Networking and Communications

• Serial transmission

  Data is sent one bit at a time over one communication line
• Parallel transmission

  Uses numerous parallel communication lines to send multiple bits simultaneously
• Advantages of serial transmission

  • Doesn't suffer from skew or crosstalk, making it more reliable especially over long distances
  • Cheap to install as it uses just one line
• Synchronous transmission

  Uses a shared clock signal between sender and receiver to time when signals are sent
• Asynchronous transmission

  Uses start and stop bits to indicate the duration of a transmission, sender and receiver don't need a shared clock
• Synchronous data transmission is used within the busses of a computer's processor in the fetch execute cycle
• Synchronous transmission is suitable for transmitting information in real time systems
• Baud rate

  Number of signal changes in the medium per second
• Bit rate


  • Number of bits transmitted over the medium per second
  • Baud Rate x No. of bits per signal
• Bandwidth


  Range of frequencies a communication medium is capable of transmitting
• Network topology

  The structure of a network
• Physical network topology


  • The actual architecture of a network
  • Networks using one physical topology will interconnect components differently to those networks that use another physical topology
• Latency


  Difference in time between an action being initiated and its effect being noticed
• Protocol


  Set of rules relating to communication between devices
• Physical star network topology

  • Each client has its own direct connection to the central hub
  • The hub receives packets for all of the clients connected to it and is responsible for delivering them to the correct recipient
• Advantages of physical star network topology

  • Packets are sent directly to their recipient, over a cable that is connected only to the recipient
  • Easy to add and remove clients to and from the network
  • Each cable has just one device communicating over it, eliminating the possibility of collisions
  • The failure of one cable does not affect the performance of the rest of the network
• Disadvantages of physical star network topology

  • Should the central hub fail, all communication over the network is stopped
  • Expensive to install thanks to the amount of cable required
• Physical bus topology


  • A single cable called a backbone connects clients
  • A device called a terminator is placed at either end of the backbone
  • No need for a central hub
• Advantages of physical bus topology

  • No central hub, reducing the chances of network failure and decreasing the cost of installation
  • Inexpensive to install as a minimum length of cable is required
• Disadvantages of physical bus topology

  • Packets are sent through the shared backbone, allowing every client on the network to see packets that aren't intended for them
  • The backbone is used for communication by multiple clients, introducing the risk of collisions
  • Should the backbone fail, the entire network becomes unusable
• Host

  A device on a network that provides services
• Client-server networking


  One or more central servers provide services to the clients on the network
• Client server networking

  • Servers are often more powerful machines than the clients
  • Clients request services from the servers, which then respond to the client with the requested service
  • Most schools, colleges and businesses use client-server networks to allow for central management of clients on the network
• Peer-to-peer networking


  Services are provided by the clients themselves and every client has equal status
• Advantages of peer-to-peer networking

  • More cost effective than client-server networking as there is no need for a powerful server to provide services
  • Easier to set up and maintain than client-server networking
• Disadvantages of peer-to-peer networking

  • All of the clients which provide services must be running in order for the network to be fully operational
• Wireless networking

  • Allows clients to communicate within a network without being physically connected to it
  • Requires WAP (wireless access point) to transmit wireless signal and wireless network adapter to connect to it
• WiFi

  A wireless local area network based on international standards
• Methods of securing a wireless network

  • Encrypting transmitted data using WPA or WPA2
  • Disabling SSID broadcast
  • Setting up a MAC address filter
• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

  A protocol used in wireless networks to avoid data collisions caused by multiple devices communicating simultaneously
• CSMA/CA process

  1. Device listens to communication channel to check if it is idle

  2. If idle, data is transmitted

  3. If busy, device waits for a random period of time before checking the channel again

  4. Exponential backoff algorithm can be used to increase the time period for waiting
• Hidden nodes

  A problem that arises when the device checking for an idle channel cannot "see" some parts of the network on which communication may be occurring
• Request to send/clear to send (RTS/CTS) protocol

  1. Transmitting device sends a "request to send" message to the server

  2. If the server is idle, it will respond with a "clear to send" message

  3. Transmitting device can then begin communication with the server
• Advantages of Parallel Data Transmission

  Faster data transfer: Since multiple bits are sent at once