AQA Alevel UNIT 4.9 Networking

AQA Alevel UNIT 4.9 Networking

1. Communication

   • TRANSMISSIONS
     • Parallel Transmission
       • Advantages: Fast (more bits per cycle)
       • Disadvantages: Less reliable over distance, more expensive (lots of wiring)
       • Description: Each bit sent a the same time along its own channel
     • Serial Transmissions
       • Advantages: Cheaper hadware, easy set up, good fro long distace, thin wires
       • Disadvantages: (Slow) only one bit per cycle)
       • Description: Each bit is sent along the same chanel, in series (one after the other
     • Synchronous Transmission
       • Description: Both sender and reciever use an external clock to indicate when to send or recieve a bit
       • Fast and reliable
     • Asynchronous Transmission
       • Description: No external clock is used, instead the sender includes timing bits to indicate the end or begginig of each 7 or 8 bit word
         • The timing data is essential for the receiving computer to know when a new transmission is beginning/ending, and gives a short gap to prepare for the next transmission
       • Useful when no external clock is available
       • Slower and less efficient because timing data must be added to the desired transmission
     • Latency
       • Description: The delay between the start of the transission, and the start of the effect of the transmission (eg, time take for first bit to be recieved)
     • Protocols
       • A set of rules relating to commuication between devices
     • Rates
       • Bit
         • Description: The number of bits of data transferred in a given time (eg. number of bits per second)
       • Baud
         • Description: A measure of the physical properties of the wire. The maximum number of signal changes per second that the wires can handle
         • If only one bit is transmitted per signal change, bitrate matches baud rate
       • Bandwidth
         • Description: Potential cpacity of a transmission channel (ie. maximum theoretical bitrate)
       • Measures of digital transmission

2. Networks

   • Topologies
     • Bus
     • Ring
     • Star
   • Types
     • Peer to Peer
     • Client-server
   • Wireless
     • A collection of connected computers which communicate with electromagnetic energy pulses through the air
     • WiFi
       • Each network requires a Wireless Access Point (WAP) to manage connections to the network
       • Each device needs a wireless adaptor to transmit and receive signals
     • Topology: Similar to a Star topology, but it has collisions in a similar way to a bus network
       • Interference due to collisions is a huge problem for wireless networks, and only one device can broadcast at a time or the signal gets corrupted. Interference can also come from nearby networks operating on the same or similar frequencies
         • Collisions can be avoided by using different frequencies for different networks, known as frequency modulation
           • Use of EM spectrum is strictly regulated, so the 2.4GHz and 5GHz bands are available for wireless networking
           • Routers automatically try to select the band with the least interference, but some will inevitably occur
     • Protocols
       • CSMA/CA
         • Carrier Sense Multiple Access / Collision Avoidance
         • Used to allow devices on the same band to communicate without collisions
         • Very simple
         • Not completely effective becuase some devices may be out of range of others on the network, meaning that the router (whic is in range of both) would be recieving two transmissions at once, causing corruption
       • RTS/CTS
         • Request To Send/Clear To Send
         • Introduced to resolve the issue with CSMA/CA by adding a checking system between the sending device and the receiving WAP ensuring that the channel is fully clear
         • Reduces collisions within the network
         • Collisions with other networks still occur
       • SSID
         • Solves the problem of knowing what network a piece of data is being transmitted on. It is essentially a network name (eg. BTWiFi-123ABC) which is sent along with each packet of data. The WAP will only repond to packets labelled with its SSID
         • Service Set IDentifier
       • MAC Filtering
         • A MAC address is the ID of each Wireless Reciever which is included in all packets of data transfer
         • WAPs can be configured to only trust certain MAC addresses. This is weak security because a hacker can alter the packet transmission to include a trusted MAC address despite not being a trusted device
       • WPA
         • Wireless Protected Access
         • A standard for wireless network encryption. Each transmitted packet is encrypted using a key, and the same key is required to decrypt the data. this means that unless the device has the network key, data cannot be sent or received
         • The only reliable security measure for Wireless networks
       • Hidden SSID
         • The WAP can be configured to not reveal its SSID so that devices will not know that it exists
         • VERY weak form of security as pacets sent across the network still contain the SSID, so an intercepted packet completely breaks down the security

3. The Internet

   • Definition of an internet: An interconnected network of computers ("a network of networks")
   • The Internet is a global internet connecting millions of neworks together
     • It can be visualised as thousands of star networks connected together, however due to its complexity there is never only one connection between these networks, everyting is connected in multiple ways at once
     • It allows the connection of computers owhich are geographically distant, but since there is no direct connection between most devices (eg. a smartphone and a web server in california), how do these devices communicate?
   • Email, the World Wide Web, VOIP and thousands of other hugely impotant services run on the internet
   • Packet Switching
     • Two computers are connected over the internet via thousands of routers, but there are many routes that data can take acoss the internet
     • Packet switching is used to keep data moving at a good speed. The method of packet switching is as follows:
       • Data is split up into chunks called packets
         • Each packet is sent independantly
           • Algorithms decide the fastest route at the time of sending each packet (each packet could take a different route)
             • Packets are reassembled at the other end
     • Packets explained: Each packets contains three main parts as shown
   • Devices
     • Routers
       • When it reads a packet, it checks if the destination is on its network. If it is, it passes it on, if not, it sends it to another router
     • Gateways
       • Gateways work like routers, but they allow a connection between two different types of network
       • When they recieve a packet, they strip out the header information, and reconstruct it in the way that the next network's protocols expect it to be configured
     • Definition: A device which directs packets to their appropriate destinations
   • IP Addresses
     • Addresses assigned to the computer by the network
     • Each server must have one in order to communicate with computers on the Internet
     • IPv4 uses 32 bit addresses, which only allows 4 billion addresses, not enough for all devices
     • Gateways are used to translate between external and internal IP addresses
       • Internal IPs are unique within a network, and Externa IPs are unique in the internet
     • Domain Names
       • Each website has its unique IP address eg. 212.58.244.68
       • Domain names are used because remembering hundreds of IP addresses would be impractical. By assigning them a domain name (eg www.bbc.co.uk), the internet becomes much easier to use
       • Domain Name System (DNS) servers are used to convert a domain name to its IP address
         • All Internet Service Providers have their own DNS which are continually updated
     • Since IPs are limited, the global registry allocates IPs to countries, and then registries allocate them to ISPs and businesses

4. TCP/IP

   • JSON and XML
     • Both standards of storing data, frequently used when transferring data over a client - server connection
     • JSON
       • Considered neater and easier to read. More space efficient. Easier to integrate with Java. Able to use arrays.
     • XML
       • Historically more widely used, although less so recently
       • Less efficient, more ambiguity in datatypes, slower to process
   • IPv4 vs IPv6

5. APIs

   • CRUD
     • REST
       • REST is an example of a CRUD design API. It allows Jave (via HTTP) to communicate with SQL
     • C - Create R - Retrieve U - Update D - Delete
   • Application Programming Interface
   • A series of rules and routins allowing different devises and software to communicate

Imágenes y archivos del mapa

• Network+Topologies+And+Types+Notes (application/pdf)
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• Tcp+Ip+Infographic (application/pdf)