1. Encoding and Transmission of Data
CSIS 625 Week 2
Encoding and Transmission of Data
Copyright 2001, Dan Oelke
Portions Copyright Dmitry Gringauz
For use by students of CSIS 625 for purposes of this class only.
CSIS 625

2. Overview Analog and Digital Signals Encoding and Modulation Vocabulary
Analog Signals
Digital Signals
Encoding and Modulation
Digital to Digital Conversion
Analog to Digital Conversion
Digital To Analog Conversion
Analog to Analog Conversion
CSIS 625

3. Analog and Digital Signals
Signal - an electromagnetic wave that transfers information
Analog Signal - Continuous set of data
Real Numbers
Digital Signals - Discrete set of data
Integer Numbers
Often binary (1 or 0 only)
Digital Signal
Analog Signal
CSIS 625

4. Periodic vs Aperiodic Signals
A signal that completes a pattern in a measureable time frame
Aperiodic Signal
A signal that does not exhibit a pattern
All aperiodic signals can be shown to be a combination of periodic signals
APeriodic Signal
Periodic Signal
CSIS 625

5. Signal definitions
Amplitude - The “height” of a signal. Measured in Volts, Amps, Watts, etc.
Period - The amount of time to complete one cycle
Frequency - The number of periods per second. Measured in Hertz (Hz)
Amplitude
Period
CSIS 625

6. Phase
The position of a sine wave relative to time zero. Measured in degrees.
0 Degrees
90 Degrees
1/4 Cycle
180 Degrees
1/2 Cycle
270 Degrees
3/4 Cycle
CSIS 625

7. Bandwidth Bandwidth - A range of frequencies Analog - measured in Hz
Bandwidth = High-Freq – Low-Freq
Spectrum - synonym - used only in analog measurements.
Bandwidth in digital realm - often used to refer to bits-per-second
CSIS 625

8. Bit Rate Most digital signals are aperiodic
Period and frequency are not appropriate to describe digital signals
Bit Interval - time to send one bit
Bit rate - number of bits send in a second. Measured in bits per second
bps - Bits Per Second
Do NOT use Hz when you mean bps or vice-versa
CSIS 625

9. Decomposing a digital signal
A digital signal can be decomposed into an infinite number of simple sine waves
It is not practical or necessary to send all of these components
Significant Bandwidth - Those frequencies necessary to recreate a digital bit pattern
Significant Bandwidth is related to bit rate
Greater bit rate = Greater significant bandwidth
CSIS 625

10. Medium Bandwidth and Significant Bandwidth
All transmission mediums have limited bandwidth
The significant bandwidth of a digital bit rate must fit within the limited bandwidth of the medium that carries it.
CSIS 625

11. Encoding
Information must often be encoded before being sent over a medium
Four basic types of encoding
Digital to Digital
Analog to Digital
Digital to Analog
Analog to Analog
Encoding schemes may be stacked
Voice to digital data to radio waves
CSIS 625

12. Digital to Digital Encoding
Using a digital signal to represent digital data
Binary data is translated to different voltage, current, or light pulses that can be transported over the medium.
Types
Unipolar - uses 1 signal level
Polar - uses 2 signal levels
Bipolar - uses 2 signal levels and 0
CSIS 625

13. Digital signal encoding formats
Unipolar
NRZL
NRZI RZ
Manchester
Differential Manchester
Bipolar-AMI
Pseudoternary
CSIS 625

14. Unipolar Encoding Simplest scheme Uses two signal levels
1’s are encoded with signal present
0’s are encoded by absence of a signal
(Sometimes inverse of the above)
Long run of 0s or 1s can’t be handled by some mediums
CSIS 625

15. Unipolar encoding - synchronization
When a signal isn’t varying, receiver can’t determine beginning and ending of each bit
Solutions:
A separate line with a clock signal
Asynchronzous Serial lines wrap each byte with start and stop bit
Scrambling of data to ensure enough transitions
Use of additional coding schemes like 8b10b
CSIS 625

16. Polar Encoding Uses a positive and a negative signal
but not a zero level
Several types of Polar encoding
NRZ - Non-Return to Zero
RZ - Return to Zero
Biphase
CSIS 625

17. Non-Return to Zero - Level
NRZL - Non-Return to Zero - Level
Simple - exactly like Polar, except
1’s are encoded with positive signal
0’s are encoded with negative signal
(Sometimes inverse of the above)
Same synchronization problems and solutions
CSIS 625

18. Non-Return to Zero - Invert on Ones
NRZI - Non-Return to Zero - Invert on Ones
A change in voltage level indicates a 1
No change in voltage level indicates a 0
Synchronization less of a problem
Every 1 bit causes a signal change
A string of 0’s still causes problems
Same synchronization solutions
CSIS 625

19. Return to Zero RZ - Return to Zero
Not strictly polar - uses 0 in addition to positive and negative
Works like NRZL, except it goes to zero between each bit.
Transition to/from zero provides for synchronization
Because there are more transisitions (2 per bit time) it has a higher significant bandwidth than NRZ
CSIS 625

20. Manchester Coding A biphase mechanism
Inversion of signal in middle of each bit
low to high transition is 1
high to low transition is 0
Mid-bit inversion provides for both data and synchronization information
May have transition between bits so that right transition can be made in middle of a bit
CSIS 625

21. Differential Manchester
A biphase mechanism
Always has a mid-bit inversion to provide timing information
Inversion at beginning of bit time provides data
Presence of inversion means 0
No inversion means 1
CSIS 625

22. Bipolar AMI Bipolar Alternate Mark Inversion
Mark comes from old telegraphy - means 1
Encoding
0 = lack of signal (0)
1 = positive or negative values alternating for successive ones
CSIS 625

23. Pseudoternary Same as Bipolar AMI, but inverts 1s and 0s Encoding
0 = positive or negative values alternating for successive zeros
1 = lack of signal (0)
CSIS 625

24. B8ZS Bipolar 8-Zero Substitution
A modification of Bipolar AMI to solve the synchronization problem that occurs when a long string of 0s occurs
Substitutes 8 consecutive 0s with fixed pattern that contains 2 AMI violations
Binary-AMI
B8ZS
V = Bipolar AMI Violation V
CSIS 625

25. HDB3 High Density Bipolar - 3 Zeros Similar to B8ZS
Substitutes 4 zeros with a pattern that contains 1 AMI violation
Binary-AMI
HDB3
V = Bipolar AMI Violation V
CSIS 625

26. Analog to Digital Encoding
Digitizing - analog to digital conversion
Approximate analog information with a digital signal
Reduces infinite number of analog values to a finite number of digital values.
Codec - Coder-Decoder
Analog to digital converter
CSIS 625

27. Pulse Amplitude Modulation (PAM)
First step to analog to digital encoding
Sample analog amplitude information at equal intervals
PAM alone not useful as measurements are still analog values
CSIS 625

28. Pulse Code Modulation (PCM)
Modifies PAM output to create completely digital signal
PCM quantizes Take the samples from PAM and assigns digital values to each measurement.
Nyquist theorem - To ensure accurate reproduction of a signal, the sample rate must be twice the highest frequency of the original signal
CSIS 625

29. PCM & Telephony
Telephony system uses 8 bits (256 levels) when quantizing
A non-linear set of quantizing levels is used so that quiet sounds are accurately reproduced
Hz is voice range.
8kHz sample rate is used to cover this range
8kHz * 8 bits/sample = 64,000 bps
CSIS 625

30. DM - Delta Modulation
Analog data is approximated using a staircase function that moves up or down by one level each sampling time.
Digital data is a stream of 1s and 0s that specify the up and down steps.
Can be implemented using simple components.
Not as good quality as PCM
Quantizing noise when slope changes slowly
Slope overload noise when slope changes fast
CSIS 625

31. Digital to Analog Conversion
ASK - Amplitude Shift Keying
FSK - Frequency Shift Keying
PSK - Phase Shift Keying
QAM - Quadrature Amplitude Modulation
combination of ASK & PSK
CSIS 625

32. Bit rate vs. Baud Rate & Carrier Signal
Bit rate is Bits per Second
Baud Rate is number of signal units per second
Baud rate is less than or equal bit rate
Don’t mix them up!
Carrier Signal
high frequency signal that is modified to carry digital signal
CSIS 625

33. ASK - Amplitude Shift Keying
Amplitude of signal varied for 1 or 0
Frequency and phase remain constant
Very susceptible to noise
On-Off-Keying - signal and no-signal
Example:
1 BIT
1 BIT 1
1 BIT
1 BIT 1
CSIS 625

34. FSK- Frequency Shift Keying
Frequency of the carrier signal is varied to represent a 1or 0.
Avoids many of the noise problems of Amplitude Shift keying
Example:
1 BIT
1 BIT 1
1 BIT
1 BIT 1
CSIS 625

35. PSK - Phase Shift Keying
The phase of the carrier signal is varied to represent a 1 or 0.
Avoids noise problems of ASK
Uses less bandwidth than FSK
Example:
1 BIT
1 BIT 1
1 BIT
1 BIT 1
CSIS 625

36. QPSK - Quadrature PSK
A type of PSK that uses 90° shifts instead of 180° shifts.
Allows for 2 bits per baud to be encoded.
CSIS 625

37. DPSK - Differential PSK
The bit pattern defines the phase change, instead of the current phase
V.22bis standard at 1200 bps uses:
00  90 Degree phase change
01  0 Degree phase change
10  180 Degree phase change
11  270 Degree phase change
CSIS 625

38. Quadrature Amplitude Modulation
The phase and amplitude of the carrier signal is varied to give several bits per baud
Number of different phases is greater than number of amplitudes
Example: 2 amplitudes & 4 phases
3 BITS
000
3 BITS
010
3 BITS
001
3 BITS
111
CSIS 625

39. Trellis Coded Modulation
Uses QAM, but includes extra data
Trellis coding is a specific type of convolutional encoding
Viterbi Decoder - a specific algorithm for decoding convolutionally encoded data.
Convolutional codes add redundancy to the data, which makes it more resistant to noise.
Resistance to noise is more important as data rates get higher.
CSIS 625

40. Constellation diagrams
Constellation diagram shows relationship between amplitude and phase of different signal levels
polar diagram,
amplitude shown as distance from center
phase shown as degrees around circle
011
010 1 1
101
100
000
001
110
ASK
PSK
111
8-QAM
16-QAM
CSIS 625

41. Bandwidth required Amplitude Shift Keying Frequency Shift Keying
bandwidth = baud rate * (1 + noise factor)
noise factor is 0 in ideal world
Frequency Shift Keying
bandwidth = (fc1 - fc0) + baud rate
Phase Shift Keying & QAM
but bit rate is higher because more than one bit per baud
CSIS 625

42. Analog to Analog Encoding
AM - Amplitude Modulation
The amplitude of the carrier is modified
Bandwidth = 2x Bandwidth of modulating signal
FM- Frequency Modulation
The frequency of the carrier is modified
Bandwidth = 10x Bandwidth of modulation signal
CSIS 625

43. Analog to Analog Encoding
Phase Modulation
The phase of the carrier is modified
Phase Modulation and FM are a special case of Angle modulation
Observing the signal, it is impossible to tell apart FM and phase modulation
CSIS 625

44. Parallel/Serial Transmission of Data
Transmission of Digital Data
Serial & Parallel transmission
Serial interfaces - DTE & DCE - Modems
CSIS 625

45. Parallel Transmission of Data
Send several bits of data at the same time, each one over a separate media link.
Typically 8 bits of data sent over 8 wires
Examples: Printer cables, SCSI, PCI bus
Allows faster transmission of data, but at the cost of multiple wires, multiple transmitters, and multiple receivers
Must keep all bits in sync
Typically uses a separate clock line
CSIS 625

46. Serial Transmission of Data
Sends all bits from node to node over a single media link.
Bits are sent one after another - or “serially”
May or may not have additional media links for clock, frame, or flow control.
Need some method of keeping track of when a byte starts and ends.
Asynchronous or Synchronous
CSIS 625

47. Serial - Asynchronous transmission
Bits are grouped together into characters
Start and stop bits frame the data bits
A start bit is sent first
Followed by the data bits
Followed by a stop bit or bits
Variable number of idle bits between characters
CSIS 625

48. Serial - Asynchronous transmission
At best - 80% efficient
8 data bits
1 start bit
1 stop bit
Allows for about a lot of timing error
Example:
Start
Data
Stop
CSIS 625

49. Serial - Synchronous transmission
Each byte of data is sent with no extra gaps between bytes.
Data is grouped into frames
Frame contains
Between frames, special idle patterns used
Much less overhead that asynchronous
Can achieve faster bit rates than asynchronous
Requires synchronization method
CSIS 625

50. Data transparency on serial links
Data transparency - the ability of a link to send any data pattern
Bit stuffing - insertion of extra bits to change a flag pattern so that data transparency is achieved
Byte stuffing - insertion of extra bytes to change a flag pattern so that data transparency is achieved
Flag character - special bit pattern to show start or end of a frame
CSIS 625

51. Serial - Synchronous transmission
Bit-oriented synchronous transmission
Uses a special bit pattern at the start and end of the frame (flag character)
Data may be any number of bits
Uses bit stuffing to replace flag pattern in data
Bit stuffing is slightly more efficient than byte stuffing
Easier to implement in hardware
CSIS 625

52. Serial - Synchronous transmission
Character oriented synchronous transmission
Uses a special byte at the start and end of the frame
Data must be an even number of 8-bit bytes
Uses byte stuffing to replace flag byte in data
Byte stuffing makes this slightly less efficient
Easier to implement in software
CSIS 625

53. DTE-DCE interface DTE - Data Terminal Equipment
A device that is a source or destination for binary digital data
DCE - Data Circuit-terminating Equipment
A device that interfaces between a DTE and a network
Modem is classic DCE example
Lots of standards specify DTE to DCE interface
More standards for DCE to DCE interface
CSIS 625

54. RS-232 Interface
Specifies the mechanical, electrical & functional characteristics of DTE-DCE interface
EIA-232 is now the official name
Tailored to Computer to modem interface
Limited to about 20 Kbps
Mechanical
less than 50 feet long cable
DB-25 connector original standard
DB-9 connector now standardized
CSIS 625

55. RS-232 Interface Electrical - Uses NRZL
0 = +3 to +15 volts
1 = -3 to -15 volts
3 pins are all that are necessary
Receive Data
Transmit Data
Ground
Other pins are often ignored
Null modem - a device that flips receive and transmit lines
CSIS 625

56. Other serial interfaces
RS uses 37 pin connector
RS uses 2-6 volt levels
40 feet Kbps
4000 feet - 1 Kbps
RS Volt balanced transmission
40 feet - 10 Mbps
CSIS 625

57. Balanced transmission
Uses two wires with a positive or negative voltage put on the line
Compared to unbalanced which using two wires, one as ground and the other as signal.
Better noise resistance than unbalanced
CSIS 625