Tài liệu Error Sources, Detection and Correction

Error Control In Networks:


Network errors are in the form of corrupted data or lost data.
Network errors occur naturally on all networks due to electrical noise and distortion and must be detected and corrected by either hardware or software.
Bit Error Rates (BERs) are calculated as the number of bits in error divided by the number of bits transmitted. A BER of 1 in 100,000 might be shown as 1:10[SUP]5[/SUP] or simply as a BER of 10[SUP]-5[/SUP].
Errors often occur in bursts where many bits in a sequence will be in error.

What are Network Errors and What Causes Errors?
Noise is any form of undesirable electrical signals introduced by equipment or natural disturbances (lightening).
Noise has the effect of causing bits to change state, extra bits or missing bits resulting in errors in a message.

Forms of Line Noise and Distortion:


White Noise
Caused by the thermal agitation of electrons in all matter. Always present (like the background hiss on radios) but not usually a problem unless the signal itself is very weak.


Impulse Noise
Noise spikes (large voltage fluctuations on the line) are a primary source of errors. Caused by telephone system equipment, lightening, fluorescent lights, poor electrical connections, etc. A 1/100 second voltage spike would damage at least 144 bits if transmission was at 14,400 bps.


Cross-talk
Signals picked up on one circuit from adjacent circuits. (We often hear this in voice telephone calls when we hear a faint conversation going on in the background.)