Cabling Terminology

Introduction

This document puts into perspective the various terms  associated with structured cabling. It is not conclusive and will evolve as the standards gain additions and new terminologies emerge. As you will see, some of the terms have yet to be explained and are presently listed as a reminder that they exist and have some importance.

Terminology

Decibels: The Intensity (I) of any kind of traveling wave, be it sound, light, water etc. is given by the formula:

I = 1/2.pw²x₀²u

 Where I is measured in watts/m², p is the density (kg/m³) of the medium in which the wave is traveling, w is given by 2n/T where n is pi and T is the period of oscillation for a Simple Harmonic wave. In addition, x₀ is the amplitude of the wave (m) and u its velocity (m/s).

W is also given by 2nf where f is the frequency of the wave. W is called the angular frequency of the wave.

For the case of sound waves, the smallest sound intensity (I₀), detectable by the human ear is approximately 10^-12 W/m². A tube train is 10¹⁰I₀, whereas a jet aircraft is 10¹⁵I₀.

So, if I = 10¹⁵I₀

Then, I/I₀ = 10¹⁵

The increase in 'loudness' of a sound is dependent upon the ratio of the intensities rather than the absolute differences in intensity. The increase in loudness is calculated by taking the logarithm of the ratio of the intensities to base 10 and this is measured in bels. So from the above equation:

The noise of a jet aircraft expressed in bels is given by log₁₀ (I/I₀) = log₁₀ 10¹⁵

Using the laws of logarithms this gives 15log₁₀ 10 = 15 bels

Normally working in bels is a bit cumbersome so the standard way of working is to use decibels, which is one tenth of a bel. So the noise of a jet aircraft is about 150 decibels, given by 10log₁₀ (I_j /I₀) relative to the lowest intensity of sound that a human ear can detect.

For an example, if the volume of a Hi-Fi system is turned down such that such that the power decreases from 1000mW to 200mW then the change in power, in decibels is given by:

10log₁₀(200/1000) = 10log₁₀2 - 10log₁₀10 = -7dB

The minus sign indicating a decrease in power.

So when one states that there has been a loss of x dB across a connector or a circuit, this means that there has been a decline in intensity of a given signal as indicated by the ratio x.

In the structured cabling world decibels are used as a measurement of noise, attenuation, and signal loss.

Length

The length measurement is achieved using Time Domain Reflectometry (TDR) techniques, a pulse is sent down the line and the time that it takes to receive the pulse back at the tester transmissions device is used to determine the length of the cable. Different type of cable will produce different results therefore a secondary parameter is necessary that distinguishes the type of cable being tested. This parameter is referred to as the Nominal Velocity of Propagation (NVP) and is expressed as a percentage of the speed of light (c.). Typical values for Category 5 cables range from 0.65c. to 0.85c. If the incorrect NVP is entered into the tester the results can be totally misleading where a 91 m link appears to pass the test whereas the true reading may be a failing 105 m link.

Cancellers

Unbalanced and Balanced Transmission

 An unbalanced circuit has one of the wires in a pair grounded at both ends. An example is EIA-232. The problem with this is that any noise produced from external sources such as voltage switching, ballast from fluorescent lights etc. will add to the signal and be seen as data. This is because any noise appearing on the ground wire is sunk to earth whilst still adding or detracting to the signal in the signal wire. This gets worst at higher data rates and over longer distances. Using a shield ground at both ends helps to alleviate the problem

A much better way to resolve this issue is to use a balanced circuit. A balun, a small transformer, isolates the wires from the circuitry and instead of passing a 0v to 2v signal, for instance, it passes a -1v to +1v signal such that each wire passes the opposite signal of the other wire's signal. In theory, the resultant EMF should be zero. The better quality of the cabling system components the more likely the signal fields cancel each other out. As the signals travel the wire the same amount of noise is picked up by both wires resulting in their being zero difference between the signals at the receiving end.

There are considered to be two types of transmission in a balanced cable:

 Differential Mode, where the conductors in a pair carry the same signal but opposite in polarity. The assumption is made that the transmission line is infinitely long and is perfectly uniform and that there is zero radiation.

 Longitudinal Mode, where the sent signal is not perfectly balanced and the twisted pair are not perfectly balanced. This results in the two signals on the conductor not completely canceling so that a net current is induced on the pair thus producing radiation. EMF from surrounding systems is coupled onto the pair.

Installation imperfections mean that some of the differential signals convert into longitudinal signals (Longitudinal Conversion Loss so both differential and longitudinal modes need to be examined when looking at EMC and cross-talk (Differential Mode NEXT). This loss is called the balance of a cable and is measured at one interface, typically the transmitter end since this is where much of the EMC is likely to occur. There is another measurement called Longitudinal Conversion Transfer Loss, which is measured between the two interfaces at the ends of a link and so takes into account the differential noise at the receiver.

Common Mode means that a current flows down both conductors in the same direction and uses the earth as the return path. Common mode noise current is often 'shunted' to earth via an earthed center tap on the receiver transformer.

Attenuation :

Attenuation is where the signal is diminished due to losses incurred throughout the transmission medium be it fiber or copper. For copper, it is normally measured in decibels per 100m, meaning the ratio of intensities between the far end of the 100m and the starting end for a given signal, a ratio of power out over power in. In copper, attenuation is largely due to copper loss and to dielectric loss from the jacket materials used around the wires. Polyethylene and Teflon are currently the best materials that minimize dielectric loss, or dissipation. Attenuation over a certain length of cable is linear for a given frequency but is different for different frequencies and temperatures

 Attenuation in fiber is far less than that for copper. Lucent Technologies guarantee that for multimode fiber there is a loss of 3.4dB/km at 850nm, whilst for single-mode Truewave fiber the loss is 0.22dB/km at 1550nm.

 Attenuation (dB) of a cable or connector is given by 10.log (received signal power without cable/received signal power with cable).

 How to measure parameters such as attenuation, NEXT and Structural Return Loss is defined in ASTM D 4566.

Insertion Loss: The maximum insertion loss allowed for a 10BASE-T link is 11.5dB at all frequencies between 5.0 and 10.0 Mhz. This includes the attenuation of the cables, connectors, patch panels, and reflection losses due to impedance mismatches to the link segment.

Nominal Velocity of Propagation (NVP)

In a conductor, electrons travel at near the speed of light. For a copper cable this speed is often expressed relative to the speed of light. For Lucent's 3071 cable the NVP is given as 0.69, meaning that electrons are traveling at 69% the speed of light. The lower the NVP, the greater the delay in signals reaching a destination, so a high NVP is required for a good quality cable. 

The NVP of a particular type of cable can vary between batches. This variation can be as much as 20% so if cable testing is carried out then it is important to implement NVP calibration of each batch and adjust the default NVP value (built in to the tester) to the measured NVP for the particular batch. NVP calibration involves using the tester to measure a known length of cable and adjusting the tester's length measurement accordingly.  

Wire Map:

The wire map test is crucial on UTP/STP systems as if this fails then it is likely that most applications will not even begin to work. The wire map test is simply a check to see that wires are connected one-to-one at each end.

 As well as shorts and open circuits, reversed pairs, crossed wires and split pairs are detected.

 Capacitance: A capacitor is a device that stores electricity and it consists of two metal conductors surrounded by a dielectric. When there is a circuit attached to the conductors (such as used by a carrier signal down two wires) a potential difference exists across the two conductors. Whilst this potential difference exists, electrons flow within the circuit to the conductor on the most negative side of the circuit whilst electrons also flow away from the most positive side. This happens until the potential difference across the conductors exactly counteracts that of the attached circuit. If the circuit is disconnected then the two conductors maintain the potential difference until they are connected together. Once this happens a current flows for a short period of time as the electrons flow in order to bring back a zero potential difference between the conductors. This is called discharging.

 Capacitance is measured in Farads and is given by Q/V, where Q is charge (measured in coulombs) and V is the potential difference (measured in volts).

 Because a farad is a very large unit of measurement you will more commonly see microfarads (10^-6) or picofarads (10^-12).

 The Mutual Capacitance of a cable is that which exists between the two conductors of a pair. The higher this is, the more likely the possibility of there being interference between the wires. EIA/TIA 568A sets an upper limit of 5.6nF/100m of Category 5 cable.

 Characteristic Impedance: Being the AC equivalent of DC resistance, impedance of a data circuit takes into account not only the resistance of the copper wire but also the reactance of the cable capacitance. For an alternating current the instantaneous voltage is given by:

 I₀R.sinwt - X_CI₀coswt

 X_C is the reactance of the capacitor and the voltage across the capacitance lags that across the resistance (R) by 90^o.

 By use of trigonometry (and without going into details) we end up with the impedance of a data circuit being given by:

 Z = V/I = (X_C² + R²)^1/2

 We can complicate matters by also including the reactance of the inductance of a cable in the maths since strictly this also makes up impedance, however to simplify matters we won't.

 The Input Impedance is the impedance of a particular circuit or cable at a specific frequency. The characteristic impedance of a cable is the specified and manufactured impedance for that cable. This value is obtained by taking plotting the input impedances for each frequency over a range and applying a smoothing function to the curve. The Characteristic Impedance should remain constant throughout the length of the cable, however variations from this characteristic impedance will be due to faults within the cable or the connecting hardware. Time Domain Reflectometry is used to find impedance defects. Category 5 cable is designed to have a characteristic impedance of 100ohms. The specifications require the impedance to not deviate from 100ohms by more than +/- 15 ohms for each frequency from 1MHz up. All components within a cable link should match each other at each frequency since an impedance mismatch will hinder coupling of the signal from one component to another and result in reflections and attenuation.

 Standards such as IEC 1156 and ATM D-4566 define impedance measurement on long cables.

 Coupling Attenuation

 Resistance Unbalance

 (See EIA/TIA 568A section 10.2.4.2)

 DC Loop Resistance: This is the resistance of the wire pair when short-circuited at the far end. This is important for Token Ring that uses relays to allow in and shut out stations from the Multi-station Access Unit.

 Attenuation Deviation: 

 Pair to Pair Near End Cross Talk (Pr-Pr NEXT)

 Cross-talk is defined as signals that have been induced, or coupled, from one active pair of wires (disturber), to another. The name comes from the effect occurring within multipair telephone cables such that other people's conversations could be heard on the line that you yourself were using. The current from the one pair of wires was inducing an emf within an adjacent pair of wires.

Pair to pair Near End Cross-talk describes the noise from the transmit pair coupled onto the receive pair of wires within the same jacket and at the near end, meaning as applied to the receive pair nearest to the transmit end. In a four pair cable, if one pair is used for transmitting a signal, then some noise will be induced on to the other pairs to varying degrees. Cable balance, twist rates and cable spacing are all factors that help reduce Cross-talk. There are also Cross-talk cancellation methods available.

NEXT is measured in decibels, and is the difference in amplitude between the actual signal and the Cross-talk signal. The NEXT effect is often expressed as Cross-talk isolation or NEXT Loss, meaning that the higher the Cross-talk isolation the less the coupling effect. NEXT is effectively a measure of the attenuation between pairs, therefore, a high NEXT Loss is good!

The Cross-talk signal is strongest at the transmit end of the pair and the next 20m. For this reason the socket at the far end is excluded from NEXT calculations as it has minimal impact. The NEXT is calculated by anti-logging each noise component and then summing the noise (measured in mV) before logging the sum. To verify the Channel quality, EIA/TIA 568A recommends testing both at the frame end and at the socket end since the far end is not included in the test itself. When this has been carried out, then you will see two sets of results, one for the near end and one for the remote end. Some testers allow this to be carried out without having to swap the tester and the remote module around.

The tighter the twists within a cable the less chance the pairs have of sharing space with other pairs and thereby incurring coupled noise. Having different twist ratios per pair results in each pair having a different lay length (the distance between full twists), the aim being to minimize, as far as possible, the distance that conductors lie next to each other.

Power Sum Near End Cross Talk (PS-NEXT)

This is the cross-talk between one pair and the combination of all the other pairs within the same jacket. This is important as more and more applications use all pairs within a jacket (e.g. Gigabit Ethernet). Power summation is not limited to 4 pair cables, it has been used in the production of larger pair count cables such as Lucent's 25pr 1061C cable.

Far End Cross Talk (FEXT)

FEXT is the coupling of the transmit signals on to the receive pair or wires at the far end, away from the transmission source on the disturbing pair. Expressed in dB, it is described as the ratio of the power induced (P_i) over the power of the original signal (P_o). FEXT = 10log P_i /P_o. The higher the FEXT isolation in decibels, the less the coupling effect. FEXT is particularly important for data systems that use parallel transmission such as Gigabit Ethernet (802.3ab) that uses dual duplex PAM-5. NEXT and Delay can be minimized with electronics, but this is not possible with FEXT.

Power Sum Far End Cross Talk (PS-FEXT)

This is the powersum equivalent of FEXT.

Equal Level Far End Cross Talk (ELFEXT)

Measured in decibels, this is the ratio of the desired receive signal strength to the strength of the noise induced by the transmit signal at the other end. This is the same as FEXT - Attenuation. The effect of attenuation on the original signal has been added to give a more realistic result.

Channel Equal Level Far End Cross Talk (Channel ELFEXT)

ELFEXT is most significant for cable, however, in the proposed Category 6 standard Channel ELFEXT will be important as this includes the FEXT of the connectors.

Power Sum EL Far End Cross Talk (PS-ELFEXT)

This is the Powersum extension of ELFEXT and is calculated by summing the antilog of the ELFEXT for each pair, and logging the sum.

De-embedded NEXT

Terminated Open Circuit (TOC)

Pair to Pair Attenuation to Cross Talk Ratio (ACR)

ACR is measured in decibels and is equal to NEXT Loss - Attenuation and is a common method of specifying a cable's performance. ACR is calculated for each pair and for each frequency measured. For Class D operation the ACR must be 4dB at 100MHz. ACR gives some latitude when looking at a cable's performance in a cabling system, which has a large variety of lengths. A short length is likely to have relatively poor Cross-talk loss but low attenuation, whereas a long length will have relatively good Cross-talk loss and high signal attenuation.

ACR is a very important measure of how well an installation carries data. At a frequency of 200MHz you should be looking at a good 6dB difference between the Attenuation and Cross-talk curves indicating a 75/25 Signal/Noise ratio. Compare this with a 4 dB difference giving a 50/50 Signal/Noise ratio.

Often, margin values, in decibels, are quoted for ACR measurements. These express how far away the ACR value is from the limits imposed by the particular standard being measured against. A positive value indicates that the ACR value is above the standard and is therefore OK.

Power Sum Attenuation to Cross Talk Ratio (PS-ACR)

Alien Near End Cross Talk

This is the noise induced on a cable by another cable running parallel to it, whether signals are being transmitted or being received. The more cables that there are within a bundle the more mixed is the induced noise. Even adjacent bundles will add to the effect.

As stated earlier Cross-talk effects are most prevalent closest to the transmit end. To this end it is important to make sure that the cable bundles leaving the patch frames are not all running in parallel to each other. This includes the bundles sitting in trays and loose laid on the floor.

It is also important to make sure that the cable management allows the patch leads to be loose laid and not be running tightly together in parallel. This loose laying of patch leads is important as it helps to attenuate the Cross talk before it enters the cable bundles. Provided that you keep to patch lead length guidelines, the longer the patch lead the better as this gives more chance for the alien Cross-talk to attenuate.

Bends and tight cable ties affects the balance of the cable and therefore makes it more susceptible to noise as the cable is more likely to radiate noise and also receive noise.

Pre-bundled cables need to be balanced with each other before being installed on site.

Channel Propagation Delay

Measured in nanoseconds, this is the time required for a signal to travel the whole channel. Quantifying delay is important as protocols such as Ethernet rely on the time it takes for frames to traverse a LAN. With CSMA/CD, if there is a collision, the circuitry of the sender waits a certain amount of time before resending. This amount of time is the time it takes the signal to travel 205m of UTP (100m there and back plus 5m). This is called the Collision Domain Parameter. A cable that has low delay characteristics is ideal for faster protocols. The new standards are looking at this being at a maximum of 535ns over 100m. TSB 67 does not define a limit, but ISO/IEC 11801 has an upper limit of 1us.

Propagation Delay Skew

Also measured in nanoseconds, parallel transmission protocols (e.g. Gigabit Ethernet, 100BaseVG and 100BaseT4) use more than one pair to transmit signals. Delay Skew is a measure of the difference in delay between two signals traveling down separate pairs. Because the number of twists is different for each pair, the length of each pair is slightly different for each cable. Other names for this are Asymmetric Skew or Differential Delay. A good quality cable will have a tightly controlled NVP per pair and the electronics associated with the particular parallel transmission technology such as Gigabit Ethernet, will be able to cope with the Delay skew provided that it falls below the limit. The new standards are looking at this being 45ns as a maximum allowable value. The current agreement after TSB 67, is 50ns as an upper limit.

Structural Return Loss

A fiber or a wire will have imperfections and variations in density that cause some of the energy to be reflected back. This is called Structural Return Loss. SRL is measured in decibels for specific frequencies and comes from the Reflection Coefficient, which is the ratio of the incident to the reflected signal amplitudes. A high dB value indicates lower reflected energy. A good indication of a quality cable is when the noise from reflections is less than a tenth that of noise from Cross talk. A high value is good!

SRL is important in bi-directional transmission schemes since the reflected signal must not so high as to make detection of the receive signal difficult.

Channel Return Loss

This tells you something of the quality of the whole network link or channel. Return Loss is the sum of all the reflected signals from patch panel, connectors and cable arriving back at the signal-originating end. This will vary across the frequency range 1 to 250MHz and is only significant for the higher speed data rates. Impedance matching of the components is critical in order to minimize return loss.

LAN components struggle with reflected energies greater than 20% (7dB). Current standards call for 3% to 10% reflected energies (15 to 10dB).

(See ISO/IEC 11801 section A.1.1.4 for detail on the measurement of Return Loss and Propagation Delay)

Cable Capacity

Cable Capacity (C)(bps) is given by the formula C = W.log2(1 + SNR) where W is bandwidth (Hz) and SNR is Signal to Noise Ratio (dB).

If the ACR goes to zero at frequency f and A_f is the attenuation at that frequency then in the data cabling world where NEXT is important C = f(0.11(-A_f ) - 2.16).

The new Class D channel is to have a pair capacity of 480Mb/s, whilst the proposed class E is to have 1092Mb/s.

Echo Cancellation

Refractive Index  

In the above diagram, a ray of light traveling in air, in one plane, hits the glass surface (incident) at an angle i to the normal. It then changes direction as it travels through the glass at an angle r to the normal. It is refracted.

The laws of refraction are:

1.The incident and refracted rays, and the normal at the point of incidence, all lie in the same plane.

2.For two given media, sin i/sin r is a constant, where i is the angle of incidence and r is the angle of refraction.

The second law is Snell's Law and defines the refractive index for light traveling from one medium (1) to another (2), this is denoted by ₁n₂.

  The absolute refractive index is defined as the value obtained when light is traveling from a vacuum to a particular medium. In this case the refractive index is denoted by n. For glass n = 1.5, for water n = 1.33 and for air n = 1.00028.

  Let's look at the trigonometry a little more closely. 

Two parallel rays of light, IO and JP hit the glass surface in the same plane. Now IO hits the surface as the ray JP reaches point X. If we say that the distance XP takes t seconds for the light to travel, then since c is the velocity of light in air, XP = ct. Similarly, if v is the velocity of light in glass, then OY = vt.

Now, sin i/sin r = sin ION/sin YOR.

Using the other parallel ray of light, sin i/sin r = sin XOP/sin OPY.

Because the sine of an angle = length of the opposite side / length of the hypotenuse

sin XOP/sin OPY = XP/OP / OY/OP = XP/OY = ct/vt = c/v

This shows that the absolute refractive index of a medium can be calculated from the following:

n = velocity of light in a vacuum / velocity of light in the medium

Now that we have established that light is refracted when traveling from one type of medium to another, let us look at the special case of light traveling from a medium of higher refractive index to a medium of lower refractive index.

There is a critical angle of incidence beyond which all the light is reflected back into the first medium, in this case the glass.

  We know that n = sin i/sin r, so for the special case where i is the critical angle c when r = 90^o and n is the refractive index of the first medium, then:

  n = sin c/sin 90^o, since sin 90^o = 1

  From this, sin c = 1/n

  The critical angle for glass to air is given by sin c = 1/1.51 = 0.667, therefore c = 41.5^o.

  In an optical fiber, the cladding has a refractive index, which is less than that of the core material, and this provides the right environment for light to continuously reflect its way down the core of the fiber with minimal loss. For multimode fiber the critical angle is 74^o whilst for single-mode fiber it is 83^o. The Cone of Acceptance is the cone produced when spinning the critical angle about the longitudinal axis of the fiber core. Provided the light rays enter the fiber at an angle N, which is greater than the critical angle (relative to the horizontal surface of the fiber end), then the light will successfully be transmitted. This Cone of Acceptance is often called the Numerical Aperture and is equal to sin N.