Introduction Of Fast
Ethernet
Introduction
Full motion
video for video conferencing requires, typically, at
least 25 Mb/sec. That means that a legacy Ethernet, at
10 Mb/sec, can only deliver poor quality real-time
video. With 100 Mb/sec, however, you can be watching a
broadcast presentation in one window while you're in
conference with three people in three other windows (for
a total of 100 megabits of bandwidth).
Consider a
file server that requires 0.6 Mb/sec (6 million bits per
second; 60% utilization on a 10 Mb/sec Ethernet). With a
100 Mb/sec Ethernet this server can now utilize
interface hardware that can pump data down the pipe at a
greatly increased rate.
It seems
clear that the evolution of the industry is moving away
from 10 Mb/sec Ethernet and towards the 100 Mb/sec (or
higher) rates of data transfer. This section of the
compendium discusses 100 Mb/sec Ethernet technology
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. Fast Ethernet is
defined for three different physical implementations.
The Implementations of Fast
Ethernet:
-
100BASE-FX: Multimode fibre
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 fibre
(10Base-F), 100BASE-FX is Fast Ethernet over fibre.
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.
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