Week6

							Communications
  & Networks


  Naomi Adams
From Analogue to Digital
   Before the 1950s computing devices
    processed data into information and
    communications devices communicated
    information over long distances.
   The two streams of technology developed
    independently of each other
   Now we have „digital convergence‟ – a merger
    of computing and communications in which the
    same information is exchanged among
    different kinds of equipment using the
    language of computers
Why did it take so long?
   The reason that the convergence
    between communication technology and
    computing technology took so long is
    that computers are digital and the world
    is analogue.
   Let‟s look at what this actually means…
What does “analogue” mean?
   Analogue data is continuous – the
    colours and sounds around us in the
    world are all continuous - varying
    smoothly from one shade or note to the
    next
   Humans experience the real world in an
    analogue form
What does “digital” mean?
   Digital describes a system based on
    discrete or discontinuous data
   This means that only a certain number of
    values can be defined
   This stems from computers using a
    binary system of 1s and 0s.
   Lets look at some examples showing the
    difference between analogue and digital
Digital and analogue




  Analogue         Digital
Digital and analogue
                      A digital photograph
                       can only show an
                       approximation of a
                       scene representing
                       continuous colour as
                       small squares of
                       distinct colour
                       (pixels), whereas
                       seeing this horse in
                       reality, one could
                       see more and more
                       detail no matter how
                       closely you looked
Digital and analogue




   Listening to music made on instruments and voices
    singing is not the same as listening to an MP3 of the
    same music – an MP3 does not contain the same
    amount of detail, in a similar way to the difference
    between really seeing something and seeing a digital
    photograph of something
Modems
   Since computers use digital signals but phone lines
    use analogue modems must translate from digital to
    analogue, send the signal along the phone line, then
    translate back from analogue to digital at the other
    end
   The process is called “modulation/demodulation”
   Modulation means to translate from digital to
    analogue
   Demodulation means to translate from analogue to
    digital
   Modems have to do all this just to use standard
    analogue voice phone lines
Networks
   Because of the convergence between
    computing and communication
    technologies, networks have become
    possible
   A network is a system of connected
    computers, telephones or other devices
    that can communicate with one another
    and share applications and data
The benefits of networks
   Sharing of peripheral devices
   Sharing of programs and data
   Better communications
   Security of information
   Access to databases (private company
    databases, or online databases)
Types of Network
   WAN – Wide Area Network
    • A communications network that covers a large
        area – like a country, or the whole world
    •   The internet is a WAN
   MAN – Metropolitan Area Network
    • A communications network that covers a city
        or a university campus
    •   Links several LANs and provides access to a
        WAN
Types of Network
   LAN – Local Area Network
    • Connects computers in a local area, like a
      building, an office or a group of buildings
      close together
   HAN – Home Area Network
    • Connects a household‟s digital devices,
      including televisions, DVDs, multiple
      computers, printers, games consoles, home
      security systems
Types of Network
   PAN – Personal Area Network
    • A wireless network used to connect
      someone‟s personal devices, like cellphone,
      printer, keyboard etc.
   Home Automation Network
    • A wireless network connecting the switches
      and sensors in a house used to control lights,
      heating, security, smoke alarms in a house
Structures of Network
   Client/Server Network
    •   Consists of clients, which are microcomputers, that
        request data, and servers, which supply data
    •   The server is a powerful computer that manages
        shared devices, it may act as a disk drive, it might
        show a database to its clients, or store programs, or
        manage email
   Peer-to-Peer Network
    •   All microcomputers in a P2P network communicate to
        each other without a server
    •   They are cheaper than a client/server network and can
        work effectively with up to 25 computers – after that
        the system will slow down a lot.
Components of a Network
   Regardless of the size of network, they will all
    have some components in common:
    •   Connections – either wired or wireless
    •   Hosts and Nodes
        host: central computer that controls a network
        node: any device that is attached to a network
    •   Packets – a fixed length block of data for transmission
    •   Protocols – a set of rules that govern the exchange of
        data over the network
    •   Network Operating System (NOS) – the operating
        system that manages the activity of the network, like
        access by multiple users and sharing of resources
TCP/IP Protocol
   TCP/IP Packets carry four types of information
    •   Sender‟s address (source IP number)
    •   Address of intended recipient (destination IP number)
    •   Number of packets the original data was broken into
         •   This happens because the amount of data the PC is sending
             can be much larger than the space in a single packet
         •   So the data has to get broken up in one or more packets
         •   Then the packets have to be assigned a number like 1 of 6, 2
             of 6, 3 of 6, 4 of 6, 5 of 6, and 6 of 6
    •   Packet number and sequence info for each packet
         •   Packets may arrive out of order (1 6 3 2 5 4 for example)
         •   This information is used to resequence the packets and put
             them back in the correct order (1, 2, 3, 4, 5, 6) so they can be
             read
   TCP/IP is the protocol used for transmitting data over the
    internet and also over LANs
Network Linking Devices
   Networks are linked together and need devices to help
    them achieve this
    •   Hubs – a central connection point for devices in a network,
        e.g. a server
    •   Bridges – an interface used to connect the same types of
        networks, for instance two LANs could be joined together
        using a bridge
    •   Gateway – an interface permitting communication between
        different types of network, e.g. between a LAN and a WAN
    •   Routers – a router is a special computer that directs
        messages when several networks are connected together
    •   Backbone – the main highway (including gateways, routers
        and other communications equipment) that connect all
        computer networks in an organisation.
Wired Communication Media
   Communications media carry signals over a communications path
   Twisted-Pair Wire
     •   2 strands of insulated copper wire twisted around each other
     •   Twisting reduces interference (crosstalk) from electrical signals
     •   Data rates are 1 – 128 Megabits per second
     •   Used for conventional telephone connections
   Coaxial Cable
     •   Insulated copper wire wrapped in a metal shield and then in an external plastic
         cover
     •   Used for cable TV and cable internet electric signals
     •   Carries voice and data up to 200 megabits per second
   Fibre-Optic Cable
     •   Fibre-optic cables contain dozens or hundreds of thin glass or plastic strands
         that transmit pulsating light instead of electricity
     •   Can transmit up to 2Gb per second
     •   Lighter, more durable and more expensive than twisted-pair or coaxial cable
Wireless Communications Media
   The electromagnetic spectrum
    encompasses all electromagnetic waves,
    from waves with very low frequencies to
    high frequencies
   This spectrum includes visible light,
    microwaves, X-rays, infrared light, and
    radio waves
   Radio waves are used in wireless
    communications
Wireless Communications Media
   Infrared Transmission
    •   Short-range communication
    •   Requires line-of-sight
    •   Found on PDAs, laptops digital cameras, usually for
        communication with a remote control
   Broadcast Radio
    •   Sends up to 2 megabits per second over long distance
    •   Uses a transmitter and a receiver
    •   Used for AM/FM radio transmission
Wireless Communications Media
   Cellular Radio
    • Used for cellphones and wireless modems
    • Uses high frequency radio waves to transmit
      voice and digital messages
   Microwave Radio
    • Superhigh frequency radio waves
    • Requires line-of-sight transmitters and
      receivers
Communication Satellites
   Communication satellites are microwave relay stations in orbit
    around the earth
   They may occupy one of three zones in space:
     •   GEO – geostationary earth orbit
          •   22,300 miles up
          •   Always in geostationary orbit above equator
     •   MEO – medium earth orbit
          •   5,000 – 10,000 miles up
     •   LEO – low earth orbit
          •   200-1,000 miles up
          •   No signal delay
          •   Cheaper to launch
   Depending on which zone satellites are in fewer or more are
    required for global coverage. If you are using LEO you need
    more satellite for global coverage than if you are using GEO
Global Positioning System
   A GPS receiver can pick up transmissions from
    any four of 24 MEO satellites and using
    triangulation pinpoint the location of the
    receiver
   Accurate to within 3 metres
   GPS uses a $10 billion infrastructure
    developed by the military
   GPS receivers contain maps that are displayed
    based on the GPS position to guide users