2.2 Fast Ethernet
2.2.1 10-Mbps vs. 100-Mbps

When Ethernet technology availed itself to users, the 10-Mbps bandwidth seemed like an unlimited resource. However, workstations have developed quite rapidly since then, and applications demand much more data in shorter amounts of time. When the data comes from remote sources rather than from a local storage device, the application needs more network bandwidth. Many new applications actually find 10 Mbps to be too slow. For example, think about a surgeon downloading an image from a server over a 10-Mbps shared-media network. He/she needs to wait for the image to download so that he/she can begin an operation. If the image is a 100-megabyte (MB) high-resolution image, it could take awhile to receive the image. Suppose the shared network makes the available user bandwidth about 500 kilobits per second (kbps) on the average. It would take the physician 27 minutes to download the image.

If you are the hospital administration, you are exposing yourself to surgical complications at worst and idle physician time at best. Obviously, this is not an ideal situation. Clearly, more bandwidth would be needed to support this medical application.

Recognizing the growing demand for higher-speed networks, the IEEE formed the 802.3u committee to begin work on a 100-Mbps technology that works over twisted-pair cables. In June 1995, IEEE approved the 802.3u specification defining a system that offered vendor interoperability at 100 Mbps.

Like 10-Mbps systems such as 10BASE-T, the 100-Mbps systems use CSMA/CD, but provide a huge improvement over legacy 10-Mbps networks. Because they operate at ten times the speed of 10-Mbps Ethernet, all timing factors scale by a factor of 10. For example, the slot time (the time it takes to transmit a 64-byte, or 512-bit, frame) for 100-Mbps Ethernet is 5.12 microseconds, one-tenth that of 10-Mbps Ethernet.

An objective of the 100BASE-X standard (here the X is a variable whose value correlates to a particular 100-Mbps standard) was to maintain a common frame format with legacy Ethernet. Therefore, 100BASE-X uses the same frame sizes and formats as 10BASE-X. Everything else scales by one-tenth because of the higher data rate. When passing frames from a 10BASE-X to a 100BASE-X system, the interconnecting device does not need to recreate the frame Layer 2 header because they are identical on the two systems.

The original Ethernet over twisted-pair cable standard, 10BASE-T supports Category 3, 4, and 5 cables up to 100 meters in length. The 10BASE-T Ethernet uses the Manchester encoding technique and signals at 20 megahertz (MHz), a level well within the bandwidth capacity of all three cable types. Because of the higher signaling rate of 100BASE-T, creating a single method to work over all cable types was highly unlikely. The encoding technologies that were available at the time forced IEEE to create variants of the standard to support both Category 3 and 5 cables. A fiber-optic version was created as well.