• Two disadvantages of packet switching:

  1. Time is spent deconstructing/reconstructing the data packets.
  2. Must wait for all packets to arrive before data is recieved.
• Circuit switching is a method commincation where a direct link is formed between two devices. The link is maintained for the duration of communication. Circuit switching requires two devices to transfer and receive data at the same rate.
• Two advantages of Circuit switching:

  1. Data arrives in a logical order, which results in a quicker reconstruction of the data
  2. Enables two users to hold communication without delays in speech
• Two disadvantages of Circuit switching:

  1. Devices must transfer and receive data at the same rate.
  2. Bandwidth is wasted during periods of time where no data is being sent.
• Firewalls prevent unauthorised access to a network. A firewall has two network interrace cards, between the user and the internet. The firewall passes the packets between these two NICs and compares them against a set of rules set by the firewall software. These rules are packet filters.
• Packet filtering limits network access in accordance with the admin rules. It works by examining the source IP, destination IP and the protocols being used as well as the ports being requested.
• When access is denied by a firewall. The packet can either be dropped or rejected. A rejected packet sends an alert to the sender to notify them of the error whereas a dropped packet will not.
• Proxy server acts as an intermediary, collecting and sending data on the behalf of the user.