Download Signal Encoding Techniques

Data Communications &
Computer Networks
Lecture 5
Signal Encoding Techniques
1
Fall 2007
Agenda
• Digital Data, Digital Signals
• Digital Data, Analog Signals
• Home Exercises
2
ACOE312 Signal Encoding Techniques
1
Encoding Techniques
• There are a number of transmission options
available today, depending on the encoding
technique
• There are four possible combinations of
encoding techniques
—Digital data, digital signal
—Digital data, analog signal
—Analog data, digital signal
—Analog data, analog signal
• We shall examine only the first two techniques
3
Digital Data
Digital Signals
4
ACOE312 Signal Encoding Techniques
2
1. Digital Data, Digital Signals
• Digital signal
—Discrete, discontinuous voltage pulses
—Each pulse is a signal element
—Binary data encoded into signal elements
5
Terms (1)
• Unipolar
—All signal elements have same sign, i.e. all positive or
all negative
• 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
—eg. For a data rate R, the bit duration is 1/R
6
ACOE312 Signal Encoding Techniques
3
Terms (2)
• Modulation rate
—Rate at which the signal level changes
—Modulation rate is measured in baud = signal
elements per second
• Mark and Space
—Mark is Binary 1, Space is Binary 0
7
Interpreting Signals
• Receiver needs to know
—Timing of bits - when they start and end
—Signal levels
• What factors determine how successful the
receiver will be interpreting the incoming signal?
—Signal to noise ratio
—Data rate
—Bandwidth
—Encoding Scheme
8
ACOE312 Signal Encoding Techniques
4
Encoding Schemes
considerations (1)
• Signal Spectrum
—Lack of high frequencies reduces required bandwidth
—Lack of dc component also desirable since it allows ac
coupling via transformer, providing electrical isolation
—Concentrate tx power in the middle of tx bandwidth
• Clocking
—Synchronizing transmitter and receiver
—External clock
—Sync mechanism based on tx signal with suitable
encoding
9
Encoding Schemes
considerations (2)
• Error detection
—Can be built in to signal encoding
• Signal interference and noise immunity
—Some codes are better than others
• Cost and complexity
—Higher signal rate (& thus data rate) lead to higher
costs
—Some codes require signal rate greater than data rate
10
ACOE312 Signal Encoding Techniques
5
Encoding Schemes
•
•
•
•
•
•
•
Return to Zero (RZ)
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar - AMI
Pseudoternary
Manchester
Differential Manchester
11
Return to zero (RZ)
• Signal amplitude varies between a positive
voltage, i.e. unipolar
• Binary 1: a constant positive voltage
• Binary 0: Absence of voltage (i.e. 0 Volts or
Ground)
• Example:
1
0
1
1
0
0
0
1
+V
0 Volts
12
ACOE312 Signal Encoding Techniques
6
Non-return to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
• Negative voltage for one value and positive for
the other, eg
—Binary 0 : Positive
—Binary 1 : Negative
• Voltage constant during bit interval
—no transition i.e. no return to zero voltage
• Example:
0
1
0
0
1
1
1
0
+V
0 Volts
-V 13
Non-return to Zero Inverted
(NRZI)
• Non-return to zero inverted on ones
• Constant voltage pulse for duration of bit time
• Data encoded as presence or absence of signal
transition at beginning of bit time
—Transition (low-to-high or high-to-low)
or
denotes a binary 1
—No transition denotes binary 0
• NRZI is an example of differential encoding
• Example:
0
ACOE312 Signal Encoding Techniques
1
1
0
1
0
0
1
+V
0 Volts
-V 14
7
NRZ-L and NRZI format
examples
0V
0V
15
Differential Encoding
• Data represented by changes rather than levels
• Benefits
—More reliable detection of transition in the presence
of noise rather than to compare a value to a
threshold level
—In complex transmission layouts it is easy to loose
sense of polarity of the signal
16
ACOE312 Signal Encoding Techniques
8
NRZ pros and cons
• Advantages
—Easy to engineer
—Make efficient use of bandwidth
• Disadvantages
—DC component
—Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission
17
Multilevel Binary
• Uses more than two levels
• Bipolar-AMI (Alternate Mark Inversion)
— zero represented by no line signal
— one represented by positive or negative pulse
— Binary 1 pulses alternate in polarity
• Benefits with respect to NRZ
— No loss of sync if a long string of ones (zeros still a problem)
— No net DC component
— Lower bandwidth
— Easy error detection
18
ACOE312 Signal Encoding Techniques
9
Pseudoternary
• Binary 1 is represented by absence of line signal
• Binary 0 is represented by alternating positive
and negative pulses
• No advantage or disadvantage over bipolar-AMI
—No loss of sync if a long string of zeros (ones still a
problem)
19
Bipolar-AMI and Pseudoternary
0
1
0
0
1
1
0
0
0
1
1
0V
0V
20
ACOE312 Signal Encoding Techniques
10
Disadvantages of Multilevel
Binary
• Not as efficient as NRZ
— Each signal element only represents one bit
— The line signal may take on one of 3 levels
— Each signal element, which could represent log23 = 1.58 bits
bears only one bit of information
• Receiver must distinguish between three levels
(+A, 0, -A) instead of two in NRZ
• Requires approximately 3dB more signal power for same
probability of bit error
— bit error for NRZ at a given SNR is much less than that for
multilevel binary
21
Biphase
• Another set of coding techniques that
overcomes NRZ limitations
• Biphase
—Manchester
—Differential Manchester
22
ACOE312 Signal Encoding Techniques
11
Manchester Encoding
0V
—Transition occurs at the middle of each bit period
—Transition serves as clock and data
—Low to high represents binary 1
—High to low represents binary 0
—Used by IEEE 802.3 (Ethernet)
23
Differential Manchester
Encoding
0V
—Midbit transition occurs always and is used for clocking only
—Transition at start of a bit period represents binary 0
—No transition at start of a bit period represents binary 1
—Note: this is a differential encoding scheme
—Used by IEEE 802.5 (token ring LAN)
24
ACOE312 Signal Encoding Techniques
12
Biphase Pros and Cons
• Advantages
—Synchronization on mid bit transition (self clocking)
—No dc component
—Error detection
• Absence of expected transition can be used to detect errors
• Disadvantages
—At least one transition per bit time and possibly two
—Maximum modulation rate is twice as that of NRZ
—Requires more bandwidth
25
Modulation Rate (1)
• Data rate or bit rate is 1/Tb, where Tb = bit duration
• Modulation rate is the rate at which signal elements are
generated
Tb
Tb
D=
R
R
=
L log2 M
where
D = modulation rate in baud
R = Data rate in bps
M = number of different signal elements = 2L
L = number of bits per signal element
26
ACOE312 Signal Encoding Techniques
13
Modulation Rate (2)
For Manchester
Encoding, the
minimum size signal
element is a pulse of
½ the duration of a
bit interval.
Bit rate = 1/Tb
For a string of all
binary 0s or all 1s, a
continuous stream of
such pulses is
generated.
Hence maximum
Modulation rate is
2/Tb
27
Digital Data
Analog Signals
28
ACOE312 Signal Encoding Techniques
14
2. Digital Data, Analog Signals
• Transmission of digital data with analog signals
• Example: Public telephone system (PSTN)
— Voice frequency range of 300Hz to 3400Hz
— Digital devices are attached to the network via a modem
(modulator-demodulator), which converts digital data to analog
signals and vice-versa
Modem
Corporate Network
Residence
PSTN
network
Server
Modem
Access Router
29
Modulation techniques
• We will examine three basic modulation
techniques
—Amplitude Shift Keying (ASK)
—Frequency Shift Keying (FSK)
—Phase Shift Keying (PSK)
30
ACOE312 Signal Encoding Techniques
15
Modulation Techniques
31
Amplitude Shift Keying (ASK)
• Values represented by different amplitudes of carrier
• Usually, one amplitude is zero
— i.e. presence and absence of carrier is used
s(t) = A cos(2πfct)
s(t) = 0
binary 1
binary 0
where fc is the carrier frequency
•
•
•
•
Susceptible to sudden gain changes
Inefficient
Up to 1200bps on voice grade lines
Used over optical fiber
32
ACOE312 Signal Encoding Techniques
16
Binary Frequency Shift Keying
• Most common form is binary FSK (BFSK)
• Two binary values represented by two different
frequencies (near carrier)
s(t) = A cos(2πf1t) binary 1
s(t) = A cos(2πf2t) binary 0
where f1, f2 are offset from carrier frequency fc by equal but opposite amounts
•
•
•
•
Less susceptible to errors than ASK
Up to 1200bps on voice grade lines
High frequency (HF) radio (3-30MHz)
Even higher frequency on LANs using co-axial
cable
33
Multiple FSK
•
•
•
•
More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents more than one bit
si(t)=A cos(2πfit), 1<i<M
where, fi=fc+(2i-1-M)fd
fc = carrier frequency
fd = difference frequency
M = number of different signal elements = 2L
L = number of bits per signal element
34
ACOE312 Signal Encoding Techniques
17
BFSK example on Voice Grade
Line
1170 Hz
2125 Hz
35
Phase Shift Keying (PSK)
• Phase of carrier signal is shifted to represent
data
• Binary PSK
—Two phases represent two binary digits
s(t) = A cos(2πfct)
s(t) = A cos(2πfct+π) = -A cos(2πfct)
binary 1
binary 0
• Differential PSK (DPSK)
—Phase shifted relative to previous transmission rather
than some reference signal
36
ACOE312 Signal Encoding Techniques
18
DPSK
Binary 0: signal of same phase as previous signal sent
Binary 1: signal of opposite phase to the preceding one
37
Quadrature PSK (QPSK)
• Quadrature means a 4-level scheme
• More efficient use by each signal element
representing more than one bit
—e.g. shifts of π/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
38
ACOE312 Signal Encoding Techniques
19
QPSK equation
• Each signal element represents
than one
s(t)=A cos(2πfct+π/4)
s(t)=A cos(2πfct+3π/4)
s(t)=A cos(2πfct-3π/4)
s(t)=A cos(2πfct-π/4)
two bits rather
11
01
00
10
39
Quadrature Amplitude
Modulation
• QAM used on asymmetric digital subscriber line (ADSL)
and some wireless standards
• Combination of ASK and PSK
• Can also be considered a logical extension of QPSK
• Send two different signals simultaneously on same
carrier frequency
— Use two copies of carrier, one shifted by 90° with respect to the
other
— Each carrier is ASK modulated
— Two independent signals over same medium
— At the receiver the two signals are demodulated and combined
to produce the original binary signal
40
ACOE312 Signal Encoding Techniques
20
QAM Levels
• Two-level ASK
—Each of two streams in one of two states
—Four state system
—Essentially QPSK
• Four-level ASK, i.e. 4 different amplitude
levels
—Combined stream in one of 16 states
• 64 and 256 state systems have been
implemented
• Improved data rate for given bandwidth
—Increased potential error rate
41
Required Reading
• Stallings chapter 5
42
ACOE312 Signal Encoding Techniques
21
Home Exercises
43
Review Questions
• List and briefly define important factors that can be used
in evaluating or comparing the various digital-to-digital
encoding techniques
• What is differential encoding?
• Contrast all digital encoding schemes listed in this
lecture (NRZL, NRZI, Bipolar AMI, Pseudoternary,
Manchester, Differential Manchester), outlining their
advantages and disadvantages
• Define the modulation rate and write an expression
which relates it with the bit rate.
• Explain the difference between ASK, FSK and PSK
modulation techniques
• What is the difference between Binary PSK, DPSK and
QPSK?
44
ACOE312 Signal Encoding Techniques
22
Exercises (1)
1. For the bit stream 01001110, sketch the
waveforms for the following codes
a)
b)
c)
d)
e)
f)
NRZ-L
NRZI
Bipolar-AMI
Pseudoternary
Manchester
Differential Manchester
Assume that:
— the most recent preceding 1 bit (AMI) has a negative voltage
— the most recent preceding 0 bit (pseudoternary) has a negative
voltage.
45
Exercises (2)
2.
The bipolar-AMI waveform representing the binary sequence
0100101011 is transmitted over a noisy channel. The received
waveform, which contains a single error, is shown in the following
figure. Locate the position of this error and explain your answer.
1
2
3
4
5
6
7
8
9
10
46
ACOE312 Signal Encoding Techniques
23