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 significance.

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 Hertz

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 measures are:

1. noise, as defined by PSNEXT,
2. headroom, as determined by Attenuation Crosstalk Ratio – ACR or PSACR,
3. signal strength, as characterized by Return Loss (RL) and Attenuation.

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.