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1. Communication System (Lathi 2.1)
1
Basic elements:
 Input transducer
 Transmitter
 Channel
 Receiver
Output transducer
Input messages:
Voice
Television picture
Email
Message
Data
Image

2. 2
Communication System
2
Input Transducer:
 Converts the message to electrical signal (baseband
signal)
 Microphone, keyboard, camera etc.
 Bandwidth of the base band signal – depends on the type
of input message
System design depends on the type of input message
Bandwidth of an information signal is the difference
between the highest and the lowest frequency contained in
that signal

3. 33
Communication System
3
Transmitter:
Process the baseband signal to a suitable form for
transmission over a channel
Consists of several sub-systems: A/D converter, modulator,
encoder etc.
Consists of oscillators, amplifiers, tuned circuits and filters,
modulators, and other circuits
 Bandwidth of the transmitted signal – depends on the
process in the transmitter

4. Communication System
Channel:
 Transmission medium that conveys the transmitted
electrical/electromagnetic signal to receiver
 Channel types: wired or wireless
 Wired: twisted copper wire (telephone, DSL), coaxial
cable (television, internet), optical fiber (backbone)
 Wireless: Microwave (Satellite and cellular), RF wave
(Cellular, WiFi, WiMax, LTE)
4

5. 5
Communication System
Channel:
 Capacity: How much information can be sent in 1 s
through a channel?
 Capacity depends on the bandwidth of the channel
 Bandwidth: Copper wire: 1 MHz, Coaxial cable: 100
MHz, Microwave/RF: GHz, Optical fiber: THz
 Attenuation, distortion, and noise are the main
impairments
 Bandwidth of a communication channel is a difference
between the highest and the lowest frequency that the
channel will allow to pass through it
 Bandwidth of a communication channel must be equal or
greater than the bandwidth of the information.
5

6. 6
Communication System
Receiver:
 Process the received signal such that the input signal
can be recovered
 Consists of several reversed sub-systems of transmitter:
D/A converter, demodulator, decoder etc.
 Consists of oscillators, amplifiers, tuned circuits and
filters, demodulators, and other circuits
6

7. 77
Communication System
Output transducer:
 Convert the demodulated signal into output message
(Voice, video, image, data, email etc.)
 Headphone, television, computer etc. are the output
transducer
7

8. Information Source
Alphabet set: A

9. 9
Mathematical Models of Sources
A source is
 stationary if {Xi} is stationary
 memoryless if Xi and Xj are independent for i ≠ j
 i.i.d. if {Xi} are i.i.d. (independent and identically
distributed)
 continuous if A is a continuous set (e.g. the real
numbers)
 discrete if A is a discrete set (e.g. the integers {0, 1,
2, . . . ,9})
 binary if A = {0, 1}

10. 10
Mathematical Models of Sources
Example: Discrete Memoryless Source (DMS)
An information source:
Alphabet A = {0, 1}
p(Xi = 1) = p
p(Xi = 0) = 1- p
 This is an example of a DMS
 Special case: when p = 0.5, the source is
called a Binary Symmetric Source (BSS)

11. 11
Measure of Information
• How much information does a message carry
from the sender to the receiver?
• Examples
–Ex.1: Imagine a person sitting in a room.
Looking out the window, she can clearly see that
the sun is shining. If at this moment she receives
a call from a neighbor saying “It is now daytime”.
Does this message contain any information?
– Ex. 2: A person has bought a lottery ticket. A
friend calls to tell her that she has won first prize.
Does this message contain any information?

12. 12
Measure of Information
– Ex.1. It does not, the message contains no information. Why?
Because she is already certain that is daytime.
– Ex. 2. It does. The message contains a lot of information,
because the probability of winning first prize is very small.
• Conclusion
The information content of a message is inversely related to
the probability of the occurrence of that event
If a message is very probable, it does not contain any
information. If it is very improbable, it contains a lot of
information.
We need quantitative measure of information

13. 13
Measure of Information
 DMS:
 Alphabet set A = {a1, a2, . . . , aN }
 Probability mass function pi = p(X = ai) for all i = 1, 2, . . . , N
Units:
Self-information:
The information revealed about each source output ai is
defined as the self-information of that output
Self-information: I(pi) = − log(pi )

14. 14
Measure of Information: Entropy
Information content (Entropy) of a source:
The weighted average of the self-information of all source outputs
Note:
 0 log 0 = 0
 H(X) is a function of the PMF of the random variable X and is, therefore, a
number
 Unit of H(X): Bits/sample

15. 15
Entropy H(X)
The binary
entropy function No information
Maximum
information

16. 16
What does H(X) Signify?
 The minimum average number of bits required to represent
each sample without distortion
 The average information obtained by observing an outcome
 The average uncertainty about X before it is observed

17. 17
What does H(X) Signifies?

18. Challenges in Communication (Lathi 1.2)
 Channel impairments: attenuation, distortion, noise
 Receiver background noise
 The magnitude of the channel impairments depends on the
type of channel
 Attenuation:
 Signal attenuation or degradation exists in all media
 Increases with distance
 Wireless medium has the highest attenuation -exponential
decay
 Optical fibers have less attenuation, eg, 0.2 dB/km
18

19. Distortion:
 Signals distort during travel through medium (why?)
 Wire: frequency dependent attenuation => lowest distortion
 Optical fiber: Delay differences in different modes, frequency dependent
attenuation, highest dispersion
 Wireless: Delay differences due to multi-path propagation, time dependent
randomness of particles, frequency dependent attenuation => highest
distortion
 Inter-symbol interference due to distortion
19
Challenges in Communication

20. Challenges in Communication
Noise:
 Channel noise/ External noise
 Random, undesirable electronic energy that enters the
communication system via the communicating medium
and interferes with the transmitted message
 Interference from nearby channels, human made noise
(automobile ignition radiation, microwave oven), natural
noise (lightning)
 External noise can be minimized with proper design
 Receiver background noise/Internal noise
 Thermal noise and random emission in electronic
devices
 One of the main problems in communication 20

21. 21
Electromagnetic Spectrum
Electromagnetic spectrum is the range of all possible
frequencies of electromagnetic radiation

22. 22
Radio Spectrum (3 kHz – 3 THz)
*International Telecommunication Union (ITU): An UN organization which allocates
global radio spectrum and satellite orbits
For more details on the application areas:
http://en.wikipedia.org/wiki/Radio_spectrum

23. 23
Signal Bandwidth
 Bandwidth: Measure of the frequency contents of
an information signal
 Bandwidth of the base band signal – depends on the type of input message
Speech
Music

24. 24
Bandwidth (for baseband signal)
1. Absolute Bandwidth: is f2 – f1, where the
spectrum is zero outside the interval f1< f <f2
along the positive frequency axis.
|X(f)|
2B
0
Absolute
Bandwidt
h = B
2. 3-dB BW (Half-power BW): is f2 – f1 , where
for frequencies inside the band f1< f < f2 , the
magnitude spectra fall no lower than 1/2 times
the maximum value, and the maximum value
occurs at a frequency inside the band.
|X(f)|
2B3dB
0
3dB
Bandwidth
B3dB
-3dB
3. Equivalent Noise Bandwidth (Beq) is the
width of a fictitious rectangular spectrum
such that the power in that rectangular
band is equal to the power associated with
the actual spectrum over positive
frequencies.
|H(f)|2
2Be
q
0
|H(0)|2

25. 25
Bandwidth (for baseband signal)
4. Null-to-null BW (zero-crossing BW) Bn: is f2 – f1, where f2 is the first null
frequency in the envelope of the magnitude spectrum above f0 and f1 is the
first null in the envelope below f0. Here f0 is the frequency where the
magnitude spectrum is a maximum. For baseband systems, f1 is zero.
|X(f)|
2Bn
0
5. Bounded Spectrum Bandwidth: is f2 – f1 where outside the band f1 < f < f2,
the PSD, which is proportional to |H(f)|2, must be down by at least a certain
amount, say 50 dB, below the maximum value of the power spectral density.
6. X% Power Bandwidth: is f2 – f1, where f1 < f < f2 defines the frequency band
in which x% of the total power resides. (100-x)% of the total power is outside
the bandwidth.

26. 26
Typical Channel Bandwidth
 Bandwidth of a communication channel is the difference between the
highest and the lowest frequency that the channel will allow to pass through it
 Bandwidth of a communication channel must be equal or greater than the
bandwidth of the information
Copper wire: 1 MHz Coaxial cable: 100 ~ 500 MHz Microwave/RF: GHz
Optical fiber: THz

27. 27
Twisted Pair Copper Cable
 Least expensive and widely used
 Two insulated copper wires arranged in regular spiral pattern
 Number of pairs are bundled together in a cable
 Limited in terms of data rate and distance
 Requires amplifiers every 5-6 km for analog signals and repeaters every 2-3 km for
digital signals
 Attenuation is a strong function of frequency: Higher frequency implies higher
attenuation
 Susceptible to interference and noise
 Twisting reduces the tendency to radiate radio frequency noise as the radiations from
twisted wires cancel each other
 Twisting also decreases the crosstalk/EMI between adjacent pairs in the cable
Unshielded twisted pair (UTP):
• Speed between 10-100 Mbps
• Susceptible to crosstalk
Shielded twisted pair (STP):
• Supports higher data rate than UTP
• Less susceptible to crosstalk

28. 28
Coaxial Cable
 Most common due to inexpensive, light, flexible and easy to work with
 Contain two conductors sharing common axis: a central conductor wire and
a surrounding outer conductor/shield serving as ground
 Less susceptible to interference and noise as the outer conductor blocks
EMI
 Support higher data rates and longer distance than twisted pair cable
 Requires amplifiers and repeaters for every few kms for analog and digital
transmissions respectively
 Superior frequency characteristics compared to twisted pair

29. 29
Optical Fiber
 Dielectric waveguide that uses pulse of light instead of electrical signals
 Thin and flexible material to guide optical rays
 Cylindrical cross-section with three concentric links: Core, Cladding and Jacket
 Advantages:
 Much higher bandwidth (theoretically 2 x 1013 Hz): can carry hundreds of Gbps
over tens of kms
 Smaller size and light weight
 Significantly lower attenuation (as low as 0.2dB/km): Greater repeater spacing
 Not affected by external EM fields, i.e., not vulnerable to interference, impulse
noise, or crosstalk
 Ruggedness and flexibility

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Wireless Media
 No use of wire: use Earth’s atmosphere to act as transmission media
 Transmission and reception are achieved using antenna
 Transmitter sends out the EM signal into the medium
 Receiver picks up the signal from the surrounding medium
 Supports mobility and flexibility
 Convenient in use
 Lower capital and operating expenditure compared to wired networks
 Connection quality vary randomly with time due to fading
 Susceptible to multi-user interference
 Less secured
 Network management more complex
High attenuation

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Channel Characteristics
Propagation constant
 Attenuation constant, α : Determines the attenuation of a
signal of frequency ω over a unit distance
 Phase constant, β: Determines the phase change (delay) in
a signal of frequency ω over a unit distance

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Channel Characteristics
For distortionless transmission:
Transmission is said to be distortion less if the input and output have
identical wave shapes: (i) amplitudes of all the frequency components are
multiplied by the same factor, and (ii) all the frequency components are
delayed by the same amount.
Thus, in distortion-less transmission, the input x(t) and output y(t) satisfy the
condition:
y(t) = Kx(t - ) => Y(ω) = KX(ω)e-jω = KX(ω)ej(ω)
where  is the delay time and K is a constant.
K
|H(ω)| = α
ω
Amplitude response
- ω
(ω) = - ω
= -β
ω
Phase response
 Equalizer can be used for minimize attenuation and phase distortions
Phase delay:
p= - (ω)/ω
= β/ω
Group delay:
g = - d(ω)/dω
= dβ/dω

33. Capacity of a Transmission Link (Lathi 1.3)
 Shannon's limit, C = B log2 (1 + SNR) bits/second
C = capacity, B = channel bandwidth
SNR = signal-to-noise ratio=Received Signal Power/
Noise Power
 Capacity increases linearly with bandwidth, but only
logarithmically with signal strength
 Shannon's limit tells us what we can achieve it tells us
nothing about how to do it
 Two primary resources in communications
 Transmitted power (should be green)
 Channel bandwidth (very expensive in the
commercial market) 33

34. Calculation of Power and SNR
SNR (dB)= 10 log10(Pr/N)
Pr = received signal power in watt
N=noise power in watt
Unit of power: watt or dBm
dBm is used for low power
Power in dBm = 10 log10 (Power in watt * 1e3)
Power in watt= 10^(Power in dBm/10)*1e-3
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35. 3535
Layering Architecture (Haykin ch 1)
Concept of layering is used in our daily life
Communication through air mail

36. 3636
Why Layering?
 Layer architecture simplifies the
communication network design
 It is easy to debug network applications in a
layered architecture network
 The communication system management is
easier
 Research/work on a layer can be done
independently

37. 3737
OSI Model
International standard organization (ISO)
established a committee in 1977 to develop an
architecture for computer communication
Open Systems Interconnection (OSI) model is the
result of this effort
 In 1984, OSI model is approved as reference
model

38. 3838
OSI Model

39. 3939
Physical layer
 Provides physical interface for transmission of
information through a medium (wired/wireless)
Covers all - mechanical, electrical, functional and
procedural - aspects for physical communication

40. 4040
Data Link Layer
 Who will transmit, when to transmit,
whom to transmit to
Attempts to provide reliable
communication over the physical layer
interface in one hop distance

41. 4141
Network Layer
The network layer is responsible for the delivery of
individual packets from the source host to the
destination host
 Determine the route for the packets

42. 4242
Upper Layers
Transport Layer
 Controls congestion
 Provides transmission reliability between source and destination
Session Layer
 The session layer is responsible for dialog
control and synchronization
Presentation Layer
 Translate, encrypt and compress data
Application Layer
 Allows access to network resource
 Make applications into data format

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43
Interaction among the layers