The Physical Layer by gjmpzlaezgx


									  Chapter 2
The Physical Layer

       The Theoretical Basis for Data
• Fourier Analysis
   – Any reasonably behaved periodic function can be written as
     Fourier series.
• Bandwidth-Limited Signals
   – How fast a signal can be transmitted depends on the
     bandwidth, general meaning of how much information can
     be carried in a given time period (usually a second) over a
     communication link, measured mostly by frequency range.
• Maximum Data Rate of a Channel

  Theory of Data Communications
• The signal (for example, measured in volts) can be
  viewed as
   a) a function of time, g(t), or
   b) a function of frequency, G(f).
• Time-Domain
   – Let g(t) denote the voltage on a wire at time t.
   – A signal, g(t), is periodic with period T if g(t+T)=g(t) for all
   – A signal is discrete if it only takes on a finite number of
   – The fundamental frequency is the inverse of the period, f =
     1/T, and is measured in cycles per second (Hz).
      Frequency-Domain Analysis
• Any "reasonably-behaved" periodic function, g(t), can
  be written as a Fourier Series - that is broken up into
  components with different frequencies.
• The time, T, required to transmit a character depends
   a) the encoding method
   b) the signalling speed or baud rate; that is, how many times
      per second the signal changes its value (voltage).
• Baud rate is not necessarily the same as bit rate. For
  example, if the values 0, 1, 2, 3, 4, 5, 6, 7 are used in a
  signal, then each signal value can represent 3 bits.
  That is 1 baud = 3 bps.
        Bandwidth-Limited Signals

• A binary signal (‘b’ = 01100010) and its root-mean-square
   Fourier amplitudes.
(b) – (c) Successive approximations to the original signal.
       Bandwidth-Limited Signals

(d) – (e) Successive approximations to the original signal.
      Frequency-Domain Analysis
• Below we will only consider 2 voltage levels, so the
  bit rate is the same as the baud rate.
   – Let b = bit rate (measured in bits per second (bps)).
   – Then, it takes 8/b seconds to send 8 bits (one character).
   – So, T = 8/b, and the fundamental frequency is b/8 Hz.
• A voice grade line is an ordinary telephone line and
  has an artificial cutoff frequency, fc, of about 3000Hz.
  So, the number of the highest harmonic that can be
  passed through is 3000/(b/8) = 24000/b. Note the
  highest harmonic has a frequency that is a multiple of
  the fundamental frequency (b/8) and the highest
  harmonic can have a frequency no more than 3000Hz.    7
Bandwidth-Limited Signals

  Relation between data rate and harmonics.
Maximum Data Rate of a Channel
• Noiseless channel: Nyquist’s Theorem – If the signal
  has V discrete levels over a transmission medium of
  bandwidth H , the maximum data rate = 2H log2 V
   – Example: a noiseless 3-kHz channel cannot transmit binary
     signals at a rate exceeding 6000 bps (= 2 x 3000 log2 2).
• Noisy Channel: Shannon’s Theorem
  maximum data rate = H log2 (1 + S/N) bits/sec H:
  bandwdith, S: signal power, N: noise power
   – S/N (Signal-to-noise ratio), usually measured as 10 log10S/N
     in db = decibels, is called thermal noise ratio.
             Physical Interfaces
• Physical layer is responsible for the generation,
  transmission, and receipt of binary data
• Generation and Receipt
  – Conversion of data between binary and analog
  – E.g. wire: voltage is applied
     • +V means a 1
     • -V means a 0
     • 0V means no data

             Physical Interfaces



     0   1      1   0   1   0      0   1

            Physical Interfaces
• Errors in physical layer:
   – Attenuation (reduced signal)
   – Distortion (wrong signal)
• Influences to error:
   – Type of Media
   – Bit Rate
   – Distance
• Finally, binary values are passed to Data Link

       Guided Transmission Data
•   Magnetic Media
•   Twisted Pair
•   Coaxial Cable
•   Fiber Optics

      Guided Transmission Data
• Magnetic Media: magnetic tape or removable
• Consider an industry standard Ultrium tape
  – It can hold 200 gigabytes.
  – A box 60x60x60 cm can hold about 1000 of these
    tapes. Total capacity is 200 terabytes or 1600
    terabits (1.6 petabits).
  – The box can be sent to anywhere in US in 24 hours.
    The effect bandwidth is 1600 terabits/86,400 sec, or
    19 Gbps.
  – If it is sent within an hour drive, the bandwidth is
    increased to over 400 Gbps. No computer network
    can even approach this.
                   Twisted Pair
• Properties
   –   A twisted pair consists of two insulated copper wires.
   –   Why twisted? Countervail the magnetic field
   –   Used in telephone and local area networking
   –   Run several kilometers
   –   The bandwidth depends on the thickness of the wire and
       distance travelled.
• Common types: UTP (Unshielded Twisted Pair)
   – Category 3: bandwidth of 16 MHz
   – Category 5: more twists per centimeter, which results in less
     crosstalk and better-quality signal over longer distance,
     bandwidth of 100 MHz
   – Category 6 and 7: 250 MHz and 600 MHz
 Twisted Pair

(a) Category 3 UTP.
(b) Category 5 UTP.
              Coaxial Cable
• 50-ohm cable for digital transmission
• 75-ohm cable for analog transmission and cable
• 1 GHz
• Local area networking and CATV

Coaxial Cable

  A coaxial cable.
                     Fiber Optics
• Glass is used instead of copper wires
• Light is transmitted instead of electrical current
• Components:
   – Light source
   – Transmission medium
   – Detector: convert light plus and electronic signal
• Single-mode fiber
   – Different rays bouncing around at different angle are said to
     be a multimode fiber.
   – If the fiber’s diameter is reduced to a few wavelength of
     light, the light can propagate in a straight line without
     bouncing, yielding a single-mode fiber.
   – 50 Gbps for 100 km                                        19
                     Fiber Optics

(a) Three examples of a light ray from inside a silica fiber impinging
     on the air/silica boundary at different angles.
(b) Light trapped by total internal reflection.
Transmission of Light through Fiber
• Three bands are used: 0.85, 1.3, 1.55 μm

     Attenuation of light through fiber in the infrared region.   21
                     Fiber Optics
• Ways to connect fibers:
   – Terminate in connectors and plugged into fiber sockets: 10 ~
     20% light lose
   – Spliced mechanically: 10% light lose
   – fused
• Comparison of fiber optics and copper wire
   – Advantages:
      • Higher bit-rates, immune to interference, hard to tap
   – Disadvantages:
      • Less familiar technology, unidirectional, easily damaged,
        expensive interfaces
              Fiber Cables

(a) Side view of a single fiber.
(b) End view of a sheath with three fibers.   23
                    Fiber Cables
• Light sources: LED (Light Emitting Diodes) and
  semiconductor lasers.
• The receiving end consists of a photodiode.

A comparison of semiconductor diodes and LEDs as light sources.
Fiber Optic Networks

A fiber optic ring with active repeaters.
     Fiber Optic Networks

A passive star connection in a fiber optics network.
          Wireless Transmission
•   The Electromagnetic Spectrum
•   Radio Transmission
•   Microwave Transmission
•   Infrared and Millimeter Waves
•   Lightwave Transmission

           Wireless Transmission
• λf = c where λ is the wavelength, f is the frequency,
  and c is the speed of light, 3 x 108 m/s
• Two basic modulation techniques used in spread
  spectrum signal transmission:
   – Frequency hopping: The transmitter hops from frequency to
   – Direct sequence: The signal is spread over a wide frequency
     band with specific coding for each channel.
       • The stream of information to be transmitted is divided
         into small pieces, each of which is allocated across to a
         frequency channel across the spectrum.
       • A data signal at the point of transmission is combined
         with a higher data-rate bit sequence (also known as a
         chipping code).                                         28
        Electromagnetic spectrum
•   LF (Low Frequency, 105 Hz): maritime
•   MF (Medium Frequency, 106 Hz): AM radio
•   HF (High Frequency, 107 Hz): radio
•   VHF (Very High Frequency, 108 Hz): FM radio, TV
•   UHF (Ultra High Frequency: 109 Hz): TV, terrestrial
•   SHF (Super High Frequency:1010 Hz): Satellite, microwave
•   EHF (Extremely High Frequency, 1011 Hz)
•   THF (Tremendously High Frequency, 1012 Hz)
•   Higher frequency: IHF?, AHF?, PHF? (Incredibly,
    Astonishingly, Prodigiously).
  The Electromagnetic Spectrum

The electromagnetic spectrum and its uses for communication.
             Radio Transmission

(a) In the VLF, LF, and MF bands, radio waves follow the
      curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.
Politics of the Electromagnetic Spectrum
• Allocate spectrum policies
   – Beauty contest requires each carrier to explain why its
     proposal serves the public interest best.
   – Lottery
   – Auction
• Open band: Frequencies are not allocated but
  restrained in a short range.

              The ISM bands in the United States.              32
           Lightwave Transmission

Convection currents can interfere with laser communication systems.
     A bidirectional system with two lasers is pictured here. 33
        Communication Satellites

•   Geostationary Satellites (GEO)
•   Medium-Earth Orbit Satellites (MEO)
•   Low-Earth Orbit Satellites (LEO)
•   Satellites versus Fiber

       Communication Satellites

• Geostationary Satellites (GEO)
  – VSAT (Very Small Aperture Terminals): 1-meter
    antennas, DirecPC
• Low-Earth Orbit Satellites (LEO)
  – Iridium: 66 satellites
  – Globalstar: 48 satellites
  – Teledesic: 30 satellites

    Communication Satellites

Communication satellites and some of their properties,
 including altitude above the earth, round-trip delay time
   and number of satellites needed for global coverage. 36
Communication Satellites

    The principal satellite bands.
Communication Satellites

      VSATs using a hub.
         Low-Earth Orbit Satellites

(a) The Iridium satellites from six necklaces around the earth.
(b) 1628 moving cells cover the earth.                        39

(a) Relaying in space: Iridium
(b) Relaying on the ground: Globalstar   40
       Satellites versus Fiber
• A single fiber has more bandwidth but is not available
  to most users.
• Satellites are possible for mobile communication.
• Satellites are cheaper for Broadcasting.
• Satellites can be deployed in places with hostile terrain
  or a poorly developed terrestrial infrastructure such as
• Satellites can be deployed in areas where obtaining the
  right for laying fiber is difficult.
• Satellites is possible for rapid military communication
Public Switched Telephone System
•   Structure of the Telephone System
•   The Politics of Telephones (FYI)
•   The Local Loop: Modems, ADSL and Wireless
•   Trunks and Multiplexing
•   Switching

 Structure of the Telephone System
• The PSTN (Public Switched Telephone Network) is the world's
  collection of interconnected voice-oriented public telephone
  networks. It's also referred to as the POTS (Plain Old Telephone

     (a) Fully-interconnected network.
     (b) Centralized switch.
     (c) Two-level hierarchy.                                43
Structure of the Telephone System

   A typical circuit route for a medium-distance call.
 Major Components of the Telephone
• 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

       The Politics of Telephones
• LATA (Local Access and Transport Area) is a
  geographic area covered by one or more local
  telephone companies, which are legally referred to as
  local exchange carriers (LECs).
• LEC (Local Exchange Carrier) is a public telephone
  company in the U.S. that provides local service. Some
  of the largest LECs are the Bell operating companies
• IXC (IntereXchange Carrier) is a company handling
  inter-LATA traffic such as AT&T, MCI, and Sprint.
• A POP (Point of Presence) is a switching office built
  to handle calls from a LATA.                        46
   The Politics of Telephones

The relationship of LATAs, LECs, and IXCs. All the
    circles are LEC switching offices. Each hexagon
        belongs to the IXC whose number is on it.     47
The Local Loop: Modems, ADSL, and
• Transmission lines suffer from three major
  – Attenuation
  – Delay distortion
  – Noise
• The square waves used in digital signals have a
  wide frequency spectrum (usually, high
  frequency) and thus are subject to strong
  attenuation and delay distortion.

The use of both analog and digital transmissions for a computer to
   computer call. Conversion is done by the modems and codecs.49
• The modulation is introduced to solve this
  – Amplitude: two different amplitudes are used to
    represent 0 and 1.
  – Frequency: different tones are used.
  – Phase: the wave is systematically shifted (45, 135,
    225, or 315º).
• A modem (modulator-demodulator) is a device
  that modulates outgoing digital signals to
  analog signals.                            50

(a) A binary signal        (c) Frequency modulation
(b) Amplitude modulation   (d) Phase modulation       51
• The number of samples/symbols per second is
  measured in baud.
• In quadrature phase-shift keying (QPSK), the
  four angles, usually out of phase by 90°, are
  used to transmit 2 bits/symbol. The bit rate is
  twice the baud rate.
• QAM-64 (Quadratrue Amplitude Modulation-
  64) allows 64 different combinations, so 6 bits
  can be transmitted per symbol.

Constellation Diagrams:
(a) QPSK.
(b) QAM-16.
(c) QAM-64.                 53
• To reduce the chance of an error, standards for higher
  speeds modems do error correction by adding extra bits
  to each sample. The schemes are known as TCM
  (Trellis Coded Modulation).
• In V.32, 14,400 bps is achieved by transmitting 6 data
  bits and 1 parity bit per sample at 2400 baud. It uses
• In V.34, the modem can run at 28,800 bps at 2400 baud
  with 12 data bits/symbol or 33,600 bps at 2400 baud
  with 14 data bits/symbol.

(a)                             (b)

  (a) V.32 for 9600 bps.
  (b) V32 bis for 14,400 bps.
• Why are 56 kbps modems in use?
  – The telephone channel is about 4000 Hz (300 ~
    3400 Hz).
  – The maximum data rate = 2 x 4000 log2 2 = 8000
  – The number of bits per sample is 8, one for control
    purpose, allowing 8000 x 7 = 56,000 bit/sec.
• V.90 provides 33.6 kbps upstream and 56 kbps
• V.92 provides 48 kbps upstream.

• A connection that allows traffic in both
  directions simultaneously is called full duplex.
• A connection that allows traffic either way, but
  only one way at a time is called half duplex.
• A connection that allows traffic only one way is
  called simplex.

  Bandwidth, Baud Rate, Bit Rate
• The bandwidth of a medium is the range of
  frequencies that pass through it with minimum
  attenuation, usually, measured in Hz.
• The baud rate is the number of samples/sec
  made. Each sample sends one symbol.
• The bit rate is the amount of information sent
  over the channel and is equal to the number of
  symbols/sec times the number of bits/symbol.

    Digital Subscriber Lines (DSL)
• xDSL is made to work by connecting to a different
  switch instead of the filter that attenuates all
  frequencies below 300 Hz and above 3400 Hz.
• The xDSL services have been designed with the
  following goals:
   – They must work over the existing category 3 twisted pair
     local loops.
   – They must not affect existing telephones and fax machines.
   – They must be faster than 56 kbps.
   – They must be always on.

      Digital Subscriber Lines

Bandwidth versus distanced over category 3 UTP for DSL.
   Digital Subscriber Lines (DSL)
• DMT (Discrete MultiTone) divides the 1.1 MHz
  spectrum available on the local loop into 256
  independent channels of 4312.5 Hz each.
  –   Channel 0: POTS
  –   Channel 1-5: not used
  –   One for upstream and one for downstream control
  –   32 channels for upstream and rest for downstream
• The ADSL standard (ANSI T1.413 and ITU G.992.1)
  allows speeds of 8 Mbps downstream and 1 Mbps
     Digital Subscriber Lines

Operation of ADSL using discrete multitone modulation.
Digital Subscriber Lines

A typical ADSL equipment configuration.
              Wireless Local Loops
• Business practice of a long-distance telephone
  company for the local phone service:
   –   It must buy or lease a building for the end office.
   –   It must fill the end office with switches.
   –   It must run a fiber between the end office and the toll office.
   –   It must acquire customer.
• How is the new local phone company to connect
  customer telephones and computers in the end office?
   – Buy the right to lay the new wires. Costly
   – Buy/lease from other local phone company. Costly
   – Use the WWL (Wireless Local Loop).
            Wireless Local Loops
• A fixed telephone using a wireless local loop is
  different from a mobile phone in three ways:
   – The wireless local loop customer often wants high-speed
     Internet connectivity.
   – A directional antenna is needs to be installed.
   – The user does not move.
• LMDS (Local Multipoint Distribution System) is a
  system for broadband microwave wireless transmission
  direct from a local antenna to homes and businesses
  within a line-of-sight radius, a solution to the so-called
  last-mile technology problem of economically bringing
  high-bandwidth services to users.
• The IEEE 802.16 can be used for wireless local loops
  standard.                                              65
Wireless Local Loops

 Architecture of an LMDS system.
         Trunks and Multiplexing
• Two categories of multiplexing schemes are used to
  multiplex many conversations over a single physical
   – In FDM (Frequency Division multiplexing), the frequency
     spectrum is divided into frequency bands. For fiber optic
     channels, WDM (Wavelength Division Multiplexing) is
   – In TDM (Time Division Multiplexing), the entire
     bandwidth is used for a chunk of time period.

Frequency Division Multiplexing

    (a) The original bandwidths.
    (b) The bandwidths raised in frequency.
    (b) The multiplexed channel.              68
Wavelength Division Multiplexing

       Wavelength division multiplexing.
        Time Division Multiplexing
• The analog signals are digitalized by a device called a codec
  (coder-decoder) producing a 7 or 8 bit number.
• PCM (Pulse Code Modulation) is a technique to digitalize
  analog data.
   – T1 carriers can handle 24 channels multiplexed together. 24 x 8 = 192
     bits + 1 bit for framing = 193 bits/frame
   – Since each analog signal must be sampled 8000 times per second, we
     must repeat this process every 1/8000 sec = 125 microseconds.
   – So, the transfer rate on the T1 carrier is: 192 bits / 0.000125 seconds =
     1.544 Mbps.
• DPCM (Differential Plus Code Modulation) is a method,
  which consists of outputting the difference between the current
  value and the previous one, to reduce the number of digitalized
  bits,                                                        70
Time Division Multiplexing

     The T1 carrier (1.544 Mbps).
Time Division Multiplexing

        Delta modulation.
Time Division Multiplexing

 Multiplexing T1 streams into higher carriers.
• SONET (Synchronous Optical NETwork) is the
  American National Standards Institute standard for
  synchronous data transmission on optical media.
• SDH (Synchronous digital hierarchy) is the
  international standard for synchronous data
  transmission on optical media.
• The goal of SONET:
   –   Possible for different carriers
   –   Unify the U.S., European, and Japanese digital systems
   –   Provide a way to multiplex multiple digital channels
   –   Provide support for operations, administration, and
       maintenance (OAM)
• Synchronous Optical Network (SONET)
  – The full specification is larger than this book.
  – It addresses both the framing and encoding
  – It multiplexes several low-speed links onto one
    high-speed link.

• SONET Frame Structure: (Synchronous Transport
  – 9 x 90 = 810 bytes
  – The first three columns are reserved for system management
  – The first 9 rows contain the overhead. Overhead has multiple
    functions: across different links, specify voice channel,
    concatenation frames.
  – The remaining 87 columns hold the user data, called the SPE
    (Synchronous Payload Envelope). The first column is the
    overhead for the sublayer.
  – STS-N frame can be thought of as consisting of N STS-1
    frames.                                                   76
Time Division Multiplexing

    Two back-to-back SONET frames.
         Time Division Multiplexing

                   SONET and SDH multiplex rates.
• STS (Synchronous Transport Signal)
• OC (Optical Carrier): OC-256 – 13.271 Gbps, OC-768 – 40 Gbps
• Synchronous Transport Modules (STM)                            78
• Circuit switching – seek out a physical path from
  sender to receiver. An end-to-end path must be
  (conceptually) established before data is sent.
• Message switching – no path is established in advance.
  The message is stored in the first switching office and
  forwarded later one hop at a time.
   – Example: store-and-forward network
   – Problem: No restriction of block size
• Packet switching – place a restriction on block size, to
  allow packets to be buffered in main memory at the
  switching office.
   – Advantages:
      Well-suited for interactive traffic
      • Improved response time and throughput         79
Circuit Switching

(a) Circuit switching.
(b) Packet switching.    80
               Message Switching

(a) Circuit switching (b) Message switching (c) Packet switching
               Packet Switching

A comparison of circuit switched and packet-switched networks.
   The Mobile Telephone System

• First-Generation Mobile Phones:
  Analog Voice

• Second-Generation Mobile Phones:
  Digital Voice

• Third-Generation Mobile Phones:
  Digital Voice and Data
Politics and Issues of Mobile Telephones
• At first, the U.S. had a single mobile phone system. In Europe
  every country devises its own system.
• Then Europe learned from mistake and standardized on a single
  system (GSM). By then, the U.S. deregulated the standard. As a
  consequence, the U.S. has two major and one minor
  incompatible system.
• Mobile phone ownership and usage in Europe is far greater than
  in the U.S.
   – A single system for all of Europe
   – In the U.S. the telephone companies charge the mobile phone owners for
     incoming call to keep callers from getting nervous about using the
• The widespread use of prepaid mobile phones in Europe (up to
  75% in some areas) and Asia.
    The Mobile Telephone System

• Every cellular system digital or analog is
  comprised of four parts.
  1. Cells and cell sites (base stations)
  2. Switching station ( mobile telephone switching
     office, MTSO )
  3. System operator and its local office
  4. Cellular telephones

    The Mobile Telephone System
• The heart of the system is made up of individual radio
  coverage areas called cells. Each cell is a self-contained
  calling area.
• Within the cell, a cell site is strategically positioned as
  a base station for receiving, sending and routing the
  radio signals of cellular phone calls.
• All cell sites are connected to the Mobile Telephone
  Switching Office (MTSO).
   – It provides connection into the Public Switched Telephone
     network ( PSTN ) - the local telephone company.
   – It provides other central functions, including call processing,
     traffic management, and transferring calls as a phone moves
     between cell sites.
The Mobile Telephone System

   The Mobile Telephone System
• Making a call
  – When a cellular user makes a call from a cellular
    phone, radio signals are transmitted to the cell site.
  – The cell site alerts the Mobile Telephone Switching
    Office (MTSO) switching station. The MTSO, in
    turn, provides an open channel ( frequency ) and
    connects the call to the Public Switched Telephone
    Network ( PSTN ).
  – The PSTN put the call through to the number to be
    reached. This process takes the same amount of time
    that it takes to make a call from a land line phone.
    The Mobile Telephone System
• Receiving a call
   – Once the MTSO receives a call, it searches for the correct
     cellular phone by sending out data over the radio waves.
   – Cellular phones in standby mode continuously scan the radio
     waves being transmitted by the MSTO. If a phone hears its
     telephone number, it sends back a signal that informs the
     closest cell site of its Electronic Serial Number (ESN) and its
     telephone number (Mobile Identification Number or MIN).
   – The cell site passes this information to the MTSO, where the
     ESN and MIN are verified and a channel (frequency) is
     assigned for the call.
   – The cellular phone receives the message directing it to tune to
     the correct voice channel. The cell site makes the voice
     channel available, and the call is completed.
    The Mobile Telephone System
• Hand-off is the transfer of a call from one cell site to
  another as the cellular phone moves through the service
  coverage area.
   – The cell site warns the MSTO that the mobile's signal
     strength is falling below a predetermined level.
   – The MTSO then alerts all cell sites bordering on the first one.
     They measure the mobile's transmitting signal and report
     back to the MTSO.
   – The MTSO, which is programmed to select the site receiving
     the strongest signal, then switches the call from the weak cell
     to the strongest cell without interrupting the call.

    The Mobile Telephone System
• Roaming is a service offered by most cellular service
  providers that allows subscribers to use cellular service
  while traveling outside their home service area.
   – When they are outside their home service area and come
     within range of another cellular system, the ROAM indicator
     on the cellular phone will light to show that they are in range.
   – When they roam (operate outside their home system), their
     cellular phone will seek service from the same type of
     cellular system as the one they subscribe to at home. But if
     that type is not available where they are roaming, the phone
     will try to obtain service from the non-home-type system. A
     blinking light indicates a non-home-type system. There is an
     extra charge for calls placed while roaming.                91
Advanced Mobile Phone System

(a) Frequencies are not reused in adjacent cells.
(b) To add more users, smaller cells can be used.   92
   Advanced Mobile Phone System
• AMPS (Advanced Mobile Phone System) is the analog
  system (1G) first developed and used in the U.S.
• The AMPS system uses FDM to separate 832 full-
  duplex channels.
   – 832 simplex transmission channels from 824 to 849 MHz
   – 832 simplex receive channels from 869 to 894 MHz
   – Each simplex channel is 30 kHz wide.
• These channels are divided into four categories:
   –   Control (base to mobile) to manage the system (21 channels)
   –   Paging (base to mobile) to alert users to calls for them
   –   Access (bidirectional) for call setup and channel assignment
   –   Data (bidirectional) for voice, fax, or data (45 channels)
• D-AMPS (Digital-AMPS) is the first digital version
  (2G) of AMPS.
   – It uses the 800 or 1900 MHz spectrum.
   – Each simplex channel is 30 kHz wide.
   – It is described in IS-54 and IS-136.
• It is also known as TDMA (Time Division Multiple
   – Several physical channels are located by dividing one
     frequency channel into several time slots.
   – The advantage of TDMA is that several channels are co-
     located on one carrier frequency, so there are less transmitters
Digital Advanced Mobile Phone System

    (a) A D-AMPS channel with three users.
    (b) A D-AMPS channel with six users.
• GSM (Global System for Mobile communications)
  is a digital voice or data cellular network used
  throughout the world.
  – The European version of GSM operates at the 900 MHz and
    1800 MHz frequencies.
  – The North American version of GSM, called GSM 1900,
    operates at the 1900 MHz frequency.
  – Each simplex channel is 200 kHz wide.
  – Connection rate is up to 9.6K bps
  – American Personal Communications (APC), a subsidiary of
    Sprint, is using GSM as the technology for a broadband
    personal communications service (PCS).
Global System for Mobile Communications

      GSM uses 124 frequency channels, each of which
                uses an eight-slot TDM system          97

A portion of the GSM framing structure.
• CDMA (Code Division Multiple Access) is a
  standard using spread spectrum transmission
  – The original CDMA standard, also known as
    cdmaOne and still common in cellular telephones in
    the U.S., offers a transmission speed of up to 14.4
    Kbps in its single channel form and up to 115 Kbps
    in an eight-channel form.
  – It operates in the 800 and 1900 MHz bands.
  – Each simplex channel is 1.25 MHz wide.
  – It can carry data at rates up to 115 kbps.
• Operation of CDMA:
  – In CDMA, the input signals are digitized and transmitted in
    coded, spread-spectrum mode over a broad range of
  – In CDMA, each bit time is subdivided into m short intervals
    called chips. Typically, there are 64 or 128 chips per bit.
  – Each station is assigned a unique m-bit code called a chip
  – To transmit a 1 bit, a station sends its chip sequence. To
    transmit a 0 bit, the station sends the one’s complement of its
    chip sequence.

CDMA – Code Division Multiple Access

    (a) Binary chip sequences for four stations
    (b) Bipolar chip sequences
    (c) Six examples of transmissions             101
    (d) Recovery of station C’s signal
       Third-Generation Mobile Phones:
            Digital Voice and Data
• Factors which drives the telephony industry:
   1. Data traffic exceeds voice traffic.
   2. Design a lightweight portable device with versatile functions
      (telephone, music player, gaming device, digital camera,
      Web interface, and more).
• IMT-2000 (International Mobile Telecommunication-
  2000) network should provide
   –   High-quality voice transmission
   –   Messaging (replace e-mail, fax, SMS, chat, etc.)
   –   Multimedia (music, videos, films, TV, etc.)
   –   Internet access (web surfing, w/multimedia.)
   Third-Generation Mobile Phones:
        Digital Voice and Data
• Two main IMT-2000 proposals (differences in
  coding methods):
  – W-CDMA (wideband code-division multiple access) by
     • W-CDMA can support communications at up from 384
       Kbps to 2 Mbps
     • A 5 MHz-wide channel is used.
     • UMTS (Universal Mobile Telecommunication System) is
       the system pushed by the EU.
  – CDMA2000 by Qualcomm.
     • CDMA2000 can support mobile data communications at
       speeds ranging from 144 Kbps to 2 Mbps.
     • A 5 MHz-wide channel is used.                   103
     2.5-Generation Mobile Phones:
         Digital Voice and Data
• EDGE (Enhanced Data rates for GMS
  Evolution) is GSM with more bits per baud.
• GPRS (General Packet Radio Service) is a data
  service that can be layered onto D-AMPS or
  – It allows mobile stations to send and receive IP
  – Each channel is 200 kHz wide.
  – Data rates of up to 115 kbps
             Cable Television

•   Community Antenna Television
•   Internet over Cable
•   Spectrum Allocation
•   Cable Modems
•   ADSL versus Cable

   Community Antenna Television
• The head end is an amplifier to strengthen the signals.
• Cable television was initially called community
  antenna television.

              An early cable television system.
             Internet over Cable
• HFC (Hybrid Fiber Coax) system is a system with fiber
  for the long-haul and coaxial cable to the houses.

       Cable television
  Internet over Cable

The fixed telephone system.
             Internet over Cable
• Spectrum Allocation
  – 5 – 42 Mhz: upstream channels
  – 54 Mhz ↑: downstream channels
  – A 6 Mhz or 8 Mhz downstream channel is modulated with
    QAM-64 or QAM-256 for the high quality cable.
  – With a 6 MHz channel and QAM-64, the net payload is 27
  – For upstream, QPSK is used because QAM-64 does not
    work well when there is too much noise.
• The head end and amplifier are upgraded to CMTS
  (Cable Modem Termination System).
    Spectrum Allocation

Frequency allocation in a typical cable TV system
               used for Internet access             110
                 Cable Modem
• A cable modem is a device
• How cable modems work?
  – Ranging: get the distance from the headend to get correct
    timing to fit in one or more minislots.
  – Acquiring upstream channel, downstream channel, and
    minislot assignments: request minislots and wait for the
    acknowledge from the headend. Otherwise, retry.
  – Sending packets to request an IP address.
  – Establishing a secret key between the head-end and modem.
  – Log in and provide its unique identifier over the secure

         Cable Modems

Typical details of the upstream and downstream
            channels in North America.           112
            ADSL versus Cable
• Which is better, ADSL or cable?
  – Theoretically, coax is hundreds of times more than
    twisted pair. But, the full capacity is not available
    for data users (See comment on Page 175).
  – In practice, ADSL providers achieve about 80% of
    the bandwidth. Cable depends on how many people
    are sharing the cable.
  – Being a point-to-point medium, ADSL is more
    secure than cable.
  – The telephone system is more reliable than cable.
  – Most ADSL providers offer a choice of ISPs
    (sometimes, required by law).                       113

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