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## Ethernets

The type of hardware most widely used throughout LANs is what is commonly known as Ethernet. It consists of a single cable with hosts being attached to it through connectors, taps or transceivers. Simple Ethernets are quite inexpensive to install, which, together with a net transfer rate of 10 Megabits per second accounts for much of its popularity.

Ethernets come in three flavors, called thick and thin, respectively, and twisted pair. Thin and thick Ethernet each use a coaxial cable, differing in width and the way you may attach a host to this cable. Thin Ethernet uses a T-shaped ``BNC'' connector, which you insert into the cable, and twist onto a plug on the back of your computer. Thick Ethernet requires that you drill a small hole into the cable, and attach a transceiver using a ``vampire tap''. One or more hosts can then be connected to the transceiver. Thin and thick Ethernet cable may run for a maximum of 200 and 500-meters, respectively, and are therefore also called 10base-2 and 10base-5. Twisted pair uses a cable made of two copper wires which is also found in ordinary telephone installations, but usually requires additional hardware. It is also known as 10base-T.

Although adding a host to a thick Ethernet is a little hairy, it does not bring down the network. To add a host to a thinnet installation, you have to disrupt network service for at least a few minutes because you have to cut the cable to insert the connector.

Most people prefer thin Ethernet, because it is very cheap: PC cards come for as little as US$50, and cable is in the range of a few cent per meter. However, for large-scale installations, thick Ethernet is more appropriate. For example, the Ethernet at GMU's Mathematics Department uses thick Ethernet, so traffic will not be disrupted each time a host is added to the network.

One of the drawbacks of Ethernet technology is its limited cable length, which precludes any use of it other than for LANs. However, several Ethernet segments may be linked to each other using repeaters, bridges or routers. Repeaters simply copy the signals between two or more segments, so that all segments together will act as if it was one Ethernet. timing requirements, there may not be more than four repeaters any two hosts on the network. Bridges and Routers are more sophisticated. They analyze incoming data and forward it only when the recipient host is not on the local Ethernet.

Ethernet works like a bus system, where a host may send packets (or frames) of up to 1500 bytes to another host on the same Ethernet. A host is addressed by a six-byte address hard-coded into the firmware of its Ethernet board. These addresses are usually written as a sequence of two-digit hex numbers separated by colons, as in aa:bb:cc:dd:ee:ff.

A frame sent by one station is seen by all attached stations, but only the destination host actually picks it up and processes it. If two stations try to send at the same time, a collision occurs, which is resolved by the two stations aborting the send, and re-attempting it a few moments later.

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