CS 1.3 Compression, Data and Networks

• Network management: Sharing resources of a remote computer such as files or printers.
• Router: Sends data to other routers to reach servers.
• WAP: Allows wireless devices to connect to a wired network.
• NIC: Is built into a device and allows it to connect to a network.
• Repeater: Strengthens signals.
• Switch: Uses MAC addresses to direct traffic to the connected devices.
• Hub: Central device to connect other devices. Sends data to all connected devices.
• WAN and LAN: LAN = small area or same site. Equipment owned by users. WAN = Large area of connected LANs. Equipment owned others.
• Standards: An agreed way of doing things, ie Ethernet. De jure: regulated by a body. De Facto: popular but unregulated. Pros: Good for product development. More choice and different combinations. ASCII and HTTPS universally recognised.
• Protocols - An agreed format or set of rules so devices can communicate.
• Networks: 2 or more computers linked to share resources, files etc.
• DNS cache - Saves frequently used IP Addresses, without going to a DNS server. Again reduces network traffic, but can save out of data information.
• Bus topology: network topology where all terminals (devices) are connected to a backbone cable.
• Star Topology: uses a central node (switch/computer) to direct the flow of data, MAC (Media Access Control) addresses identify each device.
• Mesh Topology: every node is connected to every other node. Most commonly found with wireless technology like Wi-Fi
• The Internet Structure: A network of networks. Allows computers on opposite sides of the globe to communicate with each other. The TCP/IP Stack. Transmission Control Protocol / Internet Protocol. A stack of networking protocols that work together passing packets during communication
• Application Layer


  Specifies what protocol needs to be used in order to relate the application that’s being sent
• Network Layer

  • Adds source and destination IP addresses
  • Routers operate on the network layer and the router is what uses the IP addresses to forward the packets
• Link Layer


  • The connection between the network devices
  • Adds the MAC address identifying the Network Interface Cards of the source and destination computers
• Transport Layer

  • Uses the TCP to establish an end-to-end connection between the source and recipient computer
  • Splits up data into packets
  • Labels packets with their packet number
  • Requests retransmission of any lost packets
• Circuit Switching: A method of communication where a direct link is created between two devices. Link maintained for the entire conversation. The two devices must transfer and receive data at the same rate

  Packet Switching: A method of communicating packets of data across a network. A packet is just a section of the data. Packets aren’t limited to a single route
• Data Packets :

  Segments of data. Contains various pieces of information: Header, sender and the recipient’s IP addresses, protocol being used, order of the packets, time to live, payload, the raw data, checksum.
• Firewalls: Devices designed to prevent unauthorised access to a network ● Consist of two network interface cards (NICs), in-between the user and their internet connection.
• Proxy Servers. Act as an intermediary, collecting and sending the data on behalf of the user. Protect the privacy of the user who remains anonymous. Reduce the web traffic. Can be used by administrators to prevent access to sensitive or irrelevant information at work or at school
• Client-Server: Client-Server networks have terminals known as clients connected to a server. The server is just a powerful central computer. The server holds all of the important information and has extra processing power and the clients can request to use it.
• Peer-to-Peer networks ● Computers are connected to each other so that they can share files ● Inexpensive to set up ● Allow users to share resources ● Easy to maintain
• HTML - Hypertext Markup Language

  CSS - Cascading Style Sheets
• JavaScript is a: Client side scripting, interpreted language
• id: used to specify a unique id for an HTML element. The function .onsubmit will execute JavaScript when a form is submitted
• The Hypertext Transfer Protocol (HTTP): URL, method, headers such as user credentials and cookies. In response: returns success, headers, data itself. Designed to enable communications between clients and servers.
• document.getElementById(): returns an Element object representing the element whose id property matches the specified string. Since element IDs are required to be unique if specified, they're a useful way to get access to a specific element quickly. Used almost every time you want to manipulate, or get information from, an element on your document.
• Checksum Flowchart: A checksum is a way to check if data has been changed or corrupted in transmission. The receiver calculates the checksum using the same formula as the sender. If the checksum is different, the data has been changed.
• CSS example : body  {  margin: 0px;   background-color: white;  font-family: Arial, Helvetica, sans-serif;  font-size: 18px;  text-align: center;  }  
• Server side Processing - Server side processing is when a client sends data to a server for it to be processed. This means no information is processed on the client computer.

  Is used for accessing a database, security, search engines, cloud services.

  Is good as can access secure data, and is less vulnerable to manipulation. Can tackle heavy workloads in less time.

  However client needs to wait for server, no experience customization.
• Client side Processing - Information is processed on the client computer. Is used for interactivity withput page reloads, animations, games, responsive design, offline abilities.

  Is good as no waiting for the server, more customization, can reduce delays as not using server.

  However, cannot access less secure data, can be manipulated, may take longer time to tackle a heavy workload.
• Entity - An object, person, event or thing about which data is to be recorded
• Attribute: A type of data representing a particular characteristic of each data record. Also known as a field or column.
• Relational database: A database that stores records in two or more tables. Each table describes a single entity. These relationships can be modeled by ERDs. Pros: Reduces data redundancy. Can set different access rights. Can carry out complex queries. Cons: More difficult to set up. Normalisation needed.
• Flat file: A database with a single file. Contains data on multiple entities. Pros: Easy to set up, use in code and share. Cons: Data redundancy, therefore more space is required. Sensitive data is exposed. Complex queries are more difficult.
• Primary key: An attribute that uniquely identifies every row of the table. It is automatically indexed and cannot be blank.