Maximum Network Cable Lengths

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ashleysmithd
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Maximum Network Cable Lengths

Post by ashleysmithd » Sun Jun 30, 2013 6:41 am

Hi,

I had a question regarding the maximum cable lengths you can use to network two nodes together. Coming from a broadcast background and dealing with coax, I understand high frequency attenuation due to the skinning effect whereby higher frequencies propagate towards the outer edges of the conductor thus reducing the cross sectional area in which the signal can travel down hence the resistance increases for higher frequencies.

But is it the skinning effect that's to blame for the degradation of link health in common IT networks? I got told once that it wasn't so much frequency roll off but timing. I can understand timing being an issue in layer 1 switching (ie. hubs) but I don't understand why in layer 2 (ie. switches) nodes do not accept packets of information simply because of lateness, what kind of synchronicity do networks require, and are there specifications for network timings?

If anyone can shed any light on this I'd highly appreciate it.
Many thanks.
Student Broadcast Engineer (Just started, so please go easy :P)

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techboywi
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Re: Maximum Network Cable Lengths

Post by techboywi » Sun Jun 30, 2013 1:04 pm

I believe the maximum recommended cable length for ethernet is 300 ft. I'm not sure of the technical reason behind it.. but, I do know there is a limit
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Tim Burke
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Re: Maximum Network Cable Lengths

Post by Tim Burke » Sun Jun 30, 2013 4:34 pm

Correct, 100m for Cat5 and Cat6 per the TIA/EIA 568-5-A standards, however I wouldn't do a run longer than 90m or so, accounting for patch panels, etc. If I'm not mistaken, it is indeed due to the time it takes for the packets to travel through the cable. Using a layer2/3 switch will of course allow you to extend the run.

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Re: Maximum Network Cable Lengths

Post by Baylink » Sun Jun 30, 2013 7:34 pm

The length limit was originally due to the length of the packet vs the time it took to move around the cable; collision avoidance dictated a maximum cable span -- if the cable was too long, a packet would completely leave one transceiver before the competing one started, and both packets would still collide on the wire.

In twisted-pair, this limit continued, as the span of the network *in milliseconds* still had a maximum length, since hubs were basically multiport repeaters.

In the modern switched-network environment, the only real remaining limits are due to the electronics in the line interfaces, since any given logical span only has one transceiver sending in each direction; maximum amount of interference pickup over a run, and similar things, are now the controlling factor; if you use fiber, clearly, it can be [much] longer than 100m.

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