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					S4 CIT

 Computer Network
           In the past…
 Computers are standalone machine.
 What can we do if we want to…
     share a file with someone?
     print a file but the printer is connected
      to another computer?
        Computer Network
   It is a system in which two or more
    computers are connected so that
     data communications
     resources sharing (File sharing,
      hardware sharing e.g. printer)
    among the computers become
            Types of Network
   LAN (Local Area Network)
     links computer and other peripheral
      devices over short distances.
     For examples:
        • network at home
        • network in school campuses and offices
       a LAN with an internal e-mail system,
        file server or message board is called
         Types of Network
   WAN (Wide Area Network)
     covers a large geographic area using a
      communication channel such as
      dedicated telephone lines and radio
     a WAN consists of two or more LANs.

     The Internet is the world’s largest WAN.
   Peer to Peer
     Every computer is connected directly to
      every other computer.
     Each computer can use information
      from, and provide information to each
     There is no server.

     Security is weak.

     Example is WinMX.
   Client / Server
       Individual computers (clients or workstations)
        are connected to a central computer (server).
       Server computer maintains the network user
       Clients can access programs or files stored on
        the server.
       Clients request for services whereas servers
        provide services.
       Since the server can give different levels of
        rights (權力) to different users, so security is
       Examples: School network, Playing on-line
Hardware Requirement
 for a Network - NIC
   NIC (Network Interface Card)
     also called LAN Card
     an expansion board added to the
      computer’s motherboard
     key hardware component for
      connecting the computer to a network
      (wired or wireless)
Hardware Requirement
 for a Network - NIC
   NIC can be connected to the
    computer by means of:
     PCI Bus
     USB Bus

     PCMCIA Bus (In Notebook Computer)
    Hardware Requirement
        for a Network –
     Transmission Medium
 Data and signal need to be
  transmitted over a network through
  a medium (媒體)
 It can be a cable (wired) or through
  microwave (wireless)
Transmission Medium –
  Twisted-pair Cable
 Sometimes called the UTP Cable
  (Unshielded Twisted Pair).
 Looks similar to telephone cable
 The most commonly used types of
  twisted pair wire is Cat5 cable.
 The connector of this kind of cable is
  called RJ45.
Transmission Medium –
    Coaxial Cable
 Transmit at a higher speed when
  compared with twisted-pair cables
 Can transmit signal over long
  distances so they can be laid
  underground or underwater.
     Transmission Medium –
          Optical Fibre
   Bundles of smooth, hair-thin
    strands of glass
   Transmit data with high efficiency
    in terms of light beams
   Can transmit signal over long
   Largely replaced coaxial cables
Transmission Medium –
 Electromagnetic waves which can
  wirelessly carry data through the
 It travels in straight line from one
  microwave relay station(轉發站) to
     Hardware Requirement
        for a Network –
      Connecting Devices
   Hub
     provides a central connection point for
      cables in a network
     slower than switch
     Hardware Requirement
        for a Network –
      Connecting Devices
   Switch
     advanced type of hubs
     faster than hubs
     Hardware Requirement
        for a Network –
      Connecting Devices
   Router
     connect multiple networks
     connect LANs to form a WAN

     connect a LAN to the Internet
           Wireless Network
   Usually a Wireless LAN (WLAN)
   Use microwave (Radio Frequency, RF) to
    transmit and receive data over air
   Have same features as a LAN, but without
    the limitations of a cable.
   Access Point (AP)is required in a WLAN.
   It is a device that transports data
    between a wireless LAN and a wired LAN.
   wireless NICs are also required in each
               Wireless Network
                      PCMCIA Card

Wireless NIC

Access Point
                                    USB Adapter
    Advantages of using
     Wireless network
 Save the cost in laying network
 No cluttering of cables
 More flexible when adding
 Notebook computer can access
  network at anywhere under the
  coverage area.

 MODEM – Modulator and Demodulator
 Modulate
       Convert a digital signal into analogue
   Demodulate
       Convert an analogue signal into digital

   Analogue Signal (模擬信息)
       Can be transmitted along telephone
        lines and UTP

   Digital Signal (數碼信息)
     ie: 01011010
     Computer can only process digital
              Using MODEM
        to connect to the Internet
   Traditionally,
     Use telephone lines
     Speed 56kbps

   Nowadays
     Use telephone lines & UTP Cables
     Called ADSL MODEM
     Speed 1.5Mbps, 3Mbps, 6Mbps
         Network Topology
 It is the logical layout or the way in
  which the computers and other
  devices are connected.
 Types:
     Ring
     Star

     Bus
    Ring Topology

 All devices are connected on a circular
 data flows in one direction.
 When a computer sends data, the data
  travels to each computer on the ring
  until it reaches its destination.
Star Topology

 All devices on the network connect
  to a central computer (usually a
 All data transferred from one
  computer to another passes through
  the server.
Bus Topology

 All computers and other devices are
  connected to a single central cable.
 It is flexible because
       computer can be attached to or
        detached from the network without
        disturbing the rest of the network.
                Comparison of
         different Topologies - Ring
   Ring Topology
       Advantage:
        • Easy and cheap to setup

       Disadvantage:
        • Data transmission in one direction.
        • Failure of a single device means all devices
          after cannot function.
                Comparison of
         different Topologies - Star
   Star Topology
       Advantage:
        • One device failed will not
          affect the other devices

       Disadvantage:
        • If the server breaks down, the whole
          network fails.
                Comparison of
          different Topologies - Bus
   Bus Topology
       Advantage:
        • Failure of one device
          will not affect other

       Disadvantage:
        • Failure of the cable means the whole
          network is broken down.
     more than one topology is used in a
     For example,
         in a school network,
         a bus-star topology is used with a high
          bandwidth bus as the backbone (usually a
          optical fibre).
         the backbone connects a number of star
          networks such as a LAN inside the computer
Network Operating System

 Windows 98/ME, Windows 2000/XP,
  Linux, etc include the software
  necessary to establish
  communication with a network
 Old operating system like DOS do
  not have these network features.
Brief Introduction to OS

   The most important program that runs on
    a computer.
   Every general-purpose computer must
    have an OS to run other programs.
   OS perform basic tasks
       recognizing input from the keyboard
       sending output to the display screen
       keeping track of files and directories on the
       controlling peripheral devices such as disk
        drives and printers.
Network Server Software
   multi-user
   like a traffic cop
       different programs and users running at the
        same time do not interfere with each other.
       controls file access from the server’s hard disk
       manages the print queue
   responsible for security
       ensuring that unauthorized users do not access
        the system.
       tracks user data such as user IDs and
   Examples include Windows 2000 Server
    Network Client Software
 Gathers login information
 Handles drive mapping
 Directs printouts to the network
 Examples include Windows 98/ME,
  Windows 2000/XP Home or
  Professional Edition
    Practical Consideration

 The server and the client software
  components do not necessarily have
  to match.
 A Windows NT computer in library
  and some Windows XP computer in
  classroom can login to the school’s
  Windows 2000 Server.
Common Services on Network -
       File server
  A high speed computer that stores
   the programs and data files shared
   by users.
  Most file servers also store user
   accounts and the network
  Data in a file server are important
   and must be back up regularly.
Common Services on Network -
       Mail server
  It provides e-mail services to users.
  Each user has a mailbox which stores
   the incoming mails and copies of sent
  Note the differences of Yahoo! Mail
   and Outlook Express
Common Services on Network -
       Print server
    It manages one or more printers on a
    It allows a shared printer to be accessed
     by authorized users.
    Sometimes, a Print Server may not be a
    In most case, it is connected to the main
     server in the LAN.
    But a workstation can also act as a Print
Common Services on Network -
       Web server
  A web server stores the contents of
   web pages.
  It delivers web pages according to
   the requests from other users on the
  Web pages are delivered through a
   program called web server program.
   Eg: Microsoft IIS, Apache
Common Services on Network -
      Proxy server
  It improves the efficiency of Internet
   access by temporarily storing copies
   of web pages visited by previous
   users of the same LAN.
  It also filters web pages to prevent
   users from accessing certain sites.