Computing 2 - Networks

• 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