Computing 2 - Networks

    Cards (97)

    • Star Topology

      All computers connected to a central switch
    • Star Topology - Advantages
      If one cable breaks, all other computers still work
    • Star Topology - Disadvantages
      If central switch breaks, none of the computers work. Expensive
    • Bus Topology
      One long cable, each device attached onto the cable
    • Bus Topology - Advantages
      Cheaper than star or mesh
    • Bus Topology - Disadvantages
      Slow under heavy traffic. If the cable is broken, none of the devices will work
    • Physical Vs Logical
      A physical star topology can operate just like a bus topology. To do this, you would install bus topology protocols on the devices in a physical star topology
    • Client Server Model
      Device (client) requests services from another device (server). Files and software programs can be stored on this server and multiple clients can use the same server.
    • Peer to Peer Network
      All computers have an equal status with no central server. Each computer acts as a client and a server. Cheap to setup and makes it easy to share resources such as files without needing central server
    • WiFi - Advantages
      • Easy to set up
      • Easy to use
      • Can handle a lot of users
      • Sharing of information is easier
    • WiFi - Disadvantages
      • Slower than a wired network
      • Signal easily obstructed
      • Not as secure as a wired network
      • Not everywhere has a WiFi connection - increased digital divide
    • Components Needed for Wireless Networking
      • WAP
      • Modem + Router
      • Device needs a wireless interface card
    • SSID's
      Wireless networks identified by a SSID, has to be used by all devices wanting to go on network. SSID can be hidden or protected with a password so it is more secure
    • SSID
      Service Set Identifier
    • CSMA/CA
      Carrier Sense Multiple Access with Collision Avoidance protocol
    • RTS/CTS
      Request to Send / Clear to Send
    • WiFi - How data transferred
      Uses frequencies (also called channels). Each channel has a certain range/size. Each channel overlaps slightly with the next one. A less used channel is faster
    • CSMA/CA without RTS/CTS
      • Assemble packet/frame of data to send
      • Asks if the channel is idle
      • If yes, data transmitted
      • If no, waits a period of time then checks again
    • CSMA/CA with RTS/CTS
      • Assemble data packet/frame
      • Check if channel is idle
      • If not idle, waits and then checks again
      • If it is idle, sends a RTS, if then CTS is yes, transmits data
      • Else, waits random time then checks again
    • Why use RTS/CTS?
      Hidden node problems, WAP can hear both nodes A & B but nodes A & B cannot hear each other so used to avoid collisions if they send data at the same time
    • Structure of Internet
      Intercontinental cables running at the bottom of oceans with lots of fibre optic cables
    • Internet
      Massive collection of domain name server
    • Packet Definition
      Bundle of data sent out over a network
    • Components of a Packet
      • Packet sequence number
      • Source IP Address
      • Destination IP Address
      • Source MAC Address
      • Destination MAC Address
      • Checksum
      • Data
    • Packet Switching
      Method or protocol used for sending multiple packets out across the internet
    • Packet Switching - Steps
      • Network interface receives message to send data
      • NIC splits message up into equally sized packets, numbers them and a checksum is calculated + added to each packet
      • Packets routed to local ISP, checked for errors when arrives
      • Packets sent to next route (route changes depending on congestion)
      • Packets arrive at ISP of end computer
      • Packets re-ordered by TCP into correct sequence
      • Packet stripped to just the data and sent to application that needs it
    • URL - Components
      https://www.craigndave.org/thenumbers/22017/09/cryptic_challenge.pdf
    • URL - Method
      • https
      • http
    • URL - Fully Qualified Domain Name (FQDN)
      • www - Host
      • Craigndave.org - Website Domain Name
      • FQDN = host + web domain name
    • URL - Location
      the numbers/2017/09/
    • URL - Resource

      cryptic_challenge.pdf
    • Internet Registers
      Five global organisations with worldwide databases of all domain names. Governed by Internet Corporation for Assigned Names and Numbers (ICANN)
    • (Internet Protocol) IP Address
      Unique address given to any device on the internet
    • How it Works
      • Browser requests URL
      • Browser extracts fully qualified domain name and sends to DNS Server
      • Local DNS Server maps FQDN to an IP Address + returns it to browser
      • If local DNS cannot do this, goes to regional DNS server which has bigger lookup table
      • If not found, passed on to the NSP (network service provider) which finds it and adds it to local DNS server lookup table
      • Browser then sends a get request to web server (sending the IP address) and web page is returned
    • Firewalls
      Filter all network communication in and out of a network.
    • Two types of firewall
      Software and Hardware
    • Firewall (How it works)
      Series of rules applied to all traffic and if traffic does not meet these rules, not allowed in/out. Rules could block specific web pages, messages containing certain words, anything that could be a virus
    • Proxy
      Physical computer placed between a network and an outside source so that all traffic has to go through the proxy. Has a banned list of traffic that it will not let through. (Hardware version of firewall)
    • Encryption
      Transforming data into an unreadable format so that it cannot be read without an encryption key. Cannot be read by hackers...
    • Simple Encryption
      Caesar Cipher - Shifting all letters the same number of spaces
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