TRANSMISSION IMPAIRMENT PDF

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What is Data Transmission,Attenuation, Distortion,Noise - measurement of attenuation,Attenuation Distortion,delay in distortion ,types of noise, Signal to noise ratio,etc.
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TRANSMISSION IMPAIRMENT Data Transmission  The Communication of data between two nodes by the propagation and processing signals.  The two terms “analog” and “digital” are used frequently in this context as there are two types of data transmission. Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise. Topics discussed in this section:  Attenuation  Distortion  Noise Figure 3.25 Causes of impairment

Attenuation At Attenuation, signal strength falls off with distance. It happens exponentially with the travelled distance. • Attenuation affects the propagation of waves and signals in electrical circuits, in optical fibers, as well as in air. • If the signal

attenuates too much, it becomes unintelligible, which is why most networks require repeaters at regular intervals.  Means loss of energy -> weaker signal  When a signal travels through a Medium it loses energy overcoming the resistance of the medium  Amplifiers are used to compensate for this loss of energy by amplifying the signal.

Measurement of Attenuation  To show the loss or gain of energy the unit “decibel” is used. dB = 10log10P2/P1 P1 - input signal P2 - output signal

Attenuation is often an increasing function of frequency. This leads to attenuation distortion. • It is particularly noticeable for analog signals: the attenuation varies as a function of frequency, therefore the received signal is distorted. Example Suppose a signal travels through a transmission medium and its power is reduced to one-half. This means that P2 is (1/2)P1. 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. 2. A signal travels through an amplifier, and its power is increased 10 times. This means that P2 = 10P1 . In this case, the amplification (gain of power) can be calculated as

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 measuring several points (cascading) instead of just two. In Figure 3.27 a signal travels from point 1 to point 4. In this case, the decibel value can be calculated as

Example 3.29 Sometimes the decibel is used to measure signal power in milliwatts. In this case, it is referred to as dBm and is calculated as dBm = 10 log10 Pm , where Pm is the power in milliwatts. Calculate the power of a signal with dBm = −30. Solution We can calculate the power in the signal as

Example 3.30 The loss in a cable is usually defined in decibels per kilometer (dB/km). If the signal at the beginning of a cable with −0.3 dB/km has a power of 2 mW, what is the power of the signal at 5 km? Solution The loss in the cable in decibels is 5 × (−0.3) = −1.5 dB. We can calculate the power as

Attenuation Distortion: If the strength of the signal is very low, the signal cannot be detected and interpreted properly at the receiving end. The signal strength should be sufficiently high so that the signal can be correctly detected by a receiver in presence of noise in the channel. As shown in Fig. ,an amplifier can be used to compensate the attenuation of the transmission line. So, attenuation decides how far a signal can be sent without amplification through a particular medium. Attenuation of all frequency components is not same. Some frequencies are passed without attenuation, some are weakened and some are blocked. This dependence of attenuation of a channel on the frequency of a signal leads to a new kind of distortion attenuation distortion. A square wave is sent through a medium and the output is no longer a square wave because of more attenuation of the high-frequency components in the medium.

Fig(Attenuation distortion of a square wave after passing through a medium

The effect of attenuation distortion can be reduced with the help of a suitable equalizer circuit, which is placed between the channel and the receiver. The equalizer has opposite attenuation/amplification characteristics of the medium and compensates higher losses of some frequency components in the medium by higher amplification in the equalizer. Attenuation characteristics of three popular transmission media are shown in Fig.. As shown in the figure, the attenuation of a signal increases exponentially as frequency is increased from KHz range to MHz range. In case of coaxial cable attenuation increases linearly with frequency in the Mhz range. The optical fibre, on the other hand, has attenuation characteristic similar to a band-pass filter and a small frequency band in the THz range can be used for the transmission of signal

Distortion Distortion is known as the alternation of the original signal. This may happen due to the properties of the medium. When the distortion occurs, shape of waveform is changed. • Distortion only happens in mediums like cables, wires, fibers, etc. • There are many types of distortion such as amplitude distortion, harmonic distortion, and phase distortion. • Distortion is critical for digital data since bits change into other bit. • Can be solved by equalizing circuits.

 Means that the signal changes its form or shape  Distortion occurs in composite signals  Each frequency component has its own propagation speed traveling through a medium.  The different components therefore arrive with different delays at the receiver.  That means that the signals have different phases at the receiver than they did at the source.

Delay distortion The velocity of propagation of different frequency components of a signal are different in guided media. This leads to delay distortion in the signal. For a bandlimited signal, the velocity of propagation has been found to be maximum near the center frequency and lower on both sides of the edges of the frequency band. In case of analog signals, the received signal is distorted because of variable delay of different components. In case of digital signals, the problem is

much more severe. Some frequency components of one bit position spill over to other bit positions, because of delay distortion. This leads to intersymbol interference, which restricts the maximum bit rate of transmission through a particular transmission medium. The delay distortion can also be neutralised, like attenuation distortion, by using suitable equalizers.

Noise There are different types of noise  Thermal -Thermal noise (also known as white noise) • Due to thermal agitation of electrons • It is present in all electronic devices and transmission media, and is a function of temperature. • Cannot be eliminated, and therefore places an upper bound on communications system performance. • Thermal noise increases with Temperature and Bandwidth.  Induced - from motors and appliances, devices act are transmitter antenna and medium as receiving antenna.  When signals at different frequencies share the same transmission medium, the result may be intermodulation noise. • Signals at a frequency that is the sum or difference of original frequencies or multiples of those frequencies will be produced. • the mixing of signals at f1 and f2 might produce energy at frequency f1 + f2. This derived signal could interfere with an intended signal at the frequency f1 + f2. • Comes from motors and other appliances.  Crosstalk - same as above but between two wires.  It is an unwanted coupling between signal paths. It can occur by electrical coupling between nearby twisted pairs. Typically, crosstalk is of the same order of magnitude as, or less than, thermal noise.  Happens in both wired and wireless mediums.  Two types of crosstalk 1. NEXT(Near-end Crosstalk): Interference in a wire at the transmitting end of a signal sent on a different wire. 2. FEXT(Far-end Crosstalk): Interference in a wire at the receiving end of a signal sent on a different wire  Impulse - Spikes that result from power lines, lighning, etc.

Impulse noise is non-continuous, consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude.  It is generated from a variety of cause, e.g., external electromagnetic disturbances such as lightning, power lines.  It is generally only a minor annoyance for analog data. But it is the primary source of error in digital data communication.  A small impulse can corrupt many bits.  A median filter is used to remove impulse noises.

Signal to Noise Ratio (SNR) A signal is the noise-free signal and noise is the white noise you hear when you can't tune a radio to a particular station.

A signal-to-noise ratio compares a level of signal power to a level of noise power. It is most often expressed as a measurement of decibels (dB). Higher numbers generally mean a better specification, since there is more useful information (the signal) than there is unwanted data (the noise).

SNR is also usually represented in decibels (dB) SNR=10log(P signal P noise)(dB) Or if working with the amplitudes of signal and noise

SNR=10log((A signal A noise)2)=20log(A signal A noise)(dB)

 To measure the quality of a system the SNR is often used. It indicates the strength of the signal wrt the noise power in the system.  It is the ratio between two powers.  It is usually given in dB and referred to as SNRdB.

Example The power of a signal is 10 mW and the power of the noise is 1 μW; what are the values of SNR and SNRdB ? Solution The values of SNR and SNRdB can be calculated as follows:

The values of SNR and SNRdB for a noiseless channel Are

We can never achieve this ratio in real life; it is an ideal. Figure Two cases of SNR: a high SNR and a low SNR...