CMPT 371
Data Communications and Networking
Digital Encoding
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© Janice Regan, CMPT 128, 2007-2012
Encoding
 Digital and analog data can be encoded
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as digital or analog signals.
Digital data encoded to digital signals
Analog data encoded to digital signals
Digital data encoded to analog signals
Analog data encoded to analog signals
Janice Regan © 2005-2013
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Definitions
 Data rate, R
 Rate of data transmission in bits per second
 Duration or length of a bit, tb = 1/R
 Time taken for transmitter to emit the bit
 Modulation rate, D = R/b
 Measured in baud (signal elements or
symbols per second)
 b is the number of bits per signal element
 Mark and Space
 Binary 1 and Binary 0 respectively
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Encoding and Modulation
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Stallings 2003: Figure 5.1
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Digital Signaling
 The source data is a stream of data bits, g(t) is encoded
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into voltage pulses.
The particular encoding method will determine how the
information is translated into voltage pulses
Encoding methods use multiple voltage levels
Information is carried using the voltage levels and
sometimes the transitions between voltage levels.
Factors affecting efficiency of encoding include
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Number of voltage levels
Signal bandwidth
Error detection efficiency
Maximum duration without a transition (Loss of Sync)
Amount of DC signal produced ( transformer coupling only with
no dc signal)
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Analog Signaling
 carrier signal: continuous signal with frequency, fc
 Modulation: the process of encoding the source data
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stream (baseband signal or modulating signal) onto the
carrier signal
Modulation involves superimposing variations in
amplitude, phase or frequency on the carrier signal.
These variations carry the information in the data.
The Modulated Signal (output from modulation) is
transmitted as an analog signal
The Receiver will demodulate the Modulated Signal and
extract the information
WHY? To change the signals bandwidth and frequency
so it can be transmitted on the specified limited width
communication channel.
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Digital to Digital Encoding Schemes
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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
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 Digital signal
 Discrete, discontinuous voltage pulses
 Each pulse is a signal element
 Binary data encoded into signal elements
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Nonreturn to Zero – Level (NRZI)
 Data encoding of binary ones and zeros
 Two signal levels used for encoding
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1 – Negative voltage
0 – Positive voltage
 Constant voltage pulse for duration of bit
 no return to zero voltage during pulse
 Synchronization may be lost during a long
string of zeros or ones. A change is signal level
is needed to determine where a bit starts or
ends
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Nonreturn to Zero - Level
11 1 0 0 0 1 1 0 1 1 1 1 0 0 0 1 0 1 0 1
1
0
-1
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Properties of NRZ- level signals
 Maximum possible frequency (bit rate R bps)
0
1/R
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3/R
2/R
4/R
5/R
6/R
7/R
8/R
1
0
-1
9/R
Period T=2/R, f = 1/T = R/2 (alternating 1s and 0s)
time
 Minimum possible frequency
±1
time
0
1/R
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2/R
3/R
4/R
5/R
6/R
7/R
8/R
9/R
Period T=∞, f = 1/T = 0 (all 0s or all 1s): DC component
 Bandwidth = Maximum Frequency – Minimum Frequency
= R/2 – 0 = R/2
 May have a net dc component
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Bipolar Alternate Mark Inversion (Bipolar AMI)
 Multilevel binary data encoding of 1s and 0s
 0 represented by no signal (0 voltage)
 1 represented by a signal (+ve or –ve voltage)
 Signals for ones alternate in sign (+ve and –ve)
 Constant voltage pulse for duration of bit
 No loss of sync if a long string of ones
 Sync may be lost during a long string of zeros
 Easy error detection: when 2 successive bits are
the same (+1 or -1) an error has occurred.
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Bipolar Alternate Mark Inversion (Bipolar AMI)
1 1 0 0 0 1 1 0 1 1 1 1 0 0 0 1 0 1 0 1
1
0
-1
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Properties of Bipolar AMI signals
 Maximum possible frequency (bit rate R bps)
1
0
-1
0
1/R
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3/R
2/R
4/R
5/R
6/R
7/R
8/R
9/R
Period T=2/R, f = 1/T = R/2 (all 1s)
time
 Minimum possible frequency
0
0
1/R
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3/R
2/R
4/R
5/R
6/R
7/R
8/R
9/R
Period T=∞, f = 1/T = 0 (all 0s)
time
 Bandwidth = Maximum Frequency – Minimum Frequency
= R/2 – 0 = R/2
 No net dc component (+ve and –ve bits alternate)
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Pseudoternary
1 1 0 0 0 1 1 0 1 1 1 1 0 0 0 1 0 1 0 1
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Manchester
 Biphase data encoding of binary ones and zeros
 Transition between the two possible signal levels
occurs in the middle of each bit period
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Used for clocking and to encode information
 1 - signal transition low to high
 0 - signal transition high to low
 Signal transitions may also occur at the beginning of
a bit period (to allow for the correct mid bit transition)
 Used by IEEE 802.3
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Manchester
1 1 0 0 0 1 1 0 1 1 1 1 0 0 0 1 0 1 0 1
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Properties of Manchester signals
 Maximum possible frequency (bit rate R bps)
0
1/R
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3/R
8/R
7/
Period T=1/R, f = 1/T = R (all 1s or R
all 0s)
2/R
4/R
5/R0
6/R
9/R
10/R
time
 Minimum possible frequency
0
1/R
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2/R
3/R
4/R
5/R
6/R
7/R
8/R
9/R
Period T=2/R, f = 1/T = R/2 (alternating 1s and 0s)
time
 Bandwidth = Maximum Frequency – Minimum Frequency
= R – R/2 = R/2
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Theoretical Bit Error Rate
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Stallings
2003: Figure 5.4
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Scrambling
 Use scrambling to replace sequences that cause
transmission at a constant level (voltage) for many bit
durations, and may cause synchronization problems
 Replace long constant sequences with a filling sequence
 The filling sequence must be chosen to
 Produce enough transitions to sync
 Be recognized by receiver and replaced with original data
 Same length as original
 The filling sequence should not have
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A dc component
Any long sequences of zero level line signal
Any reduction in data rate
Error detection capability
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Bipolar AMI
1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1
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Bipolar With 8 Zeros Substitution (B8ZS)
Data encoding (Based on bipolar-AMI)
 0 represented by no signal for duration of bit
 1 represented by a signal for duration of bit
 Signals for ones alternate in sign
 An octet of zeros in the data is encoded as
 000+-0-+ if the preceding voltage pulse was +ve
 000-+0+- if the preceding voltage pulse was -ve
 Transmitting station inserts these octets to replace each octet of
zeros in the data.
 Receiving station detects the octets inserted to replace sequences
of zeros and interprets each such octet as a sequence of eight
zeros
 Insertion of each octet causes two violations of the bipolar-AMI code
 These violations are unlikely to occur as a result of noise
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B8ZS
1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1
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Properties of B8ZS signals
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Maximum possible frequency (bit rate R bps)
1
0
-1
0
3/R
6/R
1/R
4/R
5/R
2/R
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Period T=2/R, f = 1/T = R/2 (all 1s)
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Minimum possible frequency
7/R
8/R
9/R
time
0
0
7/R
15/R 23/R 31/R 39/R 47/R 55/R 63/R 71/R
time
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Period T=16/R, f = 1/T = R/16 (repeating pattern of 7 0s followed by a
1)
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Bandwidth = R/2 – R/16 = 7R/16
No net dc component (+ve and –ve bits alternate, except when
substitutions occur)
Sync is maintained (need 1 bit in 8 not zero for hardware reliability)
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Used in telecom (Voip ..) on DS1 and T1 lines
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4B/5B
 Used for 100BASE-X and FDDI LANs
 4 Data Bits Encoded into 5 Code Bits
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At least 2 transitions occur in each group of
code bits
No more than 2 consecutive 0’s in a group of
code bits
No more than 3 consecutive 0’s in any coded
sequence
 80% efficiency, 125Mbaud, 100Mbps
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4B/5B: code groups
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Stallings 2003: Table 16.6
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4B/5B MLT-3
 Used for 100BASE-TX Over Twisted Pair
 Different way of encoding groups of 4B/5B
Code bits so high frequencies, which
cannot be transmitted through twisted pair
cable, are removed from the transmitted
signal
 The 4B/5B bit stream is scrambled
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4B/5B MLT-3
 The resulting bit stream is encoded
using MLT-3
 If the next input bit is zero the next output bit
is the same as the present output bit
 If the next input bit is one the next output bit
is different from the present output bit
 If the present output bit is not 0 then the
next output bit is 0
 If the present output bit is 0 the next
output bit is not zero and has a sign
opposite the previous nonzero output bit
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State Machine for MLT-3 encoding
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Stallings 2003: Figure 16.12
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MLT-3 (4B/5B 3-Level )
1 1 0 0
0 1 0 1
1 1 1 0
0 0 0 1
0 1 0 1
1 1 0 1 0 0 1 0 1 1 1 1 1 0 0 0 1 0 0 1 0 1 0 1
1
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