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William Stallings Data and Computer Communications_10_

VIEWS: 11 PAGES: 61

  • pg 1
									      Data and Computer
       Communications

             Chapter 3
         Data Transmission
Required Reading: Stallings chapter 3




                                    1
                    Physical Layer
Source node                                   Destination node

 Application                                   Application

 Presentation                                  Presentation

  Session                                       Session

  transport               Intermediate node     transport
                Packets
  Network                    Network            Network
                Frames
  Data link                  Data link          Data link

  Physical       Bits       Physical            Physical

              Signals
                                                              2
Physical / Data Link Layer Interface

      Sender                      Receiver

NL



                 HDR
DLL
                         Frame



                  ACK
 PL
                  HDR

               Transmitted Bits


                                             3
Physical Layer

Communications and Information Theory are topics
 of whole courses
We‟ll cover some theoretical basics regarding
 communications over a physical channel
We discover that there are physical limitations to
 communications over a given channel
We‟ll cover some fundamental theorems




                                                      4
Terminology (1)
Transmitter
Receiver
Medium
  Guided medium
     e.g. twisted pair, optical fiber
  Unguided medium
     e.g. air, water, vacuum




                                         5
Terminology (2)
Direct link
  No intermediate devices
Point-to-point
  Direct link
  Only 2 devices share link
Multi-point
  More than two devices share the link




                                          6
Terminology (3)
Simplex
  One direction (but in Europe means half duplex)
     e.g. Television
Half duplex
  Either direction, but only one way at a time
     e.g. police radio
Full duplex
  Both directions at the same time
     e.g. telephone




                                                     7
Electromagnetic Signals
Function of time
  Analog (varies smoothly over time)
  Digital (constant level over time, followed by a
   change to another level)
Function of frequency
  Spectrum (range of frequencies)
  Bandwidth (width of the spectrum)




                                                      8
Frequency, Spectrum and
Bandwidth
Time domain concepts
  Continuous signal
     Varies in a smooth way over time
  Discrete signal
     Maintains a constant level then changes to another constant
      level
  Periodic signal
     Pattern repeated over time
  Aperiodic signal
     Pattern not repeated over time




                                                                9
Periodic Signal Characteristics
  Amplitude (A): signal value, measured in volts
  Frequency (f ): repetition rate, cycles per second or
   Hertz
  Period (T): amount of time it takes for one repetition,
   T=1/f
  Phase (Φ): relative position in time, measured in
   degrees or radians




                                                        10
             Analog Signaling
                represented by sine waves
amplitude (volts)




                    1 cycle

                                                        phase
                                                        difference
                                                         time
                                                           (sec)


                              frequency (hertz)
                                  = cycles per second
                                                               11
             Digital Signaling
                represented by square waves or pulses
amplitude (volts)




                    1 cycle




                                                         time
                                                           (sec)


                              frequency (hertz)
                                  = cycles per second
                                                             12
Continuous & Discrete Signals




                                13
Periodic
Signals




           14
Sine Wave
Peak Amplitude (A)
  maximum strength of signal
  volts
Frequency (f)
  Rate of change of signal
  Hertz (Hz) or cycles per second
  Period = time for one repetition (T)
  T = 1/f
Phase ()
  Relative position in time

                                          15
 Varying Sine Waves



Sin2πt                 0.5Sin2πt




                      Phase Shift in
                      radians


Sin4πt
                          or

                      Phase Shift in
                      seconds 16
Wavelength ()
Distance occupied by one cycle
Distance between two points of corresponding
 phase in two consecutive cycles

Assuming signal velocity in space is equal to v
   = vT or
  f = v
  Here, V=c = 3*108 ms-1 (speed of light in free space)




                                                       17
Frequency Domain Concepts
A Signal is usually made up of many frequencies
Components are sine waves
It Can be shown (Fourier analysis) that any
 signal is made up of component sine waves
One can plot frequency domain functions
 instead of/in addition to time domain functions




                                               18
Addition of
Frequency
Components
                         (a) Sin(2πft)




                        (b) (1/3)Sin(2π(3f)t)




                                                       19
              (c) (4/π)[Sin(2πft)+(1/3)Sin(2π(3f)t)]
Frequency
Domain
 Note: For square waves,
 only odd harmonics exist
 (plus the fundamental
 component of course).


Figure a is discrete because          (a) Frequency domain function for s(t)=(4/π)[Sin(2πft)+(1/3)Sin(2π(3f)t)]
the time domain function is
periodic. Figure b is
continuous because the time
domain function is aperiodic.


See Figure 3.16 Page
103. Note that s(f) is
of the form

                                                                                                    20
                                (b) Frequency domain function for a single square pulse s(t)=1 for -X/2<t<X/2
Communications Basics
 Represent a signal as a single-valued function of time,
  g(t), to model behavior of a signal (may be voltage,
  current or other change)
 Jean-Baptiste Fourier showed we can represent a
  periodic signal (given some conditions) as the sum of a
  possibly infinite number of sines and cosines


      Period = T
                      S
      g(t) = (1/2)c + n=1 an sin(2pnft) + S bn cos(2pnft)
                                        n=1
       f = 1/T is fundamental frequency
       a & b coefficients are the amplitude of the nth harmonic
        This is a Fourier Series


                                                             21
                              Original
Time ->   Harmonic spectrum

                              As we add
                              more
                              harmonics
                              the signal
                              reproduces
                              the original
                              more closely



                                         22
 Signal Transmission

No transmission facility can transmit signals
 without losing some power
Usually this attenuation is frequency
 dependent so the signal becomes distorted
Generally signal is completely attenuated
 above some max frequency (due to medium
 characteristics or intentional filtering)
The signal is bandwidth limited



                                                 23
Signal Transmission

Time T necessary to transmit a character depends on
 coding method and signalling speed
Signaling speed = number of times per second the
 signal changes value and is measured in baud
Note that baud rate is not necessarily the same as
 the bit rate
By limiting the bandwidth of the signal we also limit
 the data rate even if a channel is perfect
Overcome this by encoding schemes




                                                    24
Spectrum & Bandwidth
Spectrum
  range of frequencies contained in signal
Absolute bandwidth
  width of spectrum
Effective bandwidth
  Often just bandwidth
  Narrow band of frequencies containing most of the
   energy
DC Component
  Component of zero frequency

                                                       25
Signal with DC Component




      (a) s(t)=1+(4/π)[Sin(2πft)+(1/3)Sin(2π(3f)t)]




                                                      26
Data Rate and Bandwidth
Any transmission system has a limited band of
 frequencies
This limits the data rate that can be carried

       See Figure 3.8 Page 79




                                                 27
Bandwidth
Width of the spectrum of frequencies that can
 be transmitted
  if spectrum=300 to 3400Hz, bandwidth=3100Hz
Greater bandwidth leads to greater costs
Limited bandwidth leads to distortion
Analog measured in Hertz, digital measured in
 baud




                                                 28
BPS vs. Baud
BPS=bits per second
Baud=# of signal changes per second
Each signal change can represent more than
 one bit, through variations on amplitude,
 frequency, and/or phase




                                              29
Analog and Digital Data
Transmission
Data
  Entities that convey meaning
Signals
  Electric or electromagnetic representations of data
Transmission
  Communication of data by propagation and
   processing of signals




                                                         30
Data
Analog
  Continuous values within some interval
  e.g. sound, video
Digital
  Discrete values
  e.g. text, integers




                                            31
Acoustic Spectrum (Analog)




                             32
Signals
Means by which data are propagated
Analog
  Continuously variable
  Various media
     wire, fiber optic, space
  Speech bandwidth 100Hz to 7kHz
  Telephone bandwidth 300Hz to 3400Hz
  Video bandwidth 4MHz
Digital
  Use two DC components

                                         33
Digital Text Signaling
Transmission of electronic pulses representing
 the binary digits 1 and 0
How do we represent letters, numbers,
 characters in binary form?
Earliest example: Morse code (dots and dashes)
Most common current form: ASCII




                                              34
ASCII Character Codes
Use 8 bits of data (1 byte) to transmit one
 character
8 binary bits has 256 possible outcomes (0 to
 255)
Represents alphanumeric characters, as well as
 “special” characters




                                                  35
Digital Image Signaling
 Pixelization and binary representation



                          Code:   00000000
                                  00111100
                                  01110110
                                  01111110
                                  01111000
                                  01111110
                                  00111100
                                  00000000




                                             36
Data and Signals
Usually use digital signals for digital data and
 analog signals for analog data
Can use analog signal to carry digital data
   Modem
Can use digital signal to carry analog data
   Compact Disc audio




                                                    37
Why Study Analog?
Telephone system is primarily analog rather
 than digital (designed to carry voice signals)
Low-cost, transmission medium (present almost
 at all places at all times
If we can convert digital information (1s and 0s)
 to analog form (audible tone), it can be
 transmitted inexpensively




                                                 38
Voice Signals
Easily converted from sound frequencies
 (measured in loudness/db) to electromagnetic
 frequencies, measured in voltage
Human voice has frequency components
 ranging from 20Hz to 20kHz
For practical purposes, the telephone system
 has a narrower bandwidth than human voice,
 from 300 to 3400Hz



                                                39
Analog Signals Carrying Analog
and Digital Data




                                 40
Digital Signals Carrying Analog
and Digital Data




                                  41
Analog Transmission
Analog signal transmitted without regard to
 content
May be analog or digital data
Attenuated over distance
Use amplifiers to boost signal
Also amplifies noise




                                               42
Digital Transmission
Concerned with content
Integrity endangered by noise, attenuation etc.
Repeaters used
Repeater receives signal
Extracts bit pattern
Retransmits
Attenuation is overcome
Noise is not amplified


                                                   43
Advantages of Digital
Transmission
 Digital technology
   Low cost LSI/VLSI technology
 Data integrity
   Longer distances over lower quality lines
 Capacity utilization
   Economical high bandwidth links
   High degree of multiplexing easier with digital
     techniques
 Security & Privacy
   Encryption
 Integration
   Can treat analog and digital data similarly
                                                      44
Transmission Media
the physical path between transmitter and
 receiver
design factors
  bandwidth
  attenuation: weakening of signal over distances
  interference
  number of receivers




                                                     45
Impairments and Capacity
Impairments exist in all forms of data
 transmission
Analog signal impairments result in random
 modifications that impair signal quality
Digital signal impairments result in bit errors (1s
 and 0s transposed)




                                                   46
Transmission Impairments
Signal received may differ from signal
 transmitted
Analog - degradation of signal quality
Digital - bit errors
Caused by
  Attenuation and attenuation distortion
  Delay distortion
  Noise




                                            47
Transmission Impairments
Attenuation
  loss of signal strength over distance
Attenuation Distortion
  different losses at different frequencies
Delay Distortion
  different speeds for different frequencies
Noise




                                                48
Attenuation


                 P1 watts        P2 watts


   transmitter                          receiver



   Attenuation              10 log10 (P1/P2) dB

   Amplification            10 log10 (P2/P1) dB




                                                   49
Attenuation
Signal strength falls off with distance
Depends on medium
Received signal strength:
  must be enough to be detected
  must be sufficiently higher than noise to be received
   without error
Attenuation is an increasing function of
 frequency



                                                       50
Delay Distortion
Only in guided media
Propagation velocity varies with frequency




                                              51
Noise (1)
Additional signals inserted between transmitter
 and receiver
Types of Noise:
Thermal
  Due to thermal excitement of electrons
  Uniformly distributed, cannot be eliminated
  White noise
Intermodulation
  Signals that are the sum and difference of original
   frequencies sharing a medium

                                                         52
Noise (2)
Crosstalk
   A signal from one line is picked up by another
 NEXT (near-end crosstalk     )
   interference in a wire at the transmitting end of a signal
     sent on a different wire

 FEXT (far-end crosstalk)
   interference in a wire at the receiving end of a signal
     sent on a different wire


Impulse
    Irregular pulses or spikes
    e.g. External electromagnetic interference
    Short duration
    High amplitude
    Less predictable                                            53
Noise

  Effect
    distorts a transmitted signal
    attenuates a transmitted signal

  signal-to-noise ratio to quantify noise


             S/Ndb =   10 log S          S= average signal power
                              N
                                             N= noise power




                                                              54
    Effect of noise


                                             Signal


                                             Noise
Logic
Threshold                                    Signal+Noise

                                             Sampling times
            0 1   1   1 1 0   0    0   0 1   Data Received
            0 1   0   1 1 0   0    1   0 1   Original data

                       Bit error
                                                          55
Channel Capacity
Data rate
  In bits per second
  Rate at which data can be communicated
Bandwidth
  In cycles per second of Hertz
  Constrained by transmitter and medium




                                            56
Maximum Data Rate

  In 1920s Nyquist (of the Nyquist Theorem)
   developed an equation for the maximum data rate of
   a noiseless channel
     For low pass filtered signal of bandwidth B
     Sampling at exactly 2B samples per sec allows
      reconstruction of the signal
     More samples are useless since the frequencies
      above B are filtered out


     C=Capacity=max data rate = 2B log2 M bits/sec
     for M discrete levels



                                                       57
Nyquist theorem


“ In a perfectly noiseless channel, if f is the
  maxmimum frequency the medium can transmit, the
  receiver can completely reconstruct a signal by
  sampling it 2*f times per second”

                                         Nyquist, 1920




                                                    58
Nyquist formula

                                       B = bandwidth
         C=      2B log2 M             M = number of discrete signal levels
          Theoretical capacity for Noiseless channel


 Example: Channel capacity calculation for voice bandwidth (~3100 Hz):

                    M          Max data rate (C)
                     2           6200 bps
                     4          12400 bps
                     8          18600 bps
                    16          24800 bps


                                                                         59
Shannon’s Law
In the „40s Shannon (of Shannon‟s Law) extended the
 equation to a channel subject to thermodynamic
 (thermal) noise
   Thermal noise measured by ratio of signal (S) power to
    noise (N) power (signal-to-noise ratio - S/N)
   But represented as: 10 log10 S/N
   These units are called decibels (dB)
   Now, for a channel with signal to noise of S/N
            Capacity=C=max bits/sec = B log2 (1 + S/N)
     Here, C=Theoretical Maximum capacity with noise

   Note: Only much lower rates are achieved since the equation
   assumes zero impulse noise and no attenuation and delay
   distortion.                                                60
Bit rate and Baud rate
 Bit rate   number of bits that are transmitted in a second


 Baud rate     number of line signal changes (variations) per second


   If a modem transmits 1 bit for every signal
     change
                 bit rate = baud rate


   If a signal change represents 2 or more or n
     bits
                bit rate = baud rate *n

                                                                        61

								
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