Mailinglist Archive: opensuse (1777 mails)

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Re: [opensuse] Moving to IPv6
  • From: James Knott <james.knott@xxxxxxxxxx>
  • Date: Tue, 14 Sep 2010 10:25:06 -0400
  • Message-id: <4C8F85C2.7030505@xxxxxxxxxx>
John Perry wrote:
Coax is much easier to build as a true constant-impedance line, but it has higher loss, especially at higher frequencies, which is the true limiting characteristic. If you can build a twisted pair to a very high standard of consistency, it will give greater ultimate range due to the lower signal loss.
Actually, it's not quite that simple. Loss depends on physical dimensions and the dielectric constant of the insulator. However, properly constructed coax generally has better shielding than Unshielded Twisted Pair (UTP).

That's also why the ancient Ethernet cables were coax, but limited essentially to 10Mbps and short distances, and we had to go to twisted pair (cat-5, cat-5E, cat-6) to get higher bandwidth and signal fidelity over longer distances. Manufacturers are building Ethernet cables much more carefully than they do short-range, low-frequency twisted pair, and for longer distances the transceivers are more complex to compensate for the poorer signal fidelity,.

It was cost that drove the switch to twisted pair. The cable, connectors etc. are much cheaper for twisted pair. Also, twisted pair, even at 10 Mb, didn't reach as far as even 10base2. 10base5 could reach even further. 10baseT (twisted pair) is rated for about 100M, 10base2 (RG58 coax) goes 200M and 10base5 (RG8) could reach about 500M. 10baseT had the further advantage that it could be used over the existing CAT 3 telephone cables that were commonly installed. Of course, fibre can go much further. I have used it to run Ethernet between two buildings that were a few kilometres apart.

Another factor that limited network distance in 10base5, 10base2 and hub connected 10baseT was the size of the collision domain. Originally, Ethernet used collision detection to arbitrate network access. In order to do this, there was a maximum permitted length, so that a collision could be detected within the first 64 bytes of the frame. At 10 Mb/s that time is 51.2 uS, which, after allowing for the speed of the signal in a cable, works out to about 5 KM (10 KM round trip), which is much greater than any permitted cable & repeater configuration. With switches and full duplex communications, there are no longer any collisions, so distances are no longer a concern. With copper you're still limited to 100M, but with fibre, microwave or other, you can now go essentially unlimited distances, limited only by what the hardware can do.

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