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					Lecture #8: Physical Basics of
        Telecommunications.
Contents
 Mathematical Results in Signaling   2
 Physical Layer Functions    6
 Guided Transmission Media:

   Electric Signal Wires        8
   Light Transmission      13



                                          1
             Mathematical Results in

                                        Signaling
    Signals’ presentation as periodical function of time:
    g(t). Period T and frequency f.
   Fourier transform - a constant + endless sum of
    sin and cos expressions (harmonics):
      derivation of sin coefficients
      derivation of cos-coefficients
      derivation of constant coefficient
   Power losses in data transmission.
   Selective harmonics’ amplitude deminission
        transmission subsiding
        filtering
   Bandwidth - frequency interval of harmonics
    (frequency components) in which the signal is       2
    transmitted
              Mathematical Results in
                           Signaling
   Impact of the number of harmonics on the
    transmitted signal shape.
                                        2/1
   Boud rate and bit rate
   Data rates and number of harmonics
     – 9.6 kb/s  2 harmonics                    2/2
     – 38.4 kb/s  0 harmonics (no transmission of binary
       signal by phone twisted pair, that has a cut-off at 3 kHz)


                                                                    3
               Mathematical Results in
                            Signaling
   Maximum data rate of a noiseless channel
    – Nyquist’s theorem: arbitrary signal passing filter
      with frequency bandwidth H can be reconstructed
      by 2H observations per second. Faster observations
      are pointless, because higher frequencies (>H) are
      filtered. And vice-versa:

      MAX(Data Rate) = 2H b/S for two-level (i.e. binary)
      signal.

      MAX(Data Rate) = 2H log2V for V-level discrete
      signal.                                               4
                Mathematical Results in
                             Signaling
   Maximum data rate of a noisy channel
    – Signal/Noise Ratio RSN=S/N (S - signal power; N -
      noise power) or usually
                    RSN = 10 log10 S /N dB
    S/N=10       S/N=100        S/N=1000
    RSN =10dB    RSN =20dB      RSN =30dB
    – Shannon’s theorem: arbitrary signal passing
      filter with frequency bandwidth H and signal-noise-
      ratio RSN has
             MAX(Data Rate) = H log2(1+S/N) b/S
    Phone lines: H=3000, RSN =30 dB 
    MAX(Data Rate)  30 kb/S
                                                          5
                             Physical Layer
 The physical layer provides mechanical,
  electrical, functional and procedural means to
  activate, maintain and deactivate physical-
  connections for bit transmission between data
  link entities.
 Physical layer entities are interconnected by

  means of a physical medium.
 Data-circuit

A communication path in the physical media for
  OSI between two physical entities, together with
  the facilities necessary in the physical layer for
                                                     6
  the transmission of bits on it.
                                     Physical layer
   Services provided to the data link layer

     –physical-connections;
     –physical-service-data-units;
     –physical-connection-endpoints;
     –data-circuit identification;
     –sequencing;
     –fault condition notification; and
     –quality of service parameters.


                                                  7
              Guided Transmission Media
   Parameters of the transmission media:
     bandwidth
     cost
     delay
     carry distance
     support devices
     general support
     durability, noise protection
     spread and popularity
   Guided media:
     conductor wires
     fiber optics
                         Light Amplification
   Unguided media:      by Stimulated Emis-
       radio waves
                         sion of Radiation
       LASER rays                             8
                                   Twisted Pair
   Pair of conductor wires that are helical twisted
   Reduction of the interference and induction
    between neighbor pairs
                               2/3a
      bandwidth: up to 1 Gb/S (not in phone lines)
      cost: low
      delay:
      carry distance: 102 -104 m without amplification
      support devices: analog and digital transmission
      general support:
      durability, noise protection :
      spread and popularity: phone systems (POTS -
       Plain Old Telephone Service)                       9
      Baseband Coaxial Cable
   Structure     2/3
   Impedance 50 ; other properties:
      bandwidth: up to 10 Mb/S
     cost: low
     delay:
     carry distance: 102 -104m without
       amplification
     support devices: analog and digital
       transmission
     general support:
     noise protection: better than twisted pair
     spread and popularity: LAN, cable TV

                                               10
            Broadband Coaxial Cable
   The bandwidth up to 300-500 MHz - for analog
    transmission and digital data modems on both ends.
   Modems allow transmission of >1b/S for each 1Hz of
    the bandwidth or
   The cable bandwidth is split into multiple 6 MHz
    channels
   Larger areas need analog amplification that defines
    transmission direction (as the amplifier has input and
    output) 
   Bi-directional transmission needs:
      dual cable connection between both ends    2/4
      single cable and frequency splitting  12         11
     Broadband Coaxial Cable
   Frequency splitting in single cable
     –   Subsplit system: 5-30 MHz for input signal and
         40-300 MHz output
     –   Midsplit system: 5-116 MHz for input signal and
         168-300 MHz output
   Parameters:
      bandwidth: up to 10 Mb/S
      cost: lower than baseband coax
      carry distance: 104 -105 m without
       amplification (analog signal)
      support devices: analog and digital transmission
      noise protection: better than twisted pair, worse
       than baseband coax
      spread and popularity: cable TV (widely
       installed), perspective for MANs                12
          Transmission Media - Fiber
                             Optics
   Evolution of the system speed scissors:
        early computer age: bottleneck is in
         intercomputer communications: data processing
         is faster than data transmission
        mature computer age: bottleneck shifted to data
         processing as the communications became faster

   Example: fiber optics transmits more than 100 Tb/S
    but the converters between electrical and optical
    signals limit the speed to 10 Gb/S.
                                                      13
                                      Fiber Optics
   Optical transmission system:
    – light source
    – light transmission media
    – light detector
   Light/electricity conversion is done by the light source
    and detector: light pulse codes “1” and generates
    electrical pulse in th detector
   Media: glass fiber (variant “fibre”) - unidirectional
    transmission (direction determined by the positions of
    the source and the detector)
   Physical ground of the light transmission:
    total internal reflection; reflection angle, boundary
    refraction; single- and multi-mode fibers.
                                                        2/5 14
                                                            Fiber Optics
     Glass transparency equals that of the clear air
     Light attenuation A[dB] per linear km,
            % of      1    10   20    30   40    50    60    70     80      90   100
    transmission
          A[dB]      20    10    7   5.2    4     3   2.2    1.5 0.95 0.45         0

     A(l) diagram, transmission bands:
       – 0.85 m-6(mm): A=0.8 i.e. 85% transmission/km                        2/6
       – 1.3 m-6 : A=0.2 i.e. 95% transmission/km                  GaAs
       – 1.85 m-6 : A=0.18 i.e. 96% transmission/km               crystal
       – bandwidth 30 THz for the three bands.

A single
soliton      Light pulses’ shape: solitons (a solitary wave that

nonlinear   propagates with little loss of energy and retains its
Schrodinger
            shape and speed after colliding with another such
surface                                                           15
                   wave)
                                         Fiber Cables
   Single core ( 50 m-6) cables and
                                                     2/7
   Multiple core cables ( 10 m-6)
   Cable Interconnections:
     – terminating connectors with fiber plugs - 20% light losses
     – mechanical junction - 5% light losses, personnel
     – termofusing - less than 1% losses, special equipment
   Light sources: LEDs or crystal lasers
   Light sensors: photo-diodes                       2/8

                                                                    16
                     Fiber Optic Networks
   Networks based on fiber optic connections can
    cower the range between LANs and WANs
   Topology always based on point-to-point
    connections e.g. ring with T-connector for each
    node:         2/9
     – passive interface: main light conductor (fiber) and
       LED/photodiode junctions for each station; high
       reliability, short distance, restricted number of computers
       in the network
     – active interface: main light fiber has a break at each
       station and the signal is regenerated by the repeater;
       repeaters are electrical (wired interface to the computer)
       or optic (fiber interface to computer); reliability depends
       on the junctions, unrestricted size in length and stations
       number, long interstation distance (km)                    17
            Fiber Optic Networks

   Passive star topology - modified ring with
    fiber interface to computer. The passive star
    is the point where every light pulse of the
    incoming fibers illuminates any of the
    outgoing fibers to the computers. 2/10
   Its properties resembles those of passive ring
    topology (limited distance and number of
    stations and independent reliability to the
    state of each station)
                                                    18
                                      Fiber vs. Wire
   Advantages:                           Drawbacks:
     Lower attenuation                    Unidirectional
     Wider bandwidth                       transmission 
     No interference between the lines     doubled conductors
     Power independence throughout the or occupied bands
      route                                More expensive
     Better protection & security against interfaces
      taps                                 Requires additional
     Lighter weight, non corrosive         staff qualification
     Lower installation cost for new
       routers
                                                                  19
2/1




      20
            Bandwidth /
            Average harmo-
 Average
            nic frequency =
harmonic                      For 3kHz
frequency   Number of har-
                              bandwidth
            monics sent




                                          21
        Twisted pair
2/3a




       (a) Category 3 UTP.
       (b) Category 5 UTP.
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posted:3/13/2012
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