Information Technology Reference
In-Depth Information
In the lead is attenuation, followed by NEXT, FEXT, ELFEXT,…, then comes
return loss, impedance, and, close behind, ACR. Well, the resulting scramble for the
top technology spot keeps us all betting on next year's network foals.
ACR—Attenuation-to-Crosstalk Ratio. The difference in dB between the meas-
ured attenuation and NEXT at a particular frequency is commonly referred to as
the attenuation-to-crosstalk ratio (ACR). The ACR is given in dB, as are the atten-
uation and NEXT of a cable. Because both those values are converted from actual
power levels, the ratio between those two power levels can be expressed in dB. Thus,
the ratio is developed by simply subtracting the two measurements. For example, an
attenuation of 37 dB and a NEXT of 40 dB yields an ACR of 3 dB. One could say
that, in essence, the remaining transmitted signal would be about 3 dB more than
the potential interference from NEXT.
Beyond Category 5 to Cat 5e, 6/AC6, and 7. Enhanced Cat 5, or Category 5e,
has now been canonized by the TIA. This recognizes the Cat 5 “levels” program,
which was originally used to specify cable and connecting hardware that exceeded
the basic Cat 5 standard. There is a compelling need to overengineer new struc-
tured cabling systems to allow for future technology upgrades. Category 6, with its
250 MHz bandwidth, certainly meets that need, and it is an integral part of TIA-
568-C. Now, you can certainly specify components that exceed Cat 5e and
Category 6. Category AC6 and Category 7 are well along the standards road, and
are soon to be a requirement for advanced network technologies. All of these have
analogs in the ISO standards as Class D, E, E-Augmented, and F cable definitions.
Looking at Table 12.6, you can see that there are substantial performance advan-
tages to the higher cable categories. For more information on these new perform-
ance categories, refer to Chapters 5 and 12 and the individual component sections
in Part Two.
Using the advanced cabling categories will clearly enhance performance.
However, the standard states that you should be able to operate Gigabit Ethernet
over simple (but performance-verified) Category 5 cabling systems. The issue for
most wiring designers is how much of a performance margin should be specified.
Category 6 specs meet this need head-on. The situation was similar for the original
Category 5 specification, where the parameter of delay skew was found to be criti-
cal for 4-pair network link implementations. Those who specified Cat 5 cable when
only Cat 3 was really required for their current application have been rewarded,
now that most network hardware needs Cat 5. Fortunately, the hardware for Cat 3
and Cat 5 is completely interchangeable, although a mixed use downgrades the link
performance expectations to the lower category.
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