Ethernet

6 Pages 1434 Words

Ethernet uses a protocol called CSMA/CD. This stands for Carrier Sense, Multiple Access, Collision Detect. The Multiple Access part means that every station is connected to a single copper wire, or a set of wires that are connected together to form a single data path. The Carrier Sense part says that before transmitting data, a station checks the wire to see if any other station is already sending something. If the LAN appears to be idle, then the station can begin to send data. An Ethernet station sends data at a rate of 10 megabits per second. That bit allows 100 nanoseconds per bit. Light and electricity travel about one foot in a nanosecond. Therefore, after the electric signal for the first bit has traveled about 100 feet down the wire, the station has begun to send the second bit. However, an Ethernet cable can run for hundreds of feet. If two stations are located, say, 250 feet apart on the same cable, and both begin transmitting at the same time, then they will be in the middle of the third bit before the signal from each reaches the other station. This explains the need for the Collision Detect part. Two stations can begin to send data at the same time, and their signals will collide. When such a collision occurs, the two stations stop transmitting, and try again later after a randomly chosen delay period. While an Ethernet can be built using one common signal wire, such an arrangement is not flexible enough to wire most buildings. Unlike an ordinary telephone circuit, Ethernet wire cannot be just spliced together, connecting one copper wire to another. Ethernet requires a repeater. A repeater is a simple station that is connected to two wires. Any data that it receives on one wire it repeats bit-for-bit on the other wire. When collisions occur, it repeats the collision as well. In common practice, repeaters are used to convert the Ethernet signal from one type of wire to another. In particular, when the connection to the d...