NETWORKS

• Symbol


  Particular pattern of bits represented by a signal
• Baud rate


  The number of signal changes in the medium per second
• Bit rate


  The number of bits transmitted over the medium per second
• Bit rate


  Baud rate x number of bits per signal
• Bandwidth


  Range of frequencies that a transmission medium is capable of transmitting, measured in Hz
• Bandwidth


  Has direct positive relationship with bit rate
• Latency


  Difference in time between an action being instantiated and its effect being noticed (e.g. keyboard lag), usually increases with distance
• Handshaking


  Exchanging signals to establish communication rules (e.g. checksums, parity bits, synchronous/asynchronous), e.g. Bluetooth
• Protocol


  Set of rules relating to communication between devices
• Serial


  Data sent 1 bit at a time over one communication line, usually metal wire but can be optical fibre or wireless
• Serial transmission used for medium/long distances (e.g. wired peripherals like mouse)
• Parallel


  Multiple parallel communication lines to send multiple bits simultaneously
• Skew


  Each communication line in parallel has slightly different electrical properties, making transmission time differ so bits aren't received together
• Parallel transmission used over short distances (e.g. between processor parts and within RAM)
• Crosstalk


  When parallel communication lines are tightly packed so signals can leak into another and corrupt data
• Parallel more expensive than serial, serial more reliable as doesn't skew/crosstalk
• Synchronous


  Clock signal between devices is synced, used within busses of processor in FDE cycle
• Synchronous transmission


  Signals sent at regular intervals and received in same order they were sent, suitable for real-time systems
• Asynchronous


  No shared clock signal, instead uses start and stop bits to indicate duration of transmission, stop is always opposite of start bit
• Asynchronous transmission


  Sender and receiver must use same baud rate, only synchronise clocks for duration of transmission
• Physical topology


  The actual physical architecture of a network
• Star topology


  Each client has its own direct connection to the central hub, which receives packets for all clients connected and is responsible for delivering them
• Star topology


  • Packets are sent directly and privately, it is easy to add and remove clients from the network, each cable has just one device communicating over it so no collisions, failure of one cable does not affect any other cables
• Star topology


  • If central hub fails all communication is halted, expensive to install due to lots of cables
• Bus topology


  All clients are connected to a single cable called a backbone with no need for a central hub
• Bus topology


  • No central hub so less risk of network failure and decreased cost of installation, inexpensive to install as minimum length of cable
• Bus topology


  • Packages sent through shared backbone so can be accessed by every client, backbone used for all client communication so can cause collisions, if backbone fails entire network fails
• Logical topology


  The flow of data
• Physical star topologies can still behave like logical bus by using bus protocol to distribute packets to all clients
• Host


  A device on a network that provides services, often a server which can provide file storage, printer sharing, internet access. Host could also be a client on the network itself
• Client-server network


  One or more central servers provide services to clients on network, connected to network in the same way as clients but often more powerful machines than clients. Services offered e.g. email management, user accounts, print queues
• Peer-to-peer network


  No shared server on network, services are provided by clients themselves and every client has equal status
• Peer-to-peer network


  • Disadvantage is all clients which provide services must be running for network to be fully functional, advantage is that it's cheaper as there is no need for a powerful server and is easier to set up and maintain
• Wireless networks


  Require WAP which connects to a wired network and a wireless network adapter in the device
• Wifi


  Used to provide wireless networks and refers to a wireless LAN that is based on international standards
• Wireless network security


  Can be secured by encrypting data using WPA or WPA2, WPA = wifi protected access, requires that a new client enters a password to connect to network
• Disabling SSID broadcast


  Method of securing wireless network, stops wireless devices within range of the network from displaying the network is available, only allowing those who know the SSID to connect
• SSID


  Service set identifier, the name that identifies a wireless network
• MAC address


  Media access control address, assigned to every wireless device by manufacturer, unique to that device
• MAC address filter


  Method of securing wireless network, whitelists only allow specific devices to connect, blacklists can block specific devices