COMP 421 /CMPET 401

1
COMP 421 /CMPET 401

• COMMUNICATIONS and NETWORKING
• CLASS 6

2
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

3
Analog Data Choices
4
Digital Data Choices
5
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

6
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

7
Encoding schemes
Analog data, Digital signal
Analog data, Analog signal
digital
analog
analog
voice
Telephone
CODEC
Digital data, Digital signal
Digital data, Analog signal
analog
digital
digital
digital
Modem
Digital transmitter
8
Encoding and Modulation
x(t)
x(t)
g(t)
g(t)
Encoder
Decoder
digital or analog
digital
t
s(f)
s(t)
m(t)
Modulator
Demodulator
m(t)
digital or analog
analog
f
fc
fc
9
Why encoding?

• Three factors determine successfulness of
  receiving a signal
• S/N (Signal to Noise Ratio)
• Data rate
• Bandwidth

10
Encoding Schemes' Evaluation Factors

• Signal spectrum
• Clocking
• Error detection
• Signal interference noise immunity
• Cost and complexity

11
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

12
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

13
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

14
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

15
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

16
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)

17
Spectral density
1.5
B8ZS,HDB3
NRZ-L, NRZI
1
AMI, Pseudoternary
0.5
Mean square voltage per unit bandwidth
Manchester, Differential Manchester
0
0
1
0.5
1.5
-0.5
Normalized frequency (f/r)
18
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

19
Encoding Schemes

• Nonreturn to Zero-Level (NRZ-L)
• Nonreturn to Zero Inverted (NRZI)
• Bipolar -AMI (Alternate Mark Inversion)
• Pseudoternary
• Manchester
• Differential Manchester
• B8ZS
• HDB3

20
Digital Data, Digital Signal
0
1
0
0
1
1
0
0
0
1
1
NRZ
NRZI
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
21
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

22
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)

23
NRZ
24
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

25
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

26
Pseudoternary

• One represented by absence of line signal
• Zero represented by alternating positive and
  negative
• No advantage or disadvantage over bipolar-AMI

27
Bipolar-AMI and Pseudoternary
28
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

29
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)

30
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)

31
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

32
Modulation Rate
The modulation Rate is at which signal elements
are generated
In General the Modulation Rate D R/b where
RData Ratebits/sec bnumber of bits per
signal element Data Rate (bit Rate 1/Tb) where Tb
is bit duration For Manchester Encoding maximum
Rate is 2/Tb
33
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

34
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.

35
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

36
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

37
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
38
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.
39
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.
40
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.

41
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 1938000
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 super-frame. 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

42
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.
43
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.
44
B8ZS and HDB3
(Assume odd number of 1s since last substitution)
See Table 5.4 for HDB3 Substitution Rules
45
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)

46
Modulation Techniques
47
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

48
ASK
Vd(t)
Vc(t)
VASK(t)
Signal power
frequency spectrum
Frequency
fc
fcf0
fc3f0
fc-f0
fc-3f0
49
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

50
FSK
Data signal
vd(t)
v1(t)
Carrier 1
v2(t)
Carrier 2
FSK(t)
Signal power
frequency spectrum
Frequency
f1
f2
51
FSK in modem (on Voice Grade Line)
frequency spectrum
52
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

53
PSK
Data Signal
vc(t)
Carrier
vc(t)
Phase coherent
vPSK(t)
Differential
vPSK(t)

• bit rate signaling rate

1800
01
Differential example for every logic 1, 180
degree phase shift
phase diagram
54
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

55
Multilevel Modulation Method
01
10
00
11
9001
18010
000
27011
0
90
180
270
4-PSK phase diagram

• bit rate n x signaling rate

56
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

57
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

58
Analog data, Digital signal

• Two principle techniques used
• PCM (Pulse Code Modulation)
• DM (Delta Modulation)

Sampling clock
PAM signal
PCM signal
Sampling Circuit
Quantizer and compander
Analog voice signal
Digitized voice signal
59
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

60
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

61
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.
62
PCM

• Sampling signal based on Nyquist theorem

Original signal
3.9
4.2
3.4
3.2
2.8
PAM pulse
1.2
4
4
PCM pulse with quantized error
3
3
3
1
011
100
011
011
001
100
011100011011001100
PCM output
63
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

64
Nonlinear Encoding
Quantizing level
15
15
14
14
13
13
12
12
11
11
10
10
9
8
9
8
7
7
6
5
6
4
5
3
4
3
2
2
1
1
0
0
Without nonlinear encoding
With nonlinear encoding
65
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.
66
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

67
Delta Modulation - example
68
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

69
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

70
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
71
V.34 Modulation
V.34 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.FC
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 modems functional capabilities in 5
seconds or less.
72
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
73
END Class