1. ITC242 – Introduction to Data Communications Week 9 Topic 14 Data Transmission Topic 15 Data Communication Fundamentals Reading 3

2. Last Week Reading 2 – Wide Area and Large-Scale Networks
Describe the basic concepts associated with wide area networks
Identify the uses, benefits, and drawbacks of WAN technologies such as ATM, FDDI, SONET, SMDS

3. Topic 14 – Data Transmission
Learning Objectives
Describe the difference between analogue and digital signals
Discuss the various transmission impairments that affect signal quality and transfer rate
Use Shannon’s formula to calculate the capacity of a channel

4. Electromagnetic Signals
Analog Signal
signal intensity varies in a smooth fashion over time. In other words, there are no breaks or discontinuities in the signal
Digital Signal
signal intensity maintains a constant level for some period of time and then changes to another constant level

5. Analog Sine Wave

6. Digital Square Wave

7. Signal Characteristics
Peak Amplitude (A)
Maximum signal value, measured in volts
Frequency (f)
Repetition rate
Measured in cycles per second or Hertz (Hz)
Period (T)
Amount of time it takes for one repetition, T=1/f
Phase ()
Relative position in time, measured in degrees

8. Frequency Domain Concepts
Spectrum of a signal is the range of frequencies that it contains
Absolute bandwidth of a signal is the width of the spectrum
Effective bandwidth contained in a relatively narrow band of frequencies, where most of signal’s energy is found
The greater the bandwidth, the higher the information-carrying capacity of the signal

9. 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

10. Voice/Audio Analog 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
Actual bandwidth used is 4kHz to isolate audio signal from adjacent bandwidths.

11. Voice Bandwidth

12. Digital Text Signals
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 forms: ASCII, UTF

13. e.g. ASCII Character Set Control Characters
Binary
Oct
Dec
Hex
Abbr
PR[1]
CS[2]
CEC[3]
Description
000 00
NUL ␀
Null character
001 1 01
SOH ␁ ^A
Start of Header
002 2 02
STX ␂ ^B
Start of Text
003 3 03
ETX ␃ ^C
End of Text
004 4 04
EOT ␄ ^D
End of Transmission
005 5 05
ENQ ␅ ^E
Enquiry
006 6 06
ACK ␆ ^F
Acknowledgment
007 7 07
BEL ␇ ^G \a
Bell
010 8 08 BS ␈ ^H \b
Backspace[4][8]
011 9 09 HT ␉ ^I \t
Horizontal Tab
012 10 0A LF ␊ ^J \n
Line feed
013 11 0B VT ␋ ^K
Vertical Tab
014 12 0C FF ␌ ^L \f
Form feed
015 13 0D CR ␍ ^M \r
Carriage return[7]

14. e.g. ASCII Character Set Printable Characters
Binary
Dec
Hex
Glyph 96 60 ` 97 61 a 98 62 b 99 63 c
100 64 d
101 65 e
102 66 f
Binary
Dec
Hex
Glyph 64 40 @ 65 41 A 66 42 B 67 43 C 68 44 D 69 45 E 70 46 F

15. Transmission Media
Physical path between transmitter and receiver (“channel”)
Design factors affecting data rate
bandwidth
physical environment
number of receivers
impairments

16. 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)

17. Transmission Impairments: Guided Media
Attenuation
loss of signal strength over distance
Attenuation Distortion
different losses at different frequencies
Delay Distortion
different speeds for different frequencies
Noise
distortions of signal caused by interference 6

18. Transmission Impairments: Unguided (Wireless) Media
Free-Space Loss
Signals disperse with distance
Atmospheric Absorption
Water vapor and oxygen contribute to signal loss
Multipath
Obstacles reflect signal creating multiple copies
Refraction
Thermal Noise

19. Types of Noise Thermal (aka “white noise”) Intermodulation Crosstalk
Uniformly distributed, cannot be eliminated
Intermodulation
When different frequencies collide (creating “harmonics”)
Crosstalk
Overlap of signals
Impulse noise
Irregular spikes, less predictable

20. Channel Capacity
The rate at which data can be transmitted over a given path, under given conditions
Four concepts
Data rate
Bandwidth
Noise
Error rate

21. Shannon’s Equation
The signal to noise ratio (SNR) sets the upper bound on the achievable data rate.
Shannon’s equation provides a theoretical maximum channel capacity given SNR.
C = B log2 (1 + SNR)
B = Bandwidth of channel in Hertz
C= Channel capacity in bps
SNR = Signal-to-noise ratio

22. Shannon’s Equation
What is the capacity of a signal operating at 5kHz with a SNR of 3?
C = B log2 (1 + SNR)
C = 5000 Hz x log2(1+3)
C = 5000 Hz x log24
C = 5000 Hz x 2
C = bps

23. Review Describe the difference between analogue and digital signals
Transmission impairments – attenuation and noise affect signal quality
Shannon’s formula provides a theoretical estimate of maximum channel capacity

24. Topic 15 – Data Communication Fundamentals
Learning Objectives
Explain the difference between analogue and digital transmission
Describe digital and analogue encoding techniques for the transmission of data
Explain the difference between asynchronous and synchronous transmission and when each technique is used
Describe the process of error detection

25. Data Communication Components
Analog: Continuous value data (sound, light, temperature)
Digital: Discrete value (text, integers, symbols)
Signal
Analog: Continuously varying electromagnetic wave
Digital: Series of voltage pulses (square wave)
Transmission
Analog: Works the same for analog or digital signals
Digital: Used only with digital signals

26. Analog DataSignal Options
Analog data to analog signal
Inexpensive, easy conversion (eg telephone)
Data may be shifted to a different part of the available spectrum (multiplexing)
Used in traditional analog telephony
Analog data to digital signal
Requires a codec (encoder/decoder)
Provides benefits of digital transmission (e.g Audio CD)

27. Digital DataSignal Options
Digital data to analog signal
Requires modem (modulator/demodulator)
Allows use of PSTN to send data
Necessary when analog transmission is used
Digital data to digital signal
Requires CSU/DSU (channel service unit/data service unit)
Less expensive when large amounts of data are involved
More reliable because no conversion is involved

28. Transmission Choices Analog transmission Digital transmission
only transmits analog signals, without regard to data content
attenuation overcome with amplifiers
signal is not evaluated or regenerated
Digital transmission
transmits analog or digital signals
uses repeaters rather than amplifiers
switching equipment evaluates and regenerates signal

29. Advantages of Digital Transmission
The signal is exact
Signals can be checked for errors
Noise/interference are easily filtered out
A variety of services can be offered over one line
Higher bandwidth is possible with data compression

30. Why Use Analog Transmission?
Already in place
Significantly less expensive
Lower attenuation rates
Sufficient for transmission of voice signals

31. Analog Encoding of Digital Data
Data encoding and decoding technique to represent data using the properties of analog waves
Modulation: the conversion of digital signals to analog form
Demodulation: the conversion of analog data signals back to digital form

32. Methods of Modulation
Amplitude modulation (AM) or amplitude shift keying (ASK)
Frequency modulation (FM) or frequency shift keying (FSK)
Phase modulation or phase shift keying (PSK)

33. Amplitude Shift Keying (ASK)
In radio transmission, known as amplitude modulation (AM)
The amplitude (or height) of the sine wave varies to transmit the ones and zeros
Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude

34. ASK Illustration 1 1

35. Frequency Shift Keying (FSK)
In radio transmission, known as frequency modulation (FM)
Frequency of the carrier wave varies in accordance with the signal to be sent
Signal transmitted at constant amplitude
More resistant to noise than ASK
Less attractive because it requires more analog bandwidth than ASK

36. FSK Illustration 1 1 1

38. Phase Shift Keying (PSK)
Also known as phase modulation (PM)
Frequency and amplitude of the carrier signal are kept constant
The carrier signal is shifted in phase according to the input data stream
Each phase can have a constant value, or value can be based on whether or not phase changes (differential keying)

39. PSK Illustration 1 1

41. Modulation Example Asymmetric Digital Subscriber Line (ADSL)
Telephone exchange can provide support for a number of ISPs,
At the exchange a combined data/voice signal is transmitted over a subscriber line
At subscriber’s site, twisted pair is split and routed to both a PC and a telephone
At the PC, an ADSL modem demodulates the data signal for the PC.
At the telephone, a microfilter passes the 4-kHz voice signal.
The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques

42. DSL Modem Layout

43. Digital Encoding of Analog Data
Evolution of telecommunications networks to digital transmission and switching requires voice data in digital form
Best-known technique for voice digitization is pulse-code modulation (PCM)
The sampling theorem: Exact reconstruction of a continuous-time baseband signal from its samples is possible if the signal is bandlimited and the sampling frequency is greater than twice the signal bandwidth
Good-quality voice transmission can be achieved with a data rate of 8 kbps
Some videoconference products support data rates as low as 64 kbps

44. Converting Samples to Bits
Quantizing
Similar concept to pixelization
Breaks wave into pieces, assigns a value in a particular range
8-bit range allows for 256 possible sample levels
More bits means greater detail, fewer bits means less detail

45. Converting Samples to Bits

46. Codec
Coder/Decoder
Converts analog signals into a digital form and converts it back to analog signals
Where do we find codecs?
Sound cards
Scanners
Voice mail
Video capture/conferencing

47. Digital Encoding of Digital Data
Most common, easiest method is different voltage levels for the two binary digits
Typically, negative=1 and positive=0
Known as NRZ-L, or nonreturn-to-zero level, because signal never returns to zero, and the value during a bit time is a level voltage

48. Differential NRZ Differential version is NRZI (NRZ, invert on ones)
A bit time: a constant-voltage pulse
A signal transition at the beginning of the bit time: Change=1, no change=0
Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level

49. Problems With NRZ
Difficult to determine where one bit ends and the next begins
In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses
Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted

50. Biphase Alternatives to NRZ
Require at least one transition per bit time, and may even have two
Modulation rate is greater, so bandwidth requirements are higher
Advantages
Synchronization due to predictable transitions
Error detection based on absence of a transition

51. Manchester Code Transition in the middle of each bit period
Transition provides clocking and data
Low-to-high=1 , high-to-low=0
Used in Ethernet

52. Differential Manchester
Midbit transition is only for clocking
Transition at beginning of bit period=0
Transition absent at beginning=1
Has added advantage of differential encoding
Used in token-ring LANs

53. Digital Encoding Illustration

54. Analog Encoding of Analog Information
Voice-generated sound wave can be represented by an electromagnetic signal with the same frequency components, and transmitted on a voice-grade telephone line.
Modulation can produce a new analog signal that conveys the same information but occupies a different frequency band
A higher frequency may be needed for effective transmission
Analog-to-analog modulation permits frequency-division multiplexing (Chapter 17)

55. Asynchronous and Synchronous Transmission
For receiver to sample incoming bits properly, it must know arrival time and duration of each bit that it receives

56. Asynchronous Transmission
uses start and stop bits to signify the beginning ait ASCII character
Data transmitted one character at a time, where each character is 5 to 8 bits in length.
Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character.
Simple and cheap but requires an overhead of 2 to 3 bits per character

57. Synchronous Transmission
Block of bits transmitted in a steady stream without start and stop codes.
Clocks of transmitter and receiver must somehow be synchronized
Provide a separate clock line between transmitter and receiver; works well over short distances,
Embed the clocking information in the data signal.
Each block begins with a preamble bit pattern and generally ends with a postamble bit pattern
The data plus preamble, postamble, and control information are called a frame

58. Review The difference between analogue and digital transmission
Digital and analogue encoding techniques:
ASK, FSK, PSK
NRZ-L, NRZI, Manchester, Differential Manchester
Asynchronous transmission
Synchronous transmission