Download CSC 335 Data Communications and Networking I

CSC535
Communication Networks I
Chapter 3b: Signal Encoding and
Conversion
Dr. Cheer-Sun Yang
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Motivation
Short distance and Long distance
communications require to encode data with
signals prior to sending the signals across
communication media. We need to discuss
the following:
• What are the communication services and
devices available today?
• How are bits encoded into electric signals?
• How are analog signals and digital signals
be converted?
2
Communication Services and
Devices
• Telephone System – switching technique
and routing methods are the two main
design issues.
• Integrated Services Digital Network
• Cellular Phones – the sender and receiver
can move
• Fax Machines
• Computers
3
Data Encoding
• ASCII (American Standard Code for
Information Interchange)
• EBCDIC (Extended Binary Coded Decimal
Interchange Code)
• Others – Baudot, morse, BCD
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Electric Current and Data Bits
The simplest electronic communication systems
use a small electric current to encode data.
Positive voltage – represents 0 (or 1)
Negative voltage – represents 1 (or 0)
A waveform diagram can be used to illustrate how
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data bits are represented and transmitted.
Electric Current and Data Bits
A waveform diagram provides a visual
representation of how an electrical signal varies
over time. For example, the diagram shows that a
longer time elapsed between the transmission of
the fourth and the fifth bits than between others.
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Digital Encoding Schemes Using
Digital Signals
•
•
•
•
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Manchester
Differential Manchester
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1
0
1
0
1
1
1
0
0
Unipolar
NRZ
Polar NRZ
NRZ-Inverted
(Differential
Encoding)
Bipolar
Encoding
Manchester
Encoding
Differential
Manchester
Encoding
Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communication
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Figure 3.25
1.2
NRZ
Bipolar
0.8
0.6
0.4
Manchester
0.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
-0.2
0.2
0
0
pow er density
1
fT
Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communication
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Figure 3.26
Nonreturn to Zero-Level (NRZL)
• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
– no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant
positive voltage for one
• More often, negative voltage for one value
and positive for the other
• This is NRZ-L
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Nonreturn to Zero Inverted
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of
signal transition at beginning of bit time
• Transition (low to high or high to low)
denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
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NRZ
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NRZ pros and cons
• Pros
– Easy to engineer
– Make good use of bandwidth
• Cons
– dc component
– Lack of synchronization capability and
hard to synchronize timing of sender and
receiver.
• Used for magnetic recording
• Not often used for signal transmission
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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
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Manchester
•
•
•
•
•
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
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Advantages of Manchester
• Synchronization: Because there is a
predictable transition during each bit
time, the receiver can synchronize on that
transition.
• Error detection: Noise on the line would
have to invert both the signal before and
affter to cause an undetected error.
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How are bits encoded into digital
signals?
• Exercise with a neighbor now.
• Draw a waveform diagram depicting the
message “Hi” using NRZL, NRZI, and
Manchester encoding schemes.
– Assume that the bit representation of “H”
is
0 1 0 0 1 0 0 0 = 0X48
– Assume that the bit representation of “i”
is
0 1 1 0 1 0 0 1 = 0X69
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Limitation
• Digital signals cannot be used to transmit
across a long distance.
• During transmitting digital signals, it is
susceptible to interference easily.
• Digital encoding schemes are widely used
in recording.
• Instead, analog signals are used to transmit
even digital data bits. How?
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Motivation on Modulation and
Demodulation
If either analog or digital signals were used
exclusively, communications would be
simplified. However, this is impossible
especially attempting to send signals across
a long distance. Digital signals cannot be
transmitted far without being converted to
analog signals. Because telephone system is
an analog device, computer signals must be
converted to analog signals.
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The Waveform of a Carrier
The wave form of an analog signal
carrier oscillates continuously even
when no signal is being sent.
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Carrier
• Researchers found that a continuous,
oscillating signal will propagate farther than
other signals.
• Instead of transmitting an electric current
that only changes when the value of a bit
changes, long-distance communication
systems send a continuously oscillating
signal, usually a sine wave, called a carrier.
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Data Modulation
• To send data, a transmitter modifies the
carrier slightly.
• Collectively, such modifications are called
modulation.
• The technique was originated for
transmitting radio or TV signals.
• Generally speaking, modulation is the
process to transform a digital signal into an
analog signal.
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Data Demodulation
• At the receiving end, the analog signal
is transformed back to digital signals.
• The process is called demodulation.
• The device to perform modulation and
demodulation is called a modem. We
will talk about modem later.
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Example of Data Modulation
The digital signal ’01’ is sent. The carrier
is reduced to 2/3 full strength to encode a
1 bit and 1/3 strength to encode a 0 bit.
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Modulation Techniques
• Amplitude shift keying (ASK)
• Frequency shift keying (FSK)
• Phase shift keying (PK)
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0
f1
fc
f2
Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communication
f
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Figure 3.27
Modulation Techniques
27
Modulation Techniques
This modulation technique is called
Amplitude Shift keying (ASK) technique.
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Information
1
0
1
1
0
1
+1
(a)
Amplitude
Shift
Keying
0
T
2T
3T
4T
5T
6T
0
T
2T
3T
4T
5T
6T
0
T
2T
3T
4T
5T
6T
t
-1
+1
(b)
Frequency
Shift
Keying
t
-1
+1
(c)
Phase
Shift
Keying
t
-1
Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communication
29
Figure 3.28
1
(a) Information
0
1
1
0
1
+A
(b) Baseband
Signal Xi(t)
T
0
2T
3T
4T
5T
t
6T
-A
+A
(c) Modulated
Signal Yi(t)
2T
T
0
3T
4T
5T
6T
t
6T
t
-A
+2A
(d) 2Yi(t) cos(2fct)
0
T
2T
3T
4T
5T
-2A
Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communication
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Figure 3.29
(a) Modulate cos(2fct) by multiplying it by Ak for (k-1)T < t <kT:
Ak
x
Yi(t) = Ak cos(2fct)
cos(2fct)
(b) Demodulate (recover) Ak by multiplying by 2cos(2fct) and lowpass filtering:
Yi(t) = Akcos(2fct)
Lowpass
Filter with
cutoff W Hz
x
2cos(2fct)
Xi(t)
2Ak cos2(2fct) = Ak {1 + cos(2fct)}
Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communication
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Figure 3.30
Modulate cos(2fct) and sin (2fct) by multiplying them by Ak and Bk respectively for
(k-1)T < t <kT:
Ak
x
Yi(t) = Ak cos(2fc t)
cos(2fc t)
Bk
x
+
Y(t)
Yq(t) = Bk sin(2fc t)
sin(2fc t)
Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communication
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Figure 3.31
Y(t)
Lowpass
Filter with
cutoff W/2 Hz
x
2cos(2fc t)
x
2sin(2fc t)
Ak
2cos2(2fct)+2Bk cos(2fct)sin(2fct)
= Ak {1 + cos(4fct)}+Bk {0 + sin(4fct)}
Lowpass
Filter with
cutoff W/2 Hz
Bk
2Bk sin2(2fct)+2Ak cos(2fct)sin(2fct)
= Bk {1 - cos(4fct)}+Ak {0 + sin(4fct)}
Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communication
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Figure 3.32
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
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Example of ASK
Bit Values
00
01
10
11
Amplitude
A1
A2
A3
A4
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Amplitude Shifting Keying (four amplitudes),
two bits per baud
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Phase Shift Keying
• Nyquist Theorem suggests that the number
of bits sent per cycle can be increased if the
encoding scheme permits multiple bits to be
encoded in a single cycle of the carrier.
• ASK and FSK work well but require at least
one cycle of a carrier wave to send a single
bit.
• PSK changes the timing of the carrier wave
abruptly to encode data. Such change is
called a phase shift.
37
Example of Phase Shift
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Phase Shift Keying
Arrows indicate points at which the carrier abruptly
jumps to a new position in the cycle. For different
code, the phase shift is different.
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Frequency Shift Keying
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QAM
• Any of the simple techniques can be used
with any number of different signals.
• More signals means a greater bit rate with a
given baud rate.
• The problem is that a higher bit rate
requires more signals and reduces the
differences among them and makes the
receiver’s job more difficult.
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QAM(cont’d)
• Another approach is to use a combination of
frequencies, amplitudes, or phase shifts,
which allows us to use a larger group of
legitimate signals while maintaining larger
differences among them.
• One technique is Quadrature Amplitude
Modulation (QAM), in which a group of
bits is assigned a signal defined by its
amplitude and phase shift.
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Quadrature PSK
• More efficient use by each signal element
representing more than one bit
– e.g. shifts of /4 (45o)
– Each element represents three bits
– Can use 4 phase angles and have two
amplitudes
– 9600bps modem use 12 angles , four of
which have two amplitudes
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Signal Associations for QAM
44
Two amplitudes and four phases are used to send
three bits per baud.
45
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
• In the presence of noise, bit error rate of PSK and
QPSK are about 3dB superior to ASK and FSK
46
Analog-to-Digital Conversion
• Usually, it is the reverse of what we have
just discussed. A modem examines the
incoming signals for amplitude,
frequencies, and phase shifts and generates
digital signals. This works for signals
having constant characteristics.
• What about analog signals whose
characteristics change continually for
example voice ?
47
Pulse Code Modulation
• One way of making the signal truly digital
is to assign amplitudes from a predefined
set to the sample signals.
• This process is called PCM.
48
49
The pulse amplitude is divided into eight values or 23 values.
50
Accuracy of PCM
1. The sampling frequency
2. The number of amplitudes chosen: in Fig
2.47, the resulted signal becomes
distorted.
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Modem
• Modem = modulator + demodulator
• A modem converts digital signals to analog
signals before sending them across a phone
line.
• Another modem converts analog signals back
to digital signals before passing them to a
receiver.
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Illustration of Dial-up Modem
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Modems
• Intelligent Modems (Hayes Compatible)
– A user can enter commands such as continuing dialing,
beeping when disconnected, etc.
– Hayes Modem allows a user to enter AT command to
request for connection.
– ATDT5551234: AT represents AT command; D stands
for dial; T stands for tone dialing.
• Cable Modems – connects to cable TV
carrier from a PC and a TV.
• Null Modems – used for connecting two
local PC’s together. (will be discussed again
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in next chapter)
Summary of Modem
A pair of modem is required for long-distance
communication across a leased line; each
modem contains separate circuitry to send and
receive digital data. To send data, a modem
emits a continuous carrier wave, which it then
modulates according to the values of the bits
being transferred. To receive data, a modem
detects modulation in the incoming carrier, and
uses it to recreate the data bits.
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2-D signal
Bk
Bk
2-D signal
Ak
Ak
4 “levels”/ pulse
2 bits / pulse
2W bits per second
16 “levels”/ pulse
4 bits / pulse
4W bits per second
Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communication
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Figure 3.33
Bk
Bk
Ak
Ak
4 “levels”/ pulse
2 bits / pulse
2W bits per second
16 “levels”/ pulse
4 bits / pulse
4W bits per second
Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communication
58
Figure 3.34
Reading Assignments
• Read Chapter 3.5, 3.6
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