Data and Computer Communications Ninth Edition by William Stallings Chapter 3 – Data Transmission Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011

Data Transmission What we've got here is failure to communicate. What we've got here is failure to communicate. Paul Newman in Cool Hand Luke Paul Newman in Cool Hand Luke

Data Transmission The successful transmission of data depends on two factors: 1) quality of the signal being transmitted 2) characteristics of the transmission medium

Transmission Terminology Data transmission occurs between transmitter and receiver over some transmission medium. Communication is in the form of electromagnetic waves. Guided media twisted pair, coaxial cable, optical fiber Unguided media (wireless) air, vacuum, seawater

TransmissionTerminology Transmission Terminology

SSSSimplex signals transmitted in one direction eg. Television HHHHalf duplex both stations transmit, but only one at a time eg. police radio FFFFull duplex simultaneous transmissions eg. telephone

Frequency, Spectrum and Bandwidth aanalog signal signal intensity varies smoothly with no breaks ddigital signal signal intensity maintains a constant level and then abruptly changes to another level pperiodic signal signal pattern repeats over time aaperiodic signal pattern not repeated over time Time Domain Concepts

Analog and Digital Signals

Periodic Signals

Sine Wave  peak amplitude (A) maximum strength of signal maximum strength of signal typically measured in volts typically measured in volts  frequency (f) rate at which the signal repeats rate at which the signal repeats Hertz (Hz) or cycles per second Hertz (Hz) or cycles per second period (T) is the amount of time for one repetition period (T) is the amount of time for one repetition T = 1/f T = 1/f  phase (  ) relative position in time within a single period of signal relative position in time within a single period of signal (periodic continuous signal)

Varying Sine Waves s(t) = A sin(2  ft +  )

Wavelength ( ) the wavelength of a signal is the distance occupied by a single cycle can also be stated as the distance between two points of corresponding phase of two consecutive cycles assuming signal velocity v, then the wavelength is related to the period as = vT or equivalently f = v especially when v=c c = 3*10 8 ms -1 (speed of light in free space)c = 3*10 8 ms -1 (speed of light in free space) For copper: v=0.55c to 0.77cFor copper: v=0.55c to 0.77c

Frequency Domain Concepts  signals are made up of many frequencies  components are sine waves  Fourier analysis can show that any signal is made up of components at various frequencies, in which each component is a sinusoid  can plot frequency domain functions

Addition of Frequency Components (T=1/f) (c) is the sum of f & 3f  sound/sound.html sound/sound.html sound/sound.html  ava/index.php?topic=17 ava/index.php?topic=17 ava/index.php?topic=17

Frequency Domain Representations  frequency domain function of Fig 3.4c  frequency domain function of single square pulse

Spectrum & Bandwidth

Signal with dc Component

Data Rate and Bandwidth any transmission system has a limited band of frequencies this limits the data rate that can be carried on the transmission medium square waves have infinite components and hence an infinite bandwidth most energy in first few components limiting bandwidth creates distortions There is a direct relationship between data rate and bandwidth.

Analog and Digital Data Transmission  data entities that convey information entities that convey information  signals electric or electromagnetic representations of data electric or electromagnetic representations of data  transmission communication of data by propagation and processing of signals communication of data by propagation and processing of signals

Acoustic Spectrum (Analog) -> définition du décibel -> définition du décibel -> définition du décibel

Analog and Digital Transmission (1080i vs 1080p :

(Digital Data) Examples:TextCharacter strings IRA (International reference alphabet)

Advantages & Disadvantages of Digital Signals

Audio Signals  frequency range of typical speech is 100Hz-7kHz  easily converted into electromagnetic signals  varying volume converted to varying voltage  can limit frequency range for voice channel to Hz

Video Signals  to produce a video signal a TV camera is used  USA standard is 483 lines per frame, at a rate of 30 complete frames per second actual standard is 525 lines but 42 lost during vertical retrace actual standard is 525 lines but 42 lost during vertical retrace  horizontal scanning frequency is 525 lines x 30 scans = lines per second (63.5 μ s per line but 11 μ s is lost during horizontal retrace -> 52.5 μ s )  max frequency reached if line alternates black and white (450 columns)  max frequency of 4.2MHz (450/2 cycles in 52.5 μ s)

Video Signals 

Analog Signals Modulator/demodulator

Digital Signals Similar to modem but for voice signal Coder/decoder Transmitter/receiver

(Analog and Digital Transmission)

Transmission Impairments  signal received may differ from signal transmitted causing: analog -> degradation of signal quality analog -> degradation of signal quality digital -> bit errors digital -> bit errors  most significant impairments are attenuation attenuation attenuation distortion (some freq. are attenuation distortion (some freq. are more attenuated than others) more attenuated than others) delay distortion (velocity of waves depends delay distortion (velocity of waves depends on their frequencies) on their frequencies) noise noise

ATTENUATION Received signal strength must be: strong enough to be detected sufficiently higher than noise to be received without error Strength can be increased using amplifiers or repeaters. Equalize attenuation across the band of frequencies used by using loading coils or amplifiers.  signal strength falls off with distance over any transmission medium  varies with frequency

Attenuation Distortion

Delay Distortion  occurs because propagation velocity of a signal through a guided medium varies with frequency  various frequency components arrive at different times resulting in phase shifts between the frequencies  particularly critical for digital data since parts of one bit spill over into others causing intersymbol interference

Delay Distortion

Noise unwanted signals inserted between transmitter and receiver is the major limiting factor in communications system performance

Categories of Noise Intermodulation noise produced by nonlinearities in the transmitter, receiver, and/or intervening transmission medium effect is to produce signals at a frequency that is the sum or difference of the two original frequencies (sosmat.com)sosmat.com

Categories of Noise Crosstalk: a signal from one line is picked up by another can occur by electrical coupling between nearby twisted pairs or when microwave antennas pick up unwanted signals Impulse Noise: caused by external electromagnetic interferences noncontinuous, consisting of irregular pulses or spikes short duration and high amplitude minor annoyance for analog signals but a major source of error in digital data

Channel Capacity Maximum rate at which data can be transmitted over a given communication channel under given conditions Channel characteristics : data rate in bits per second bandwidth in cycles per second or Hertz noise average noise level over path error rate rate of corrupted bits limitations due to physical properties main constraint on achieving efficiency is noise

Nyquist Bandwidth In the case of a channel that is noise free:  if rate of signal transmission is 2B then can carry signal with frequencies no greater than B given bandwidth B, highest signal rate is 2B given bandwidth B, highest signal rate is 2B  for binary signals, 2B bps needs bandwidth B Hz  can increase rate by using M signal levels  Nyquist Formula is: C = 2B log 2 M  data rate can be increased by increasing M however this increases burden on receiver however this increases burden on receiver noise & other impairments limit the value of M noise & other impairments limit the value of M

Shannon Capacity Formula  considering the relation of data rate, noise and error rate: faster data rate shortens each bit so bursts of noise corrupts more bits faster data rate shortens each bit so bursts of noise corrupts more bits given noise level, higher rates mean higher errors given noise level, higher rates mean higher errors  Shannon developed formula relating these to the signal-to-noise ratio -> SNR= (signal power) / (noise power)  capacity C = B log 2 (1+SNR) (ex. SNR=0 -> C=0) theoretical maximum capacity theoretical maximum capacity get much lower rates in practice get much lower rates in practice  SNR db = 10 log 10 (SNR)

Summary  transmission concepts and terminology guided/unguided media guided/unguided media  frequency, spectrum and bandwidth  analog vs. digital signals  data rate and bandwidth relationship  transmission impairments attenuation/delay distortion/noise attenuation/delay distortion/noise  channel capacity Nyquist/Shannon Nyquist/Shannon