Testing

Several different tests are conducted on the permanent link and channel to ensure that an accurate evaluation of the performance of the installed cabling is achieved. The following text provides a brief description of the characteristics measured by these tests and the methodology involved in the testing process.The tests conducted include the following:

Near End Crosstalk

Crosstalk is undesirable signal transmission from one signal pair to another in close proximity. Crosstalk can cause communication problems in networks. The most significant characteristic of LAN cabling performance is crosstalk. High levels of crosstalk will prevent a LAN from performing properly.

The NEXT test measures crosstalk by applying a test signal to one signal pair and measuring the amplitude of the crosstalk signals received by the other signal pairs. The crosstalk value is computed as the difference in amplitude between the test signal and the crosstalk signal as measured on the pair under test, from the same end of the cable. This difference is called Near End Crosstalk (NEXT) and is expressed in decibels (dB). Lower NEXT values correspond to better system performance.

All signals transmitted through a cable are affected by attenuation, therefore crosstalk occurring at the far end of a cable contributes less to NEXT than crosstalk occurring at the near end of a cable.

Powersum Near End Crosstalk (PSNEXT)

Powersum Near End Crosstalk (PSNEXT) is an extension of conventional NEXT. Powersum NEXT plays an important role in determining whether the cabling system is capable of running protocols that utilise multiple pairs in the same sheath concurrently. There are 6 NEXT combinations, but only four Powersum combinations, one each for the blue, orange, green and brown pairs. Powersum is determined mathematically from the six conventional NEXT tests.

Attenuation to Crosstalk Ratio (ACR)

ACR (attenuation to crosstalk ratio) is the difference between NEXT in dB and attenuation in dB. The ACR value indicates how the amplitude of signals received from a far-end transmitter compare to the amplitude of NEXT produced by near-end transmissions. A high ACR value means that the received signals are much larger than thecrosstalk. In terms of NEXT and attenuation values, a high ACR value corresponds to low NEXT and low attenuation.

Powersum ACR (PSACR)

Powersum ACR is to ACR as PSNEXT is to NEXT. Instead of the ACR values being measured for all six pair combinations they are calculated for the four pairs in the cable. Modern protocols utilise more than one pair to achieve their high bit rates. In situations such as these more than one signal in each direction could be transmitted at any one time.

Attenuation is the decrease in the strength of a signal over the length of the cabling link and channel. This is caused by the loss of electrical energy due to the resistance of the conductors, and by leakage of energy from the link and channel. This loss of energy is expressed in decibels (dB). Lower attenuation values correspond to better link and channel performance.

For example, when comparing the performance of two cables at a particular frequency, a link and channel with an attenuation of 10 dB performs better than a link and channel with an attenuation of 20 dB.

Link and channel attenuation is determined by the cable and cross connect construction, length and the frequencies of the signals transmitted through the link and channel. At higher frequencies, skin effect, inductance and capacitance cause attenuation to increase.

Return Loss (RL) is the difference between the power of a transmitted signal and the power of the signal reflections caused by variations in link and channel impedance.

A return loss plot indicates how well the link and channel's impedance matches its rated impedance over a range of frequencies. High return loss values mean a close impedance match, which results in greater differentiation between the powers of transmitted and reflected signals. Links and channels with high return loss values are more efficient at transmitting LAN signals as less of the signal is lost in reflections.

Equal Level Far End Crosstalk (ELFEXT)

Equal Level Far End Crosstalk (ELFEXT) is a new type of test that has been introduced to ensure modern cabling systems are capable of transmitting modern protocols. The latest protocols utilise multiple pairs and signals can travel in opposite directions at the same time. It is no longer sufficient to simply test for crosstalk at the near end, far end crosstalk tests must also be completed.

The test signal is transmitted on a pair at one end of the cable link and channel, and measured on a different pair at the other. The transmitted signal is attenuated along the length of the cable and that attenuation is added back to the final measurement to give Equal Level FEXT.

Powersum Equal Level Far End Crosstalk (PSELFEXT)

As with all crosstalk measurements (including ACR) there is also a Powersum ELFEXT (PSELFEXT). These are calculated values expected for multi-pair simultaneous full duplex transmissions.