Wire Propagation Effects Propagation Effects Signal changes as

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

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 Spike Signal Strength Error Noise Floor Time

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 Signal Noise Floor Distance

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 Signal SNR Noise Floor Distance

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 OK Noise Spike Low Speed (Long Duration) One Bit Average Noise During Bit Error Noise Spike One Bit High Speed Average Noise (Short During Bit Duration)

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 Interference Time

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 New – Poor connections – Cross-talk interference