Transmission Impairments: Bad Things that Can Happen to Signals

1. Transmission Impairments:Bad Things that Can Happen to Signals Based on Chapter 3 of William Stallings, Data and Computer Communication, 8th Ed. Kevin BoldingElectrical EngineeringSeattle Pacific University

2. Why Your Signal Doesn’t Come Through • Bad things can (will) happen to your signal • Analog signals will be degraded • Results in loss of signal quality • Digital signals will have errors introduced • Bits flipped • Main types of distortion: • Attenuation – Reduction of signal power • Delay distortion – “Spreading out” of signal in time • Random noise • Non-random noise – spikes and correlated noise

3. Signal Scaling vs. Signal Distortion vs. Noise • Attenuation scales down the signal • Linear attenuation makes an exact, lower-power copy • In the absence of other noise, can be restored by a noise-free amplifier (Good luck!) • Distortion changes the shape of the signal • Changes affect the signal non-linearly • Difficult to un-do unless the engineer has a strong understanding of the exact distortion process • Noise/Interference – Added to your signal • Signal remains the same, but has extra “stuff” added to it • Difficult to remove for analog signals

4. Signal strength reduced according to the distance traveled For guided media, generally logarithmic Signal strength proportional to 1/2d For example, signal strength halves every 100m Attenuation guided media = cables, wires, fibers, etc. • For unguided media, proportional to the square of the distance • Signal strength proportional to 1/d2 • At a given distance, signal spread out over a surface with radius d. Area is proportional to d2 unguided media = air, water, earth, space, etc.

5. Attenuation occurs at different rates for different frequencies Signal is distorted, with some frequencies arriving stronger than others Attenuation Distortion

6. Delay Distortion • Different frequencies travel at different speeds through guided media • Higher- or lower-frequency components arrive at the destination delayed from the middle-frequency components Digital signals: Results in“pulse spreading” (High freq. components arrive late) Serious issue

7. Noise • Undesired signals that creep into the transmitted signal • Thermal Noise – Noise generated by electrons agitated by heat • N0 = kT (units are Joules) • k = Boltzman’s constant = 1.3803 x 10-23 J/K • T = temperature in K • Thermal noise increases with: • Temperature • Bandwidth • N = kTB (Watts) for a particular bandwidth B • Practical systems always filter the signal to a finite B

8. Non-uniform noise • Intermodulation noise • When mixing components at frequencies f1 and f2, some noise appears at f1+f2 • Crosstalk / Interference • Electro-magnetic coupling of signals in media near each other (wires or wireless) • Impulse Noise • Spikes of noise (generally unpredictable) • Caused by external events (lightning, switching of circuits, etc.)

9. Effects of Noise • Analog systems • Other signals mixed with the intended signal • Crosstalk on telephone lines • Crackling/White specks from impulse noise • Digital systems • Errors due to bit flips • Impulse noise is especially troublesome