Computer Communication & Networks Lecture # 09 Computer Communication & Networks

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

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

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

Transmission Impairents Attenuation

Transmission Impairents Attenuation

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

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

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

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

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

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

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

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)

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

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

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

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"

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.

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) = 29.901 kbps Thus, the so-called 56 kbps modems have this capacity limit