Transcript Chapter 5 - William Stallings, Data and Computer

FIT 1005 Networks & Data Communications
Lecture 4 – Signal Encoding Techniques
Reference: Chapter 5
Data and Computer Communications
Eighth Edition
by William Stallings
Lecture slides by Lawrie Brown
Modified Slides: http://users.monash.edu.au/~amkhan/fit1005
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Data
Types
Signal
Encoding
Analog Signal
&
Digital Signals
Signal
Encoding
Digital Data Analog Data
Digital Data
Analog Data
to
Summary
to
to
To
Digital Signal Analog Signal Analog Signal Digital Signal
Type of
• Data
– digital
– analog
• Signal
– digital
– analog
• Data Encoding (Schemes)
–
–
–
–
digital to digital
digital to analog
analog to digital
analog to analog
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Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Type of
• Data
– digital
– analog
• Signal
– digital
– analog
• Data Encoding (converted into)

–
–
–
–
Converting source data  into communication signal
digital to digital
digital to  analog
analog to  digital
analog to  analog
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Summary
Data
Types
Signal
Encoding
Analog
Signal
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
Data can be encoded in either form of signals
Analog Signals
Analog Data
Voice
(sound waves)
Telephone
Analog Signal
Digital Data
Binary Voltage
pulses
Modem
Analog Signal
(Carrier frequency)
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Data
Types
Signal
Encoding
Digital
Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Data can be encoded in either form of signals
Digital Signals
Analog Data
Analog data
CODEC
Digital Signal
Digital Data
Digital data
Digital
Transmitter
Digital Signal
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Signal Encoding Techniques (1)
• Digital signaling: For digital signaling, a data source g(t),
which may be either digital or analog, is encoded into a
digital signal x(t).
• Analog signaling: The basis for analog signaling is a
continuous constant-frequency fc signal known as the Carrier
signal. E.g. AM or FM
• Baseband signal: The input signal may be analog or digital
and is called the modulating signal or baseband signal.
– Baseband signals are the fundamental group of frequencies in an analog or
digital waveform that may be transmitted along a channel. Examples of a
digital baseband signal may be Ethernet signals operating over a LAN
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Encoding is required because
• Signals are carried by the physical medium
• The performance of medium will vary depending on
the kind of signal, with varying characteristics in
terms of
–
–
–
–
Attenuation
Error rate
Distance
etc….
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Encoding
signal
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Modulation
signal
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
Signal Encoding Techniques (2)
• The electromagnetic signal is generated at the physical layer
• The electromagnetic signal could be analog or digital signals that
must carry the data(message).
• The data must be encoded into signals:
– Data Encoding is done to produce a analog or digital signal
– Modulation of a analog/digital data is used to produce an analog
signal
• This analog signal can then be transmitted over a
network/communication link.
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Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Which one of the 4 combinations to choose?
• Digital data/Digital signal
– Equipment for encoding is less complex
– Digital transmission has less errors
• Analog data/Digital signal
– Digital transmission can be done on the existing analog medium –
good return on investment (ROI)
– Digital transmission has less errors
• Digital data/Analog signal
– Some high data rate mediums are analog (e.g. optical fiber)
– most of the unguided media are analog (e.g. Wireless)
• Analog data/Analog signal
– Can be transmitted easily and cheaply; different position on the
spectrum can be shared on the same media (e.g. Frequency-division
multiplexing)
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Data to
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Data Encoding
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Data Encoding
•
•
•
•
digital data to digital signal
analog data to analog signal
digital data to analog signal
analog data to digital signal
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Digital Data, Digital Signal
• Digital signal
– discrete, discontinuous voltage pulses
– each pulse is a signal element
– binary data encoded into signal elements
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Properties that need to be considered while encoding
• At least 5 properties need to be considered
when encoding any form of data to any signals
1.
2.
3.
4.
5.
Signal spectrum requirement
Signal synchronization capability
Signal error-detecting capability
Signal interference and noise immunity
Cost and complexity of the encoding/decoding
equipment.
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Property 1
• Signal spectrum
• digital signal can contain infinite frequencies  high
bandwidth is required
• Conversely lack of high frequency component means
less bandwidth required for transmission (cheaper)
• No DC component - otherwise the decoding equipment
can be expensive
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
What is DC Component?
• A non-zero constant value of the Fourier series
transformation of a signal is the value of the DC
component
• If the signal varies between positive and negative
voltages, then a non-zero difference in the area above
and below the zero voltage line implies that the signal
has a DC component
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
What is DC Component?
0
0
+ dc component
0
0
- dc component
0
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Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Property 2
• Signal synchronization capability
• Need to determine the beginning and end of each bit
position.
– Not an easy task
– May need a separate clock between sender and receiver to
synchronize them or
– embedded
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Property 3
• Signal Error-detecting capability
• can the encoding technique identify (and correct) errors
when the signal is corrupted?
• Parity or CRC
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Property 4
• Signal interference and noise immunity
• Certain encoded signals can be decoded correctly when
the signal is corrupted due to interferences or
inducement of noise.
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Encoding
• Unfortunately we will not be able to achieve all the first
four properties at the cheapest cost by a single encoding
technique.
• Hence cost is an last important factor that needs to be
considered as well
• Each encoding technique will satisfy (either fully or
partially) only a sub-set of the properties.
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Signal element versus data element
Encoding requires to distinguish between Signal element and data element
Ratio r is defined as number of data elements carried by each signal elements
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Some Popular Encoding Schemes of
Digital data to Digital signals
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
Some Terms used w.r.t Digital data to  Digital signals
• Unipolar  all signal elements have either +ve or –ve
voltage (same sign)
• Bipolar  one logic state is represented by a +ve voltage
and the other by a –ve voltage
• data rate  rate of data transmission in bits per second
• duration or length of a bit  time taken for the transmitter
to emit a bit = 1/data rate
• modulation rate  rate at which the signal level changes,
measured in baud = number of signal elements per
second
• mark and space  another term for binary 1 and binary 0
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Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Encoding
Schemes
Has more
desirable
properties
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
Nonreturn to Zero-Level (NRZ-L)
Zero /
positive
volts
•
two different voltages for 0 and 1 bits
Presence
of -ve voltage
 0 bit  0 / +ve voltage
 1 bit  -ve voltage
•
•
voltage constant during bit interval
– no mid-bit transition - i.e. no return to zero voltage
– one can use absence of voltage for 0, constant positive voltage for 1 but
becomes a DC component problem
– more often, negative voltage for 1 (mark) and positive voltage for 0 (space)
NRZ-L is typically the code used to generate or interpret digital data by
terminals and other devices
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Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Nonreturn to Zero Invert on ones (NRZI)
• 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 binary 1
– no transition denotes binary 0
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Nonreturn to Zero Invert on ones (NRZI)
• 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 binary 1
– no transition denotes binary 0
• example of differential encoding since we have
– data represented by changes rather than levels
– more reliable detection of transition rather than level
– easy to lose sense of polarity & synchronization
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
NRZ Pros & Cons
• Pros
– easy to design encoder and decoder
– make good use of bandwidth
• Cons
– dc component
– lack of synchronization capability
• used for magnetic recording
• not often used for signal transmission
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Multilevel Binary Bipolar-AMI
• Use more than two levels
• Bipolar-AMI
 zero represented by no line signal
 one represented by positive or negative pulse







one pulses alternate in polarity
no loss of sync if a long string of ones
long runs of zeros still a problem
no net dc component
lower bandwidth
easy error detection because of the alternation of the polarity
Used on a T-carrier communication links
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Multilevel Binary Pseudoternary
• one represented by absence of line signal
• zero represented by alternating positive and negative
• no advantage or disadvantage over Multilevel binary
bipolar
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Multilevel Binary Issues
• synchronization with long runs of 0’s (in the case BipolarAMI) or 1’s (in the case of pseudoternary)
– can insert additional bits to force transitions, ( as in ISDN )
– Insertion causes  wasted bit transmission  leads to reduced
transmission rate
• not as efficient as NRZ
– each signal element only represents one bit
> receiver distinguishes between three levels: +A, 0, -A
– a 3 voltage level system could represent log23 = 1.58 bits
– requires approx. 3dB more signal power for same probability of
bit error
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Manchester Encoding
•
•
•
•
•
•
has transition in the middle of each bit period
transition serves as clock and data (property 2)
low to high represents one
high to low represents zero
An example of a bipolar encoding method
Specified for the IEEE 802 –Ethernet standards
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Differential Manchester Encoding
1
• Mid-bit transition can be used for clocking
• transition at start of bit period representing 0
• no transition at start of bit period representing 1
– this is a differential encoding scheme
• used by IEEE 802.5 - token ring LANs
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
What does differential encoding mean?
• In differential encoding, the signal is decoded by
– comparing the polarity of adjacent signal elements
– rather than determining the absolute value of a signal element.
• In other words it looks for a transition rather than the
absolute value.
• The above would require synchronisation with respect to
the sender.
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Differential Manchester Encoding
• At least 4 properties need to considered when encoding
digital data to digital signals
• 1. Signal spectrum requirement - less DC component and
less frequency spectrum 
• 2. Signal synchronization capability – by the mid-bit
transition 
• 3. Signal error-detecting capability 
• 4. Signal interference and noise immunity –signal is
decoded by comparing the polarity of adjacent signal
elements rather then the absolute value of the signal
element 
• 5. Cost and complexity of the encoding/decoding
equipment.  - partially met
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
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)
– has no dc component
– has error detection
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Scrambling
• Scrambling replaces long sequences of 0..0’s that
would produce constant voltage by a special codes.
• these filling sequences must
– produce enough transitions to provide synchronization
– Should be recognized by receiver & replaced with original data
– be of same length as original sequence and hence no date rate
reduction
• design goals
–
–
–
–
have no dc component
have no long sequences of zero level line signal
have no reduction in data rate
give error detection capability
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
B8ZS and HDB3
- ve
000- +0+-
B8ZS
(Bipolar with 8-zeros
substitution)
If an octet of all zeros occurs
last voltage pulse is positive,
Encoded as 000+–0–+
If an octet of all zeros occurs
Last voltage pulse is negative,
Encoded as 000–+0+–
HDB3 (High-Density Bipolar 3zeros)
If string of Four zeros sequences
occurs
The fourth zero(0) is replaced
with a code violation(+- or -+
pulses).
Successive violations are
replaced with alternate polarity.
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Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Data Encoding
•  digital data to digital signal
• analog data to analog signal
• digital data to analog signal
• analog data to digital signal
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Analog data, Analog Signal
• By modulating the data (baseband) with a high carrier
signal fc.
• Why?
– higher frequency fc can give more efficient transmission & BW
– permits frequency division multiplexing (chapter 8)
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Modulation
• is a process of encoding an analog (or digital) data on to
an analog carrier signal whose frequency, say is f c
Where f c is a high frequency
• Input – analog (digital) data – called baseband or
modulating signal m(t)
• Output - analog signal - carried modulated signal s(t)
Amplitude Modulated Signal
M(t) ʘ Fc(t)
Analog Data
modulating Signal m(t)
High Frequency Signal
Fc(t)
Communication channel
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Modulation Techniques
• is to vary one of the following three aspects of the carrier
signal
• Amplitude of the carrier (Amplitude modulation (AM))
• Frequency of the carrier (Frequency modulation (FM))
• Phase of the carrier (Phase modulation(PM))
• Example – AM and FM radio stations for the first two
respectively
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
Analog Data
to
to
Digital Signal Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Analog
Modulation
Techniques
Amplitude Modulation
Phase Modulation
Frequency Modulation
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Data Encoding
• digital to digital
• analog to analog
• digital to analog
• analog to digital
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Digital Data, Analog Signal
• Use the same modulation techniques discussed for
analog data, analog signals
• main use is public telephone system
– has freq range of 300Hz to 3400Hz
– use modem (modulator-demodulator)
• encoding techniques
– Amplitude shift keying (ASK)
– Frequency shift keying (FSK)
– Phase shift keying (PK)
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Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
Digital Data, Analog Signal – Modulation Techniques
Digital Data
Amplitude
Shift keying
Frequency
Shift keying
Phase
Shift keying
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Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
Amplitude Shift Keying
• encode 0/1 by different carrier amplitudes
– usually have one amplitude zero
• susceptible to sudden gain changes
• inefficient
• used for
– up to 1200bps on voice grade lines
– very high speeds over optical fiber
s(t) = A * Sin (2π f t + θ)
A = Amplitude
f = carrier frequency
θ = Phase Angle
“0” = 0 * Sin (2π f t + 0)
“1” = A * Sin (2π f t + 0)
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Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
Binary Frequency Shift Keying
• most common is binary FSK (BFSK)
• two binary values represented by two different
frequencies (near carrier)
s(t) = A * Sin (2π f t + θ)
• less susceptible to error than ASK
A = Amplitude
• used for
f = carrier frequency
– up to 1200bps on voice grade lines
θ = Phase Angle
– high frequency radio
– even higher frequency on LANs using co-axial cable
“0” = A * Sin (2π f1 t + 0)
“1” = A * Sin (2π f2 t + 0)
www.infotech.monash.edu
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Multiple FSK
• Each signalling element can also represent more than
one bit
– then more than two frequencies are required (e.g:2 bits – 4
frequencies for 00, 01, 10 and 11)
• More bandwidth efficient
• However more prone to error
www.infotech.monash.edu
Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
Phase Shift Keying
• phase of carrier signal is shifted to represent
data
s(t) = A * Sin (2π f t + θ)
A = Amplitude
• binary PSK
f = carrier frequency
– two phases represent two binary digits
θ = Phase Angle
• differential PSK
– phase shifted relative to previous transmission rather
than some reference signal
“0” = A * Sin (2π f1 t + 0)
“1” = A * Sin (2π f1 t + 180)
www.infotech.monash.edu
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Quadrature PSK
• get more efficient use if each signal element represents
more than one bit
– e.g.. shifts of /2 (90o)
– each element represents two bits
– split input data stream in two & modulate onto carrier & phase
shifted carrier
• can use 8 phase angles & more than one amplitude
– 9600bps modem uses 12 angles, four of which have two
amplitudes
“11” = A * Cos (2π fc t + 45)
“10” = A * Cos (2π fc t + 135)
“00” = A * Cos (2π fc t + 225)
“01” = A * Cos (2π fc t + 315)
www.infotech.monash.edu
Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Quadrature Amplitude Modulation
• QAM used in asymmetric digital subscriber line (ADSL) and
some wireless communication
• combination of ASK and PSK
• logical extension of QPSK
• send two different signals simultaneously on same carrier
frequency
– uses two copies of carrier fc, one shifted by a phase angle of 90°
– each carrier is ASK modulated
– two independent signals over same medium
– demodulate and combine for original binary output
www.infotech.monash.edu
Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
QAM Modulator
send two different signals simultaneously on same carrier frequency
uses two copies of carrier fc, one shifted by a phase angle of 90°
http://www.inue.unieach carrier is ASK modulated
stuttgart.de/german/lehre/lesungen/uet2/applet/QAM16e.html
two independent signals over same medium
demodulate and combine for original binary output
ASK – signal-1
www.infotech.monash.edu
ASK – signal-2
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Data Encoding
• digital data to digital signal
• analog data to analog signal
• digital data to analog signal
• analog data to digital signal
www.infotech.monash.edu
Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Analog Data, Digital Signal
• Need to convert
– analog data into  digital Data
> called digitization
– then digital data to  digital signal
• Digitization Principle: Take the amplitude of the
signal at different intervals and convert the
signal value into digital data.
www.infotech.monash.edu
Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Analog Data, Digital Signal
• Once the analog data is converted into digital data then
we can use any of the encoding techniques that we talked
earlier to convert it to digital signals
• For example we can then:
– transmit using NRZ-L, NRZI or using Manchester or Differential
Manchester coding techniques
– can be converted to analog signal
www.infotech.monash.edu
Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Digitizing Analog Data
www.infotech.monash.edu
Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Analog Data, Digital Signal
• analog to digital conversion is done
using a codec (coder-decoder)
• Conversion is done at least in two
different ways
– pulse code modulation (PCM)
– delta modulation (DM)
www.infotech.monash.edu
Summary
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
An example
t1
t2
t3
t4
t5
t6
•
Let the analog data be as
shown in the left picture
•
Find the amplitude of the
signal at regular intervals (say
at t1, t2, t3, t4, t5, t6 in the
figure)
•
Express each amplitude in a
binary form.
e.g. a signal voltage level, say
at t2 is represented as binary
code
•
code # [15] in base10= [1111]2
which is binary - digital data
www.infotech.monash.edu
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
An Example (cont’d)
Voltage levels
(M)
Binary 4-bit
code
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
1111
1110
1101
1100
1011
1010
1001
1000
0111
0110
0101
0100
0011
0010
0001
0000
www.infotech.monash.edu
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
How often you need to sample the analog data?
• If you want to decode the original signal from its
digital data, then the sampling theorem specifies
the minimum rate at which you need to sample the
analog signal
– “If a signal f(t) is sampled at regular intervals of time at a rate
higher than twice the highest signal frequency, then the
samples contain all the information of the original signal”
(Stallings DCC8e)
– e.g.. 4000Hz voice data, requires 8000 sample per sec
– Note that these are analog samples, referred to as pulse
amplitude modulation (PAM) samples
– The PAM samples must be assigned to a binary code to
complete the digitization of the original analog signal
www.infotech.monash.edu
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
PCM Block Diagram
www.infotech.monash.edu
Summary
Delta Modulation
• analog input is approximated by a staircase function
– can move up or down one level () at each sample interval
• has binary behavior
– since function only moves up or down at each sample interval
– hence can encode each sample as single bit
– 1 for up or 0 for down
www.infotech.monash.edu
Delta Modulation Example
www.infotech.monash.edu
Delta Modulation Operation
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Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
PCM versus Delta Modulation
•
•
•
•
•
DM has simplicity compared to PCM
DM is easier to implement compared to PCM
DM has worse SNR compared to PCM
PCM has better SNR compared to DM
PCM has issue of bandwidth being used
– e.g. for good voice reproduction with PCM
> want 128 levels (7 bit) & voice bandwidth of 4khz
> need 8000 x 7 = 56kbps
• data compression can improve on BW issues..
• PCM is the choice and still growing demand for digital signals
– Due to use of repeaters, TDM(no intermodulation noise), and efficient
switching
• PCM is preferred to DM for analog signals
www.infotech.monash.edu
Data
Types
Signal
Encoding
Analog Signal &
Digital Signals
Signal
Encoding
Digital Data
to
Digital Signal
Analog Data
to
Analog Signal
Digital Data
to
Analog Signal
Analog Data
To
Digital Signal
Summary
• We looked at signal encoding techniques
– digital data to digital signal
– analog data to analog signal
– digital data to analog signal
– analog data to digital signal
www.infotech.monash.edu
Summary