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GLOSSARY
- Attenuation
Attenuation defines a signal's loss over
distance. As a signal travels down a cable, it loses strength. The
unit "dB," or decibel, defines the amount of attenuation
by measuring a signal's strength. Factors that influence attenuation
include conductor size, material type, and frequency of operation
and distance. Lower values are better.
- Power
Sum NEXT
Power Sum NEXT (near-end crosstalk) indicates
the effect that the energized pairs have on the non-energized pair.
Good numbers are critical in applications such as Gigabit Ethernet
that require 4-pair technology (full-duplex mode).
- FEXT
FEXT (far-end crosstalk) is crosstalk
measured at the end of the cable opposite the signal source. ELFEXT
measurements compensate for the effects of varying cable lengths so
that all cable can be certified to the same limit. Without good
ELFEXT numbers, a channel less than 100 meters can suffer increased
collisions, retries and server utilization, slowing a network and
causing lost sessions or network drop-offs.
- ACR
ACR (attenuation-to-crosstalk ratio) shows
the usable bandwidth of a system. Also expressed in decibels, larger
ACR numbers indicate more usable bandwidth.
- RETURN
LOSS
Return loss indicates how much signal will be
lost due to reflected energy, also known as echo. Return loss
becomes a factor in full-duplex transmission because a portion of
the signal is "reflected" back to the transmitting end,
disrupting transmission. Return loss can result in signal jitters,
meaning dropped packets, more transmissions, slower speeds and
frustrated users.
- PROPAGATION
DELAY
Propagation delay is the time interval
required for the signal to be transmitted from one end of the
circuit to the other.
- DELAY
SKEW
Delay skew defines the timing difference of
the slowest and fastest pairs in a multi-pair signal traveling down
a cable. When all four pairs are activated, it is important that the
signals arrive close to the same time. When the difference in time
is too great, the receiving device is unable to reassemble the
signal. This will ultimately cause errors and lost data. |
