Linux Ethernet-Howto: Cables, Coax, Twisted Pair

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## 5. Cables, Coax, Twisted Pair

If you are starting a network from scratch, you will probably be using Cat5 wire for 10/100baseT (twisted pair telco-style cables with RJ-45 eight wire `phone' connectors). If you stumble across some old surplus 10Base2 thin ethernet (RG58 co-ax cable with BNC connectors) it might be suitable for linking a few machines together in a home ethernet. The old-fashioned thick ethernet, RG5 or RG8 cable with N connectors is really obsolete and rarely seen anymore.

See Type of cable... for an introductory look at cables. Also note that the FAQ from comp.dcom.lans.ethernet has a lot of useful information on cables and such. FTP to rtfm.mit.edu and look in `/pub/usenet-by-hierarchy/` for the FAQ for that newsgroup.

## 5.1 Thin Ethernet (thinnet)

Thinnet (10Base-2) is pretty much obsolete now. It is fine for somebody playing around with a home network and old ISA cards. There are two main drawbacks to using thinnet. The first is that it is limited to 10Mb/sec - 100Mb/sec requires twisted pair. The second drawback is that if you have a big loop of machines connected together, and some bonehead breaks the loop by taking one cable off the side of his tee, the whole network goes down because it sees an infinite impedance (open circuit) instead of the required 50 ohm termination. Note that you can remove the tee piece from the card itself without killing the whole subnet, as long as you don't remove the cables from the tee itself. And if you are doing a small network of two machines, you still need the tees and the 50 ohm terminators -- you can't just cable them together! It is also vital that your cable have no `stubs' -- the `T' connectors must be attached directly to the ethercards.

## 5.2 Twisted Pair

Twisted pair networks require active hubs, which start around $50. You can pretty much ignore claims that you can use your existing telephone wiring as it is a rare installation where that turns out to be the case.

On the other hand, all 100Mb/sec ethernet proposals use twisted pair, and most new business installations use twisted pair. The wiring should be listed as Category 5. Anything less than Cat 5 is useless.

If you are only connecting two machines, it is possible to avoid using a hub by purchasing or making a special cross-over or null cable. But note that some cards that try to sense autonegotiation and so on expect to be talking to a hub and not another card, and thus may not work in this configuration.

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