Unit 1 Physical Layer.pptx of Tushar rohila

Position of the physical layer

Signals
Digital Transmission
Analog Transmission
Multiplexing
Transmission Media
Circuit Switching and Telephone Network
High Speed Digital Access

To be transmitted, data must be
transformed to electromagnetic
signals.

Analog and Digital
Analog and Digital Data
Analog and Digital Signals
Periodic and Aperiodic Signals
Simple and Composite Signals

Signals can be analog or digital.
Analog signals can have an infinite
number of values in a range; digital
signals can have only a limited
number of values.

Comparison of analog and digital signals

In data communication, we commonly
use periodic analog signals and
aperiodic digital signals.

Analog Signals
Sine Wave
Phase
Examples of Sine Waves
Time and Frequency Domains
Composite Signals
Bandwidth

Frequency and period are inverses of
each other.

Units of periods and frequencies
Units of periods and frequencies
Unit Equivalent Unit Equivalent
Seconds (s) 1 s hertz (Hz) 1 Hz
Milliseconds (ms) 10–3
s kilohertz (KHz) 103
Hz
Microseconds (ms) 10–6
s megahertz (MHz) 106
Hz
Nanoseconds (ns) 10–9
s gigahertz (GHz) 109
Hz
Picoseconds (ps) 10–12
s terahertz (THz) 1012
Hz

Example 1
Example 1
Express a period of 100 ms in microseconds, and express
the corresponding frequency in kilohertz.
Solution
Solution
From Table 3.1 we find the equivalent of 1 ms.We make
the following substitutions:
100 ms = 100  10-3
s = 100  10-3
 10
s = 105
s
Now we use the inverse relationship to find the
frequency, changing hertz to kilohertz
100 ms = 100  10-3
s = 10-1
s
f = 1/10-1
Hz = 10  10-3
KHz = 10-2
KHz

Frequency is the rate of change with
respect to time. Change in a short span
of time means high frequency. Change
over a long span of time means low
frequency.

If a signal does not change at all, its
frequency is zero. If a signal changes
instantaneously, its frequency is
infinite.

Phase describes the position of the
waveform relative to time zero.

Relationships between different phases

Example 2
Example 2
A sine wave is offset one-sixth of a cycle with respect
to time zero. What is its phase in degrees and radians?
Solution
Solution
We know that one complete cycle is 360 degrees.
Therefore, 1/6 cycle is
(1/6) 360 = 60 degrees = 60 x 2 /360 rad = 1.046 rad

Sine wave examples (continued)

An analog signal is best represented in
the frequency domain.

Time and frequency domains (continued)

A single-frequency sine wave is not
useful in data communications; we
need to change one or more of its
characteristics to make it useful.

When we change one or more
When we change one or more
characteristics of a single-frequency
characteristics of a single-frequency
signal, it becomes a composite signal
signal, it becomes a composite signal
made of many frequencies.
made of many frequencies.

According to Fourier analysis, any
composite signal can be represented as
a combination of simple sine waves
with different frequencies, phases, and
amplitudes.

The bandwidth is a property of a
The bandwidth is a property of a
medium: It is the difference between
medium: It is the difference between
the highest and the lowest frequencies
the highest and the lowest frequencies
that the medium can
that the medium can
satisfactorily pass.
satisfactorily pass.

We use the term bandwidth to refer to
We use the term bandwidth to refer to
the property of a medium or the width
the property of a medium or the width
of a single spectrum.
of a single spectrum.

Example 3
Example 3
If a periodic signal is decomposed into five sine waves
with frequencies of 100, 300, 500, 700, and 900 Hz,
what is the bandwidth? Draw the spectrum, assuming all
components have a maximum amplitude of 10 V.
Solution
Solution
B = fh  fl = 900  100 = 800 Hz
The spectrum has only five spikes, at 100, 300, 500, 700,
and 900 (see Figure 13.4 )

Example 4
Example 4
A signal has a bandwidth of 20 Hz. The highest
frequency is 60 Hz. What is the lowest frequency? Draw
the spectrum if the signal contains all integral frequencies
of the same amplitude.
Solution
Solution
B = f
B = fh
h 
 f
fl
l
20 = 60
20 = 60 
f
fl
l
f
fl
l = 60
= 60 
20 = 40 Hz
20 = 40 Hz

Example 5
Example 5
A signal has a spectrum with frequencies between 1000
and 2000 Hz (bandwidth of 1000 Hz). A medium can
pass frequencies from 3000 to 4000 Hz (a bandwidth of
1000 Hz). Can this signal faithfully pass through this
medium?
Solution
Solution
The answer is definitely no. Although the signal can have
The answer is definitely no. Although the signal can have
the same bandwidth (1000 Hz), the range does not
the same bandwidth (1000 Hz), the range does not
overlap. The medium can only pass the frequencies
overlap. The medium can only pass the frequencies
between 3000 and 4000 Hz; the signal is totally lost.
between 3000 and 4000 Hz; the signal is totally lost.

Digital Signals
Digital Signals
Bit Interval and Bit Rate
As a Composite Analog Signal
Through Wide-Bandwidth Medium
Through Band-Limited Medium
Versus Analog Bandwidth
Higher Bit Rate

Example 6
Example 6
A digital signal has a bit rate of 2000 bps. What is the
duration of each bit (bit interval)
Solution
Solution
The bit interval is the inverse of the bit rate.
Bit interval = 1/ 2000 s = 0.000500 s
= 0.000500 x 106
s = 500 s

A digital signal is a composite signal
A digital signal is a composite signal
with an infinite bandwidth.
with an infinite bandwidth.

Bandwidth Requirement
Bandwidth Requirement
Bit
Rate
Harmonic
1
Harmonics
1, 3
Harmonics
1, 3, 5
Harmonics
1, 3, 5, 7
1 Kbps 500 Hz 2 KHz 4.5 KHz 8 KHz
10 Kbps 5 KHz 20 KHz 45 KHz 80 KHz
100 Kbps 50 KHz 200 KHz 450 KHz 800 KHz

The bit rate and the bandwidth are
The bit rate and the bandwidth are
proportional to each other.
proportional to each other.

Analog versus Digital
Analog versus Digital
Baseband and Broadband Transmission
Low-pass versus Band-pass
Digital Transmission
Analog Transmission

Figure 3.19 Low-pass and band-pass

The analog bandwidth of a medium is
The analog bandwidth of a medium is
expressed in hertz; the digital
expressed in hertz; the digital
bandwidth, in bits per second.
bandwidth, in bits per second.

Digital transmission needs a
Digital transmission needs a
low-pass channel with an infinite or
low-pass channel with an infinite or
very wide bandwidth.
very wide bandwidth.

Analog transmission can use a band-
Analog transmission can use a band-
pass channel.
pass channel.

Data Rate Limit
Data Rate Limit

Example 7
Example 7
Consider a noiseless channel with a bandwidth of 3000
Hz transmitting a signal with two signal levels. The
maximum bit rate can be calculated as
Bit
Bit Rate = 2
Rate = 2 
 3000
3000 
 log
log2
2 2 = 6000 bps
2 = 6000 bps

Example 8
Example 8
Consider the same noiseless channel, transmitting a signal
with four signal levels (for each level, we send two bits).
The maximum bit rate can be calculated as:
Bit Rate = 2 x 3000 x log
Bit Rate = 2 x 3000 x log2
2 4 = 12,000 bps
4 = 12,000 bps

Example 9
Example 9
Consider an extremely noisy channel in which the value
of the signal-to-noise ratio is almost zero. In other words,
the noise is so strong that the signal is faint. For this
channel the capacity is calculated as
C = B log
C = B log2
2 (1 + SNR) = B log
(1 + SNR) = B log2
2 (1 + 0)
(1 + 0)
= B log
= B log2
2 (1) = B
(1) = B 
 0 = 0
0 = 0

Example 10
Example 10
We can calculate the theoretical highest bit rate of a
regular telephone line. A telephone line normally has a
bandwidth of 3000 Hz (300 Hz to 3300 Hz). The signal-
to-noise ratio is usually 3162. For this channel the
capacity is calculated as
C = B log
C = B log2
2 (1 + SNR) = 3000 log
(1 + SNR) = 3000 log2
2 (1 + 3162)
(1 + 3162)
= 3000 log
= 3000 log2
2 (3163)
(3163)
C = 3000
C = 3000 
 11.62 = 34,860 bps
11.62 = 34,860 bps

Example 11
Example 11
We have a channel with a 1 MHz bandwidth. The SNR
for this channel is 63; what is the appropriate bit rate and
signal level?
Solution
Solution
C = B log
C = B log2
2 (1 + SNR) = 10
(1 + SNR) = 106
6
log
log2
2 (1 + 63) = 10
(1 + 63) = 106
6
log
log2
2 (64) = 6 Mbps
(64) = 6 Mbps
Then we use the Nyquist formula to find the
number of signal levels. Shannon gives upper limit,
we can chosesomething lower, 4Mbps, for example
4 Mbps = 2
4 Mbps = 2 
 1 MHz
1 MHz 
 log
log2
2 L
L 
 L = 4
L = 4
First, we use the Shannon formula to find our upper
First, we use the Shannon formula to find our upper
limit.
limit.

Transmission Impairment
Transmission Impairment
Attenuation(loss of energy)
decible(dB)=10*log10(P2/P1))
20*log10(V2/V1))
Distortion(change in shape)
Noise(thermal, induced, impulse, crosstalk)

Example 12
Example 12
Imagine a signal travels through a transmission medium
and its power is reduced to half. This means that P2 = 1/2
P1. In this case, the attenuation (loss of power) can be
calculated as
Solution
Solution
10 log
10 log10
10 (P2/P1) = 10 log
(P2/P1) = 10 log10
10 (0.5P1/P1) = 10 log
(0.5P1/P1) = 10 log10
10 (0.5)
(0.5)
= 10(–0.3) = –3 dB
= 10(–0.3) = –3 dB

Example 13
Example 13
Imagine a signal travels through an amplifier and its
power is increased ten times. This means that P2 = 10 *
P1. In this case, the amplification (gain of power) can be
calculated as
10 log
10 log10
10 (P2/P1) = 10 log
(P2/P1) = 10 log10
10 (10P1/P1)
(10P1/P1)
= 10 log
= 10 log10
10 (10) = 10 (1) = 10 dB
(10) = 10 (1) = 10 dB

Example 14
Example 14
One reason that engineers use the decibel to measure the
changes in the strength of a signal is that decibel numbers
can be added (or subtracted) when we are talking about
several points instead of just two (cascading). In Figure a
signal travels a long distance from point 1 to point 4. The
signal is attenuated by the time it reaches point 2.
Between points 2 and 3, the signal is amplified. Again,
between points 3 and 4, the signal is attenuated. We can
find the resultant decibel for the signal just by adding the
decibel measurements between each set of points.

Figure Example 14
dB = –3 + 7 – 3 = +1

More About Signals
More About Signals
Throughput(how fast actually send data
through a network)
Latency(how long it takes to completely arrive
the message at receiver end)
Latency=prop time+tran time+que time+proc time
Transmission Time(tran time=msg size/B)
Propagation Speed
Propagation Time(time required to travel
one bit from source to target)
(prop time =distance/prop speed)
Wavelength

Unit 1 Physical Layer.pptx of Tushar rohila

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