How It Works: Ethernet

Ethernet: a standard for physically connecting computers into a local area network and a communications protocol that lets those computers share data.

• Most widely implemented networking standard.
  
  
• Best suited for businesses.
  
  
• Works with all modern operating systems.
  
  
• Requires special hardware and wiring.
  
  

If you have multiple computers in the same location, ethernet helps multiply their productivity. By creating an ethernet network, you can transfer files between PCs and file servers, print documents at printers hundreds of feet from your desk, run applications stored on other computers, and share high-speed access to the Internet. By far the most popular networking technology for large and small businesses, ethernet's widespread availability and fast transfer speeds have made it the de facto standard for networking.

The ethernet specification describes how data can be sent between computers in close physical proximity, in what's called a local area network, or LAN. To be a part of the LAN, every computer needs a network interface--which bundles data into chunks that travel across the network--and a connection point, or port, for the special wiring that connects all the PCs. This port, built into the motherboard or an add-in network interface card, sends data to the network and receives data sent from other computers on the network.

But ethernet is more than hardware: The standard also dictates the communications protocol, or how connected computers send data. Computers linked by ethernet send data along the wire in small chunks called packets. Think of a packet as a suitcase, tagged to travel to a different city, moving along a conveyor belt in an airport: In addition to the data itself, each packet carries a destination address and your computer's "home" address.

The ethernet interface uses a protocol called Carrier Sense Multiple Access With Collision Detection to send packets. That mouthful means that the computer first looks for a lull in activity--like a gap where there's no suitcase on the conveyor belt--before it sends out a packet. Every time a packet reaches its destination the sender gets confirmation while the computer waits for a gap to open to shoot off another packet. Devices along the way read the address and pass the packet along to the next device nearest its destination. Occasionally, two devices send a packet into the same gap at the same time, resulting in a "collision" and the loss of both packets, for the moment. When packets collide, the PCs that sent them are instantly notified, and each chooses a random interval to wait before it resends the packet. This approach helps prevent network gridlock.

Inside the LAN, packets flow along wires; the arrangement of the wires and connected computers is called the topology of the network. Though the ethernet specification accounts for many topologies, the most popular configuration is called the star topology. In this kind of network, each computer (or node) connects directly to a centralized device (or hub). Hubs receive packets from one node and distribute them to other nodes. Passive hubs simply pass every incoming packet out to every connected node; switched hubs send packets to only the node to which the packet is addressed.

You can also run ethernet over a bus topology, in which the computers branch off from one main line. Star topologies are easier to manage than bus topologies and use less expensive wiring. But you don't have to create a whole LAN to use ethernet. If you simply want to connect two computers, you can create a peer-to-peer network by connecting a "crossover" cable between the two PCs' network ports. Ethernet is also commonly used to connect PCs to high-speed Internet devices, such as cable or DSL modems.

The specification also details how fast data can travel and the types of wires that must be used. For a long time, 10Base-T ethernet, capable of passing 10 megabits of data per second, was the fastest and most popular implementation. As people used ethernet for larger and more complex networks, and as file sizes grew, 100Base-T ethernet (also known as Fast Ethernet), with ten times the data transfer speed, became the ethernet of choice. To get that much more speed, Fast Ethernet uses higher-quality cabling that sends the packets more quickly without degrading the signal. Still newer, but not as widely implemented, is a version that sends data at 1 gigabit per second. And the 10-gbps version is on the horizon. These superfast connections are often used to create large networks, as they prevent data bottlenecks, or gridlock.