Data and Computer Communications

1
Data and Computer Communications

• Chapter 2
• Data Communication

2
Intro

• Transmission falls under three categories
• Simplex- signals are transmitted in only one
  direction
• One host is the transmitter and the other is
  receiver.
• Half-duplex- both hosts may transmit, but only
  one at a time.
• Full-duplex both hosts may transmit
  simultaneously.
• The medium is carrying signals in both directions
  at the same time.

3
The concepts of Frequency, Spectrum and Bandwidth

• Use electromagnetic signals to transmit data.
• Signal is generated by the transmitter and
  received by the receiver over the transmission
  medium.
• The signal is a function of time, anyway it can
  also be expresses as a function of frequency- the
  signal consists of components of different
  frequencies.
• Two concepts will be discussed
• Time domain concepts
• Frequency domain concepts

4
Time domain concepts

• Viewed as a function of time, an electromagnetic
  signal can be either analog or digital.
• Analog signal
• Signal intensity/strength various in a smooth way
  over time
• No break or discontinuities in the signal.
• Digital signal
• Signal intensity maintains a constant level then
  changes to another constant level

5
Periodic and Aperiodic signal

• Both analog and digital signals can take of two
  forms periodic and aperiodic.
• Periodic signal
• Pattern repeated over time
• Aperiodic signal
• Pattern not repeated over time

6
Analogue Digital Signals
7
PeriodicSignals
8
Sine Wave- The fundamental of the periodic signal

• Can be represented by three parameters
• Peak Amplitude (A)
• maximum strength of signal over time
• Measured in volts
• Frequency (f)
• The number of periods in one second
• Rate of change of signal
• Hertz (Hz) or cycles per second
• Period time for one repetition (T)
• T 1/f

9
Continue

• Phase (?)
• Relative position in time
• Describes the position of the waveform relative
  to time zero.
• Measured in degrees or radian (3600 / 2? rad)

10
Varying Sine Wavess(t) A sin(2?ft ?)
11
Wavelength

• Another characteristic of a signal traveling
  through a transmission medium.
• It binds the period/frequency of a simple sine
  wave to the propagation speed of the medium.
• It depends on frequency and medium.
• Distance occupied by one cycle
• Distance between two points of corresponding
  phase in two consecutive cycles
• Symbol used ?
• Assuming signal velocity/speed/rate v
• ? vT
• ?f v
• c 3108 ms-1 (speed of light in free space)

12
Frequency Domain Concepts

• Signal usually made up of many frequencies
• Components are sine waves
• Can be shown (Fourier analysis) that any signal
  is made up of component sine waves
• Can plot frequency domain functions

13
Addition of FrequencyComponents(T1/f)
14
FrequencyDomainRepresentations
15
Spectrum Bandwidth

• Spectrum
• range of frequencies contained in signal
• Figure 2.1 above the spectrum extends from f to
  3f
• DC (direct current) Component
• Component of zero frequency

16
Signal with DC Component
17
Bandwidth

• The range of frequencies that a medium can pass
  without losing one-half of the power contained in
  that signal.
• Exp If a medium can pass frequencies between
  1000 and 5000 without losing most of the power
  contained in this range, its bandwidth is
  5000-10004000.
• Exp voice has a spectrum of 300 to 3300 Hz ( a
  bandwidth of 3000 Hz). If the transmission line
  is using having only 1000 Hz bandwidth, thus some
  frequencies in voice will be missing cannot be
  recognizable.
• Absolute bandwidth
• width of spectrum
• Effective bandwidth
• Often just bandwidth
• Narrow band of frequencies containing most of the
  energy

18
Data Rate and Bandwidth

• Any transmission system has a limited band of
  frequencies
• This limits the data rate that can be carried

19
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

20
Analog and Digital Data

• Analog
• Continuous values within some interval
• e.g. sound, video
• Digital
• Discrete values
• e.g. text, integers

21
Acoustic Spectrum (Analog)
22
Analog and Digital 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
• a sequence of voltage pulses that may be
  transmitted over a wire medium
• Exp a constant positive voltage level binary 0
  and
• a constant negative voltage level
  binary 1

23
Advantages Disadvantages of Digital

• Cheaper
• Less susceptible to noise
• Greater attenuation
• Pulses become rounded and smaller
• Leads to loss of information

24
Attenuation of Digital Signals
25
Example on Analog signal Speech

• Frequency range (of hearing) 20Hz-20kHz
• Speech 100Hz-7kHz
• Easily converted into electromagnetic signal for
  transmission
• Sound frequencies with varying volume converted
  into electromagnetic frequencies with varying
  voltage
• Limit frequency range for voice channel
• 300-3400Hz

26
Conversion of Voice Input into Analog Signal
27
Video Components

• USA - 483 lines scanned per frame at 30 frames
  per second
• 525 lines but 42 lost during vertical retrace
• So 525 lines x 30 scans 15750 lines per second
• 63.5?s per line
• 11?s for retrace, so 52.5 ?s per video line
• Max frequency if line alternates black and white
• Horizontal resolution is about 450 lines giving
  225 cycles of wave in 52.5 ?s
• Max frequency of 4.2MHz

28
Binary Digital Data

• From computer terminals etc.
• Two dc components
• Bandwidth depends on data rate

29
Conversion of PC Input to Digital Signal
30
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

31
Analog Signals Carrying Analog and Digital Data
32
Digital Signals Carrying Analog and Digital Data
33
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

34
Digital Transmission

• Concerned with content
• Integrity endangered by noise, attenuation etc.
• Repeaters used
• Repeater receives signal
• Extracts bit pattern- recovers the pattern of 1s
  and 0s
• Retransmits
• Attenuation is overcome
• Noise is not amplified

35
Advantages of Digital Transmission

• Digital technology
• Low cost LSI/VLSI technology
• Data integrity
• Longer distances over lower quality lines
• Capacity utilization
• High bandwidth links economical
• High degree of multiplexing easier with digital
  techniques
• Security Privacy
• Encryption
• Integration
• Can treat analog and digital data similarly

36
Transmission Impairments

• Signal received may differ from signal
  transmitted
• Analog - degradation of signal quality
• Digital - bit errors may be introduced
• Caused by
• Attenuation and attenuation distortion
• Delay distortion
• Noise

37
Attenuation

• Signal strength falls off with distance
• Depends on medium- happened in both guided and
  unguided media
• 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
• For analog signal, attenuation varies as a
  function of frequencies, the received signal is
  distorted.
• To overcome need to equalize the attenuation
  across a band of frequencies.
• Or use amplifiers that amplify high frequencies
  more than lower frequencies.

38
Delay Distortion

• Only in guided media
• Due to propagation velocity/strength varies with
  frequency
• The received signal is distorted due to varying
  delays experienced at its constituent /basic
  frequencies.
• Critical for digital data
• Exp A sequence of bits is being transmitted,
  because of delay distortion , some of the signal
  components of one bit position spill over other
  bit position unknown data.

39
Noise (1)

• Additional signals inserted between transmitter
  and receiver
• 4 categories of noise
• Thermal
• Due to thermal agitation of electrons
• Present in all electronic devices and
  transmission media.
• Is a function of temperature
• Uniformly distributed across the bandwidths
• Referred as white noise
• Cannot be eliminated effect the communication
  performance
• Particularly significant for satellite
  communication

40
Noise(2)

• Intermodulation
• When a signal at different frequencies sharing
  the same transmission medium may result in
  intermodulation noise.
• Signals that are the sum /difference of original
  frequencies sharing a medium
• Exp 2 signals, f1 and f2 f1f2, interfere with
  an intended signal at the frequency f1f2
• Produced by nonlinearities in the transmitter,
  receiver and/or intervening transmission medium.
• Crosstalk
• A signal from one line is picked up by another.
• Can occur by electrical coupling between nearby
  twisted pair/coax cable lines carrying multiple
  signals.

41
Noise (3)

• Impulse
• Noncontinuous, consist of
• Irregular pulses or spikes of short duration and
  relatively high amplitude
• e.g. External electromagnetic interference,
  lightening
• Minor annoyance for analog data, major for
  digital data.
• Exp a sharp spike of energy of 0.01s duration
  would not destroy any voice data but would wash
  out about 560 bits of data being transmitted at
  56 kbps.

42
Channel Capacity.

• Refer to the maximum rate at which data can be
  transmitted over a given communication
  path/channel
• 4 concepts need to be link to understand this.

43
Channel Capacity(1)

• 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
• Noise
• The average level of noise over the
  communications path
• Error rate
• the rate at which errors occur, an error is the
  reception of a 1 when a 0 was transmitted and
  vice versa.

44
Nyquist Bandwidth

• The limitation on the data rate in on bandwidth
  only.( consider channel is noise free)
• If rate of signal transmission is 2B then signal
  with frequencies no greater than B is sufficient
  to carry signal rate
• Given bandwidth B, highest signal rate is 2B
• Given binary signal, data rate supported by B Hz
  is 2B bps
• Can be increased by using M signal levels
• M no of discrete signal or voltage levels.
• C 2B log2M
• For a given bandwidth, the data rate can be
  increased by increasing the number of different
  signal elements.
• Places a burden at the receiver.

45
Shannon Capacity Formula

• Consider data rate, noise and error rate
• Faster data rate shortens each bit so burst of
  noise affects more bits
• The present of noise can corrupt one/more bits.
• At given noise level, high data rate means higher
  error rate
• Signal to noise ration (in decibels)
• SNRdb10 log10 (signal/noise)
• Capacity CB log2(1SNR)
• This is error free capacity

46
Signal-to-noise ratio(SNR)

• is the ratio of the power in a signal to the
  power contained in the noise that is present at a
  particular point in the transmission.
• Measured at the receiver- since at this point the
  signal need to be process and recover the data
• Often stated as decibels
• SNR db 10 log 10 signal power/noise power
• High SNR mean high-quality signal and low number
  of required intermediate repeaters.
• Important in digital data transmission since it
  sets the upper bound on the achievable data rate.

47
Encoding

• Recap- Chapter 3 explain the distinction between
  analog digital data , analogdigital signal.
• Either form of data could be encoded into either
  form of signal.
• Figure 5.1- for digital signaling, a data source
• g(t) which may be either digital/analog is
  encoded into a digital signal x(t)
• The actual form of x(t) depends on encoding
  technique.

48
Data encoding

• Both analog and digital data can be encoded as
  either analog or digital signals.
• Encoding Techniques
• Digital data, digital signal
• Analog data, digital signal
• Digital data, analog signal
• Analog data, analog signal

49
Terms (1)

• Unipolar
• All signal elements have same sign
• Polar
• One logic state represented by positive voltage
  the other by negative voltage
• Data rate
• Rate of data transmission in bits per second
• Duration or length of a bit
• Time taken for transmitter to emit the bit
• For a data rate ,R the bit duration is 1/R.

50
Terms (2)

• Modulation rate
• Rate at which the signal level changes
• Measured in baud signal elements per second
• Mark and Space
• Binary 1 and Binary 0 respectively
• Refer to table 5.1 pg 132 for Key Data
  Transmission Terms.

51
Unipolar

• Use only one voltage level
• If the signal elements all have the same sign,
  i.e ve or ve thus the signal is unipolar
• Drawing on the whiteboard.
• 2 problems make it undesirable.
• A dc component- the average amplitude of unipolar
  encoded signal is nonzero, this will create a dc
  component.
• Lack of synchronization if the data contain a
  long sequence of 0s/1s, there is no change in the
  signal during this duration that can alert the
  receiver to potential synchronization problems.

52
Synchronization

• To correctly interpret the signal received from
  the sender, the receivers bit interval must
  correspond exactly to the senders bit intervals.
• A self-synchronization digital signal includes
  timing info in the data being transmitted.
• This can be achieved if there are transitions in
  the signal that alert the receiver to the
  beginning , middle /end of the pulse.

53
Polar

• Use 2 voltage level , ve, -ve
• The average voltage level on the line is reduced
  and the dc component problem occurred in unipolar
  is improved,
• This class, we study on 2 polar encoding
• Nonreturn to Zero (NRZ)
• Return to Zero (RZ)

54
Interpreting Digital Signals-Receiver

• Need to know
• Timing of bits - when they start and end
• Signal levels high/low
• Factors affecting successful interpreting of
  signals
• Signal to noise ratio
• Data rate
• Bandwidth

55
Continue..

• Another factor that can used to improve
  performance-encoding scheme.
• What is it?
• It is the mapping from data bits to signal
  elements.

56
Comparison of Encoding Schemes (1)

• Signal Spectrum
• Lack of high frequencies reduces required
  bandwidth needed for transmission.
• Lack of dc component allows ac coupling via
  transformer, providing electrical isolation and
  therefore reduce the interference.
• Concentrate power in the middle of the bandwidth
• Clocking
• Synchronizing transmitter and receiver
• Sync mechanism based on signal transmitted
• Can be achieve using suitable encoding

57
Comparison of Encoding Schemes (2)

• Error detection
• Responsibility of data link layer
• Useful to have some error detection capability
  built into the physical signaling encoding
  scheme.- permit detection of error earlier.
• Signal interference and noise immunity
• Some codes are better than others, in the
  presence of noise.
• Cost and complexity
• Higher signal rate ( thus data rate) lead to
  higher costs
• Some codes require signal rate greater than data
  rate

58
Encoding Schemes

• Return to Zero (RZ)
• Nonreturn to Zero-Level (NRZ-L)
• Nonreturn to Zero Inverted (NRZI)
• Bipolar -AMI
• Pseudoternary
• Manchester
• Differential Manchester
• B8ZS
• HDB3
• Note in this course we touch only RZ and NRZ)

59
Nonreturn to Zero-Level (NRZ-L)

• In NRZ the value of the signal is always either
  ve/-ve
• In NRZ-L-the level of the signal depends on the
  type of bit it represents.
• ve voltage-gtbit 0, -ve voltage -gtbit 1.
• The level of the signal depend on the state of
  the bit.
• Voltage constant during bit interval (gap)
• no transition I.e. no return to zero voltage
• Problem Contain data which comprise of long
  stream of 0s or 1s. The receiver receives a
  continuous voltage and determines how many bits
  are sent by relying on its clock, which may not
  be synchronized with the sender clock.

60
Nonreturn to Zero Inverted (NRZ-I)

• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of signal
  transition at beginning of bit time
• Transition (low to high or high to low) denotes a
  binary 1
• No transition denotes binary 0
• An example of differential encoding
• In here, the signal is inverted if a 1 is
  encountered.

61
Continue

• Refer to diagram on slide 62
• In NRZ-L sequence, ve,-ve voltages have specific
  meanings ve0 and ve 1.
• NRZ-I sequence, the voltages per se are
  meaningless. The receiver looks for changes from
  one level to another as its basis for recognition
  of 1s.

62
NRZ
63
Differential Encoding

• Data represented by changes rather than levels
• More reliable detection of transition rather than
  level
• In complex transmission layouts it is easy to
  lose sense of polarity (refer to text pg 125)

64
NRZ pros and cons

• Pros
• Easy to engineer
• Make good use of bandwidth
• Cons
• dc component
• Lack of synchronization capability

65
Continue..

• Anytime the original data contains strings of
  consecutive 1s or 0s, the receiver can lose its
  place.
• Solution need to include synchronization in the
  encoded signal like NRZ-I-only on sequence of 1s.
• To ensure synchronization, signal must change for
  each bit.
• The receiver can use these changes to build-up,
  update and sync its clock.
• To change every bit requires more than 2 values.

66
Return to Zero (RZ)

• Use 3 values positive, negative, zero
• The signal change during each bit
• A positive voltage means 1 and negative voltage
  means 0, halfway through each bit interval the
  signal returns to zero.
• A 1 bit is represented by positive-to-zero and 0
  bit by negative-to-zero.
• Purpose to provide synchronization between
  receiver and transmitter.

67
RZ (continue)

• Draw RZ encoding in class.
• Main disadvantage
• Requires two signal changes to encode 1 bit and
  therefore occupies more bandwidth.
• Most effective compared to the previous one.

68
Multiplexing

• Bandwidth of a medium ling two devices gt than the
  bandwidth needs of the devices, the link can be
  shared.
• Multiplexing set of technologies that allows
  the simultaneous transmission of multiple signals
  across a single data link.
• Refer to diagram at slide 69.

69
Multiplexing

• The n inputs line direct their transmission
  streams to a multiplexer (MUX)- combines all the
  n inputs to a single stream.
• At the receiving end, that stream will be fed
  into a demultiplexer (DEMUX) and directs them to
  their corresponding lines.

70
Frequency Division Multiplexing

• FDM-an analog technique that can be applied when
  the bandwidth of a link (hertz) is greater than
  the combined bandwidths of the signals to be
  transmitted.
• Each signal is modulated to a different carrier
  frequency
• Carrier frequencies separated so signals do not
  overlap (guard bands)
• e.g. broadcast radio
• Channel allocated even if no data

71
Frequency Division MultiplexingDiagram
72
FDM System
73
FDM of Three Voiceband Signals
74
Wavelength Division Multiplexing

• Multiple beams of light at different frequency
• Carried by optical fiber
• A form of FDM
• Each color of light (wavelength) carries separate
  data channel
• 1997 Bell Labs
• 100 beams
• Each at 10 Gbps
• Giving 1 terabit per second (Tbps)

75
WDM Operation

• Same general architecture as other FDM
• Number of sources generating laser beams at
  different frequencies
• Multiplexer consolidates sources for transmission
  over single fiber
• Optical amplifiers amplify all wavelengths
• Typically tens of km apart
• Demux separates channels at the destination

76
Synchronous Time Division Multiplexing

• Data rate of medium exceeds data rate of digital
  signal to be transmitted
• Multiple digital signals interleaved in time
• May be at bit level of blocks
• Time slots preassigned to sources and fixed
• Time slots allocated even if no data
• Time slots do not have to be evenly distributed
  amongst sources

77
Time Division Multiplexing
78
TDM System
79
TDM Link Control

• No headers and trailers
• Data link control protocols not needed
• Flow control
• Data rate of multiplexed line is fixed
• If one channel receiver can not receive data, the
  others must carry on
• The corresponding source must be quenched
• This leaves empty slots
• Error control
• Errors are detected and handled by individual
  channel systems

80
Statistical TDM

• In Synchronous TDM many slots are wasted
• Statistical TDM allocates time slots dynamically
  based on demand
• Multiplexer scans input lines and collects data
  until frame full
• Data rate on line lower than aggregate rates of
  input lines

81
Statistical TDM Frame Formats
82
Performance

• Output data rate less than aggregate input rates
• May cause problems during peak periods
• Buffer inputs
• Keep buffer size to minimum to reduce delay