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lecture4

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                                                                                     Review of lecture 3
                          EEC-682/782                                                Physical Layer
                       Computer Networks I                                                Public switched telephone network
                                                                                          Mobile telephone system
                                       Lecture 4                                          Cable television
                                                                                     Course web page
                                 Wenbing Zhao                                             http://academic.csuohio.edu/zhao_w/teaching/eec682.htm
                              w.zhao1@csuohio.edu
                       (Lecture nodes are based on materials supplied by
                                                                                     Textbook online:
                          Dr. Louise Moser at UCSB and Prentice-Hall)
                                                                                          http://proquest.safaribooksonline.com/0130661023
                                                                                     No class next week

                                                                                                           EEC-682: Computer Networks I
                                                                               Spring Semester 2005              - Wenbing Zhao                        2




   Review of Lecture 3                                                            Fourier Analysis

      Theoretical basis for data communication                                       Info is transmitted by varying voltage or current
                                                                                     Let f(t) be value of voltage or current at time t, any well-
      Guided transmission media                                                      behaved periodic function g(t) with period T can be represented
                                                                                     as Fourier series
      Wireless transmission
      Communication satellites
                                                                                     where f=1/T, the fundamental frequency, an and bn are sine and
                                                                                     cosine amplitudes of nth harmonics (terms)
                                                                                     The amplitudes and constant are given by




                                  EEC-682: Computer Networks I                                             EEC-682: Computer Networks I
Spring Semester 2005                    - Wenbing Zhao                     3   Spring Semester 2005              - Wenbing Zhao                        4
  Bandwidth-Limited Signals
                                                                         Maximum Data Rates of a Channel
   A binary signal and its root-mean-square Fourier amplitudes.
   (b) – (c) Successive approximations to the original signal.
                                                                            Theorem (Nyquist 1924) for noiseless channels
                                                                                 If an arbitrary signal is run through a low-pass filter of
                                                                                 bandwidth H, then the filtered signal can be completely
                                                                                 reconstructed by making on 2H samples per second
                                                                                 Max data rate = 2H log2V bits/sec, where signal consists of
                                                                                 V discrete lines
                                                                                 Ex: H = 3000 Hz, V = 2 (binary)
                                                                                 max data rate = 2*3000*log22 = 6000 bits/sec
                                                                                 Ex: H = 3000 Hz, V = 64
                                                                                 max data rate = 2*3000*log264 = 36,000 bits/sec




                               EEC-682: Computer Networks I                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao               5   Spring Semester 2005               - Wenbing Zhao                        6




   Maximum Data Rates of a Channel                                       Guided Transmission Data

      Theorem (Shannon 1948) noisy channels                                      Magnetic Media
           Amount of thermal noise = signal to noise ratio                       Twisted Pair
           = signal power / noise power = S/N
               Decibel (dB): 10 log10S/N
                                                                                 Coaxial Cable
           Max data rate = H log2(1+S/N) bits/sec                                Fiber Optics
           Ex: H = 3000 Hz, S/N = 30dB = 1000
           max data rate = 3000*log2(1+1000) = 30,000
           upper bound is hard to reach, 9600 bits/sec is good




                               EEC-682: Computer Networks I                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao               7   Spring Semester 2005               - Wenbing Zhao                        8
   Wireless Transmission                                                                       The Electromagnetic Spectrum

      The electromagnetic spectrum
      Radio transmission
      Microwave transmission
      Infrared and milimeter waves
      Lightwave transmission



                                                                                                   Radio, microwave, infrared, visible light
                                                                                                        Parts of electromagnetic spectrum used to transmit info by modulating
                                                                                                        amplitude, frequency, phase
                                                                                                   UV, X-ray, gamma rays are hard to produce and modulate, do not
                                                                                                   propagate well, are harmful to living things

                                 EEC-682: Computer Networks I                                                                 EEC-682: Computer Networks I
Spring Semester 2005                   - Wenbing Zhao                                   9    Spring Semester 2005                   - Wenbing Zhao                              10




   Communication Satellites                                                                     Communication Satellites
      Contains one or more transponders, each
           Listens to some part of spectrum
           Amplifies incoming signal
           Rebroadcasts at another frequency to avoid interference with
           incoming signal
               Downward beams can be broad, or
               Narrow, covering an area only hundreds of km in diameter, this mode of
               operation is known as a bent pipe
      Where to place satellites
           Van Allen belts – layers of highly charged particles trapped by the
           earth’s magnetic field
           Three regions in which satellites can be placed safely
               Geostationary Satellites (GEO)
               Medium-earth orbit satellites (MEO)                                                    Communication satellites and some of their properties,
               Low-earth orbit satellites (LEO)
                                                                                                    including altitude above the earth, round-trip delay time and
                                                                                                          number of satellites needed for global coverage.

                                 EEC-682: Computer Networks I                                                                 EEC-682: Computer Networks I
Spring Semester 2005                   - Wenbing Zhao                                   11   Spring Semester 2005                   - Wenbing Zhao                              12
   Public Switched Telephone System                                  Structure of the Telephone System
        Structure of the telephone system
        The politics of telephones
        The local loop: modems, ADSL and wireless
        Trunks and multiplexing
        Switching




                                                                                    (a) Fully-interconnected network.
                                                                                    (b) Centralized switch.
                                                                                    (c) Two-level hierarchy.
                        EEC-682: Computer Networks I                                            EEC-682: Computer Networks I
Spring Semester 2005          - Wenbing Zhao                 13   Spring Semester 2005                - Wenbing Zhao                   14




                                                                     Major Components of the Telephone System
   Structure of the Telephone System

       A typical circuit route for a medium-distance call.                 Local loops
                                                                               Analog twisted pairs going to houses and businesses
                                                                           Trunks
                                                                               Digital fiber optics connecting the switching offices
                                                                           Switching offices
                                                                               Where calls are moved from one trunk to another




                        EEC-682: Computer Networks I                                            EEC-682: Computer Networks I
Spring Semester 2005          - Wenbing Zhao                 15   Spring Semester 2005                - Wenbing Zhao                   16
                                                                                            The Local Loop: Modems, ADSL, and Wireless
   The Politics of Telephones
                                               LATA – Local Access and Transport Areas       The use of both analog and digital transmissions for a computer to
                                               LEC – Local Exchange Carrier
                                               IXC – IntereXchange Carrier                     computer call. Conversion is done by the modems and codecs.
                                               POP – Point of Presence




      The relationship of LATAs, LECs, and IXCs. All the circles
        are LEC switching offices. Each hexagon belongs to the                              Transmission lines suffer from three major problems
                        IXC whose number is on it.                                          Attenuation, delay distortion, and noise
                               EEC-682: Computer Networks I                                                           EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                                  17   Spring Semester 2005              - Wenbing Zhao                        18




   Modems                                                                                    Modems
       Modem – device used between                                                             Constellation Diagrams – legal combinations of amplitude and phase
       digital computer and analog
       telephone system. It converts
       digital bit stream into
       modulated analog signal and
       vice verse
       Codec – inverse of a modem.
       It is a device that converts a
       continuous analog signal into a
       digital bit stream
       Baud – number of samples per
       second. During each baud, one
       symbol is sent. One symbol
       can carry multiple bits
                                                 (a) A binary signal
                                                 (b) Amplitude modulation                              (a) QPSK: Quadrature Phase Shift Keying
                                                 (c) Frequency modulation                              (b) QAM-16: Quadrture Amplitude Modulation
                                                 (d) Phase modulation
                                                                                                       (c) QAM-64.
                               EEC-682: Computer Networks I                                                           EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                                  19   Spring Semester 2005              - Wenbing Zhao                        20
   Modems                                                                     Digital Subscriber Lines

                                                                                   Bandwidth versus distanced over category 3 UTP for DSL.


                                                                                                   Higher connection speed => smaller serviceable area




                  (a)                                           (b)
                       (a) V.32 for 9600 bps.
                       (b) V32 bis for 14,400 bps.


                                 EEC-682: Computer Networks I                                         EEC-682: Computer Networks I
Spring Semester 2005                   - Wenbing Zhao                 21   Spring Semester 2005             - Wenbing Zhao                           22




   Digital Subscriber Lines                                                   Digital Subscriber Lines

          Operation of ADSL using discrete multitone modulation.              A typical ADSL equipment configuration.
                                                                                                                                     NID – Network
                                                                                                                                         Interface
                                                                                                                                         Device
                                                                                                                                     Splitter – an analog
                                                                                                                                         filter that
                                                                                                                                         separates the
                                                                                                                                         voice band from
                                                                                                                                         data




                                 EEC-682: Computer Networks I                                         EEC-682: Computer Networks I
Spring Semester 2005                   - Wenbing Zhao                 23   Spring Semester 2005             - Wenbing Zhao                           24
   Wireless Local Loops                                                                Trunks and Multiplexing
                  Architecture of an LMDS system.
                                                                                          Idea: multiplex many conversations over single
                                                                                          physical channel with high bandwidth
                                                  IEEE 802.16 – wireless MAN              FDM – Frequency Division Multiplexing
                                                                                               Frequency spectrum divided into logical channel
                                                                                               Each user has exclusive use of own frequency band
                                                                                          TDM – Time Division Multiplexing
                                                                                               Time divided into slots each user has time slot
                                                                                               Users take turns in round robin fashion




                              EEC-682: Computer Networks I                                                       EEC-682: Computer Networks I
Spring Semester 2005                - Wenbing Zhao                             25   Spring Semester 2005               - Wenbing Zhao              26




  Frequency Division Multiplexing                                                      Wavelength Division Multiplexing




                 (a) The original bandwidths.
                 (b) The bandwidths raised in frequency.
                 (b) The multiplexed channel.
                              EEC-682: Computer Networks I                                                       EEC-682: Computer Networks I
Spring Semester 2005                - Wenbing Zhao                             27   Spring Semester 2005               - Wenbing Zhao              28
  Time Division Multiplexing
      T1 Carrier (1.544 Mbps)
                                                                                                 Time Division Multiplexing
          24 voice channels multiplexed together, each channel inserts 8 bits (7 data,
          1 control) into output stream in round-robin fashion
                                                                                                    Encoding systems for digitizing analog signals – use
          Why 125 microsecond?
              Telephone channel bandwidth - 4000 Hz                                                 statistical techniques to reduce number of
              Nyquist thm – codecs make 8000 samples/sec, enough to capture all info                bits/channel (signal changes slowly compared to
              This technique is called PCM (Pulse Code Modulation)
                                                                                                    sampling frequency)
                                                                                                         Differential pulse code modulation – output difference
                                                                                                         between current value and previous value rather than
                                                                                                         digitized amplitude
                                                                                                         Predictive encoding – Extrapolate previous few values to
                                                                                                         predict next value. Encode difference between actual signal
                                                                                                         and predicted one
                                                                                                         Delta modulation – Requires each sampled value to differ
                                                                                                         from its predecessor by +/-1

                                   EEC-682: Computer Networks I                                                            EEC-682: Computer Networks I
Spring Semester 2005                     - Wenbing Zhao                                  29   Spring Semester 2005               - Wenbing Zhao                        30




   Time Division Multiplexing                                                                    Time Division Multiplexing

                                 Delta modulation.                                                       Multiplexing T1 streams into higher carriers.




                                   EEC-682: Computer Networks I                                                            EEC-682: Computer Networks I
Spring Semester 2005                     - Wenbing Zhao                                  31   Spring Semester 2005               - Wenbing Zhao                        32
  Time Division Multiplexing
                                                                                      Time Division Multiplexing
      SONET – Synchronous Optical NETwork
           Standard TDM system at physical layer                                       SONET and SDH (Synchronous Digital Hierarchy) multiplex rates
           (started 1984, standardized 1989)
                                                Two back-to-back SONET frames.




                               EEC-682: Computer Networks I                                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                           33   Spring Semester 2005                  - Wenbing Zhao                34




   Switching                                                                          Circuit Switching

      Two types of switching
           Circuit switching – physical path set up from source to
           destination before any data transmitted, e.g., phone system
               Adv: no congestion problem, only delay is propagation time
               Disadv: unused bandwidth on allocated circuit is wasted
           Packet switching – store-and-forward, one hop at a time,
           uses pipelining, each packet has limited size
               Adv: low overhead – no setup required, high utilization
               Disadv: packets may be received out of order, packets may be
               lost due to buffer overflow


                                                                                                          (a) Circuit switching.
                                                                                                          (b) Packet switching.
                               EEC-682: Computer Networks I                                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                           35   Spring Semester 2005                  - Wenbing Zhao                36
  Message Switching
                                                                           Circuit Switching vs. Packet Switching




(a) Circuit switching (b) Message switching (c) Packet switching
                                EEC-682: Computer Networks I                                           EEC-682: Computer Networks I
Spring Semester 2005                  - Wenbing Zhao               37   Spring Semester 2005                 - Wenbing Zhao                             38




                                                                          Advanced Mobile Phone System (AMPS)
   The Mobile Telephone System
                                                                              Transmitter/receiver on different frequency levels
            First-Generation Mobile Phones                                         Region divided into cells (10-20km diameter), each using some set
                                                                                   of frequencies
                Analog Voice
                                                                                   Frequencies reused in non-adjacent cells
            Second-Generation Mobile Phones                                        Small cells - Less power, small cheap devices
                Digital Voice                                                      Reuse – multiple simultaneous calls on each frequency in widely
                                                                                   separated cells
            Third-Generation Mobile Phones
                Digital Voice and Data


                                                                                                                                      (a) Frequencies are
                                                                                                                                      not reused in
                                                                                                                                      adjacent cells.
                                                                                                                                      (b) To add more
                                                                                                                                      users, smaller cells
                                                                                                                                      can be used.
                                EEC-682: Computer Networks I                                           EEC-682: Computer Networks I
Spring Semester 2005                  - Wenbing Zhao               39   Spring Semester 2005                 - Wenbing Zhao                             40
     Advanced Mobile Phone System
                                                                                         Advanced Mobile Phone System
       Base station – at the center of each cell, to which all phones in
       the cell transmit                                                                    Uses FDM
           Consists of computer and transmitter/receiver connected to an
           antenna                                                                               832 transmission channels, 30kHz wide
           Connected to Mobile Telephone Switching Office (MTSO), or                             832 receive channels, 30kHz wide
           Mobile Switching Center (MSC)
       Handoff – the process of breaking up connection with an old cell                     The 832 channels are divided into four categories
       and reestablish connection with a new cell                                                Control (base to mobile) to manage the system
           At any time, mobile phone is logically in one cell and under control of                   21 channels
           that cell’s base station
                                                                                                 Paging (base to mobile) to alert users to calls for them
           When mobile leaves a cell, its base station notices the phone signal
           fading and asks surrounding base stations how much power they’re                      Access (bidirectional) for call setup and channel assignment
           getting from the phone                                                                Data (bidirectional) for voice, fax, or data
           Base station then transfers ownership to the cell getting the
           strongest signal
           Phone is then informed of its new base station and switches to a
           new channel
                               EEC-682: Computer Networks I                                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                              41   Spring Semester 2005               - Wenbing Zhao                         42




   Advanced Mobile Phone System                                                          Advanced Mobile Phone System

      Call management                                                                       Making a call
           Each mobile phone has 32-bit serial number, 10-digit phone                            Mobile user switches on phone, enters number to be called
           number, in its PROM                                                                   on keypad, hits SEND button
           When switched on, mobile scans list of 21 control channels                            Phone sends number to be called and its own id on access
           to find most powerful signal                                                          channel
           Mobile then broadcasts its serial number and phone number                             When base station gets request, it informs MTSO
           When base station hears this, it tells MTSO, which records                            MTSO looks for idle channel for call and sends channel
           the new customer and informs the home MTSO of new                                     number back on control channel
           customer’s location                                                                   Mobile switches to selected voice channel and waits for
           Mobile reregisters every 15 minutes                                                   called party to pick up phone




                               EEC-682: Computer Networks I                                                        EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                              43   Spring Semester 2005               - Wenbing Zhao                         44
                                                                                     Digital Advanced Mobile Phone System (D-AMPS)
   Advanced Mobile Phone System
                                                                                            2nd generation of AMPS system
                                                                                                Can co-exist with AMPS
      Receiving a call
                                                                                                Void signal digitized and compression at the phone
           All idle phones continuously listen to paging channel to
           detect messages for them                                                             Multiple users per channel, using TDM
           When a call is placed to a mobile phone, a packet is sent to
           mobile’s MTSO, which sends a packet to mobile’s base
           station
           Base station broadcasts on paging channel
           Mobile responds on control channel
           Base station tells mobile phone which channel to switch to
           Mobile switches to channel and starts ringing

                                                                                        (a) A D-AMPS channel with three users.       (b) A D-AMPS channel with six users.


                               EEC-682: Computer Networks I                                                          EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                            45   Spring Semester 2005                   - Wenbing Zhao                                   46




                                                                                      Global System for Mobile Communications (GSM)
   Digital Advanced Mobile Phone System
                                                                                          D-AMPS widely used in US. Everywhere else uses GSM
                                                                                          To 1st approximation, GSM is similar to D-AMPS
      MAHO – Mobile Assisted Handoff
                                                                                              Both use FDM, transmitting on one frequency, receiving on a higher
           In D-AMPS, 1/3 of the time a mobile is neither sending nor                         frequency; a single frequency pair is split by TDM into time slots shared
           receiving. It uses the idle slots to measure the line quality.                     by multiple mobiles
           When it discovers that the signal is waning, it complains to MTSO              Diff: GSM channels are much wider than AMPS channels
           MTSO then break the connection                                                     200kHz vs. 30kHz
           In the meantime, the mobile can try to tune to a stronger signal               GSM uses 124 frequency channels, each of which uses an eight-slot
           from another base station                                                      TDM system
           Still takes ~300msec




                               EEC-682: Computer Networks I                                                          EEC-682: Computer Networks I
Spring Semester 2005                 - Wenbing Zhao                            47   Spring Semester 2005                   - Wenbing Zhao                                   48
   GSM                                                                                    CDMA – Code Division Multiple Access
              A portion of the GSM framing structure.                                        Works completely differently from TDM and FDM
                                                                                                  Each station transmits over entire frequency spectrum all
                                                                                                  the time
                                                                                                  Each station is assigned a unique m-bit code, called a chip
                                                                                                  sequence
                                                                                                      To transmit a 1, it sends its chip sequence
                                                                                                      To transmit a 0, it sends the one’s complement of its chip
                                                                                                      sequence
                                                                                                      Assumes multiple signals add linearly – colliding frames are not
                                                                                                      garbled!
                                                                                                      Chip sequences for different stations are othogonal




                          EEC-682: Computer Networks I                                                                  EEC-682: Computer Networks I
Spring Semester 2005            - Wenbing Zhao                                    49   Spring Semester 2005                   - Wenbing Zhao                               50




                                                                                         Third-Generation Mobile Phones
   CDMA – Code Division Multiple Access
                                                                                            IMT-2000 – ITU issued a blue print for 3rd Generation mobile
                                        (a) Binary chip sequences for four                  telcomm in 1992, hoping to go service in 2000
                                        stations                                                IMT - International Mobile Telecommunications
                                        (b) Bipolar chip sequences (+1 for 1,
                                                                                                Basic services an IMT-2000 network should provide
                                        and -1 for 0)
                                                                                                    High-quality voice transmission
                                        (c) Six examples of transmissions
                                                                                                    Messaging (replace e-mail, fax, SMS, chat, etc.)
                                        (d) Recovery of station C’s signal
                                                                                                    Multimedia (music, videos, films, TV, etc.)
                                                                                                    Internet access (web surfing, w/multimedia.)
                                                                                                Technology based on CDMA
                                                                                                    W-CDMA from Ericsson and CDMA2000 from Qualcomm
                                                                                                Did not happen in year 2000
                              Let S be m-chip vector for station S
                              All chip sequences are orthogonal                                 2.5G services
                                  Normalized inner product of any two                               EDGE (Enhanced Data rates for GSM Evolution) – GSM with more bits per
                                  distinct chip seq, S and T, is 0, i.e., S*T=0                     baud
                              S*S = 1                                                               GPRS (General Packet Radio Service) – overlay packet network on top of D-
                                                                                                    AMPS or GSM

                          EEC-682: Computer Networks I                                                                  EEC-682: Computer Networks I
Spring Semester 2005            - Wenbing Zhao                                    51   Spring Semester 2005                   - Wenbing Zhao                               52
   Cable Television                                           Community Antenna Television

                                                                                  An early cable television system.
             Community Antenna Television
             Internet over Cable
             Spectrum Allocation
             Cable Modems
             ADSL versus Cable




                       EEC-682: Computer Networks I                                        EEC-682: Computer Networks I
Spring Semester 2005         - Wenbing Zhao           53   Spring Semester 2005                  - Wenbing Zhao           54




   Internet over Cable                                        Internet over Cable

                                                              The fixed telephone system.




                       EEC-682: Computer Networks I                                        EEC-682: Computer Networks I
Spring Semester 2005         - Wenbing Zhao           55   Spring Semester 2005                  - Wenbing Zhao           56
   Spectrum Allocation                                                         Cable Modems

                  Frequency allocation in a typical cable                                     Typical details of the upstream and
                    TV system used for Internet access                                           downstream channels in North
                                                                                                             America.




                            EEC-682: Computer Networks I                                                EEC-682: Computer Networks I
Spring Semester 2005              - Wenbing Zhao                       57   Spring Semester 2005              - Wenbing Zhao           58




                          Cable Modems

      When cable modem is switched on, it scans the downstream
      channels looking for a special packet periodically sent out by
      the headend to provide system parameters
      Cable modem then announces its presence and the headend
      responds by assigning the modem to its upstream and
      downstream channels
      Modem then determines the distance from headend (ranging)
      Upstream channels are divided in time in minislots. Modem must
      request minislots before sending
      Headend also assign each modem to a minislot for upstream
      bandwidth request
      All data traffic is encrypted


                            EEC-682: Computer Networks I
Spring Semester 2005              - Wenbing Zhao                       59

								
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