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11.commSys11.ppt

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  • pg 1
									 We look at the basic elements
of a communications systems,
and the various services
available.
  Overview

    1. What is a Communications System?
    2. Bandwidth
    3. Network Topology
    4. Protocols
    5. Packet Switching
    6. Circuit Switching
    7. Analog <-> Digital Conversion
    8. Telephone Services
    9. Cable-based Services
    10. Wireless
000-209 Intro to CS.
11/Comm. Systems                           2
 1. What is a Communications System?

      A communications system is a combination of
       hardware, software, and connecting links that
       transport data between a sender and a receiver.

      A sender and receiver
       are linked by a
       communications channel
        e.g.telephone lines,
         fiber-optic cable

000-209 Intro to CS.
11/Comm. Systems                                         3
   2. Bandwidth

      The amount of data that can be sent over a network
       in a certain period of time.
      Usually measured in bits per second (bps), kilobits
       per second (kbps), or megabits per second (mps).
      Two main types:
        broadband       (high capacity)
              e.g. by using fibre-optic cable
        narrowband        (less capacity)
              e.g when using the telephone system
000-209 Intro to CS.
11/Comm. Systems                                             4
   3. Network Topology

      The shape of the interconnections
       in a communications system
       is its topology.

      Star topology - the communications lines fan out
       from a central location
        every   connection is dedicated to one user



000-209 Intro to CS.
11/Comm. Systems                                       continued 5
      A bus topology provides a common or shared
       communications link
        used   by cable TV companies




000-209 Intro to CS.
11/Comm. Systems                              continued 6
      A ring topology connects devices in a continuous
       loop
        used   by older local area networks




000-209 Intro to CS.
11/Comm. Systems                                          7
   4. Protocols

      A communications protocol is a set of rules
       about how a sender and receiver should
       communicate.

      The rules specify such things as
        data representation, signalling, authentication, and
         error detection



000-209 Intro to CS.
11/Comm. Systems                                                8
   5. Packet Switching

      Data is separated into
       small packets.
      Each packet is sent through the network using the
       best route available at that time.
      At the receiving end, the packets are reassembled
       into the original data.

      Packet switching is used by the Internet.
000-209 Intro to CS.
11/Comm. Systems                                   continued 9
                           2. Each packet is
                           addressed to its
                           destination.

                                                    3. A packet might
1. A message is                                     travel the shortest
divided into                                        path to its
packets.                                            destination.


        4. If a route is                       5. When the
         congested or                          packets arrive at
   inoperable, packets                         their destination,
    can be rerouted to                         they are
           other links.                        reassembled.


  000-209 Intro to CS.
  11/Comm. Systems                                                   10
   Advantages of Packet Switching

      Packet switching uses network bandwidth
       efficently.

      It minimizes transmission latency
        the   time it takes for data to pass across the network


      Packet switching can deal with network failure.

000-209 Intro to CS.
11/Comm. Systems                                                   11
6. Circuit Switching

      A dedicated communications path is established
       between two devices through one or more
       switching nodes.

      Unlike packet switching, digital data is sent as a
       continuous stream of bits.

      The telephone system uses circuit switching.
000-209 Intro to CS.
11/Comm. Systems                                            12
   Advantages of Circuit Switching

      Bandwidth is guaranteed.

      Any communication delay is only due to
       propagation time.

      Primary advantage of the telephone system for
       computer communications is that it's cheap for
       users.
000-209 Intro to CS.
11/Comm. Systems                                        13
       Twisted Pair Cable

          Telephone systems use miles and miles of
           twisted-pair cables.
Twisted-pair cable
terminates with a
plastic RJ-45
connector.




   000-209 Intro to CS.
   11/Comm. Systems                                   14
   7. Analog <-> Digital Conversion

      Old phone lines use analog sound signals.
      Modern systems use digital signals:
        lesssusceptible to noise
        require simpler circuitry



      It's possible to convert analog signals into
       digital signals (and vice versa).


000-209 Intro to CS.
11/Comm. Systems                                      15
   Analog and Digital Signals




000-209 Intro to CS.
11/Comm. Systems                16
                                                                                Central office
        The Conversion Process                                                  PoP - incoming
                                                                                digital may be
                                                                                converted to
                                                                                analog
              Transmission to
              neighborhood
              switch box
                                     Neighborhood switch
              (within 1800
              yards)                 If not analog already,
                                     converted at this point

                                                                             National Backbone,
                                                                             All digital networks
Modem in computer
Converts analog to digital
                                                                        Central office PoP
                                      Neighborhood switch               Definitely digital now
               Transmission to        May convert to digital.
               neighborhood           May remain analog
               Switch box                                       Transmission to city
                                                                Point of Presence
   000-209 Intro to CS.
               (within 1800 yards)
                                                                central switch
   11/Comm. Systems                                                                                 17
   8. Telephone-based Services

      POTS (plain old telephone service) is an analog
       service.

      Voiceband modem - converts digital pulses into
       analog tones to send digital computer data over a
       POTS line.




000-209 Intro to CS.
11/Comm. Systems                                  continued 18
    ISDN
     a   standard for digital transmission of voice and
       data
      uses circuit switching with ordinary telephone
       wire (and other media, such as fibre-optic cable)
      it requires hardware adapters at both ends of the
       transmission
      basic (64 kbps); enhanced (128 kbps)



000-209 Intro to CS.
11/Comm. Systems                                 continued 19
     ADSL (Asymmetric Digital Subscriber Line)
       fortransmitting digital information at a high
        bandwidth on existing phone lines

     ADSL offers asymmetric data rates
       1.5 to 9 Mbps when receiving (the downstream rate)
       16 to 640 Kbps when sending (the upstream rate)



     Speeds depends on distance from the telephone
      company office
000-209 Intro to CS.
       at most ~3
11/Comm. Systems miles away                             continued 20
      T1 Line - provides 1.544 Mbps send and
       receive capacity over a dedicated line

      T3 Line - uses fiber-optic cables to provide
       service with a capacity of 44.736 Mbps
        theequivalent of 28 T1 lines
        enough to show full-screen, full-motion video



000-209 Intro to CS.
11/Comm. Systems                                         21
   Fiber-Optic Cable

      Fiber-optic cable is a bundle of extremely small
       tubes of glass called optical fibers.
        thinner   than human hair


    Fiber-optic cable is replacing
    twisted-pair cable where high
    bandwidth is required



000-209 Intro to CS.
11/Comm. Systems                                 continued 22
      How fiber-optic cable works:
        miniature  lasers send pulses of light
        each fiber is a one-way communications
         channel
        light signals encounter little resistance




000-209 Intro to CS.
11/Comm. Systems                                     23
   9. Cable TV-based Services

      Cable TV companies in the US have installed
       miles of high-bandwidth coaxial cables.
        carrying   capacity far in excess of POTS lines


      A cable modem is a device designed to
       demodulate a signal from the
       cable and translate it back
       into Internet data.

000-209 Intro to CS.
11/Comm. Systems                                           24
   Coaxial Cable

      Coaxial cable - high-capacity communications
       cable consisting of a copper wire conductor
        common         use is to carry television signals


   Coaxial cable
   contains shielding
   which increases
   bandwidth.


000-209 Intro to CS.
11/Comm. Systems                                             continued 25
      Coaxial cable:
        has excellent bandwidth, but not as good as
         fibre-optic cable
        more expensive
        more difficult to work with than twisted-pair




000-209 Intro to CS.
11/Comm. Systems                                         26
   10. Wireless Communications

      Radio-based systems that allow transmission of
       information without a physical connection,
        no   need for copper wire or fiber-optic


      Cellular, infrared, microwave, and satellite
       broadcasting are forms of wireless communication.

      Common wireless standards include 802.11b, and
       Bluetooth.
000-209 Intro to CS.
11/Comm. Systems                                           27
   Infrared Wireless

      Infrared transmissions occur below the
       visible light of spectrum.
        requires    line-of-sight communication
           sender   must be able to see the receiver




000-209 Intro to CS.
11/Comm. Systems                                        28
   Microwaves

      High-frequency radio waves used for point-to-
       point and one-directional communication of
       audio and data.
      Requires line-of-sight, and ground stations must
       be within 30 miles of each other.
      Many communications systems transmit
       microwave signals between a land-based ground
       station and a satellite.

000-209 Intro to CS.
11/Comm. Systems                                          29
   Satellite

      Direct satellite service (DSS) uses a
       geosynchronous or low-earth orbit satellite to
       send television, voice or computer data directly
       to a satellite dish.
        transmits in one direction – downstream
        requires a standard modem and phone line for
         upstream transmission



000-209 Intro to CS.
11/Comm. Systems                                          30
   801.11

      802.11 is a family of specifications developed by
       the IEEE for wireless communications.
        IEEE = Institute of Electrical and Electronics
         Engineers


      802.11b: provide data rates of up to 11 Mbps per
       second at distances up to approximately 300 feet.
        alsocalled Wi-Fi
       line-of-sight not required
000-209 Intro to CS.
11/Comm. Systems                                           31
   Bluetooth

      An open standard for short-range (30 feet) and
       low speed (up to 1Mbps) wireless transmission of
       digital voice and data.
                     not required
        line-of-sight
        low-power (unlike 802.11b), so suitable for portable
         devices




000-209 Intro to CS.
11/Comm. Systems                                                32

								
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