Wire Propagation Effects Propagation Effects –Signal changes as it travels –If change is too great, receiver may not be able to recognize it Distance Original Signal Final Signal

Wire Propagation Effects: Attenuation Attenuation: Signal Gets Weaker as it Propagates –May become too weak for receiver to recognize Signal Strength Distance

Wire Propagation Effects: Distortion Distortion: Signal changes shape as it propagates –Adjacent bits may overlap –May make recognition impossible for receiver Distance

Wire Propagation Effects: Noise Noise: Thermal Energy in Wire Adds to Signal –Noise floor is average noise energy –Random energy, so noise spikes sometimes occur Signal Strength Time Signal Noise Spike Noise Floor Error

Wire Propagation Effects Noise and Attenuation –As signal attenuates, gets closer to noise floor –So noise errors increase with distance, even if the average noise level is constant Signal Strength Distance Signal Noise Floor

Wire Propagation Effects: SNR Want a high Signal-to-Noise Ratio (SNR) –Signal strength divided by average noise strength –As SNR falls, errors increase Signal Strength Distance Signal Noise Floor SNR

Wire Propagation Effects: Noise & Speed Noise and Speed –As speed increases, each bit is briefer –Noise fluctuations do not average out as much –So noise errors increase as speed increases One Bit Noise Spike Average Noise During Bit Low Speed (Long Duration) One Bit Noise Spike Average Noise During Bit High Speed (Short Duration) OK Error

Wire Propagation Effects: Interference Interference –Energy from outside the wire (nearby motors, other wires, etc.) –Adds to signal, like noise –Often intermittent (comes and goes), so hard to diagnose Signal Strength Time Signal Interference

Wire Propagation Effects: Cross- Talk Interference Cross-Talk Interference –Often, multiple wires in a bundle –Each radiates some of its signal –Causes “cross-talk” interference in nearby wires

Wire Propagation Effects:Cross Talk Wire Usually is Twisted –Usually, several twists per inch –Interference adds to signal over half twist, subtracts over other half –Roughly cancels out –Simple but effective Single Twist Interference -+ Signal

Wire Propagation Effects:Cross Talk Terminal Cross-Talk Interference –Wire must be untwisted at ends to fit into connectors –So cross-talk interference is high at termination –Problems severe if untwist more than about 1.25 cm (1/2 inch) –Usually the biggest propagation effect Terminal Cross Talk

Practical Issues in Propagation Effects Distance limits in standards prevent serious propagation effects –For instance, usually 100 meters maximum for ordinary copper wire –If stay within limits, usually no serious problems Problems usually occur at connectors –Crossed wires –Poor connections –Cross-talk interference New