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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.
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