Transcript 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?
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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
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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
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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
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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
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f
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Figure 3.27
Modulation Techniques
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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
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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
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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)}
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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)
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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.
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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
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Two amplitudes and four phases are used to send
three bits per baud.
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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
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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 ?
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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.
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The pulse amplitude is divided into eight values or 23 values.
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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
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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
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Figure 3.34
Reading Assignments
• Read Chapter 3.5, 3.6
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