Information Technology Reference
In-Depth Information
tested before Addendum 5 to TIA-568-A, you should use the additional testing
requirements of TSB-95 to verify the cable for gigabit performance levels (as
described later in this section).
The original specifications for Category 5 components (connecting hardware
and cable) were developed with the view that 155 Mbps was about the limit for the
technology. Also, it was assumed (rashly, in hindsight) that most implementations
would use only two pairs of the 4-pair cable, as was the case with 100BaseTX. So
the test parameters based on a 100 MHz limit tested two pairs at a time, to allow
for the expected signaling structures. This allows for a third harmonic component
of a 33.3 MHz fundamental signal, essentially within the requirements for the 100
and 155 Mbps technologies.
In order to place gigabit rates on the same type of cable, all four pairs are
required. The net effect is that the rate is split by four, and through further encoding,
reduced to allow operation within the 100 MHz-tested Channel. But just barely! It
is a little risky to place the reliable operation of your network on an infrastructure
that just meets the minimum criteria for operation on day one. This means that there
can be absolutely no room for any aging of any of the components, and no deterio-
ration of the connections for the life of the installation. That borders on the absurd.
The solution has been to provide a series of enhanced-performance categories
or levels to make certain that installed wiring could meet the needs of Gigabit
Ethernet, and other technologies. So-called enhanced Category 5e cable became
available almost immediately. To sort out the various claims, the TIA began studies
to supplement and revise the TIA-568-A standard.
At the same time, Anixter, the wiring component distributor that began the sys-
tem that resulted in the TIA's component category definition, set up a new levels sys-
tem for Category 5 cable. This system recognizes that “better” can be quantified
(and, no doubt, priced as well). See Table 12.5.
The result was Category 5e (enhanced Cat 5). The most obvious difference in
Cat 5e cable performance is seen through crosstalk. A cable with a lower NEXT
(and FEXT, the far-end measurement) will naturally perform better at all the fre-
quencies. As a matter of fact, the point at which a link ceases to function for a par-
ticular frequency range is where the attenuation and the NEXT are equal. For early
constructions of Cat 5 cable, this point was engineered to just over 100 MHz.
So, let's look first at several of the critical component parameters at gigabit
speeds. Then, we will discuss the advances beyond Cat 5 that can get you reason-
able performance margins at gigabit and beyond.
Attenuation at Gigabit Rates. As a signal travels along a cable, the resistance,
capacitance, and inductance of the copper wires act to reduce the amplitude of the
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