This specification was driven by the need to support higher performance
/ higher speed LAN systems, specifically Gigabit Ethernet. The
TIA clearly defined the requirements for Enhanced Category 5 with
its publication last November.
TIA 568-A-5 adjusts an existing characteristic, Near-End Crosstalk
(NEXT) so that it is 3 dB more stringent than before. This increases
the attenuation to crosstalk ratio (ACR) of the cable, effectively
reducing the noise level and making the data signal easily detected
at the receiving end.
The addendum also adds several other characteristics. These are
Equal Level Far-End Crosstalk (ELFEXT), Power Sum ELFEXT (PSELFEXT),
Power Sum NEXT (PSNEXT), and Return Loss.
ELFEXT is a measure of signal transfer from one pair to another
at the receiving end of the cable, with the effects of attenuation
normalized. Most signal transfer occurs at the sending end, where
the signal is strongest (the NEXT characteristic). However, as
high performance network systems become more sensitive to noise
in the cable and channel, ELFEXT becomes a noise source of greater
With both NEXT and ELFEXT, a "worst pair" measurement
is taken. In other words, each pair in the cable is tested one
at a time against every other pair in the cable. The pair-to-pair
combination with the poorest performance characterizes the cable’s
NEXT and ELFEXT capabilities. PSNEXT and PSELFEXT differ from
these measurements in a substantial way. While one pair is being
tested, all the other pairs are energized with signals. This creates
a transmission environment with maximum potential for crosstalk
noise. The reason this is important is that Gigabit Ethernet uses
a technique known as full duplex transmission. Most LAN systems
in use today only use two pairs, one to send and one to receive.
In a full duplex system, all 4 pairs are used simultaneously to
send and receive signals. The power sum test environment simulates
this real-life application.
Return Loss is a measurement of dimensional consistency within
the cable. It is important because variations in geometry in the
cable can result in impedance variations, which in turn cause
signal reflections. A cable with poor geometric symmetry similarly
contributes to signal degradation and loss of signal energy as
frequencies increase and bit encoding schemes pack more bits per
Attenuation, while not changed for 568-A-5, is also an important
characteristic in that it defines the resulting signal strength.
While all of the aforementioned characteristics have a role in
defining the performance of the cable, when it comes to protecting
the channel installation against external noise, high temperatures
or imperfect installation or termination practices, the defining
- noise, as defined by PSNEXT,
- headroom, as determined by Attenuation Crosstalk Ratio – ACR
- signal strength, as characterized by Return Loss (RL) and
Headroom, for ACR/PSACR margin over spec.) defines how high the
signal level is relative to the noise level and thus provides
the simplest measure of how well the cable will provide a strong
signal profile, which is critical under adverse conditions.
With good Return Loss values, the cable is reducing signal reflections
that result in attenuation of the signal and cause echoes that
become noise. As with ACR, good RL values mean a robust signal
compared to the noise.
With emphasis on PSNEXT, NEXT, ACR and Return Loss, the Berk-Tek
LANmark 350 enhanced Category 5 cables are designed to offer an
exceptional level of margin over CAT 5e specifications that matter
most in the real world.
NOTE: Channel performance can be adversely affected by improper
installation practices, poor termination methods, mismatched components,
external noise, and even by elevated temperatures.