1. Computer Communication & Networks
Lecture # 09
Computer Communication & Networks

2. Transmission Impairments
Signals travel through the transmission media, which are not perfect
The imperfections cause impairment in signal
This means that the signal at the beginning and end of the medium are not same
There are three types of impairment usually occur
Attenuation
Distortion
Noise

3. Transmission Impairments
For analog signals, these impairments introduce various random modifications that degrade the signal quality
For digital signals, bit errors are introduced

4. Transmission Impairents
Attenuation
Attenuation means loss of energy (weak signal)
It is the progressive reduction in amplitude of a signal as it travels farther from the point of origin
When a signal travels through a medium, it loses some of its energy so that it can overcome the resistance of the medium
That is why a wire carrying electrical signals gets worm, if not hot, after a while
Some of the electrical energy in signal is converted to heat
To compensate for this loss, the amplifiers are used to amplify the signal

5. Transmission Impairents
Attenuation

6. Transmission Impairents
Attenuation

7. Transmission Impairents
Attenuation
To show the loss or gain of energy the unit “decibel” is used.
10log10 powerin/powerout
Pin = 100mW
Pout = 10mW
attenuation = 10log10 (100/10) = 10 dB

8. Transmission Impairents
Attenuation
Suppose a signal travels through a transmission medium and its power is reduced to one-half. This means that Pout is (1/2)Pin
In this case, the attenuation (loss of power) can be calculated as;
A loss of 3 dB (–3 dB) is equivalent to losing one-half the power

9. Transmission Impairents
Attenuation
A signal travels through an amplifier, and its power is increased 10 times. This means that Pout = 10Pin . In this case, the amplification (gain of power) can be calculated as

10. Transmission Impairents
Distortion
Distortion means that signal changes its form or shape
Distortion occurs in a composite signal that is made of different frequencies
Each signal component has its own propagation speed through a medium and, therefore its own delay in arriving at the final destination
That means that the signals have different phases at the receiver than they did at the source

11. Transmission Impairents
Distortion
The distortion is caused by the fact that the velocity of propagation of a signal through a medium varies with frequency
Thus, various frequency components of a signal will arrive at the receiver at different times
This effect is referred to as delay distortion, as the received signal is distorted due to variable delay in its components

12. Transmission Impairents
Noise is another problem occurred during the transmission of data
Noise is any signal that is not useful
Original Signal
Noise
Output Signal

13. Transmission Impairents
There are different types of noise
Thermal: The random motion of electrons in a wire which creates an extra signal not originally sent by the transmitter
Induced: Noise that comes from motors and appliances, devices act are transmitter antenna and medium as receiving antenna
Crosstalk: It is the effect of one wire on other, it is same induced but between two wires
Impulse: Irregular disturbances, such as lightning or power line spikes etc. It is a primary source of error in digital data

14. Signal to Noise Ratio
In analog and digital data communications, signal-to-noise ratio, often written S/N or SNR, is a measure of signal strength relative to background noise
The ratio is usually measured in decibels (dB)
If the strength of incoming signal is P and the noise in the channel is N then the signal-to-noise ratio, S/N, is given by the formula;
S/N = Power/Noise
and in decibels;
S/N = 10 log10(Power/Noise)

15. Signal to Noise Ratio
If Power = Noise, then S/N = 0;
In this situation, the signal borders on unreadable, because the noise level severely competes with it
In digital communications, this will probably cause a reduction in data speed because of frequent errors that require the source computer or terminal to resend some packets of data
If Power is less than Noise, then S/N is negative;
In this type of situation, reliable communication is generally not possible unless steps are taken to increase the signal level and/or decrease the noise level at the destination computer or terminal
Ideally, Power is greater than Noise, so S/N is positive

16. Transmission Imparients
The value of SNR for a noiseless channel is;
We can never achieve this ratio in real life; it is an ideal

17. Transmission Imparients
As an example, suppose that Power = 10 W and Noise = 1 W, then
S/N = 10log10(10/1) = 10dB
which results in the signal being clearly readable
If the signal is much weaker but still above the noise, say 1.30 W, then
S/N = 10log10(1.30) = 1.14 dB
which is a marginal situation, there might be some reduction in data speed under these conditions

18. Shannon’s Bandwidth
If we had such a thing as an infinite-bandwidth, noise-free channel we could transmit unlimited amounts of error-free data over it per unit of time
However real life signals have both bandwidth and noise-interference limitations
In electronic communication channels, Shannon capacity is the maximum amount of information that can pass through a channel without error,
i.e., it is a measure of its "goodness"

19. Shannon’s Bandwidth
Shannon's Law says that the highest obtainable error-free data speed, expressed in bits per second (bps), is a function of the bandwidth and the signal-to-noise ratio
Let c be the maximum obtainable error-free data speed in bps that a communications channel can handle
Let b be the channel bandwidth in hertz
Let s represent the signal-to-noise ratio
Then Shannon's law is stated as follows:
c = b log2 (1 + s)
The function log2 represents the base-2 logarithm. All logarithms are exponents. The base-2 logarithm of a number x is the number y such that 2y = x.

20. Shannon’s Bandwidth
The telephone channel has a bandwidth of 3 kHz and a signal-to-noise ratio exceeding 30 dB
The maximum capacity error free data rate a modem can produce for this channel
C = 3×103log2(1+103)
= kbps
Thus, the so-called 56 kbps modems have this capacity limit