Electrical Parameters for Category Cables

Crosstalk

Crosstalk is defined as the unwanted induction of signal from one circuit to another.

Insertion Loss (Attenuation)

Insertion Loss or Attenuation is the degree of signal amplitude decrease (or loss), measured in Decibels.
Electrical signals transmitted by a link lose some of their energy as they travel along the link. Insertion loss measures the amount of energy that is lost as the signal arrives at the receiving end of the cabling link. The insertion loss measurement quantifies the effect of the resistance the cabling link offers to the transmission of the electrical signals.

The insertion loss in a cable is largely dependent upon the gauge of wire used in constructing the pairs. 23 AWG wires will have less insertion loss than the same length 24 AWG (thinner) wires. Also, stranded cabling will have 20-50% more insertion loss than solid copper conductors. Field test equipment will report the worst value of insertion loss and margin, where the margin is the difference between the measured insertion loss and the maximum insertion loss permitted by the standard selected. Hence a margin of 4 dB is better than 1 dB.

Causes of Insertion Loss

Excessive length is the most common reason for failing insertion loss. Fixing links that have failed insertion loss normally involves reducing the length of the cabling by removing any slack in the cable run.
Excessive insertion loss can also be caused by poorly terminated connectors / plugs. A poor connection can add significant insertion loss.
Higher Temperatures also affects insertion loss in some cables. For this reason, standards bodies tend to specify insertion loss requirements adjusted for 20C. Cables operating in temperature extremes can be subject to additional insertion loss and where this is likely, the design of the cabling system should take this into consideration.

Near End Cross Talk (NEXT)

NEXT is one of the most important cable measurements. NEXT measures the signal interference from one pair to another pair on the cable end nearest (near end) the test device. Crosstalk occurs between adjacent wire pairs (pair to pair NEXT). High NEXT readings are often the result of the connector or its method of termination.

If wires are not tightly twisted, the result is Near End Crosstalk (NEXT). Most of us have experienced a telephone call where we could hear another conversation faintly in the background. This is crosstalk. In fact, the name crosstalk derives from telephony applications where 'talk' came 'across'. In LANs, NEXT occurs when a strong signal on one pair of wires is picked up by an adjacent pair of wires. NEXT is the portion of the transmitted signal that is electromagnetically coupled back into the received signal.

Since NEXT is a measure of difference in signal strength between a disturbing pair and a disturbed pair, a larger number (less crosstalk) is more desirable than a smaller number (more crosstalk). Because NEXT varies significantly with frequency, it is important to measure it across a range of frequencies, typically 1 100 MHz.

Causes of Near End Cross Talk

In many cases, excessive crosstalk is due to poorly twisted terminations at connection points.

Power Sum Near-End Crosstalk (PS-NEXT)

PS-NEXT is defined as the sum of the total NEXT (near-end crosstalk) of 3 wire pairs as they affect the 4th pair in a 4 pair cable such as Cat5e and Cat6. PS NEXT is an important measurement for qualifying cabling intended to support 4 pair transmission schemes such as Gigabit Ethernet.

Since PS NEXT is a measure of the difference in signal strength between disturbing pairs and a disturbed pair, a larger number (less crosstalk) is more desirable than a smaller number (more crosstalk). Because PS NEXT varies significantly with frequency, it is important to measure it across a range of frequencies, typically 1 - 100 MHz. If you look at the PS NEXT on a 50 meter segment of twisted pair cabling, it has a characteristic "roller coaster" shape. That is, it varies up and down significantly, while generally increasing in magnitude. This is because twisted pair coupling becomes less effective for higher frequencies. Typically, PS NEXT results are around 3 dB lower than the worst-case NEXT result at each end of the link.

Since PS NEXT is a calculation based on NEXT measurements, troubleshooting for PS NEXT failures reduces troubleshooting for NEXT problems. Once you have isolated and repaired the NEXT problem, PS NEXT will automatically improve.

ACR (Attenuation to Crosstalk Ratio)

[This parameter is not required by the ANSI standards but may be required in order to obtain the premise wiring vendor's warranty]

ACR provides an indication of bandwidth for the two wire-pair network applications. ACR is a computed parameter that is analogous to ELFEXT and expresses the signal to noise ratio for a two wire-pair system. This calculation yields 12 combinations - six from each end of the link.

Return Loss

Return Loss (RL) measures the total energy reflected on each wire pair. Return Loss is to be measured from both ends of the link-under-test for each wire pair. This parameter is also to be measured form 1 through 100 MHz in frequency increments that do not exceed the maximum step size defined in the standards as shown in Table 1, column 2.

Minimum test results documentation (summary results): Identify the wire pair that exhibits the worst case margin and the wire pair that exhibits the worst value for Return Loss. These wire pairs must be identified for the tests performed from each end. Each reported case shall include the frequency at which it occurs as well as the test limit value at this frequency.

Attenuation Crosstalk Ratio Far-End (ACR-F) (formally called ELFEXT)

The new nomenclature ACR-F as defined in ANSI/TIA-568-C.2 as well as various European and International Standards. ACR-F is an acronym for Attenuation Crosstalk Ratio Far-End. ACR-F is a calculated result, rather than a measurement. It is derived by subtracting the Insertion Loss of the disturbing pair from the Far End Crosstalk (FEXT) this pair induces in an adjacent pair. Another way to understand ACR-F is to think of far-end Attenuation Crosstalk Ratio (ACR) as the same thing. ACR-F that is too high is indicative of either excessive Insertion Loss, higher than expected FEXT, or both.

Power Sum Equal-Level Far-End Crosstalk (PS-ELFEXT)

PS-ELFEXT measures the total sum of all interference from pairs on the far end onto a pair on the near end without the effects of attenuation.

PSACR

[This parameter is not required by the ANSI standards but may be required in order to obtain the premise wiring vendor's warranty]

The Power Sum version of ACR is based on PSNEXT and takes into account the combined NEXT disturbance of all adjacent wire pairs on each individual pair. This calculation yields 8 combinations - one for each wire pair from both ends of the link.

Alien Crosstalk (ANEXT)

Alien Crosstalk (ANEXT) is defined as unwanted signal coupling from one balanced twisted-pair component, channel, or permanent link to another. Alien crosstalk is the most significant transmission parameter impacting 10GBASE-T performance. Alien Crosstalk measures the crosstalk included in a wire pair by wire pairs in the adjacent cables.

Since alien crosstalk is only caused by differential (or balanced) signal coupling, alien crosstalk is not adversely impacted by common noise such as noise from motors, transformers, or florescent lights that are present in the environment.

Causes of Alien Crosstalk

The amount of ANEXT depends on a number of factors, including the proximity of adjacent cables and connectors, the cable length, cable twist density, and EMI. Patch panels and connecting hardware are also affected by Alien Crosstalk.

Propagation Delay

Propagation delay is the time required for the signal to travel from one of the link to the other. This measurement is to be performed for each of the four wire pairs. Propagation delay, or delay, is a measure of the time required for a signal to propagate from one end of the circuit to the other. Delay is measured in nanoseconds (nS). Typical delay for category 5e UTP is a bit less than 5 nS per meter (worst case allowed is 5.7 nS/m). Delay is the principle reason for a length limitation in LAN cabling. In many networking applications, such as those employing CSMA/CD, there is a maximum delay that can be supported without losing control of communications.

ANSI/TIA-1152 and ISO/IEC 11801, which refers testing to IEC 61935-1, all require the measurement to be made at 10 MHz with field testers. Most structured wiring standards expect a maximum horizontal delay of 570 nS. If design specifications allow, higher delay can be acceptable. Since each pair in the cable has its own unique twist ratio, the delay will vary in each pair. This variance (delay skew, discussed in the next section) should not exceed 50 nS on any link segment up to 100 meters. Standards require all pairs to meet the requirement. It is possible to report just the worst case pair. This will be the pair with the highest propagation delay.

Excessive propagation delay can have only one cause: the cable is too long. If you fail propagation delay, check to ensure that the pass/fail criteria match the design specifications. If so, the cable is too long. In many cases, a cable up to 25% too long (125m for Category 5e) will still support most LAN applications. However, the installation will fail most structured wiring standards, such as those published by CENELEC, ISO/IEC, and the TIA. In some cases, if the customer insists on the location of the terminal equipment, and an excessive length cannot be avoided, you can verify other cable parameters. If they pass, you can provide information that indicates the cable meets frequency-dependent parameters but is non-compliant with overall standards due to excessive length.

Nominal Velocity of Propagation (NVP)

Nominal Velocity of Propagation (NVP) is much different from Propagation Delay. NVP refers to the inherent speed of signal travel relative to the speed of light in a vacuum (designated as a lower case c). NVP is expressed as a percentage of c, for example, 72%, or 0.72c. All structured wiring cables will have NVP values in the range of 0.6c to 0.9c.

Delay Skew

Propagation (skew) is the difference between the propagation delay on the fastest and slowest pairs in a UTP cable. Some cable constructions employ different types of insulation materials on different pairs. This effect contributes to unique twist ratios per pair and to skew.

Skew is important because several high-speed networking technologies, notably Gigabit Ethernet, use all four pairs in the cable. If the delay on one or more pairs is significantly different from any other, then signals sent at the same time from one end of the cable may arrive at significantly different times at the receiver. While receivers are designed to accommodate some slight variations in delay, a large skew will make it impossible to recombine the original signal without a video skew compensator device.

Well-constructed and properly installed structured cabling should have a skew less than 50 nanoseconds (nSec) over a 100-meter link. Lower skew is better. Anything under 25 nSec is excellent. Skew between 45 and 50 nanoseconds is marginally acceptable. If the skew is high, provided the intended application is a 2-pair application such as 10BASE-T or token ring, the application should still perform. If one pair is much higher or lower in delay than the others, very high skew may result. Examine the delay results for each pair. If one pair exhibits uncharacteristically high or low delay, re-examine the installation.

Impedance

A figure of merit Measured in Ohms. A uniform transmission line (cable) of an arbitrary length of cable will have no standing waves or reflections from the end and a constant frequency at every point on the cable. Impedance is made up of Resistance, Inductance, Capacitance and Conductance inherent in a cable.

Capacitance

Capacitance is the property of an electrical charge between positive and negative conductors. This is measured by the amount of separated electrical charge that can be stored. Measured in Pico-Farad. In our case it relates the measurement between the conductor and the shield. A low capacitance is what we require.