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Chapter 5Data Encoding

Review

- Information Numeric Data, characters, voice,

pictures, codes or any massage that can be read

by and has meaning to human and machine.

Review

- For transmission
- Information must be converted into binary first.
- ASCII table
- Unicode
- Information must be encoded into electromagnetic

signals. (Analog or digital)

Review

- Digital Signal
- A digital signal is a sequence of discrete

discontinuous voltage pulses. - Each pulse is a signal element
- In its simplest form each signal element

represents a binary 0 or 1.

Data Encoding

- Both analog and digital information can be

encoded as either analog or digital. (Function of

media and communication ) - Digital data, digital signal
- Digital data, analog signal
- Analog data, digital signal
- Analog data, analog signal

Terminology (digital signal)

- Unipolar encoding If the signal elements all

have the same algebraic signs, all positive or

all negative, the signal is called unipolar. - Polar encoding One logical state is represented

by positive voltage and the other by the negative

voltage level.

Terminology (digital signal)

- Data rate The rate in bits per second that the

data is transmitted. (R) - Bit duration The amount of time for one bit

transmission (1/R) - Modulation rate The rate at which the signal

level is changed. (baud rate, signal levels per

second)

Terminology

- Encoding scheme The mapping from data bits to

signal elements - Spectrum The spectrum of a signal is the range

of frequencies that it contains. - Absolute bandwidth The width of the spectrum
- Effective bandwidth The are of the bandwidth

where most of the energy of the signal is

concentrated.

Terminology

- DC (direct current)component A component of a

signal with the frequency of zero. - Example
- S(t)1(4/?)sin(2 ? ft) .

Evaluation of Various Encoding Techniques

(affecting factors)

- Signal spectrum
- Lack of high frequency components means less

bandwidth required for transmission - DC component It is desirable to have no DC

component. (easier implementation) - Clocking The beginning and end of each bit

position must be determined. - Providing separate clocking information.
- Implementation of some other ways of

synchronization

Evaluation of Various Encoding Techniques

(affecting factors)

- Error detection
- To detect errors more quickly, some error

detection techniques must be built into signaling

encoding methods. - Signal interference and noise immunity
- Some signal encoding techniques provide better

error rate (BER) than others - Cost and complexity

Data Encoding

- Digital data, analog signal
- A modem converts digital data to analog data
- Amplitude shift keying (ASK)
- Frequency shift keying (FSK)
- Phase shift keying (PSK)

Data Encoding

- Analog data, Digital signals
- Pulse code modulation (PCM)
- Samples analog data periodically
- Quantizing (limiting the possible values to

discrete set of values) the samples

Data Encoding

- Digital data, digital signal
- Simplest form of digital encoding
- Two voltage level required
- It can be enhanced to improve performance.

Digitalto-Digital Encoding Schemes

- Unipolar
- Uses only one level of voltage (almost obsolete)
- Polar
- Uses two level of voltage
- Bipolar
- Uses theree level of voltage

Unipolar Encoding

- Presence and absence of a voltage level is used

for two binary digits. - The absence of voltage could represent zero.
- A constant positive voltage could represent 1.

Unipolar

- Amplitude

0

1

0

0

0

Time

Unipolar Encoding Issues

- Synchronization A major issue
- Example For a bit rate of 1000 bps, the

receiving device must measure each bit for 0.005

s. - DC Component
- The average amplitude of a unipolar encoded

signal is not zero. - This creates a DC component ( a component with

zero frequency). - DC component can not travel through some media

that can not handle DC component

Polar Encoding

- Polar encoding uses tow voltage levels (positive

and negative)

Polar

NRZ

RZ

Biphase

Differential Manchester

NRZ-L

NRZ-I

Manchester

Variation of Nonreturn to Zero (NRZ)

- NRZ-L, Nonreturn to Zero-level (polar)
- The level of the signal depends on the type of

the bit it represents (a positive voltage usually

represents bit 0 and negative voltage represents

the bit 1 (or vice versa) - The problem exist when receiver needs to

interpret long streams of 1 or zero. - Or NRZ-I (Nonreturn to Zero Invert on ones)

Nonreturn to Zero-Level

Amplitude

1

1

1

0

1

0

0

0

Time

Variation of Nonreturn to Zero (NRZ)

- NRZ-I (Nonreturn to Zero Invert on ones)
- An inversion of voltage level represents a 1 bit.
- The transition between a positive and negative

voltage represents a 1 not the voltage level

itself. - A 0 is represented by no change
- Still a string of zeros is a problem.

Nonreturn to Zero, invert on ones

Amplitude

1

1

0

1

0

1

0

0

0

Time

Nonreturn to Zero-Level Nonreturn to Zero, invert

on ones

Amplitude

0

1

0

0

1

1

1

0

Time

0

1

0

0

0

1

1

1

0

Return to Zero

- One solution to synchronization issue of NRZ-L

and NRZ-I is using RZ (Return to Zero) encoding

schemes. - It uses three values positive, negative and

zero. - In RZ, the signal changes during each bit.
- A 1 bit is represented by positive-to zero and a

0 bit by negative-to-zero.

Return to Zero

It requires two signal changes to encode one

bit. (uses more bandwidth)

0

1

0

0

1

1

1

Time

These transitions can be used for synchronization

NRZ pros and cons

- Pros
- Easy to engineer
- Make good use of bandwidth
- Cons
- dc component
- Lack of synchronization capability
- Used for magnetic recording
- Not often used for signal transmission

Polar

NRZ

RZ

Biphase

Differential Manchester

NRZ-L

NRZ-I

Manchester

Biphase Encoding

- The most popular encoding to deal with the

synchronization problem. - The signal changes at the middle of the bit

interval and continues to the opposite pole (dose

not return to zero). - Types of biphase encoding
- Manchester
- Differential Manchester

Biphase Encoding

- Manchester Encoding
- The inversion at the middle of each bit is used

for both synchronization and bit representation - i.e. Transition serves as clock and data
- Low to high represents one
- High to low represents zero
- Used by IEEE 802.3

Manchester Encoding

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

Biphase Encoding

- Differential Manchester
- Transition at the middle of bit interval is used

for clocking only. - Transition at the start of a bit period

represents zero. - No transition at start of a bit period represents

one. - Note this is a differential encoding scheme
- Used by IEEE 802.5.

Differential Manchester Encoding

Presence of transition at the beginning of the

bit interval represents zero. Absence of

transition at the beginning of the bit interval

represents one.

Biphase Pros and Cons

- Con
- At least one transition per bit time and possibly

two - Maximum modulation rate is twice NRZ
- Requires more bandwidth
- Pros
- Synchronization on mid bit transition (self

clocking) - No dc component
- Error detection
- Absence of expected transition

Multilevel Binary

- Use more than two levels
- Bipolar-AMI (Alternate mark inversion)
- Pseudoternary (variation of Bipolar-AMI)

Bipolar Encoding

- Uses there voltage levels
- Positive, negative, and zero
- Zero level represents binary 0
- Ones are represented by alternating positive and

negative voltages

Types of Bipolar Encoding

- Bipolar Alternate Mark Inversion (AMI)
- Bipolar 8-zero substitution (B8ZS)
- High density bipolar 3 (HDB3)

Types of Bipolar Encoding

Bipolar Alternate Mark Inversion (AMI)

- Mark comes from telegraphy (meaning 1)
- Zero voltage represents zero
- Binary 1s are represented by alternating

positive and negative voltages

Bipolar Alternate mark inversion (AMI)

Bipolar-AMI and Pseudoternary

Types of Bipolar Encoding

- Pros
- DC component is zero
- A long sequence of 1s is always synchronized.
- Lower bandwidth
- Easy error detection
- Cons
- No mechanism for synchronization of long string

of zeros

Variation of AMI

- Bipolar 8-zero substitution (B8ZS)
- (implemented in US)
- High Density bipolar 3 (HDB3)
- (implemented in Europe)
- In both methods the original pattern is modified

in the case of multiple consecutive zeros.

Bipolar 8-zero substitution (B8ZS)

- It works similar to BMI
- Whenever 8 or more consecutive zeros occurs,

signal level is forced to change.

Pseudoternary

- One represented by absence of line signal
- Zero represented by alternating positive and

negative - No advantage or disadvantage over bipolar-AMI

Trade Off for Multilevel Binary

- Not as efficient as NRZ
- Each signal element only represents one bit
- In a 3 level system could represent log23 1.58

bits - Receiver must distinguish between three levels

(A, -A, 0) - Requires approx. 3dB more signal power for same

probability of bit error

Scrambling

- Use scrambling to replace sequences that would

produce constant voltage - Filling sequence
- Must produce enough transitions to sync
- Must be recognized by receiver and replace with

original - Same length as original
- No dc component
- No long sequences of zero level line signal
- No reduction in data rate
- Error detection capability

B8ZS

- Bipolar With 8 Zeros Substitution
- Based on bipolar-AMI
- If octet of all zeros and last voltage pulse

preceding was positive encode as 000-0- - If octet of all zeros and last voltage pulse

preceding was negative encode as 000-0- - Causes two violations of AMI code
- Unlikely to occur as a result of noise
- Receiver detects and interprets as octet of all

zeros

HDB3

- High Density Bipolar 3 Zeros
- Based on bipolar-AMI
- String of four zeros replaced with one or two

pulses

B8ZS and HDB3

Digital Data, Analog Signal

- Public telephone system
- 300Hz to 3400Hz
- Use modem (modulator-demodulator)
- Amplitude shift keying (ASK)
- Frequency shift keying (FSK)
- Phase shift keying (PK)

Digital to Analog Encoding

Amplitude Shift Keying

- Values represented by different amplitudes of

carrier - Usually, one amplitude is zero
- i.e. presence and absence of carrier is used
- Susceptible to sudden gain changes
- Inefficient
- Up to 1200bps on voice grade lines
- Used over optical fiber

Modulation Techniques (ASK)

Binary 1

Binary 0

ASK

Modulation Techniques(ASK)

Frequency Shift Keying

- Values represented by different frequencies (near

carrier) - Less susceptible to error than ASK
- Up to 1200bps on voice grade lines
- High frequency radio
- Even higher frequency on LANs using co-ax

Modulation Techniques (ASK)

Binary 1

Binary 0

f1 and f2 are offset from fc by equal but

opposite amount

FSK on Voice Grade Line

FSK

Modulation Techniques(FSK)

Phase Shift Keying

- Phase of carrier signal is shifted to represent

data - Differential PSK
- Phase shifted relative to previous transmission

rather than some reference signal

Modulation Techniques (PSK)(Differential PSK)

Binary 1

Binary 0

The phase shift is is in reference to previous

bit transmitted Rather than to some constant

reference signal.

PSK

PSK Constellation

Quadrature PSK

- More efficient use by each signal element

representing more than one bit - e.g. shifts of ?/2 (90o)
- Each element represents two bits
- Can use 8 phase angles and have more than one

amplitude - 9600bps modem use 12 angles , four of which have

two amplitudes

Modulation Techniques (PSK)(Differential QPSK)

Binary 11

Binary 10

Binary 00

Binary 01

4-PSK

4-PSK Constellation

8-QAM Signal

8-PSK Constellation

Have a great day . See you on Friday.

PSK Bandwidth

4-QAM and 8-QAM Constellation

Bandwidth for ASK

Bandwidth for FSK

16-QAM Constellation

Bit Rate and Baud Rate

Bit Rate and Baud Rate

Modulation Techniques(FSK)

Performance of Digital to Analog Modulation

Schemes

- Bandwidth
- ASK and PSK bandwidth directly related to bit

rate - FSK bandwidth related to data rate for lower

frequencies, but to offset of modulated frequency

from carrier at high frequencies - (See Stallings for math)
- In the presence of noise, bit error rate of PSK

and QPSK are about 3dB superior to ASK and FSK

Analog Data, Digital Signal

- Digitization
- Conversion of analog data into digital data
- Digital data can then be transmitted using NRZ-L
- Digital data can then be transmitted using code

other than NRZ-L - Digital data can then be converted to analog

signal - Analog to digital conversion done using a codec
- Pulse code modulation
- Delta modulation

Pulse Code Modulation(PCM) (1)

- If a signal is sampled at regular intervals at a

rate higher than twice the highest signal

frequency, the samples contain all the

information of the original signal - (Proof - Stallings appendix 4A)
- Voice data limited to below 4000Hz
- Require 8000 sample per second
- Analog samples (Pulse Amplitude Modulation, PAM)
- Each sample assigned digital value

Pulse Code Modulation(PCM) (2)

- 4 bit system gives 16 levels
- Quantized
- Quantizing error or noise
- Approximations mean it is impossible to recover

original exactly - 8 bit sample gives 256 levels
- Quality comparable with analog transmission
- 8000 samples per second of 8 bits each gives

64kbps

Nonlinear Encoding

- Quantization levels not evenly spaced
- Reduces overall signal distortion
- Can also be done by companding

Delta Modulation

- Analog input is approximated by a staircase

function - Move up or down one level (?) at each sample

interval - Binary behavior
- Function moves up or down at each sample interval

Delta Modulation - example

Delta Modulation - Operation

Delta Modulation - Performance

- Good voice reproduction
- PCM - 128 levels (7 bit)
- Voice bandwidth 4khz
- Should be 8000 x 7 56kbps for PCM
- Data compression can improve on this
- e.g. Interframe coding techniques for video

Analog Data, Analog Signals

- Why modulate analog signals?
- Higher frequency can give more efficient

transmission - Permits frequency division multiplexing (chapter

8) - Types of modulation
- Amplitude
- Frequency
- Phase

Analog Modulation

Spread Spectrum

- Analog or digital data
- Analog signal
- Spread data over wide bandwidth
- Makes jamming and interception harder
- Frequency hoping
- Signal broadcast over seemingly random series of

frequencies - Direct Sequence
- Each bit is represented by multiple bits in

transmitted signal - Chipping code

Required Reading

- Stallings chapter 5

Review

Atmospheric and Extraterrestrial Noise

- Lightning It is a major source of noise, caused

by the static discharge of thunderclouds. - Several million volts
- Currents exceeding 20,000 amps.
- Solar Noise Ionized gases of the sun produces a

wide range of frequencies that penetrate the

Earths atmosphere. - Cosmic Noise Radiation of noise by distant stars

penetrating the Earths atmosphere.Long haul

telecommunications service (1500 km support

20,000 to 60,000 voice channels) - An alternative to fiber optic and coaxial cable
- Short point-to-point links between buildings

(closed-circuit TV or data link)