Digital Transmission

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Chapter 4 Digital Transmission
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4-1 DIGITAL-TO-DIGITAL CONVERSION
In this section, we see how we can represent
digital data by using digital signals. The
conversion involves three techniques line
coding, block coding, and scrambling. Line coding
is always needed block coding and scrambling may
or may not be needed.
Topics discussed in this section
Line Coding Line Coding SchemesBlock
Coding Scrambling
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Components of Data Communication

• Data
• Analog Continuous value data (sound, light,
  temperature)
• Digital Discrete value (text, integers, symbols)
• Signal
• Analog Continuously varying electromagnetic wave
• Digital Series of voltage pulses (square wave)
• Transmission
• Analog Works the same for analog or digital
  signals
• Digital Used only with digital signals

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Analog Data--gtSignal Options

• Analog data to analog signal
• Inexpensive, easy conversion (eg telephone)
• Used in traditional analog telephony
• Analog data to digital signal
• Requires a codec (encoder/decoder)
• Allows use of digital telephony, voice mail

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Digital Data--gtSignal Options

• Digital data to analog signal
• Requires modem (modulator/demodulator)
• Necessary when analog transmission is used
• Digital data to digital signal
• Less expensive when large amounts of data are
  involved
• More reliable because no conversion is involved

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Transmission Choices

• Analog transmission
• only transmits analog signals, without regard for
  data content
• attenuation overcome with amplifiers
• signal is not evaluated or regenerated
• Digital transmission
• transmits analog or digital signals (ie.
  digitizing analog)
• uses repeaters rather than amplifiers

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Figure 4.1 Line coding and decoding
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Figure 4.2 Signal element versus data element
r number of data elements / number of signal
elements
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• Data Rate Vs. Signal Rate
• Data rate the number of data elements (bits)
  sent in 1s (bps). Its also called the bit rate
• Signal rate the number of signal elements sent
  in 1s (baud). Its also called the pulse rate,
  the modulation rate, or the baud rate.

• We wish to
• 1. increase the data rate (increase the
  speed of transmission)
• 2. decrease the signal rate (decrease the
  bandwidth requirement)
• Worst case, best case, and average case of r
• S c N / r baud

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Baseline wandering Baseline running average of
the received signal power
DC Components Constant digital signal creates low
frequencies
Self-synchronization Receiver Setting the clock
matching the senders
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Figure 4.3 Effect of lack of synchronization
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Figure 4.4 Line coding schemes
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Figure 4.5 Unipolar NRZ scheme
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Digital Encodingof Digital Data

• Most common, easiest method is different voltage
  levels for the two binary digits
• Typically, negative1 and positive0
• Known as NRZ-L, or nonreturn-to-zero level,
  because signal never returns to zero, and the
  voltage during a bit transmission is level

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Differential NRZ

• Differential version is NRZI (NRZ, invert on
  ones)
• Change1, no change0
• Advantage of differential encoding is that it is
  more reliable to detect a change in polarity than
  it is to accurately detect a specific level

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Problems With NRZ

• Difficult to determine where one bit ends and the
  next begins
• In NRZ-L, long strings of ones and zeroes would
  appear as constant voltage pulses
• Timing is critical, because any drift results in
  lack of synchronization and incorrect bit values
  being transmitted

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Figure 4.6 Polar NRZ-L and NRZ-I schemes
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Figure 4.7 Polar RZ scheme
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Manchester Code

• Transition in the middle of each bit period
• Transition provides clocking and data
• Low-to-high1 , high-to-low0
• Used in Ethernet

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Differential Manchester

• Midbit transition is only for clocking
• Transition at beginning of bit period0
• Transition absent at beginning1
• Has added advantage of differential encoding
• Used in token-ring

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Figure 4.8 Polar biphase Manchester and
differential Manchester schemes
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• High0, Low1
• No change at begin0, Change at begin1
• H-to-L0, L-to-H1
• Change at begin0, No change at begin1

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Figure 4.9 Bipolar schemes AMI and pseudoternary
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Multilevel Schemes

• In mBnL schemes, a pattern of m data elements is
  encoded as a pattern of n signal elements in
  which 2m Ln
• m the length of the binary pattern
• B binary data
• n the length of the signal pattern
• L number of levels in the signaling

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Figure 4.10 Multilevel 2B1Q scheme
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Figure 4.11 Multilevel 8B6T scheme
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Figure 4.13 Multitransition MLT-3 scheme
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Table 4.1 Summary of line coding schemes
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Block Coding

• Redundancy is needed to ensure synchronization
  and to provide error detecting
• Block coding is normally referred to as mB/nB
  coding
• it replaces each m-bit group with an n-bit group
• m lt n

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Figure 4.14 Block coding concept
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Figure 4.15 Using block coding 4B/5B with NRZ-I
line coding scheme
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Table 4.2 4B/5B mapping codes
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Figure 4.16 Substitution in 4B/5B block coding
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Figure 4.17 8B/10B block encoding
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Scrambling

• It modifies the bipolar AMI encoding (no DC
  component, but having the problem of
  synchronization)
• It does not increase the number of bits
• It provides synchronization
• It uses some specific form of bits to replace a
  sequence of 0s

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Figure 4.19 Two cases of B8ZS scrambling
technique
B8ZS substitutes eight consecutive zeros with
000VB0VB
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Figure 4.20 Different situations in HDB3
scrambling technique
HDB3 substitutes four consecutive zeros with 000V
or B00V depending on the number of nonzero pulses
after the last substitution.
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4-2 ANALOG-TO-DIGITAL CONVERSION
The tendency today is to change an analog signal
to digital data. In this section we describe
two techniques, pulse code modulation and delta
modulation.
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Figure 4.21 Components of PCM encoder
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Figure 4.22 Three different sampling methods for
PCM
Analog signal is sampled every Ts s, where Ts is
the sample interval or period. The inverse of Ts
is called sampling rate or sampling frequency and
donoted by fs
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According to the Nyquist theorem, the sampling
rate must be at least 2 times the highest
frequency contained in the signal.
What can we get from this 1. we can sample a
signal only if the signal is band-limited 2.
the sampling rate must be at least 2 times the
highest frequency, not the bandwidth
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Figure 4.23 Nyquist sampling rate for low-pass
and bandpass signals
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Figure 4.24 Recovery of a sampled sine wave for
different sampling rates
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Figure 4.25 Sampling of a clock with only one
hand
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Example
An example related is the seemingly backward
rotation of the wheels of a forward-moving car in
a movie. This can be explained by
under-sampling. A movie is filmed at 24 frames
per second. If a wheel is rotating more than 12
times per second, the under-sampling creates the
impression of a backward rotation.
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Example
A complex low-pass signal has a bandwidth of 200
kHz. What is the minimum sampling rate for this
signal?
Solution The bandwidth of a low-pass signal is
between 0 and f, where f is the maximum frequency
in the signal. Therefore, we can sample this
signal at 2 times the highest frequency (200
kHz). The sampling rate is therefore 400,000
samples per second.
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Example
A complex bandpass signal has a bandwidth of 200
kHz. What is the minimum sampling rate for this
signal?
Solution We cannot find the minimum sampling rate
in this case because we do not know where the
bandwidth starts or ends. We do not know the
maximum frequency in the signal.
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Figure 4.26 Quantization and encoding of a
sampled signal
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Contribution of the quantization error to SNRdb
SNRdb 6.02nb 1.76 dB nb bits per sample
(related to the number of level L)
What is the SNRdB in the example of Figure 4.26?
Solution We have eight levels and 3 bits per
sample, so SNRdB 6.02 x 3 1.76 19.82 dB
Increasing the number of levels increases the
SNR.
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Example
A telephone subscriber line must have an SNRdB
above 40. What is the minimum number of bits per
sample?
Solution We can calculate the number of bits as
Telephone companies usually assign 7 or 8 bits
per sample.
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PCM decoder recovers the original signal
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The minimum bandwidth of the digital signal is nb
times greater than the bandwidth of the analog
signal. Bmin nb x Banalog
We have a low-pass analog signal of 4 kHz. If we
send the analog signal, we need a channel with a
minimum bandwidth of 4 kHz. If we digitize the
signal and send 8 bits per sample, we need a
channel with a minimum bandwidth of 8 4 kHz
32 kHz.
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DM (delta modulation) finds the change from the
previous sample Next bit is 1, if amplitude of
the analog signal is larger Next bit is 0, if
amplitude of the analog signal is smaller
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Figure 4.29 Delta modulation components
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Figure 4.30 Delta demodulation components
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4-3 TRANSMISSION MODES
1. The transmission of binary data across a link
can be accomplished in either parallel or serial
mode. 2. In parallel mode, multiple bits are
sent with each clock tick. 3. In serial mode, 1
bit is sent with each clock tick. 4. there are
three subclasses of serial transmission
asynchronous, synchronous, and isochronous.
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Figure 4.31 Data transmission and modes
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Figure 4.32 Parallel transmission
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Figure 4.33 Serial transmission
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Asynchronous transmission 1. We send 1 start bit
(0) at the beginning and 1 or more stop bits (1s)
at the end of each byte. 2. There may be a gap
between each byte. 3. Asynchronous here means
asynchronous at the byte level, but the bits
are still synchronized, their durations are the
same.
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Synchronous transmission In synchronous
transmission, we send bits one after another
without start or stop bits or gaps. It is the
responsibility of the receiver to group the bits.