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COMP 421 /CMPET 401
COMMUNICATIONS and NETWORKING
CLASS 6
Encoding Techniques

Digital data, digital signal
– Easy encoding / Less Complex Less Expensive

Analog data, digital signal
– Can transmit data over Digital Network

Digital data, analog signal
– Modems / Fiber / Unguided Media

Analog data, analog signal
– Cheap & Easy Baseband Transmission / FDM
Analog Data Choices
analog line
analog voice
telephone
analog voice
digitized voice
Codec
01101000110
Codec: coder and decoder
digital line
Digital Data Choices
analog line
moduated data
modem
data
data
DSU
01101000110
DSU: data service unit
digital line
Transmission Choices

Analog transmission
– Only transmits analog signals, without regard
for data content
– Attenuation overcome with amplifiers

Digital transmission
– Transmits analog or digital signals
– Uses repeaters rather than amplifiers
Advantages of Digital Transmission
The signal is exact
 Signals can be checked for errors
 Noise/interference are easily filtered out
 A variety of services can be offered over
one line
 Higher bandwidth is possible with data
compression

Encoding schemes
Analog data, Analog signal
voice
analog
CODEC
analog
Modem
digital
analog
Telephone
Digital data, Analog signal
digital
Analog data, Digital signal
Digital data, Digital signal
digital
Digital
transmitter
digital
Encoding and Modulation
x(t)
x(t)
g(t)
digital
or
analog
Encoder
digital
g(t)
Decoder
t
s(f)
s(t)
m(t)
Modulator
digital
or
analog
m(t)
Demodulator
analog
fc
fc
f
Why encoding?

Three factors determine successfulness of
receiving a signal:
– S/N (Signal to Noise Ratio)
– Data rate
– Bandwidth
Encoding Schemes' Evaluation Factors

Signal spectrum

Clocking

Error detection

Signal interference & noise immunity

Cost and complexity
Digital Data, Digital Signal / Characteristics

Digital signal
– Uses discrete, discontinuous, voltage pulses
– Each pulse is a signal element
– Binary data is encoded into signal elements
Terms (1)

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
Terms (2)

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
Interpreting Signals

Need to know
– Timing of bits - when they start and end
– Signal levels

Factors affecting successful interpretation
of signals:
– Signal to noise ratio
– Data rate
– Bandwidth
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
– It is important to concentrate power in the
middle of the bandwidth
Comparison of Encoding Schemes (2)
Clocking issues
•Synchronizing transmitter and receiver is essential
•External clock is one way used for synchronization
•Synchronizing mechanism based on signal is also
used & preferred (over using an external clock)
Mean square voltage per unit bandwidth
Spectral density
1.5
1
NRZ-L,
NRZI
B8ZS,HDB3
AMI, Pseudoternary
0.5
Manchester,
Differential Manchester
0
-0.5
0
0.5
1
1.5
Normalized frequency (f/r)
Comparison of Encoding Schemes (3)

Error detection
– Can be built into 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
Encoding Schemes
Nonreturn to Zero-Level (NRZ-L)
 Nonreturn to Zero Inverted (NRZI)
 Bipolar -AMI (Alternate Mark Inversion)
 Pseudoternary
 Manchester
 Differential Manchester
 B8ZS
 HDB3

Digital Data, Digital Signal
0 1 0 0
NRZ
NRZI
Bipolar -AMI
Pseudoternary
Manchester
Differential
Manchester
1 1 0 0
0 1 1
Nonreturn to Zero-Level (NRZ-L)

Two different voltages:
– 0 - Low Level
– 1 - High Level

Voltage constant during bit interval

Most often, negative voltage for one value
and positive for the other
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 (Data
represented by changes rather than levels)

NRZ
NRZ pros and cons

Pros
– Easy to engineer
– Makes good use of bandwidth

Cons
– dc component
– Lack of synchronization capability
Used for magnetic recording
 Not often used for signal transmission

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 happens (zeros
still a problem)
– No net dc component  Can use a transformer for
isolating transmission line
– Lower bandwidth
– Easy error detection
Pseudoternary

One represented by absence of line signal

Zero represented by alternating positive
and negative

No advantage or disadvantage over
bipolar-AMI
Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary

Not as efficient as NRZ
– With multi-level binary coding, the line signal
may take on one of 3 levels, but each signal
element, which could represent log23 = 1.58
bits of information, bears only one bit of
information
– Receiver must distinguish between three
levels : (+A, -A, 0)
– Requires approx. 3dB more signal power for
same probability of bit error
Biphase

Manchester
– Transition in middle of each bit period
– Transition serves as clock and data
– One is represented by a transition from low to high
– Zero is represented by a transition from high to low

Used by IEEE 802.3 (Ethernet)
Differential Manchester
•Always a transition in the middle of the interval for clocking
•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 (Token Ring)
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 points to error in
transmission
Modulation Rate
The modulation Rate is at which signal elements are generated
In General the Modulation
Rate D = R/b where
R=Data Rate=bits/sec
b=number of bits per signal element
Data Rate (bit Rate 1/Tb)
where Tb is bit duration
For Manchester Encoding
maximum Rate is: 2/Tb
Scrambling Techniques

Used to reduce signaling rate relative to the
data rate by replacing sequences that would
produce constant voltage for a priod of time
with a filling sequence that accomplishes the
following goals:
– Must produce enough transitions to maintain
synchronization
– Must be recognized by receiver and replaced with
original data sequence
– is same length as original sequence
Scrambling Techniques
•No dc component
•No long sequences of zero level line signal
•No reduction in data rate
•Error detection capability
•As an example, fax machines use the
modified Huffman code to accomplish this.
B8ZS

B8ZS: Abbreviation for bipolar with eight-zero substitution

Same as Bipolar AMI 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
This is unlikely to occur as a result of noise
Receiver detects and interprets the sequence as octet
of all zeros






B8ZS
•A one is sent on a T1 by sending a pulse, as opposed to not sending a pulse.
•The alternating mark rule means that if the last pulse sent was of a positive
going polarity, the next pulse sent must be negative going.
•If a T1 device receives two pulses in a row and they are of the same polarity a
bipolar violation (BPV) has occurred.
•In B8ZS a specific combination of valid pulses and bipolar violations is used
to represent a string of eight zeroes, whenever the user data contains eight
zeroes in a row
B8ZS
Since a T1 uses a single pair of wires in each direction and the only signals on
those wires are the pulses which represent data; the only way to recover
clock and retain synchronization on a T1 is by detecting the rate at which
pulses are being received. All of the equipment in a T1 circuit must operate
at the same rate because all of the equipment must sense the T1 at the
correct time in order to determine if a pulse (1) or no pulse (0) has been
received at each bit time.
Since only ones are sent as pulses and zeroes are represented by doing
nothing, if too many zeroes are sent at a time there will be no pulses on the
T1 at all and the clock circuitry in all of the hardware will rapidly fall out of
synchronization. Thus the design of AMI requires that a certain ONES
DENSITY be maintained, that a certain minimum of the bits over a certain
period of time be guaranteed to be a ONE (pulse). This is why AMI circuits
require DENSITY enforcement
B8ZS
Briefly stated; on average one bit in eight must be a one and no more than
(varies according to specific standard) so many zeroes may be sent in a row.
In order to be able to satisfy the ones density requirement on an AMI T1 one
bit out of every eight is taken away from the user, not available for voice or
data traffic, and that 1 bit in 8 is always sent as a one. Once this has been
done the requirement for ones density is satisfied and the user is free to send
any data pattern in the remaining bandwidth.
B8ZS
The rate of a T1 is 1.544 megabits per second. 8K is used for
framing leaving 1.536MBPS. The 1.536 is usually divided into
24 timeslots (DS0s) or "channels" each being inherently
64KBPS. By taking the 1 bit in 8 that is reserved to satisfy
ones density the user is left with 56K per timeslot.
AMI
•AMI = Alternate Mark Inversion. This is the original method of
formatting T1 data streams. In AMI a zero is always sent by doing
nothing, at the time when a pulse might otherwise be sent, a pulse is not
sent to represent a zero.
•A one is sent on an AMI T1 by sending a pulse, as opposed to not
sending a pulse.
•The alternating mark rule means that if the last pulse sent was of a
positive going polarity, the next pulse sent must be negative going.
•If an AMI T1 device receives two pulses in a row and they are of the
same polarity a bipolar violation (BPV) has occurred.
•Thus AMI has a rudimentary error checking capability with a 50%
probability of detecting altered, inserted or lost bits end to end.
ESF
Extended Super Frame
A DS level and framing specification for synchronous digital streams
over circuits in the North America. A DS1 "frame" is composed of 24
eight-bit bytes plus one framing bit (193 bits). 8000 bytes per second
come from each source, and thus 8000 frames per second are transported
by the DS1 signal. The result is 193*8000 = 1,544,000 bits per second.
In the original standard, the framing bits continuously repeated the
sequence 110111001000, and such a 12-frame unit is called a superframe. In voice telephony, errors are acceptable (early standards allowed
as much as one frame in six to be missing entirely), so the least
significant bit in two of the 24 streams was used for signaling between
network equipments. This is called robbed bit signaling
ESF
To promote error-free transmission, an alternative called the
extended super-frame (ESF) of 24 frames was developed. In this
standard, six of the 24 framing bits provide a six bit cyclic
redundancy check(CRC-6), and six provide the actual framing.
The other 12 form a virtual circuit of 4000 bits per second for use
by the transmission equipment, for call progress signals such as
busy, idle and ringing. DS1 signals using ESF equipment are
nearly error-free, because the CRC detects errors and allows
automatic re-routing of connections.
HDB3



High Density Bipolar 3 Zeros
Based on bipolar-AMI
String of four zeros replaced with one or two
pulses
Note: The following is the explanation for the HDB3 code example on the
next slide (see rules in Table 5.4, page 142):
Assuming that an odd number of 1's have occurred since the last substitution,
since the polarity of the preceding pulse is "-", then the first 4 zeros are
replaced by "000-". For the next 4 zeros, since there have been no Bipolar
pulses since the 1st substitution, then they are replaced by"+00+" since the
preceding pulse is a "-". For the 3rd case where 4 zeros happen, 2 (even)
Bipolar pulses have happened since the last substitution and the polarity of
the preceding pulse is "+", so "-00-" is substituted for the zeros.
B8ZS and HDB3
(Assume odd number of 1s
since last substitution)
See Table 5.4 for HDB3 Substitution Rules
Digital Data, Analog Signal





Transmitting digital data through PSTN (Public
telephone system)
– 300Hz to 3400Hz bandwidth
– modem (modulator-demodulator) is used to
convert digital data to analog signal and vice
versa
Three basic modulation techniques are used:
Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PSK)
Modulation Techniques
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

ASK
Vd(t)
Vc(t)
VASK(t)
Signal
power
frequency spectrum
fc-3f0 fc-f0
fc fc+f0
fc+3f0
Frequency
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 (3-30 MHz)
 Higher frequency on LANs using co-ax

FSK
Data
signa
l
Carrier 1
v1(t)
Carrier 2
v2(t)
vd(t)
FSK(t)
Signal
power
frequency spectrum
Frequency
f1
f2
FSK in modem (on Voice Grade Line)
Amplitude
PSTN bandwidth
400
1180 1650
(1270) (2025)
1850
980
(2225)
(1070)
3400
Frequency(Hz)
frequency spectrum
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
PSK
Data
Signal
vc(t)
Carrier vc(t)
Phase
coherent vPSK(t)
Differential
v’PSK(t)

bit rate = signaling rate
180=0
Differential example: for every logic 1,
180 degree phase shift
0=1
phase diagram
Quadrature PSK

More efficient use by each signal element
representing more than one bit
– e.g. shifts of /2 (90o)
– Each element represents two bits
– Can use 8 phase angles and have more than
one amplitude
– 9600bps modems use 12 angles , four of
which have two amplitudes
Multilevel Modulation Method
00
+90°=01
01
10
11
+180°=10
0°=00
+270°=11
0°
4-PSK phase diagram

+90° +180°
+270°
bit rate = n x signaling rate
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
– Requires more analog bandwidth than ASK
In the presence of noise, bit error rate of PSK and
QPSK are about 3dB superior to ASK and FSK
Analog Data, Digital Signal

Digitization
– Conversion of analog data into digital data
– Digital data can then be transmitted using NRZ-L
or using other codes
– Digital data can then be converted to analog signal
– Analog to digital conversion done using a CODEC
– Pulse code modulation
– Delta modulation
Analog data, Digital signal

Two principle techniques used
– PCM (Pulse Code Modulation)
– DM (Delta Modulation)
Sampling
clock
Analog
voice
signal
Sampling
Circuit
PAM signal
PCM signal
Quantizer
and compander
Digitized
voice
signal
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
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
Pulse Code Modulation(PCM) (3)
The process starts with an analog signal, which is
sampled by PAM sample. the resulting pulse are
quantized to produced PCM pulses and then encoded
to produce bit stream. At the receiver end, the process
is reversed to reproduce the analog signal.
PCM

Sampling signal based on Nyquist theorem
Original signal
PAM pulse
PCM pulse
with quantized error
3.2
3
4
2.8
3.4
3
3
4.2
1.2
4
1
011
PCM output
3.9
100
011
011
001
100
011100011011001100
Nonlinear Encoding
Quantization levels are not necessarily
equally spaced. The problem with equal
spacing is that the mean absolute error for
each sample is the same, regardless the
signal level. Lower amplitude values are
relatively more distorted.
 Nonlinear encoding reduces overall signal
distortion
 Can also be done by companding

Nonlinear Encoding
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Quantizing level
15
14
13
Strong signal
Weak signal
12
11
10
9
8
76
5
4
3
2
1
0
Without nonlinear encoding With nonlinear encoding
Nonlinear Encoding
Prior to the input signal being sampled and converted by ADC into a
digital form, it is passed through a circuit known as a compressor.
Similarly, at the destination, the reverse operation is perform on the
output of the DAC by a circuit known as expander.
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
Delta Modulation - example
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
Analog Data, Analog Signals

Why modulate analog signals?
– Higher frequency can give more efficient
transmission
– Permits frequency division multiplexing

Types of modulation
– Amplitude
– Frequency
– Phase
Multilevel Modulation Method
Quadrature Amplitude Modulation (QAM)
Combines differential phase and amplitude
shifts to achieve 16 distinct states, thereby
allowing 4 bits to be represented by a
single signal
16-QAM phase diagram
V.34 Modulation
V.34
V.FC
Also known as V.FAST. It will allow modems to operate at 28Kb/s. Uses
multidimensional trellis coding and line probing equalization, power control
and framing.
Adaptive Pre-Emphasis or Precoding is a new form of adaptive equalization
that modifies the transmitted signal as well as the receiver.
Trellis Coding in more complex forms (64-state 4D, 32-state 4D, etc.) make
more efficient use of constellation space. Non-linear encoding wraps the
constellation space to bring the inner points closer and increase the distance
between the outer points.
Shell Mapping forms circular constellations which are optimum shape.
Shaping distributes consolation points nearer the center, which is less sensitive
to noise.
Adaptive Power Control changes the levels to produce the best performance
over impaired channels. This capability may also improve performance over
analog cellular services.
Scaling maintains the best transmit power levels when different modulation
technologies are employed.
Framing encodes bits over multiple symbols. This increases the systems ability
to support different combinations of symbol and data rates and makes it possible
to integrate a secondary channel.
V.FAST Class developed by Rockwell International. It is based on the V.34 proposed
design, but it is an interim solution. It does not support the V.8 handshaking
mechanism for full V.34 compatibility (it will require a software modification)
V.8 negotiation using a modulated calling tone and answer tone transfers information
about two modem’s functional capabilities in 5 seconds or less.
The 56K Modem
The V.90 modulation uses PAM. Each symbol is a different voltage level. 128
symbols multiplied by 8000 symbols per second, gives a 56,000 bits per second
downstream rate.
If the environment is noisy, less voltage levels are used. For example, if 64 are in use,
then the speed will be 48,000 bits per second in a 56Kbps connection, the server is a
digital modem. The PAM modulation requires at least 45dB SNR. The minimum RX
level a receiver can pull in is 34db below TX.
For upstream transmission, the information is transmitted in the old way, analog,
using QAM, A2D, through the PSTN, D2A and analog again. The upstream rate is
limited to 31.2Kbps
END Class