Computer Netowrks Shift Keying Aplitudee

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Digital-to-analog conversion is the process of changing one of the
characteristics of an analog signal based on the information in digital
data.
DIGITAL DATA, ANALOG SIGNALS

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• Transmitting digital data through the public telephone network.
• Telephone network was designed to receive, switch, and transmit analog
signals in the voice-frequency range of about 300 to 3400 Hz.
• Digital devices are attached to the network via a modem (modulator-
demodulator), which converts digital data to analog signals, and vice versa

.
An analog signal carries 4 bits per signal element. If 1000
signal elements are sent per second, find the bit rate.
Bit Rate=Number of bits per signal element×
Signal elements per second
Bits per signal element = 4
Signal elements per second = 1000
Bit Rate=4×1000=4000 bits per second (bps)

.
An analog signal has a bit rate of 8000 bps and a baud rate of 1000
baud.
1. How many data elements are carried by each signal element?
2.How many signal elements do we need?

.
An analog signal has a bit rate of 8000 bps and a baud rate of 1000
baud.
1. How many data elements are carried by each signal element?
2.How many signal elements do we need?
The number of data elements per signal element, N is the ratio of the bit
rate to the baud rate

.
Carrier Signal (Modulated signal) : High-frequency signal.
A carrier signal is a high-frequency sinusoidal waveform that is used to carry
or transmit information (like binary data) over a communication channel.
Message signal (Modulating signal) :
The message signal, also called the modulating signal, is the original data
or information that we want to transmit. In digital modulation techniques like
ASK, this is typically a binary sequence consisting of 1s and 0s.

.
• Digital information changes the carrier signal by modifying one or more of its
characteristics (amplitude, frequency, or phase).
• This kind of modification is called modulation (shift keying)
• Modulation techniques :
Amplitude shift keying (ASK),
Frequency shift keying (FSK),
Phase shift keying (PSK).
Quadrature Amplitude Modulation (QAM)

.
• In amplitude shift keying, the amplitude of the carrier signal is varied to create
signal elements.
• Both frequency and phase remain constant while the amplitude changes.
Amplitude Shift Keying
Binary Amplitude Shift Keying
• The peak amplitude of one signal level is 0; the other is the same as the
amplitude of the carrier frequency

.
When the amplitude of the NRZ signal is 1, the amplitude of the carrier frequency is held;
when the amplitude of the NRZ signal is 0, the amplitude of the carrier frequency is zero
• The resulting transmitted signal is
• where the carrier signal is

.
• The ASK technique is used to transmit digital data over optical fiber.
• For LED transmitters, one signal element is represented by a light pulse while
the other signal element is represented by the absence of light.
• For Laser transmitters, low level represents one signal element, while a higher-
amplitude light wave represents another signal element.

.
• Given a bandwidth of 10,000 Hz (1000 to 11,000 Hz), draw the full-duplex
ASK diagram of the system.
• Find the carriers and the bandwidths in each direction. Assume there is no
gap between the bands in the two directions.

.
For full-duplex ASK, the bandwidth for each direction is
BW = 10000 / 2 = 5000 Hz
The carrier frequencies can be chosen at the middle of each band
fc (backward) = 1000 + 5000/2 = 3500 Hz
fc (forward) = 11000 – 5000/2 = 8500 Hz

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Frequency Shift Keying
• In frequency shift keying, the frequency of the carrier signal is varied to represent
data.
• The frequency of the modulated signal is constant for the duration of one signal
element, but changes for the next signal element if the data element changes.
• Both peak amplitude and phase remain constant for all signal elements.
• The most common form of FSK is binary FSK (BFSK), in which the two binary
values are represented by two different frequencies.

.
Frequency Shift Keying
• The resulting transmitted signal for one bit time is
where f1 and f2 are typically offset from the carrier frequency fc by equal but
opposite amounts.
• BFSK is less susceptible to error than ASK.
• Telephone lines.
• It is also commonly used for high-frequency (3 to 30 MHz) radio transmission.

.
Multiple FSK (MFSK)
element
signal
per
bits
of
number
L
elements
signal
different
of
number
M
frequency
difference
the
f
frequency
carrier
the
f
f
M
i
f
f
where
M
i
t
f
A
t
s
L
d
c
d
c
i
i
i




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


2
)
1
2
(
1
),
2
cos(
)
( 
More than two frequencies (M frequencies) are used

.
Multiple FSK (MFSK)
Period of signal element
period
bit
T
period
element
signal
T
LT
T b
s
b
s :
:
,

Minimum frequency separation
)
(
2
/
1
2
)
/(
1
2
/
1 rate
bit
Lf
T
f
LT
f
T d
b
d
b
d
s 




MFSK signal bandwidth:
d
d
d Mf
f
M
W 2
)
2
( 


.
Multiple Frequency Shift Keying
period
bit
T
period
element
signal
T
LT
T b
s
b
s :
:
,


.
Multiple FSK (MFSK)
For fc=250KHz, fd=25KHz, and M=8 (L=3 bits), find
1. The frequency assignment for each of the 8 possible 3-bit data
combinations.
2.Bandwidth
3.Datarate
d
d
d Mf
f
M
W 2
)
2
( 

)
(
2
/
1
2
)
/(
1
2
/
1 rate
bit
Lf
T
f
LT
f
T d
b
d
b
d
s 




period
bit
T
period
element
signal
T
LT
T b
s
b
s :
:
,

d
c
i f
M
i
f
f )
1
2
( 



Solution :

.
Multiple FSK (MFSK)
With fc=250KHz, fd=25KHz, and M=8 (L=3 bits), what are the
frequency assignment for each of the 8 possible 3-bit data
combinations:
KHz
Mf
W
bandwidth
KHz
f
KHz
f
KHz
f
KHz
f
KHz
f
KHz
f
KHz
f
KHz
f
d
s 400
2
425
111
375
110
325
101
275
100
225
011
175
010
125
001
75
000
8
7
6
5
4
3
2
1
































This scheme can support a data rate of:
Kbps
Hz
bits
Lf
T d
b 150
)
25
)(
3
(
2
2
/
1 



.
We need to send data 3 bits at a time at a bit rate of 3 Mbps. The carrier
frequency is 10 MHz. Calculate the number of levels (different frequencies),
the baud rate, and the bandwidth.
Also find different frequencies.
d
c
i f
M
i
f
f )
1
2
( 



d
d
d Mf
f
M
W 2
)
2
( 

)
(
2
/
1
2
)
/(
1
2
/
1 rate
bit
Lf
T
f
LT
f
T d
b
d
b
d
s 




period
bit
T
period
element
signal
T
LT
T b
s
b
s :
:
,


.
We need to send data 3 bits at a time at a bit rate of 3 Mbps. The carrier
frequency is 10 MHz. Calculate the number of levels (different frequencies),
the baud rate, and the bandwidth.
Also find different frequencies
We can have L=3,M = 23
= 8.
The baud rate is D=R/L
D= 3 Mbps/3 = 1 Mbaud.
The bandwidth is :
T=Bit period=1/3 micro sec
TS=LTb=3*(1/3) =1 micro sec
2fd=1/TS =1MHz
Bandwidth=2Mfd= 8MHz

.
d
c
i f
M
i
f
f )
1
2
( 




.
Phase Shift Keying
• The phase of the carrier signal is shifted to represent data.
• Two-Level PSK : Uses two phases to represent the two binary digits
and is known as binary phase shift keying.
• Phase of the carrier is varied to represent digital data (binary 0 or 1)
• Amplitude and frequency remains constant.
• If phase 0 deg to represent 0, 180 deg to represent 1. (2-PSK)

.
Phase Shift Keying
• The resulting transmitted signal is
• Because a phase shift of 180° is equivalent to flipping the wave or
multiplying it by -1.

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Differential PSK (DPSK)
• In DPSK, the phase shift is with reference to the previous bit transmitted
rather than to some constant reference signal
• Binary 0:signal burst with the same phase as the previous one
• Binary 1:signal burst of opposite phase to the preceding one

.
Phase Shift Keying
Four-Level PSK
• More efficient use of bandwidth can be achieved if each signaling element
represents more than one bit.
• Quadrature phase shift keying (QPSK), uses phase shifts separated by
multiples of
Each signal element represents two bits rather than one.

.
Quadrature Amplitude Modulation
• PSK is limited by the ability of the equipment to distinguish small differences in
phase.
• This factor limits its potential bit rate.
• By combining ASK and PSK it is possible to obtain higher data rate.
• Quadrature amplitude modulation is a combination of ASK and PSK.

.
Performance
The transmission bandwidth
• r depends on modulation and filtering process.
0<= r <=1
• R is bit rate
• M is number of different signal element.
• L is number of bits

Compare Bandwidths for ASK and MPSK
Given: =100kbps, =0.5, =8, Find: Which modulation (ASK or
𝑅 𝑟 𝑀
MPSK) uses less bandwidth?

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• The communication media usually have much higher bandwidth.
• On the other hand individual users have lesser data to send.
• The two communicating stations do not utilize the full capacity of a data
link.
• Multiplexing is the set of techniques that allow the simultaneous
transmission of multiple signals across a single data link.
• Allows several transmission sources to share a larger transmission
capacity.
Multiplexing

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Figure 8.1 Multiplexing
• Multiplexer : Combines (multiplexes) data from the n input lines and transmits over a
higher-capacity data link.
• Demultiplexer : Accepts the multiplexed data stream, separates (demultiplexes) the data
according to channel, and delivers data to the appropriate output lines.

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• Three types of multiplexing techniques.
• Frequency division multiplexing (FDM)
– The most heavily used
– Familiar to anyone who has ever used a radio or television set.
• Time division multiplexing (TDM)
– known as synchronous TDM.
– Commonly used for multiplexing digitized voice streams and data streams.
• Statistical TDM.
– To improve on the efficiency of synchronous TDM by adding complexity to the
multiplexer.
– Also known as asynchronous TDM, and intelligent TDM.

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FREQUENCY DIVISION MULTIPLEXING (FDM)
used with analog signals.
A number of signals are carried simultaneously on the same medium by
allocating to each signal a different frequency band.
Modulation equipment is needed to move each signal to the required
frequency band, and
Multiplexing equipment is needed to combine the modulated signals.
FDM is possible when the useful bandwidth of the transmission medium
exceeds the required bandwidth of signals to be transmitted.

.
(a) Frequency division multiplexing
• A number of signals can be carried simultaneously if each signal is modulated
onto a different carrier frequency and the carrier frequencies are sufficiently
separated that the bandwidths of the signals do not significantly overlap.

.
• The spectrum of signal mi(t) is shifted to be centered on fi
• fi must be chosen so that the bandwidths of the various signals do not significantly overlap.
Figure 8.4 FDM System

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Multiplexing Process
Each source generates a signal of a similar frequency range.
Inside the multiplexer, these similar signals modulate different carrier
frequencies ( f1, f2, and f3).
The resulting modulated signals are then combined into a single
composite signal that is sent out over a media link.

.
Demultiplexing Process
The demultiplexer uses a series of filters to decompose the multiplexed
signal into its constituent component signals.
The individual signals are then passed to a demodulator that separates
them from their carriers and passes them to the output lines.

.
Assume that a voice channel occupies a bandwidth of 4 kHz. We
need to combine three voice channels into a link with a bandwidth
of 12 kHz, from 20 to 32 kHz. Show the configuration, using the
frequency domain. Assume there are no guard bands.
Solution
modulate each of the three voice channels to a different bandwidth.
Use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz
bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the
third one. Then we combine them .

.
Five channels, each with a 100-kHz bandwidth, are to be multiplexed together.
What is the minimum bandwidth of the link if there is a need for a guard band
of 10 kHz between the channels to prevent interference?
Solution
For five channels, we need at least four guard bands. This means that the required
bandwidth is at least
5 × 100 + 4 × 10 = 540 kHz,

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• Multiplexor accepts input from devices in a round-robin fashion and transmit the
data.
• Synchronous TDM is called synchronous because the time slots are pre
assigned to sources and fixed.
• The slot length equals the transmitter buffer length, typically a bit or a byte
(character).
• The time slots for each source are transmitted whether or not the source has
data to send
• it is possible for a synchronous TDM device to handle sources of different data
rates. For example, the slowest input device could be assigned one slot per
cycle, while faster devices are assigned multiple slots per cycle
SYNCHRONOUS TIME DIVISION MULTIPLEXING

.
• The multiplexer has six inputs that might each be, say, 9.6 kbps.
• A single line with a capacity of at least 57.6 kbps (plus overhead capacity) could
accommodate all six sources.

.

.
Figure 8.6 Synchronous TDM System
• The data are organized into frames.
• In each frame, one or more slots are dedicated to
each data source.
• The sequence of slots dedicated to one source,
from frame to frame, is called a channel.

.
• The time slots for each source are transmitted whether or not the source has data
to send.
• The capacity is wasted to achieve simplicity of implementation.

.
Even with fixed assignment it is possible for a synchronous TDM device to handle
sources of different data rates.
For example, the slowest input device could be assigned one slot per cycle, while
faster devices are assigned multiple slots per cycle.

.
STATISTICAL TIME DIVISION MULTIPLEXING
• A statistical multiplexor transmits only the data from active
workstations.
• If a workstation is not active, no space is wasted on the
multiplexed stream.
• A statistical multiplexor accepts the incoming data streams and
creates a frame containing only the data to be transmitted.

.
Figure 8.12 contrasts statistical and synchronous TDM.
Figure 8.12 Synchronous TDM Compared with Statistical TDM

.
• The statistical multiplexer does not send empty slots if there are data to send.
• However, the positional significance of the slots is lost.
• Address information is required to assure proper delivery.
• There is more overhead per slot for statistical TDM because each slot carries
an address as well as data.

.
Figure 8.13 shows two possible formats.
Figure 8.13 Statistical TDM Frame Formats
• only one source of data is included per frame. That source is identified by an address.
• The length of the data field is variable, and its end is marked by the end of the overall frame.
• This scheme work well under light load but is quite inefficient under heavy load.

.
• Path between transmitter and receiver in a data communication system.
Transmission Media
Guided Media: Waves are guided along the solid medium.
Unguided Media : Provides a means for transmitting electromagnetic signals through but
do not guide them. Also called wireless transmission.
data rate depends on bandwidth, signal to noise ratio

.
Quality of transmission
• Characteristics and quality of data transmission are determined by
media and signal characteristics.
• For Guided media, the medium is more important in determining the
limitations of the transmission.
ie bandwidth, signal to noise ratio, attenuation.
• For unguided media, the bandwidth of the signal produced by the
transmission antenna is more important than the medium.
ie antenna will play a key role.

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Twisted pair
• Twisted pair consists of two insulated copper wires arranged in a regular spiral
pattern.
• Number of pairs are bundled together into a cable by wrapping them in a tough
protective sheath.

.
Why twisting
• Signal passing in one pair of wire may induce signal on another pair of wire.
• Twisting decreases the cross talk interference between adjacent pairs in a
cable.
• Tighter twisting produce much better performance, but also increases the
cost.
Twisted Pair Categories
Category 3 : 3-4 twists per feet
Category 5 : 3-4 twists per inch

.
Twisted Pair common types
Unshielded Twisted Pair (UTP)
• Ordinary telephone lines
• Subject to electromagnetic interfernce. (spark, lightening)
Shielded Twisted Pair (STP)
• Has a metal foil or braided mesh covering that
encases each pair of insulated conductor.
• Expensive compare to UTP
• Not popular in our day to day applications like
telephone, LAN etc

.
Coaxial cable
• Consists of outer cylindrical conductor that surrounds a single inner wire
conductor
• The inner conductor is held in place by either regularly spaced insulating
rings or a solid dielectric materials.
• The outer conductor is covered with a jacket.
• Due to its shielding, coaxial cables are much less susceptible to interference
or crosstalk than twisted pair.

.
Optical fiber
A fiber-optic cable is made of glass or plastic and transmits signals in the
form of light.
An optical fiber cable has a cylindrical shape and consists of three
concentric sections: the core, the cladding, and the jacket .

.
Optical Fiber
An optical fiber is made of three sections:
• The core carries the light signals
• The cladding keeps the light in the core
• The coating protects the glass
Optical fiber transmits a signal-encoded beam of light by means of total internal reflection
Two different types of light source are used:
light emitting diode (LED) and
injection laser diode (ILD).

.
Characteristics of Optical Fiber
• Greater capacity:
– Data rate is high compared to twisted pair and coaxial cable.
– Data rates of hundreds of Gbps over tens of kilometers.
• Smaller size and lighter weight:
– Thinner than coaxial cable or bundled twisted-pair cable
• Lower attenuation:
– Attenuation is lower for optical fiber than for coaxial cable or twisted pair.
• Electromagnetic isolation:
– Optical fiber systems are not affected by external electromagnetic fields.
• Greater repeater spacing:
– Fewer repeaters mean lower cost and fewer sources of error.

.
Wireless Transmission
Communication of data through the air without the use of physical conductors like
cables or wires.
Uses electromagnetic waves such as radio waves, microwaves, and infrared
waves to transmit signals over the air.
There are three major categories of wireless transmission:
Radio Waves : Used for long-distance communication like AM/FM radio, TV
broadcasts, and mobile phones.
Microwaves : Used in satellite communication and point-to-point links such as
microwave towers.
Infrared : Used in short-range communication like remote controls and wireless
keyboards.

.
Wireless Transmission
Advantages:
• Enables mobility and remote access.
• Easy and fast deployment.
• Useful in areas where wired infrastructure is impractical.
Disadvantages:
• More prone to interference and signal degradation.
• Security risks due to open transmission medium.

.
Antennas
Device used to transmit or receive electromagnetic waves.
Acts as a bridge between a wired communication system (like a
transmitter or receiver) and the air through which wireless signals travel.
Types of Antennas:
Omnidirectional Antennas : Radiate signals in all directions (e.g., Wi-Fi
routers).
Directional Antennas : Focus signals in a specific direction (e.g.,
satellite dishes).
Functions of an Antenna:
Converts electrical signals into electromagnetic waves during
transmission.
Converts received electromagnetic waves into electrical signals during
reception.

.
Wireless Propagation
• A signal radiated from an antenna travels along one of three routes:
– Ground wave,
– Sky wave,
– Line of sight (LOS).
.

Example
We need to use synchronous TDM and combine 20
digital sources, each of 100 Kbps. Each output slot
carries 2 bit from each digital source, but one extra bit
is added to each frame for synchronization. Answer
the following questions:
a. What is the size of an output frame in bits?
b. What is the output frame rate?
c. What is the duration of an output frame?
d. What is the output data rate?
e. What is the efficiency of the system (ratio of useful bits to
the total bits).

Example
• Ten sources, six with a bit rate of 200 kbps and
four with a bit rate of 400 kbps are to be
combined using TDM with no synchronizing
bits. Answer the following questions about the
final stage of the multiplexing:
a) What is the size of a frame in bits?
b) What is the frame rate?
c) What is the duration of a frame?
d) What is the data rate?
[Each output slot carries 1 bit from each digital source]

Example
• Show the contents of the five output frames for
a synchronous TDM multiplexer that combines
four sources sending the following characters.
Note that the characters are sent in the same
order that they are typed. The third source is
silent.
a) Source 1 message: HELLO
b) Source 2 message: HI
c) Source 3 message:
d) Source 4 message: BYE

Example
• A character-interleaved time division multiplexer is used
to combine the data streams of a number of 110-bps
asynchronous terminals for data transmission over a
2400-bps digital line. Each terminal sends
asynchronous characters consisting of 7 data bits, 1
parity bit, 1 start bit, and 2 stop bits. At least 3% of the
line capacity is reserved for pulse stuffing to
accommodate speed variations from the various
terminals.
a) Determine the number of bits per character.
b) Determine the number of terminals that can be
accommodated by the multiplexer.

a) n = 7 + 1 + 1 + 2 = 11 bits/character
b) Available capacity = 2400 × 0.97 = 2328
bps
If we use 20 terminals sending one
character at a time in TDM, the total
capacity used is:
20x110 bps=220bps<2328
21 × 110 bps = 2310 bps available capacity
22 x 110bps=2420>2328

Computer Netowrks Shift Keying Aplitudee

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