Data Communications

1
Data Communications

• Chapter 3
• Data and Signals

2
Terminology (1)

• Transmitter
• Receiver
• Medium
• Guided medium
• e.g. twisted pair, optical fiber
• Unguided medium
• e.g. air, water, vacuum

3
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

4
Terminology (3)

• Simplex
• One direction
• 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

5
Frequency, Spectrum and Bandwidth

• Time domain concepts
• Continuous signal
• Infinite number of points at any given time
• Discrete signal
• Finite number of points at any given time
  maintains a constant level then changes to
  another constant level
• Periodic signal
• Pattern repeated over time
• Aperiodic signal
• Pattern not repeated over time

6
Continuous Discrete Signals
7
Periodic Signals
8
Sine Wave

• Peak Amplitude (A)
• Maximum strength of signal
• Typically 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

9
Varying Sine Waves
10
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

11
Addition of FrequencyComponents
12
FrequencyDomain
13
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

14
Signal with DC Component
15
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

16
Data

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

17
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
• Uses two (or more) DC components

18
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

19
Analog Signals Carrying Analog and Digital Data
20
Digital Signals Carrying Analog and Digital Data
21
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

22
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

23
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

24
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

25
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

26
Attenuation

• Signal strength is measured in decibels (dB)
• dB is a relative measure of loss (or gain)
• NdB 10 log10 (P2 / P1)
• P2 ending power level in watts
• P1 beginning power level in watts
• Example P1 10 watts, P2 5 watts
• Even easier remember ½ rule
• Losses and gains are additive

27
Attenuation

• Signal to noise ratio shows the ratio of signal
  power to noise power
• S/NdB 10 log10 (signal power/noise power)
• Example 1 Signal power 1000 watts, noise
  power 20 mw
• Example 2 Signal power 100 w, noise power
  0.000002w

28
Delay Distortion

• Only in guided media
• Propagation velocity varies with frequency

29
Noise (1)

• Additional signals inserted between transmitter
  and receiver
• Thermal
• Due to thermal agitation of electrons
• Uniformly distributed
• White noise
• Intermodulation
• Signals that are the sum and difference of
  original frequencies sharing a medium

30
Noise (2)

• Crosstalk
• A signal from one line is picked up by another
• Impulse
• Irregular pulses or spikes
• e.g. External electromagnetic interference
• Short duration
• High amplitude

31
Data Rate and Bandwidth

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

32
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
• Baud rate
• Frequency with which the components change

33
BPS vs. Baud

• Data rate rarely the same as baud rate
• Examples

34
Channel Capacity

• Nyquist
• Maximum data rate of a noiseless channel
• 2 H log2 V bps
• Where H frequency
• V the number of discrete levels
• Example H 4000 Hz, V 2

35
Channel Capacity

• Shannon (which includes noise)
• Maximum data rate (in bps) H log2 (1 S/N)
• H frequency
• S signal power in watts
• N noise power in watts
• Example H 3400 Hz, S 0.2 w, N 0.0002 w
• Max data rate 3400 log2 (11000)
• 3400 x 9.97
• 33898 bps

36
Review Questions

• List the advantages and disadvantages of analog
  and digital
• What is the difference between data and signals?
• List the three components of a signal
• What is a composite signal?
• How do you calculate a dB?

37
Review Questions

• How do you use the Nyquist formula?
• 4000 Hz, 8 signal levels, data rate?
• 50,000 bps data rate, 4000 Hz, how many signal
  levels?
• How do you use the Shannon formula?
• 8000 Hz, signal power 20w, noise power
  0.002w, what is the data rate?
• 5000 Hz, signal power 50w, data rate
  20000bps, what is the possible noise power?