How Ethernet Works

Ethernet is a local area network (LAN) technology with networks traditionally operating within a single building, connecting devices in close proximity. At most, Ethernet devices should have only a few hundred yards of cable between them.

For two devices on a network to successfully communicate, they must both understand the same protocol. Ethernet follows a simple set of rules that govern its basic operation. To better understand these rules, it is important to understand Ethernet terminology.

• Medium —a path along which the electronic signals travel.
• Segment —a single shared medium.
• Node —a device that attaches to a segment.
• Frame —variably sized chunks of information.

The Ethernet protocol specifies a set of rules for constructing frames. There are explicit minimum and maximum lengths for frames, and a set of required information that must appear in the frame. Each frame must include destination and source addresses, which identify the recipient and the sender of the message. The address uniquely identifies the node. No two Ethernet devices can have the same address. Since a signal on the Ethernet medium reaches every attached node, the destination address is critical to identify the intended recipient of the frame. A frame with a broadcast address is intended for every node on the network.

"Carrier sense multiple access with collision detection" (CSMA/CD) is how the Ethernet protocol regulates communication among nodes. "Multiple access" means that when one Ethernet station transmits, all the stations on the medium hear the transmission. "Carrier sense" means that before a station transmits, it "listens" to the medium to determine if another station is transmitting. If the medium is quiet, the station recognizes that this is an appropriate time to transmit.

Ethernet nodes listen to the medium while they transmit to ensure that they are the only station transmitting at that time. If the stations hear their own transmission returning garbled, then they know that a collision occurred. A single Ethernet segment is sometimes called a collision domain because no two stations on the segment can transmit at the same time without causing a collision. When stations detect a collision, they cease transmission, wait a random amount of time, and attempt to transmit when they detect silence on the medium.

The random pause and retry is an important part of the protocol. If two stations collide when transmitting once, then both will need to transmit again. At the next appropriate chance to transmit, both stations involved with the previous collision will have data ready to transmit. If they transmitted again at the first opportunity, they would most likely collide, and continue to collide indefinitely. Instead, the random delay makes it unlikely that any two stations will collide more than a few times in a row.

Modern Ethernet networks use twisted pair wiring or fiber optics to connect stations in a radial pattern. Although legacy Ethernet networks transmitted data at 10 megabits per second (Mbps), modern networks can operate at 100 or even 1000 Mbps.

Switched networks replace the shared medium of legacy Ethernet with a dedicated segment for each station. These segments connect to a switch, which acts much like an Ethernet bridge. Some switches today can support hundreds of dedicated segments. Since the only devices on the segments are the switch and the end station, the switch picks up every transmission before it reaches another node. The switch then forwards the frame over the appropriate segment. But since any segment contains only a single node, the frame reaches only the intended recipient. This feature allows many transmissions to occur simultaneously on a switched network.

Full duplex refers to the ability to send and receive data at the same time. Legacy Ethernet is half duplex. In a switched network, nodes communicate only with the switch and never directly with each other. Switched networks also employ twisted pair or fiber optic cabling, both of which use separate conductors for sending and receiving data. In this type of environment, Ethernet stations can forgo the collision detection process and transmit at will, since they are the only potential devices that can access the medium. This ability allows end stations to transmit to the switch at the same time that the switch transmits to them, achieving a collision-free environment.