FIT1005

1

• FIT1005
• FIT Monash University
• Data and Signals
• Reference
• Chapter 3 -Stallings

2
Transmission Media

• Data transmission occurs between a transmitter
  and receiver over some transmission medium
• Guided media, the waves are guided along a
  physical path Twisted pair, Coaxial Cable,
  Fibre Optic
• Unguided media, also called wireless, provide a
  means of transmitting waves, but do not guide
  them Radio, Microwave, Infrared
• In both cases, communication is in the form of
  electromagnetic waves

3
Twisted Pair
UTP Cat 5E 4 pair
4
Coaxial Cable
5
Optical Fiber
6
Links

• The term direct link is used to refer to the
  transmission path between two devices in which
  signals propagate directly from a transmitter to
  receiver
• There are no intermediate devices other than
  amplifiers or repeaters used to increase signal
  strength
• This term can apply to both guided and unguided
  media
• If there are intermediate devices, it is an
  indirect link
• A guided transmission medium is point-to-point if
  it provides a direct link between two devices, if
  more than two devices share the medium it is
  multipoint

7
Links
8
Transmission Modes

• Simplex
• Signals are transmitted in only one direction
• One station is the transmitter and the other is
  the receiver
• Half duplex
• both stations may transmit, but only one at time
• Full-duplex
• Both stations may transmit simultaneously
• The medium is carrying signals in both directions
  at the same time

9
Transmission Modes

• commercial radio
• CB radio
• television
• smoke signals
• classroom discussion
• family arguments
• ocean tides
• shortwave radio communications

10
Signals

• Data communications is concerned with
  electromagnetic signals used to transmit data
• An electromagnetic signal can be either analog or
  digital

11
Analog Data

• take on continuous values in some time interval
• For example
• Voice and video are continuously varying patterns
  of sound intensity
• Most data collected by sensors, such as
  temperature and pressure, are continuous values

12
Digital Data

• Digital data take on discrete values, letters
  (A,B,C ) and integers (0,1,2 )
• While textual data are most convenient for human
  beings
• Data processing and communication systems are
  designed using binary data (0,1)
• A number of codes (ASCII, Unicode) have been
  devised by which characters (A,B,C 0,1,2 )
  are represented by a sequence of binary bits

13
Signals

• An Analog signal
• is one in which the signal intensity varies in a
  smooth fashion over time
• There are no breaks or discontinuities in the
  signal
• A Digital signal
• is one in which the signal intensity maintains a
  constant level for some period of time and then
  changes to another constant level
• It is discrete and discontinuous
• The discrete signal might represent binary 1s and
  0s

14
Signal Representation in Time Domain

• We can measure the amplitude, frequency, and
  phase of a signal in relationship to time

15
Signal Representation in the Time Domain
Analog Signal
Digital Signal
16
Simple Signal Models

• Analog Signal Model the sine wave, an example
  of a periodic continuous signal
• Digital Signal Model the square wave, example
  of a periodic discrete signal
• These are periodic signals, in which the same
  signal pattern repeats over time

17
Simple Signal Models
18
The Sine Wave ModelCharacteristics

• Peak amplitude (A), the peak amplitude is the
  maximum value or strength of the signal over time
• frequency (f), the frequency is the rate (in
  cycles per sec or Hertz) at which the signal
  repeats
• period (T), which is the amount of time taken for
  one repetition
• phase (f), phase is a measure of the relative
  position in time within a single period of a
  signal
• Wavelength (?), the wavelength is the distance
  (in metres) occupied by a single cycle

19
The Sine Wave Model
20
Signal Representation in Frequency Domain

• An electromagnetic signal is made up of many
  frequencies
• We can identify each of the signal frequencies
  and measure its power (Amplitude)

21
Signal Representation in the Frequency Domain
22
Signalling versus Transmission

• Signalling
• is the physical propagation of an electromagnetic
  signal along a suitable medium
• Transmission
• is the communication of data by the propagation
  and processing of electromagnetic signals

23
Human Speech as an Analog Signal

• Human speech is an acoustic (science of sound)
  analog signal, with frequency components in the
  range of 20Hz - 20kHz
• This acoustic signal can easily be converted to
  an electromagnetic signal for transmission
• The amplitude of sound frequencies is measured in
  loudness while the amplitude of the converted
  electromagnetic frequency is measured in volts

24
Human Speech as an Analog Signal

• In the case of human speech, the data can be
  represented directly by an electromagnetic signal
  occupying the same spectrum (frequency range).
• However, there is a need to compromise between
  the fidelity of sound transmitted electrically
  and the cost of transmission
• The spectrum of speech is between 100Hz and 7kHz.
  A much narrower bandwidth will produce
  acceptable voice reproduction.

25
Human Speech as an Analog Signal

• The standard spectrum for a voice channel is
• 300-3400Hz, with bandwidth 3400 300 3100Hz.
• This is adequate for speech transmission
• Minimises required transmission capacity
• Allows the use of inexpensive telephone sets
• The telephone transmitter converts the incoming
  acoustic voice signal into an electromagnetic
  signal over the range 300-3400Hz
• This signal is transmitted through the telephone
  system (PSTN) to a receiver

26
Analog and Digital Signals

• Analog signals can be used to represent analog
  data
• Digital signals can be used to represent digital
  data
• Digital data can also be represented by analog
  signals
• Use a modem (modulator/ demodulator)
• The modem converts a series of binary voltage
  pulses into an analog signal by encoding the
  digital data onto a carrier frequency
• At the other end of the line, another modem
  demodulates the signal to recover the original
  data

27
Analog and Digital Signals
28
Signal Attenuation

• Attenuation - the loss of signal strength (power)
• A signal will become weaker (attenuate) as the
  signal propagates (through the medium) over
  distance
• To achieve longer distances,
  the
  analog signal must be amplified
  and a digital
  signal must be regenerated.

29
Attenuation of Digital Signals
30
Signal Attenuation

• Amplifiers
• Boost the energy in the analog signal
• Unfortunately, amplifiers boost noise components
  as well
• With amplifiers cascaded to achieve long
  distances, the signal becomes more and more
  distorted
• For analog data (voice), quite a bit of
  distortion can be tolerated before data becomes
  unintelligible
• For digital data, cascaded amplifiers will
  introduce errors

31
Signal Attenuation

• Repeaters
• Regenerates the digital signal
• A repeater receives the digital signal,
  recovers the
  pattern of 1s and 0s
  and retransmit a new (noise is
  removed) signal, thereby overcoming attenuation

32
Digital versus Analog Transmission

• Which is the preferred method of transmission?
• The answer supplied by the telecommunications
  industry and its customers is digital
• Both long-haul telecommunications facilities
  and intra-building services have
  moved to digital transmission and,
  where
  possible digital signalling techniques

33
Digital TransmissionThe Way to GO

• Digital technology being more cost and size
  effective
• The advent of LSI and VLSI technology has caused
  a continuing drop in the cost and size of digital
  circuitry
• Higher data integrity
• With the use of repeaters rather than amplifiers,
  the effects of noise and other signal impairments
  are not cumulative
• Thus it is possible to transmit data longer
  distances over lower quality lines using digital
  means while maintaining the integrity of the data
  

34
Digital TransmissionThe Way to GO

• Better capacity utilisation
• It is economical to build transmission links of
  very high bandwidth
• A high degree of multiplexing is needed to
  utilise such capacity effectively
• This is more easily and cheaply achieved with
  digital (time division) rather than analog
  (frequency division) techniques
• Better security and privacy
• Encryption techniques readily applied to digital
  data

35
Digital TransmissionThe Way to GO

• Integration benefits
• By treating both analog and digital data
  digitally, all signals have the same form and can
  be treated similarly
• As a result, it is more economical and convenient
  to deal with voice, video, and digital data by
  integrating them

36
Transmission Impairments

• With any communications system,
  the signal that is received
  may differ from the signal that is transmitted
  due to various transmission impairments
• Analog signal, the impairments can degrade the
  quality of the signal,
• Digital signal, bit errors may be introduced

37
Transmission Impairments

• Distortion distorts the shape of the signal
• Noise adds unwanted signals

38
Transmission Impairments

• Attenuation
• Attenuation distortion
• Delay distortion
• Noise
• Thermal noise
• Intermodulation noise
• Crosstalk
• Impulse noise

39
Attenuation Issues

• A received signal must have sufficient strength
  so that the receiver can detect it
• The signal must maintain a level sufficiently
  higher than noise to be received without error
• Attenuation increases with an increase in
  frequency, leading to Attenuation Distortion

40
Signal Strength

• For a point-to-point link, the signal strength of
  the transmitter must be strong enough to be
  received intelligibly
• It should also be not so strong as to overload
  the circuitry of the transmitter or receiver,
  which could cause distortion
• More complex problem for multipoint lines where
  the distance from a transmitter to receiver is
  variable

41
Attenuation Distortion

• Has a greater impact on analog signals
• The amount of signal attenuation that occurs
  increases with an increase in frequency
• As a result the received signal is distorted,
  reducing intelligibility
• To overcome this problem, techniques are
  available for equalising attenuation across a
  band of frequencies
• One such approach is to use amplifiers that
  amplify high frequencies more than lower
  frequencies

42
Delay Distortion

• Occurs because the velocity of signal propagation
  through a guided medium varies with frequency
• The received signal is distorted due to the
  varying delays experienced by its constituent
  frequencies
• Delay distortion is particularly critical for
  digital data
• Some of the signal components of one bit position
  will spill over into other bit positions, causing
  intersymbol interference

43
Noise

• For any data transmission event, the received
  signal will consist of the transmitted signal
  plus additional unwanted signals
• These unwanted signals are inserted somewhere
  between transmission and reception
• These signals are referred to as noise
• Noise is the major limiting factor in
  communication systems performance

44
Thermal Noise

• Is due to thermal agitation of electrons
• It is present in all electronic devices and
  transmission media
• It is a function of temperature
• It is uniformly distributed across the bandwidth
  used in communication systems and hence is
  referred to a white noise
• It cannot be eliminated and therefore places an
  upper bound on communication systems performance

45
Intermodulation Noise

• Occurs when signals of different frequencies
  share the same transmission medium
• Mixing of signals at frequencies f1 and f2
  might produce energy
  at the frequency f1f2,
  which could interfere
  
  with an intended signal at the frequency f1f2

46
Crosstalk

• Crosstalk is an unwanted coupling between signal
  paths
• It can occur by electrical coupling between
  nearby twisted pairs or, coax cables lines
  carrying multiple signals
• Fibre Optic cables are not affected by crosstalk

47
Impulse Noise

• Variety of causes, including external
  electromagnetic disturbances, such as lightening,
  and faults and flaws in communications system
  can cause Impulse noise
• Impulse noise is noncontinuous, consisting of
  irregular pulses or noise spikes of short
  duration and of relatively high amplitude
• Impulse noise is generally only a minor annoyance
  for analog data
• It is the primary source of error in digital data
  communication

48
Channel Capacity

• Is maximum rate at which data can be transmitted
  over a given communication channel
• Need to consider
• Data rate, bits per second (bps), is the rate at
  which data can be communicated
• The bandwidth of channel, cycles per second or
  Hertz
• Noise, average level over channel
• Error rate on channel

49
Channel Capacity

• Nyquist (1924)
• In a noise free channel, the channel capacity, in
  bps, of the channel is twice the bandwidth of the
  channel
• On a telephone channel with a frequency range
  from 300Hz to 3400Hz, the bandwidth is
• 3400 300 3100Hz
• Hence, the channel capacity is
• 2 x 3100 6200 bps
• Using two level signaling

50
Channel Capacity

• Multilevel signalling the Nyquist formulation
  becomes
• C 2B log2M
• Where M is the number of discrete signal or
  voltage
• levels
• For M8, log28 3 ,C 18600bps, for B 3100Hz
• For a given B, C can be increased by increasing
  the number of different signal elements M
• Noise and other impairments will limit the
  practical value of M

51
Channel Capacity

• For a given level of noise a greater signal
  strength would improve the ability to receive
  data correctly
• This can be expressed as the signal-to-noise
  ratio
• S/N eg 1000/1
• It can be convert to decibels via

52
Channel Capacity

• Shannon (1949)
• The channel capacity on a noisy channel, in
  bps is given by
• C Blog2(1SNR)
• C capacity of the channel in
  bps
• B bandwidth of channel in Hz
• Shannon formula represents the theoretical
  maximum channel capacity that can be achieved
• Noise is thermal

53
The slides following this are for your interest
only
54
Time Domain Concepts Contd.

• When a signal is travelling at a velocity v, the
  wavelength is related to the period as
• ? vT or equivalently v f?
• Of particular relevance to this unit is the case
  where
• v c, the speed of light in free space, which
  is approximately 3 108 m / s

55
Frequency Domain Concepts Contd.

• The period of the total signal is equal to the
  period of the fundamental frequency
• It can be shown, using a discipline known as
  Fourier analysis, that any signal is made up of
  components at various frequencies in which each
  component is sinusoid
• That is, by adding together enough sinusoidal
  signals, each with the appropriate amplitude,
  frequency and phase, any electromagnetic signal
  can be constructed
• That is, there is a frequency domain function
  S(f) that specifies the peak amplitude of
  constituent frequencies

56
Frequency Domain Concepts Contd.
57
Frequency Domain Concepts Contd.

• The spectrum of a signal is the range of
  frequencies that it contains
• For the previous signal, which had components of
  frequencies f and 3f, has a spectrum extending
  from f to 3f
• The absolute bandwidth of a signal is the width
  of the spectrum
• In the above example the bandwidth is 2f
• Many signals (such as square waves) have infinite
  bandwidth, although most of the energy is
  contained in a relatively narrow band of
  frequencies
• This band is referred to as the effective
  bandwidth, or just bandwidth

58
Frequency Domain Concepts Contd.

• If a signal includes a component of zero
  frequency, that component is a direct current
  (dc) or constant component
• With no dc component, a signal has an average
  amplitude of zero, as can be seen in the time
  domain
• With dc component, it has a frequency term at f0
  and a nonzero average amplitude

59
Frequency Domain Concepts Contd.
60
Relationship Between Data Rate and Bandwidth

• Although a given waveform may contain frequencies
  over a wide range, practically any transmission
  system can accommodate only a limited band of
  frequencies
• This in turn, limits the data rate that can be
  carried on transmission medium
• When we add additional odd multiples of base
  frequency f, the resulting waveform approaches
  that of a square wave more and more closely
• It can be shown that the frequency components of
  the square wave with amplitudes A and (-A) can
  be expressed as follows

61
Relationship Between Data Rate and Bandwidth

• As can be seen, this waveform has an
  infinite
• number of frequency components and hence an
  infinite bandwidth
• However, the peak amplitude of the kth
  frequency component kf is only 1/k
• So most of the energy in this waveform is in
  the first few frequency components

62
Relationship Between Data Rate and Bandwidth
Contd.

• Suppose that we have A1, k1,3,5 and f1MHz
  106Hz for an approximated square wave signal
• Then the bandwidth of the signal is (5106) 106
  4MHz
• The period of the fundamental frequency is T
  1/106 1µs
• If we consider this waveform as a bit string of
  1s and 0s, one bit occurs every 0.5 µs for a data
  rate of 2 106 2Mbps
• Thus, for a bandwidth of 4 MHz, a data rate of 2
  Mbps is achieved with signal accuracy is limited
  to k lt 5

63
Relationship Between Data Rate and Bandwidth
Contd.

• In general, any digital waveform will have
  infinite bandwidth
• When this waveform is transmitted as a signal
  over any medium, the transmission system will
  limit the bandwidth that can be used
• Further, greater the bandwidth transmitted, the
  greater the cost
• Thus on the one hand, economic and practical
  reasons dictate that digital information be
  approximated by a signal of limited bandwidth

64
Relationship Between Data Rate and Bandwidth

• On the other hand, limiting the bandwidth creates
  distortions, which makes the task of interpreting
  the signal more difficult
• As a general rule, for a data rate of a digital
  signal W bps, a very good representation can be
  achieved with a bandwidth of 2W Hz
• The higher the data rate of a signal, the greater
  is its required effective bandwidth

65
Relationship Between Data Rate and Bandwidth
Contd.

• We may think that bandwidth of a signal as being
  centred about some frequency referred to as the
  centre frequency
• Higher the centre frequency, the higher the
  potential bandwidth and therefore the higher the
  potential data rate
• For example, if a signal is centred at 2 MHz, its
  maximum bandwidth is 4 MHz

66
Analog and Digital Data Transmission

• The term analog and digital corresponds, roughly,
  to continuous and discrete
• These terms are used in data communications in at
  least three contexts
• Data, signalling and transmission
• Data are defined as entities that convey meaning
• Signals are electric or electromagnetic
  representations of data

67
Analog and Digital Data Transmission Contd.

• Today the most commonly used text code is the
  International Reference Alphabet (IRA)
• Each character in this code is represented by a
  unique 7.bit pattern thus 128 different
  characters can be represented
• IRA-encoded characters are almost always stored
  and transmitted using 8 bits per character
• The eighth bit is a parity bit used for error
  detection

68
Analog and Digital Transmission Contd.

• The same technique may be used with an analog
  signal if it is assumed that the signal carries
  digital data
• At appropriately spaced points, the transmission
  system has repeaters rather than amplifiers
• The repeater recovers digital data from the
  analog signal and generates a new, clean analog
  signal
• As a result, noise is not cumulative

69
Transmission Impairments Contd.

• Attenuation is weakening of signal strength with
  distance that could happen over any transmission
  medium
• For guided media, attenuation is generally
  exponential and expressed as a constant number
  of decibels per unit distance
• For unguided media, attenuation is a more complex
  function of distance and make up of the
  atmosphere
• The problem of attenuation is overcome by using
  amplifiers and repeaters

70
Transmission Impairments Contd.

• Thermal noise cannot be eliminated and therefore
  places an upper bound on communications systems
  performance
• The amount of thermal noise to be found in a
  bandwidth of 1 Hz in any device or conductor is
  given by
• N0 kT (W/Hz)
• Where
• N0 noise power density in watts per 1
  Hz of bandwidth
• k Boltzmanns constant 1.38 10-23 J/K
• T Temperature in kelvins

71
Channel Capacity Contd.

• Noise is the average level of noise over the
  communication path
• Error rate is the rate at which errors occur
• As all transmission channels of any practical
  interest are of limited bandwidth, we like to
  make as efficient use as possible of a given
  bandwidth
• For digital data, this means that we would like
  to get as high a data rate as possible at a
  particular limit of error for a given bandwidth

72
Channel Capacity Contd.

• As a standard quality measure for digital
  communications system performance the ratio of
  signal energy per bit to noise power density per
  Hz is used
• That is, Eb/N0
• Where Eb STb and N0 kT (from slide 48)
• S Signal power
• Tb the time required to send one bit
• Further, the data rate R 1/ Tb

73
Channel Capacity Contd.

• This gives
• In decibel notation, it gives

74
Channel Capacity Contd.

• The Eb/N0 ratio is important as the bit error
  rate for digital data is a decreasing function of
  this ratio
• Given a value of the ratio needed to achieve a
  desired error rate, the parameters in the
  preceding formula may be selected
• As the bit rate R increases, the transmitted
  signal power, relative to noise, must increase to
  maintain the required Eb/N0

75
Channel Capacity Contd.

• Nyquist formula indicates that, all other things
  being equal, doubling the bandwidth doubles the
  data rate
• Now consider the relationship among data rate,
  noise and error rate
• If the data rate is increased , the bits become
  shorter, thereby affecting more bits by a given
  pattern of noise
• Thus at a given noise level, the higher the data
  rate, the higher the error rate