David <
[email protected]> wrote:
Back in ancient history network cables had to be twisted to stop interference.
I think.
Now Ethernet cables are sometimes flat.
Is this a new thing?
Is it because both ends can cope with more interference?
Just idly puzzling.
Pairs are run close together to minimise the area of the inductive loop the signal current flows around (transmitter+ => wire+ => receiver => wire- => transmitter-)
In an external magnetic field, the induced curent is proportional to the
loop area. Make the loop as small as possible => small induced current => minising interference from induction
Twisted pairs are a way to achieve a very small loop area - the tighter the twist the smaller the loop. They also take out imbalances, eg one wire
being closer to the edge of the cable than the other.
Some flat cables have the pairs twisted, so all is fine.
On those where the pairs aren't twisted, they're run in parallel. The loop area is still kept small, but there's a risk of unbalance, eg capacitive crosstalk from wires in adjacent pairs. To remedy this they are kept short (capacitance is proportional to area of the plates - length of adjacent
wires in this case). They are just patch cables, and not used for wiring up
an office block.
Theo
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