Ethernet

Ethernet is a LAN media type that functions at the data link layer. Ethernet uses the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) mechanism to send information in a shared environment. Ethernet was initially developed with the idea that many devices would be connected to the same physical piece of wiring. The acronym CSMA/CD describes the actual process of how Ethernet functions.

In a traditional, or hub-based, Ethernet environment, only one NIC can successfully send a frame at a time. All NICs, however, can simultaneously listen to information on the wire. Before an Ethernet NIC puts a frame on the wire, it will first sense the wire to ensure that no other frame is
currently on the wire. If the cable uses copper, the NIC can detect this by examining the voltage levels on the wire. If the cable is fiber, the NIC can also detect this by examining the light frequencies on the wire.

The NIC must go through this sensing process, since the Ethernet medium supports multiple access—another NIC might already have a frame on the wire. If the NIC doesn’t sense a frame on the wire, it will go ahead and transmit its own frame; otherwise, if there is a frame on the wire, the NIC will wait for the completion of the transmission of the frame on the wire and then transmit its own frame.

If two or more machines simultaneously sense the wire and see no frame, and each places its frame on the wire, a collision will occur. In this situation, the voltage levels on a copper wire or the light frequencies on a piece of fiber get messed up. For example, if two NICs attempt to put the same voltage on an electrical piece of wire, the voltage level will be different than if only one device does so. Basically, the two original frames become unintelligible (or undecipherable). The NICs, when they place a frame on the wire, examine the status of the wire to ensure that a collision does not occur: this is the collision detection mechanism of CSMA/CD.

If the NICs see a collision for their transmitted frames, they have to resend the frames. In this instance, each NIC that was transmitting a frame when a collision occurred creates a special signal, called a jam signal, on the wire, waits a small random time period, and senses the wire again. If no frame is currently on the wire, the NIC will then retransmit its original frame. The time period that the NIC waits is measured in microseconds, a delay that can’t be detected by a human. Likewise, the time period the NICs wait is random to help ensure a collision won’t occur again when these NICs retransmit their frames.

The more devices you place on a segment, the more likely you are to experience collisions. If you put too many devices on the segment, too many collisions will occur, seriously affecting your throughput. Therefore, you need to monitor the number ofc ollisions on each of your network segments. The more collisions you experience, the less throughput you’ll get. Normally, if your collisions are less than one percento f your total traffic, you are okay. This is not to say that collisions are bad—they are just one part of how Ethernet functions.

Because Ethernet experiences collisions, networking devices that share the same medium (are connected to the same physical segment) are said to belong to the same collision, or bandwidth, domain. This means that, for better or worse, traffic generated by one device in the domain can affect other devices.