Transcript Solution First 4B/5B block coding increases the bit rate to.

Chapter 4
Digital Transmission
4.1
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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 Schemes
Block Coding
Scrambling
4.2
Figure 4.1 Line coding and decoding
4.3
Figure 4.2 Signal element versus data element
4.4
Example 4.1
A signal is carrying data in which one data element is
encoded as one signal element ( r = 1). If the bit rate is
100 kbps, what is the average value of the baud rate if c is
between 0 and 1?
Solution
We assume that the average value of c is 1/2 . The baud
rate is then
4.5
Figure 4.3 Effect of lack of synchronization
4.8
Example 4.3
In a digital transmission, the receiver clock is 0.1 percent
faster than the sender clock. How many extra bits per
second does the receiver receive if the data rate is
1 kbps? How many if the data rate is 1 Mbps?
Solution
At 1 kbps, the receiver receives 1001 bps instead of 1000
bps.
At 1 Mbps, the receiver receives 1,001,000 bps instead of
1,000,000 bps.
4.9
Figure 4.4 Line coding schemes
4.10
Nonreturn to Zero-Level (NRZ-L)
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Two different voltages for 0 and 1 bits
Voltage constant during bit interval
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no transition I.e. no return to zero voltage
e.g. 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
Figure 4.5 Unipolar NRZ scheme
4.12
Figure 4.6 Polar NRZ-L and NRZ-I schemes
4.13
Nonreturn to Zero Inverted
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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
Note
In NRZ-L the level of the voltage
determines the value of the bit.
In NRZ-I the inversion
or the lack of inversion
determines the value of the bit.
4.15
Differential Encoding
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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
NRZ pros and cons
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Pros
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Cons
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Easy to engineer
Make good use of bandwidth
dc component
Lack of synchronization capability
Used for magnetic recording
Not often used for signal transmission
Multilevel Binary
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Use more than two levels
Bipolar-AMI
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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
Pseudoternary
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One represented by absence of line signal
Zero represented by alternating positive
and negative
No advantage or disadvantage over
bipolar-AMI
Figure 4.9 Bipolar schemes: AMI and pseudoternary
4.24
Trade Off for Multilevel Binary
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Not as efficient as NRZ
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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
Biphase
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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(ethernet) standard for baseband
coaxial cable and twisted-pair CSMA/CD bus LANs
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 token ring LAN, using shielded
twisted pair
Manchester Encoding
Differential Manchester
Encoding
Biphase Pros and Cons
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Con
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At least one transition per bit time and
possibly two
Maximum modulation rate is twice NRZ
Requires more bandwidth
Pros
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Synchronization on mid bit transition (self
clocking)
No dc component
Error detection
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Absence of expected transition
Figure 4.7 Polar RZ scheme
4.30
Figure 4.8 Polar biphase: Manchester and differential Manchester schemes
4.31
Note
In Manchester and differential
Manchester encoding, the transition
at the middle of the bit is used for
synchronization.
4.32
Table 4.1 Summary of line coding schemes
4.39
Note
Block coding is normally referred to as
mB/nB coding;
it replaces each m-bit group with an
n-bit group.
4.40
Figure 4.14 Block coding concept
4.41
Figure 4.15 Using block coding 4B/5B with NRZ-I line coding scheme
4.42
Table 4.2 4B/5B mapping codes
4.43
Figure 4.16 Substitution in 4B/5B block coding
4.44
Figure 4.18 AMI used with scrambling
4.47
Figure 4.19 Two cases of B8ZS scrambling technique
4.48
Note
B8ZS substitutes eight consecutive
zeros with 000VB0VB.
4.49
Figure 4.20 Different situations in HDB3 scrambling technique
4.50
Note
HDB3 substitutes four consecutive
zeros with 000V or B00V depending
on the number of nonzero pulses after
the last substitution.
4.51
4-2 ANALOG-TO-DIGITAL CONVERSION
We have seen in Chapter 3 that a digital signal is
superior to an analog signal. 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.
Topics discussed in this section:
Pulse Code Modulation (PCM)
Delta Modulation (DM)
4.52
Figure 4.21 Components of PCM encoder
4.53
Note
According to the Nyquist theorem, the
sampling rate must be
at least 2 times the highest frequency
contained in the signal.
4.55
Figure 4.24 Recovery of a sampled sine wave for different sampling rates
4.58
Example 4.9
Telephone companies digitize voice by assuming a
maximum frequency of 4000 Hz. The sampling rate
therefore is 8000 samples per second.
4.62
Figure 4.26 Quantization and encoding of a sampled signal
4.65
Example 4.14
We want to digitize the human voice. What is the bit rate,
assuming 8 bits per sample?
Solution
The human voice normally contains frequencies from 0
to 4000 Hz. So the sampling rate and bit rate are
calculated as follows:
4.68
4-3 TRANSMISSION MODES
The transmission of binary data across a link can be
accomplished in either parallel or serial mode. In
parallel mode, multiple bits are sent with each clock
tick. In serial mode, 1 bit is sent with each clock tick.
While there is only one way to send parallel data, there
are three subclasses of serial transmission:
asynchronous, synchronous, and isochronous.
Topics discussed in this section:
Parallel Transmission
Serial Transmission
4.74
Figure 4.31 Data transmission and modes
4.75
Figure 4.32 Parallel transmission
4.76
Figure 4.33 Serial transmission
4.77
Note
In asynchronous transmission, we send
1 start bit (0) at the beginning and 1 or
more stop bits (1s) at the end of each
byte. There may be a gap between
each byte.
4.78
Note
Asynchronous here means
“asynchronous at the byte level,”
but the bits are still synchronized;
their durations are the same.
4.79
Figure 4.34 Asynchronous transmission
4.80
Note
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.
4.81
Figure 4.35 Synchronous transmission
4.82
Asynchronous Transmission
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Data transmitted 1
character at a time
Character format is 1
start & 1+ stop bit, plus
data of 5-8 bits
Character may include
parity bit
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Timing needed only
within each character
Resynchronization each
start bit
Uses simple, cheap
technology
Wastes 20-30% of
bandwidth
Synchronous Transmission
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Large blocks of bits
transmitted without
start/stop codes
Synchronized by clock
signal or clocking data
Data framed by preamble
and postamble bit
patterns
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More efficient than
asynchronous
Overhead typically below
5%
Used at higher speeds
than asynchronous
Requires error checking,
usually provided by
HDLC
Synchronization Choices
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Low-speed terminals and PCs commonly
use asynchronous transmission
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inexpensive
“burst” tendency of communication
reduces impact of inefficiency
Large systems and networks commonly
use synchronous transmission
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overhead too expensive; efficiency
necessary
error-checking more important