Fast Ethernet

When Ethernet technology availed itself to users, the 10 Mbps bandwidth seemed like an unlimited resource. (Almost like when we had 640k of PC seemed we would never need more!) Yet workstations have developed rapidly since then, and applications demand more data in shorter amounts of time. When the data comes from remote sources rather than from a local storage device, this amounts to the application needing more network bandwidth. New applications find 10 Mbps to be too slow. Consider a surgeon downloading an image from a server over a 10 Mbps shared media network. He needs to wait for the image to download so that he can begin/continue the surgery. If the image is a high resolution image, not unusually on the order of 100 MB, it could take a while to receive the image. What if the shared network makes the available user bandwidth about 500 kbps (a generous number for most networks) on the average? It could take the physician 26 minutes to download the image:

100 MB x 8/500 kbps = 26 minutes

If that were you on the operating table waiting for the image to download, you would not be very happy! 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 a good situation. Sadly, many hospital networks function like this and consider it normal. Clearly, more bandwidth is needed to support this 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 twistedpair cables. In June of 1995, IEEE approved the 802.3u specification defining a system that offered vendor interoperability at 100 Mbps.

Like 10 Mbps systems such as 10BaseT, the 100 Mbps systems use CSMA/CD, but provide a tenfold improvement over legacy 10 Mbps networks. Because they operate at 10 times the speed of 10 Mbps Ethernet, all timing factors reduce by a factor of 10. For example, the slotTime for 100 Mbps Ethernet is 5.12 microseconds rather than 51.2 microseconds. The IFG is .96 microseconds. And because timing is one tenth that of 10 Mbps Ethernet, the network diameter must also shrink to avoid late collisions.

An objective of the 100BaseX standard was to maintain a common frame format with legacy Ethernet. Therefore, 100BaseX uses the same frame sizes and formats as 10BaseX. Everything else scales by one tenth due to the higher data rate. When passing frames from a 10BaseX to a 100BaseX system, the interconnecting device does not need to re-create the frame's Layer 2 header because they are identical on the two systems.

10BaseT, the original Ethernet over twisted-pair cable standard, supports Category 3, 4, and 5 cables up to 100 meters in length. 10BaseT uses a single encoding technique, Manchester, and signals at 20 MHz well within the bandwidth capability of all three cable types. Because of the higher signaling rate of 100BaseT, creating a single method to work over all cable types was not likely. The encoding technologies that were available at the time forced IEEE to create variants of the standard to support Category 3 and 5 cables. A fiber optic version was created as well.

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