Networking Technologies Overview


Local Area Networks: Ethernet and Fast Ethernet

Ethernet has been around since the late 1970s and remains the leading network technology for local area networks (LANs). (A LAN is a network contained in a building or on a single campus.) Ethernet is based on carrier sense multiple access with collision detection (CSMA/CD) (click here on {Token Ring} to learn about another basic style of network communication). Simply put, an Ethernet workstation can send data packets only when no other packets are traveling on the network ? when the network is "quiet." Otherwise, it waits to transmit, as a person might wait for another to speak during a conversation.

If multiple stations sense an opening and start sending at the same time, a "collision" occurs. Then, each station waits a random amount of time and tries to send its packet again. After 16 consecutive failed attempts, the original application that sent it has to start over again. As more people try to use the network, the number of collisions, errors, and subsequent retransmits grows quickly, causing a snowball effect.

Collisions are normal occurrences, but too many can start to cause the network to slow down. When more than 50 percent of the network?s total bandwidth is used, collision rates begin to cause congestion. Files take longer to print, applications take longer to open and users are forced to wait. At 60 percent or higher, the network can slow dramatically or even grind to a halt.

As noted in the previous section, Ethernet?s bandwidth or data-carrying capacity (also called throughput) is 10 megabits per second (Mbps). Fast Ethernet (or 100Base-T) works the same way ? through collision detection ? but it provides 10 times the ?bandwidth ? 100 megabits per second.

Shared Ethernet is like a single lane highway with a 10 Mbps speed limit. Shared Fast Ethernet is like a much wider highway with a 100 Mbps speed limit ? more room for cars and they can travel at higher speeds. What would Switched Ethernet look like? A multi-lane highway with a speed limit of 10 Mbps in each lane. Switched Fast Ethernet also would be a multi-lane highway, but with a speed limit of 100 Mbps in each lane.

Token Ring

Token Ring is a "token-passing" technology and an alternative to Ethernet?s collision-detection method. A token travels through the network, which must be set up in a closed ring, and stops at each workstation to ask whether it has anything to send. If not, the token continues to the next point on the network. If there is data to send, the token grabs it and proceeds to the destination without stopping at the other computers along the way. Then it returns to the sending computer to acknowledge the transmission, before beginning its rounds again. Token Ring networks operate at either 4 or 16 Mbps, but with the low cost, ease of use and easy migration to higher performance in Ethernet networks, Token Ring is rarely used for new network installations.

High-Speed LAN Technologies

Today?s growing, fast-changing networks are like growing communities ? the traffic they create tends to cause congestion and delays. To alleviate these problems, you can install higher-speed LAN technologies in your network, which move traffic more quickly and offer greater data-carrying capacity than Ethernet, Fast Ethernet and Token Ring. Fiber Distributed Data Interface (FDDI) is another "token-passing" technology, operating at 100 Mbps. But since it requires different wiring (fiber) and different hubs and switches from Ethernet, FDDI is losing ground to Fast Ethernet and other high-speed technologies. Asynchronous Transfer Mode (ATM) operates at a range of speeds up to 622 Mbps at present. It is a popular choice for the backbones of extremely demanding or large networks, and it has special features, such as the ability to carry voice and video traffic along with data, and it can be used for wide area networks connecting geographically separated sites. Gigabit Ethernet operates at 1000 Mbps and is fully compatible with Ethernet and Fast Ethernet wiring and applications.