Introduction to Fast Ethernet

Virtually everyone who uses Ethernet has wished from time to time that their network had a higher bandwidth. When Ethernet was being designed in the late 1970s, 10Mbps seemed immense. With today's bandwidth-intensive multimedia applications, or even with just the departmental server, that number sometimes is barely adequate. Yes, faster network technologies were available, but they were complicated and expensive. Then came Fast Ethernet.

Anyone who understands classic Ethernet already understands much about Fast Ethernet. Fast Ethernet uses the same cabling and access method as 10Base-T. With certain exceptions, Fast Ethernet is simply regular Ethernet, just ten times faster. Whenever possible, the same numbers used in the design of 10Base-T were used in Fast Ethernet, just multiplied or divided by ten.

The Implementations of Fast Ethernet

Fast Ethernet is defined for three different physical implementations:

• 100BASE-TX: Category 5
• 100BASE-FX: Multimode fiber
• 100BASE-T4: Category 3

Probably the most popular form of Fast Ethernet is 100BASE-TX. 100BASE-TX runs on EIA/TIA 568 Category 5 unshielded twisted pair, sometimes called UTP-5. It uses the same pair and pin configurations as 10Base-T, and is topologically similar in running from a number of stations to a central hub.

As an upgrade to 10Mbps Ethernet over Multimode fiber (10Base-F), 100BASE-FX is Fast Ethernet over fiber. Single duplex runs are supported up to 400m, and full duplex runs are supported for up to 2km.

Fast Ethernet is possible on Category 3 UTP with 100BASE-T4. There is a popular misconception that Fast Ethernet will only run on Category 5 cable. That is true only for 100BASE-TX. If you have Category 3 cable with all four pairs (8 wires) connected between station and hub, you can still use it for Fast Ethernet by running 100BASE-T4. 100BASE-T4 sends 100Mbps over the relatively slow UTP-3 wire by fanning out the signal to three pairs of wire. This "demultiplexing" slows down each byte enough that the signal won't overrun the cable. Category 3 cable has four pairs of wire, eight wires total, running from point to point. 10Base-T only uses four wires, two pairs. Some cables only have these two pairs connected in the RJ-45 plug. If the category 3 cabling at your site has all four pairs between hub and workstation, you can use Fast Ethernet by running 100BASE-T4.