Physical

1
Chapter 6
Physical Layer
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OBJECTIVES
After reading this chapter, the reader should be
able to
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6.1
DIGITAL AND ANALOG
4
Figure 6-1
Digital and analog entities
5
Figure 6-2
Digital data
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Figure 6-3
Analog data
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Figure 6-4
Digital signal
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Figure 6-5
Bit and bit interval
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Technical Focus Units of Bit Rate

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Figure 6-6
A sine wave
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Figure 6-7
Amplitude
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Figure 6-8
Period and frequency
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Technical Focus Units of Frequency

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Technical Focus Frequency and Change

The concept of frequency is similar to the
concept of change. If a signal (or data) is
changing rapidly, its frequency is higher. If it
changes slowly, its frequency is lower. When a
signal changes 10 times per second, its
frequency is 10 Hz when a signal changes 1000
times per second, its frequency is 1000 Hz.
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Figure 6-9
Phase
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Figure 6-10
Zero frequency and infinite frequency
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Note
Phase describes the position of a waveform
relative to other waveforms.
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Business Focus Two Familiar Signals
A familiar signal in our daily lives is the
electrical energy we use at home and at work. The
signal we receive from the power company has an
amplitude of 120 V and a frequency of 60 Hz (a
simple analog signal). Another signal familiar to
us is the power we get from a battery. It is an
analog signal with an amplitude of 6 V (or 12 or
24) and a frequency of zero.

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Business Focus The Bandwidth of Telephone Lines
The conventional line that connects a home or
business to the telephone office has a bandwidth
of 4 kHz. These lines were designed for carrying
human voice, which normally has a bandwidth in
this range. Human voice has a frequency that is
normally between 0 and 4 kHz. The telephone lines
are perfect for this purpose. However, if we try
to send a digital signal, we are in trouble. A
digital signal needs a very high bandwidth
(theoretically infinite) it cannot be sent using
these lines. We must either improve the quality
of these lines or change our digital signal to a
complex signal that needs only 4 kHz.

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6.2
TRANSFORMING DATA TO SIGNALS
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Figure 6-11
Transforming data to signals
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Figure 6-12
Digital-to-digital encoding
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Note
A digital signal has a much higher bandwidth than
an analog signal. There is a need for a better
media to send a digital signal.
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Note
Most LANs use digital-to-digital encoding because
the data stored in the computers are digital and
the cable connecting them is capable of carrying
digital signals.
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Figure 6-13
Digital encoding methods
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Technical Focus Average Values in Digital
Signals

With one exception, all of the signals in Figure
16.3 have an average value of zero (the positive
and negative values cancel each other in the
long run). The first signal, unipolar, has a
positive average value. This average value,
sometimes called the residual value, cannot pass
through some devices (such as a transformer). In
this case, the receiver receives a signal that
can be totally different from the one sent and
results in an erroneous interpretation of data.
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Technical Focus Synchronization in Digital
Signals

To correctly interpret the signals received from
the sender, the receivers bit intervals must
correspond exactly to the senders bit
intervals. If the receiver clock is faster or
slower, the bit intervals are not matched and
the receiver will interpret the signals
differently than the sender intended. A
self-synchronizing digital signal includes
timing information in the data being transmitted.
This can be achieved if there are transitions in
the signal that alert the receiver to the
beginning, middle, or end of the bit interval. If
the receivers clock is out of synchronization,
these alerting points can reset the clock.
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Figure 6-14
Digital-to-analog modulation
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Figure 6-15
ASK
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Figure 6-16
FSK
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Figure 6-17
PSK
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Technical Focus Understanding Bit Rate and Baud
Rate

A transportation analogy can clarify the concept
of bauds and bits. A baud is analogous to a car
a bit is analogous to a passenger. A car can
carry one or more passengers. If 1000 cars go
from one point to another each carrying only one
passenger (the driver), then 1000 passengers are
transported. However, if each car carries four
passengers (car pooling), then 4000 passengers
are transported. Note that the number of cars,
not the number of passengers, determines the
traffic and, therefore, the need for wider
highways. Similarly, the number of bauds
determines the required bandwidth, not the
number of bits.
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Technical Focus Capacity of a Channel

We often need to know the capacity of a channel
that is, how fast can we send data over a
specific medium? The answer was given by
Shannon. Shannon proved that the number of bits
that we can send through a channel depends on
two factors the bandwidth of the channel and
the noise in the channel. Shannon came up with
the following formula
C B log2 (1 signal-to-noise ratio) C is the
capacity in bits per second B is the bandwidth.
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Figure 6-18
Analog-to-digital conversion
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Figure 6-19
PCM
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Technical Focus Sampling Rate and Nyquist
Theorem

As you can see from the preceding figures, the
accuracy of any digital reproduction of an analog
signal depends on the number of samples taken. So
the question is, how many samples are sufficient?
This question was answered by Nyquist. His
theorem states that the sampling rate must be at
least twice the highest frequency of the
original signal to ensure the accurate
reproduction of the original analog signal. So if
we want to sample a telephone voice with a
maximum frequency of 4000 Hz, we need a
sampling rate of 8000 samples per second.
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6.3
TRANSMISSION MODES
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Figure 6-20
Data transmission
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Figure 6-21
Parallel transmission
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Figure 6-22
Serial transmission
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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.
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Note
Asynchronous here means asynchronous at the byte
level, but the bits are still synchronized
their durations are the same.
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Figure 6-23
Asynchronous transmission
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Note
In synchronous transmission, we send bits one
after another without start/stop bits or gaps. It
is the responsibility of the receiver to group
the bits.
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Figure 6-24
Synchronous transmission
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6.4
LINE CONFIGURATION
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Note
Line configuration defines the attachment of
communication devices to a link.
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Figure 6-25
Point-to-point line configuration
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Figure 6-26
Multipoint line configuration
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6.5
DUPLEXITY
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Figure 6-27
Half-duplex mode
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Figure 6-28
Full-duplex mode
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6.6
MULTIPLEXING SHARING THE MEDIA
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Figure 6-29
Multiplexing versus no multiplexing
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Figure 6-30
Categories of multiplexing
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Figure 6-31
FDM
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Note
FDM can only be used with analog signals.
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Technical Focus Use of FDM in Telephone Systems

ATT uses a hierarchical system to multiplex
analog lines
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Figure 6-32
Prisms in WDM multiplexing and demultiplexing
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Figure 6-33
TDM
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Note
TDM can be used only with digital signals.
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Figure 6-34
Synchronous TDM
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Technical Focus Use of TDM in Telephone Systems

ATT uses a hierarchical system to multiplex
digital lines
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Figure 6-35
Asynchronous TDM
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Figure 6-36
Multiplexing and inverse multiplexing
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Technical Focus Use of TDM in ATM Networks

Asynchronous TDM is used today in the ATM
network, a wide area network that we discuss in
Chapter 11. ATM is acell network the packets
traveling through the networkare small packets
called cells.