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. |
