Causes of noise

I would like to know what noise in a network is. Also its causes and perhaps remedies.
Thank you.
budusonAsked:
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sirbountyConnect With a Mentor Commented:
I believe you're referring to crosstalk.  It's basically interference from an outside source.  
Remedies include using proper cabling and/or higher quality cabling...

[Crosstalk] is caused by signal crossover resulting from poorly twisted, internal wiring
[Ambient noise] is caused by interference from an external source, such as another electronic device
[Attenuation] is caused by the degradation of a signal over the length of the transmission or as a result of crosstalk or ambient noise
ref: http://www.cdwg.com/webcontent/editorialg/networking/011503_Decipherthecablingpuzzle.asp

Crosstalk: A disturbance, caused by electromagnetic interference, along a circuit or a cable pair. A telecommunication signal disrupts a signal in an adjacent circuit and can cause the signals to become confused and cross over each other. ref: http://www.webopedia.com/TERM/C/crosstalk.html

ACR: Also known as headroom, attenuation crosstalk ratio (ACR) is the difference between attenuation and crosstalk at a given frequency along a cable. Measured in decibels, ACR is a calculation used in networking transmission to assure that a signal transmitted across a twisted-pair cable is stronger at the receiving end than any interference signals imposed on that same pair by crosstalk from adjacent pairs. ref: http://www.webopedia.com/TERM/A/attenuation_crosstalk_ratio.html
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RWrigleyCommented:
Noise is anthing that disrupts the smooth flow of electrons (or light, if you're using Fibre) down the wire.

Most likely source is badly terminated CAT 5 wire.  Check where the cable enteres the RJ45 connector and make sure that the wires go all the way to the top of connector, and that there is a portion of the sheath underneat the crimp (so that you can't wiggle the cable back and forth in the connector).  Also, make sure that the cable was terminated properly...the second pair of wires should be split around the first pair.

Second most likely source is that someone ran the cable over a flourescnet light, or underneath a monitor, or beside an electric heater, or something like that.  Most inter-office networking is done with Unshielded Twisted Pair wire.  Being unshielded, its somewhat vulnerable to strong EMI (electro-magnetic interference).  The devices I listed above are all big EMI generators.

Also, make sure you're using CAT 5 or Cat 5e wire.  Cat 3 wire or lower won't support 100mb data properly, particularily over longer distances.
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