communications

• serial transmission

  bits are sent one after the other along the same data line
• parallel transmission

  bits are sent simultaneously across separate data channels
• disadvs. of parallel transmission

  skew - different wires have slightly different properties, so bits travel at different speeds. crosstalk - electromagnetic interference between adjacent wires. Increases as speed of transmission increases. If data is corrupted, it has to be retransmitted. Is only suitable for distances up to 2m
• synchronous transmission

  data is sent in packets at certain time intervals (timed with internal clock pulse). No gaps in transmission. Receiver counts the bits and reconstructs into bytes
• asynchronous transmission

  data is sent when it is available. Each byte has a start bit and a stop bit. Transmitter and receiver are not synchronised
• latency

  time delay between sending and receiving of first packet of a communication
• protocol

  set of rules regarding the communication between devices, e.g data format, error checking procedures, rate of transmission
• bit rate

  amount of bits transferred per second
• baud rate

  amount of signal changes per second
• baseband

  where one bit is transferred per each signal change. bit rate = baud rate
• bandwidth

  maximum capacity of communication channel. The range of frequencies at which data can be transmitted through a channel.
• physical topology

  physical arrangement of connections between devices
• logical topology

  how data flows around a network
• bus topology

  nodes are connected to a single communication line. Each end has a terminator to stop signals bouncing back. Only one computer can successfully transmit at a time
• star topology

  nodes are connected to a central device. Usually a hub (transmits message to every computer on the network) or a switch (transmits message to intended computer only).
• features of peer to peer network?

  No central server, all computers can see all files, direct communication, if a computer is turned off data cannot be retrieved
• features of client server network?

  server - powerful computer which provides services. Client - computers which send requests. Servers control backups and client access.
• features of cloud computing?

  uses remote servers. Data is remotely accessible. Automatic backups. Does not need to be downloaded.
• examples of network security measures?

  WPA, WPA-2 (generates a new 128 bit key for each packet it sends), MAC whitelisting
• Which collision avoidance method works when nodes are hidden from each other?

  CSMA/CA with RTS/CTS - sends request to send and waits for "clear" response
• two ways of increasing network security?

  Disabling SSID broadcast (hides network), using WPA (encrypts transmission)
• relationship between bit rate and bandwidth?

  directly proportional
• what is WAP?

  wireless access point - enables wireless devices to connect to a local network
• what is SSID?

  service set identifier - identifies the network by a unique name. Must be used by all devices on the network.
• what is wireless network adapter?

  installed in device so that it can access wireless networks
• what is WiFi?

  wireless local area network based on international standards
• define the internet:

  a global wide area network formed from the connection of many other networks, uses TCP/IP protocol
• what is a packet?

  container in which data is transmitted over networks
• what is a router?

  a device that receives packets (from host /router) and uses their destination IP address to pass them (correctly formatted) to another host/router
• what is a gateway?

  a device used to connect networks which use different protocols so that info can be transferred
• what is packet switching?

  a message is split into packets, each packet is transmitted via the best route (individual to each packet) and reassembled by the recipient
• explain time to live:

  when a packet passes through a router, this is a hop. each packet can only go through a certain amount of hops before it is dropped - this is its time to live
• components of a packet:

  sender's address, receiver's address, time to live, packet contents, sequence number
• what is the domain name system?

  how domain names are translated to their IP addresses, using the domain name server (a table of domain names and their IP address)
• what is packet filtering?

  accepts or blocks packets depending on their source IP address or their protocol, can be done by network admin or automatically
• what is stateful inspection?

  looking at packet's contents to decide whether to accept it
• what is a proxy server?

  server that sits between public and private network, manages every packet that passes through
• how do firewalls provide anonymity for private networks?


  when a packet is sent from a private network, it goes through a firewall, so firewall address becomes sender address when it reaches the public network
• what is encryption?

  using an algorithm and a key to convert a message into a form that is only understandable with the key
• symmetric encryption:

  sender and receiver share private key, which is used for encryption and decryption. Before transmission, a key exchange must occur