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16.546 Computer Telecommunications:
Modulation and Data Encoding
Professor Jay Weitzen
Electrical & Computer Engineering Department
The University of Massachusetts Lowell
1
Data Encoding at the PL
Source node
Destination node
Application
Application
Presentation
Presentation
Session
Session
Intermediate node
transport
Network
Packets
transport
Network
Network
Data link
Data link
Physical
Physical
Frames
Data link
Physical
Bits
Signals
2
Network A Node
7
6
5
We Need to Encode PL Frame
Application
AL-Hdr
Presentation
PL-Hdr
Session
4
Transport
3
Network
2
Data Link
1
Physical
SL-Hdr
DLL-Hdr
PL-Hdr
Presentation Layer Msg
Session Layer Msg
TL-Hdr
NL-Hdr
Application Layer Msg
Transport Layer Msg
Network Layer Msg
Data Link Layer Msg
Physical Layer Msg
3
Encoding Techniques
Digital data, digital signal
Analog data, digital signal
Digital data, analog signal
Analog data, analog signal
4
Digital Data, Digital Signal
Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
5
Terminology
Unipolar
– All signal elements have same sign
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
Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
Mark and Space
– Binary 1 and Binary 0 respectively
6
Interpreting Signals
Need to know
– Timing of bits - when they start and end
– Signal levels
Factors affecting successful interpreting of signals
– Signal to noise ratio
– Data rate
– Bandwidth
7
Comparison of Encoding Schemes (1)
Signal Spectrum
– Lack of high frequencies reduces required bandwidth
– Lack of dc component allows ac coupling via
transformer, providing isolation
– Concentrate power in the middle of the bandwidth
Clocking
– Synchronizing transmitter and receiver
– External clock
– Sync mechanism based on signal
8
Comparison of Encoding Schemes (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
9
Encoding Schemes
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
4B/5B, MLT-3, 8B/10 Schemes
10
Nonreturn to Zero-Level (NRZ-L)
Two different voltages for 0 and 1 bits
Voltage constant during bit interval
– no transition, i.e., no return to zero voltage
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
11
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
12
NRZ
13
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
14
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
15
Multilevel Binary
Use more than two levels
Bipolar-AMI (Alternate Mark Inversion)
–
–
–
–
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 (zeros still a
problem)
– No net dc component
– Lower bandwidth
– Easy error detection
16
Pseudoternary
One represented by absence of line signal
Zero represented by alternating positive and
negative
No advantage or disadvantage over bipolar-AMI
17
Bipolar-AMI and Pseudoternary
18
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
19
Biphase
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
Differential Manchester
–
–
–
–
–
Midbit transition is clocking only
Transition at 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
20
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
21
Modulation Rate
22
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
23
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
24
HDB3
High Density Bipolar 3 Zeros
Based on bipolar-AMI
String of four zeros replaced with one or two
pulses
25
B8ZS and HDB3
26
Digital Signal Encoding For LANs
4B/5B-NRZI
– Used for 100BASE-X and FDDI LANs
– Four Data Bits Encoded into Five Code Bits, 80%
MLT-3
– 100BASE-TX & FDDI Over Twisted Pair
8B/6T
– Uses Ternary Signaling (Pos, Neg, Zero Voltages)
– Eight Data Bits Encoded into 6 Ternary Symbols
8B/10B
– Used for Fibre Channel & Gigabit Ethernet
27
10 Gigabit Ethernet (1 of 2)
• IEEE 802.3ae
• MAC: it’s just Ethernet
– Maintains 802.3 frame format and size
– Full duplex operation only
– Throttled to 10.0 for LAN PHY or 9.58464 Gb/s for WAN PHY
• PHY: LAN and WAN phys
– LAN PHY uses simple encoding mechanisms to transmit data on dark fiber
and dark wavelengths
– WAN PHY adds a SONET framing sublayer to utilize SONET/SDH as layer 1
transport
• PMD: optical media only
–
–
–
–
850 nm on MMF to 65m
1310 nm, 4 lambda, WDM to 300 m on MMF; 10 km on SMF
1310 nm on SMF to 10 km
1550 nm on SMF to 40 km
28
10 Gigabit Ethernet (2 of 2)
• Supports dark wavelength and SONET/TDM with
unlimited reach
• Several Coding Schemes (64b/66b; 8B/10B;
Scramblers)
• Three optional interfaces: XGMII; XAUI; XSBI
• Extension of MDIO interface
• Continues Ethernet’s reputation for cost effectiveness
and simplicity (goal 10X performance for 3X cost)
• Expected target for ratification in Spring 2002
29
802.3ae to 802.3z Comparison
10 Gigabit Ethernet
1 Gigabit Ethernet
• CSMA/CD + Full
Duplex
• Carrier Extension
• Optical/Copper Media
• Leverage Fibre Channel
PMD’s
• Reuse 8B/10B Coding
• Support LAN to 5 km
•
•
•
•
Full Duplex Only
Throttle MAC Speed
Optical Media Only
Create New Optical
PMD’s From Scratch
• New Coding Schemes
• Support LAN to 40
km; Use SONET/SDH
as Layer 1 Transport
30
Converting From Analog To Digital
31
Pulse Code Modulation: a digital
encoding scheme used in TDM
In this modulation technique, an analog signal is
digitized, and interleaved with other digitized
voice signal to create a single bit stream
At the receiving end, the bit stream is decomposed
into separate digital streams of lower frequencies,
each stream is then converted back into what
resembles the original voice signal.
32
Steps Required to Generate PCM
Streams
Sampling: periodic measurement of the analog
signals at regular intervals
Quantizing: assigning discrete values to samples
Coding: assigned binary codes to samples using
what is known as the PCM code word
33
Sampling
(a)
(b)
(c)
Figure 2.2 : creating a PAM wave for a single sinusoid.
(a) is a sinusoid signal, (b) a pulse train, (c) the result of
passing (a) and (b) through a point by point multiplier.
34
Sampling
Sampling rate: how often should we take
measurements of the analog signal
at least at twice the rate of its highest frequency
component
For a voice channel with a frequency range
between 300 Hz and 3400 Hz (bandwidth of 3100
Hz) we need to take a sample at least at a rate of 2
X 3100 = 6200 Hz or every 1/6200 second
35
Sampling
In practical system, we sample multiple channel,
we combine the samples of all channels into a
single signal called the PAM signal (Pulse
Amplitude Modulation signal)
In American systems we sample 24 channels
In the European systems 30 channels are sampled
36
Quantization
To represent samples by a fixed number of bits
For example if the amplitude of the PAM signal
range between -1 and +1 there can be infinite
number of values. For instance one value can be 0.2768987653598364834634
For practicality, we may use 20 different discrete
values between -1 and +1 volts
Each value at a 0.1 increment
37
Quantization: the binary world
Because we live in a binary world, we select the
total number of discrete values to be binary
number multiple (i.e., 2, 4, 8, 16, 32, 64, 128, 256,
and so on)
This facilitate binary coding
For instance, if there were 4 values they would be
as follows: 00, 01, 10, 11
This is a 2-bit code
38
Quantization:
16 coded quantum steps
Between -1 and + 1 volts signal
16 discrete steps
each step at 0.125 volts increment or decrement
from the adjacent step
0 0000
0v 3
0011 0.375v
1 0001 0.125v
4
0100 0.500v
2 0010 0.25v
5
0101 0.625v
39
Quantization: 16 quantum steps
(-1 to + 1 volts)
+1
Range of standard
values (V)
0
-1
15 : 1111
14: 1110
13: 1101
12: 1100
11: 1011
10: 1010
9: 1001
8: 1000
7: 0111
6: 0110
5: 0101
4: 0100
3: 0011
2: 0010
1: 0001
0: 0000
8 9 10 1112 13 12 11 10.. 6 .........
Coded values
40
Quantization Distortion
Quantization error is the different between the quantum
value and the true value
More steps reduce quantizing distortion in linear
quantization
This will require higher bandwidth, since we need more
bits for each code word
Voice represent a problem because of the wide dynamic
range, the level from the loudest syllable of the loudest
talker to the lowest syllable of the quietest talker
S/D = 6n + 1.8 dB EX: 7 bit PCM cod 6.7 + 1.8 = 43.8
practical system S/D = 30 - 33 dB
41
Companding
Compression/Expanding
Non-linear
The voltage level between the loudest and the
lowest is segmented in non-linear manor
The voltage range of each segment varies
according to the level of the voltage
42
Non-linear Quantization
Segment #
Voltage levels
5.0
1
3.0
2
1.5
3
0.5
4
5
0
6
7
8
43
Non-linear Quantization
Compressed Output
Voltage
Segment 2 has 3 steps like all
of the other segments
Segment 4
Segment 3
Segment 2
Segment 1
-5.0
0.5
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Input Voltage
4.5 5.0
44
Coding for Modern PCM systems
Non-linear
Logarithmic
A-Law
u-Law
45
A-Law
AX
Y
1 log A
Y
1 _ log( AX
1 _ log A
for
for
V
0v
A
V
v V
A
46
U-Law
log( 1 u | X |)
| Y |
log( 1 u)
47
Coding for Modern PCM systems
i
v
Y
X
Where = instantaneous input voltage
V
B
V = maximum input voltage for which peak
limitation is
absent
i = number of quantization steps starting from the center
of the range
B = number of quantization steps on each side of the
center of the range.
48
13-segment A-Law Curve
Segment
(Chord)
Code
6
112
5
96
4
80
1101XXX
3
64
1100XXX
2
48
1011XXX
1111XXX
1/2
1110XXX
1/4
1/8
POSITIVE
1/16
1/32
1
32
1010XXX
1/64
1001XXX
0
1000XXX
0
1/4
2/4
3/4
1
(V)
0000XXX
0001XXX
0010XXX
0011XXX
NEGATIVE
0100XXX
0101XXX
0110XXX
0111XXX
Figure 2.7: 13-segment approximation of the A-law
curve used with E1 PCM equipment
49
PCM Code Word
Sign
S
Segment
Number
Level
Value
A B C
D
Figure 2.8: PCM Code Example
50
S/D for A-law & u-Law
For A = 87.6: S/D = 37.5 dB
u = 255: S/D = 37
51
Modems: Modulator/Demodulator
Used to Package bits for transport over
broadband media
– 3 ways to encode information on a carrier
- Phase
- Frequency
- Amplitude
52
Definition of Modulation
Let m(t) be an arbitrary modulating (information)
waveform. (could be either analog or digital)
Let c(t)=cos(wct +f(t)) be the carrier
The argument of the sinusoid is the instantaneous phase
(wct +f( t ))
The instantaneous frequency (2pfi)is given by d/dt (wct
+f( t )) = wc +d/dt(f(t))2pfi
53
Types of Modulation
If c(t)=m(t) cos(wct +f), the information is
transported in the amplitude of the carrier. We
call this Amplitude Modulation (AM)
If fi(t)=km(t), the information is transported in
the instantaneous frequency. We call this
frequency modulation (FM).
If f( t )=km(t) the information is carried in the
instantaneous phase, and we call this phase
modulation (PM).
54
Modulation Techniques
55
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
56
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
57
Frequency Modulation
FM Used for high fidelity audio broadcast and
digital transmission.
Uses Shannon concept of bandwidth expansion.
58
FSK on Voice Grade Line
59
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
60
Phase Modulation
Generally used for digital modulation
61
Quadrature PSK
More efficient use by each signal element
representing more than one bit
– e.g. shifts of p/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
62
Constellation Space
Create 2-axis (e.g. sine and cosine) actually it could be a ndimensional hyper-plane
Express digital modulation alphabet as points in the
hyper-plane. The farther apart the points are in the space,
the more immunity there is against noise and interference.
More distance, better error performance. Keep this in
mind.
The maximum power is the length of the longest vector.
The average transmitter power is the average distance
squared of all the points.
63
Case Study 1: ASK
• If m(t) = {0,1} and we amplitude modulate a carrier with m(t)
then the modulation is called on/off keying (OOK) or 2-amplitude
shift keying (2-ASK)
• 2-ASK, (points are at (0,0), and (0,1), in the 2 dimensional (sine,
cosine plane). Minimum distance between points is 1 for 1 unit of
power, and 1 bit per symbol.
• Distance between points corresponds to error performance
64
Case Study 2: Multi-Level ASK
•If maximum power is normalize to 1 then points are at
(0,0), (0,1/3), (0,2/3), (0,1). Distance is reduced from
2-ASK and performance is worse. Requires 3x or 9x
power to maintain 1 unit of distance.
• From Shannon, as we add more information in a fixed
bandwidth, it becomes increasingly expensive in terms
of SNR to add more data.
65
Case 3: Orthogonal FSK
•Frequencies are chosen so that the waveforms are orthogonal
over the period of the bit T.
• Points are at (0,1) and (1,0) for 2-FSK. Distance is sqrt(2).
Error performance better than 2-ASK but not as good as others.
66
Case 4: QPSK and PSK
y(t)
y(t)
A
-A
-A
A
A
x(t)
x(t)
-A
Example signal constellation
diagram for BPSK signal.
67
Higher Order Modulations Very
Inefficient in terms of Power
68
Case 6: QAM
Beyond 3 bits/symbol, PSK too power inefficient. Must use
hybrid amplitude and phase modulation called QAM
69
Example V.32 Constellation
70
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
71
Coherent vs. Non-Coherent
Detection
Coherent detection requires a copy of the carrier
to be recovered from the received signal for use in
the detection process. It is more efficient because
it uses all phase information, but requires added
complexity
Non-coherent detection using an envelope
detector is much easier to implement, but less
efficient because it uses only the envelope
information and not the phase information.
72
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)
73
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
74
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
75
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
76
Nonlinear Encoding
Quantization levels not evenly spaced
Reduces overall signal distortion
Can also be done by companding
77
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
78
Delta Modulation - example
79
Delta Modulation - Operation
80
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
81
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
82
Analog
Modulation
83
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
84
Encoding Schemes - WAN Techniques
1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0
AMI
0 0 0 0 V B 0 V B
B8ZS
0 0 0 V B 0 0 V
B 0 0 V
HDB3
Both are well suited to characteristics of WAN channels
85
Encoding Schemes - Spectral Density
B8ZS,
HDB3
1.2
1.0
Mean
Square
Voltage .8
per Unit
Bandwidth.6
NRZ-L
NRZI
AMI, Pseudoternary
Manchester,
Diff. Manchester
.4
.2
.2
.4
.6
.8
1.0 1.2 1.4 1.6 1.8 2.0
Normalized Frequency (f/R)
86
Communications Interface
Destination WS
SourceWS
Information Exchange
•Content Material
•Acquisition
•Conversion
•Compression
•Buffering
•Media Access
•Protocol
•Segmentation
•Streaming
Transmission
Or
Network
•Packet Routing
•Node Switching
•Buffering
(Network Delay & Transmission Jitter)
•Content Material
•Acquisition
•Conversion
•Compression
•Buffering
•Media Access
•Protocol
•Reassembly
•Synchronization
87
Asynchronous and Synchronous
Transmission
Timing problems require a mechanism to
synchronize the transmitter and receiver
Two solutions
– Asynchronous
– Synchronous
88
Asynchronous
Data transmitted one character at a time
– 5 to 8 bits
Timing only needs maintaining within each
character
Resync with each character
89
Asynchronous (diagram)
90
Asynchronous - Behavior
In a steady stream, interval between characters is uniform
(length of stop element)
In idle state, receiver looks for transition 1 to 0
Then samples next seven intervals (char length)
Then looks for next 1 to 0 for next char
Simple
Cheap
Overhead of 2 or 3 bits per char (~20%)
Good for data with large gaps (keyboard)
91
Synchronous - Bit Level
Block of data transmitted without start or stop
bits
Clocks must be synchronized
Can use separate clock line
– Good over short distances
– Subject to impairments
Embed clock signal in data
– Manchester encoding
– Carrier frequency (analog)
92
Synchronous - Block Level
Need to indicate start and end of block
Use preamble and postamble
– e.g. series of SYN (hex 16) characters
– e.g. block of 11111111 patterns ending in 11111110
More efficient (lower overhead) than async
93
Synchronous (diagram)
94
Line Configuration
Topology
– Physical arrangement of stations on medium
– Point to point
– Multi point
• Computer and terminals, local area network
Half duplex
– Only one station may transmit at a time
– Requires one data path
Full duplex
– Simultaneous transmission and reception between two stations
– Requires two data paths (or echo canceling)
95
Traditional Configurations
96
Interfacing
Data processing devices (or data terminal equipment,
DTE) do not (usually) include data transmission facilities
Need an interface called data circuit terminating
equipment (DCE)
– e.g. modem, NIC
DCE transmits bits on medium
DCE communicates data and control info with DTE
– Done over interchange circuits
– Clear interface standards required
97
Characteristics of Interface
Mechanical
– Connection plugs
Electrical
– Voltage, timing, encoding
Functional
– Data, control, timing, grounding
Procedural
– Sequence of events
98
V.24/EIA-232-F
ITU-T v.24
Only specifies functional and procedural
– References other standards for electrical and mechanical
EIA-232-F (USA)
–
–
–
–
–
RS-232
Mechanical ISO 2110
Electrical v.28
Functional v.24
Procedural v.24
99
Mechanical Specification
100
Electrical Specification
Digital signals
Values interpreted as data or control, depending
on circuit
More than -3v is binary 1, more than +3v is
binary 0 (NRZ-L)
Signal rate < 20kbps
Distance <15m
For control, more than-3v is off, +3v is on
101
Local and Remote Loopback
102
Procedural Specification
E.g. Asynchronous private line modem
When turned on and ready, modem (DCE) asserts DCE
ready
When DTE ready to send data, it asserts Request to Send
– Also inhibits receive mode in half duplex
Modem responds when ready by asserting Clear to send
DTE sends data
When data arrives, local modem asserts Receive Line
Signal Detector and delivers data
103
Dial Up Operation (1)
104
Dial Up Operation (2)
105
Dial Up Operation (3)
106
Null Modem
107
ISDN Physical Interface Diagram
108
ISDN Physical Interface
Connection between terminal equipment (c.f.
DTE) and network terminating equipment (c.f.
DCE)
ISO 8877
Cables terminate in matching connectors with 8
contacts
Transmit/receive carry both data and control
109