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Industrial Ethernet Book Issue 72 / 37
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Screened and shielded cabling: the physics, facts and myths

Screened and shielded twisted–pair copper cabling has been around a long time. A global standard in the 1980s, many market sectors migrated largely to unshielded cables. However, 10GBase–T Ethernet over copper cabling has re–established the commercial viability of screened and shielded systems in a previously UTP–centric market. Valerie Maguire explores this competitive landscape, clarifying the many confusing and often contradictory messages which confound both cabling experts and end–users alike.

LAN CABLING emerged to support the first commercial computer networks which began to appear in the 1980s. These first networks were typically supported by IBM Token Ring transmission, which was standardised as IEEE 802.5 in 1985. Cabling for the Token Ring network consisted of 'IBM Type 1' cable mated to unique hermaphroditic connectors. The cable itself comprised two loosely twisted, foil shielded, 150 Ω pairs surrounded by an overall braid as may be seen in Figure 1.


Fig.1.IBM Type 1 cable

This media was a good choice in supporting the first generation LAN topologies for several reasons. Its design took advantage of the twisted–pair's transmission line physics to maximise both distance – Token Ring served distances up to 100m – and data rate using affordable transceivers. In addition, the foils and braid improved crosstalk and electromagnetic compatibility (EMC) performance to levels that could not yet be realised by early generation twisted–pair design and manufacturing capability. Not surprisingly, a handful of buildings are still supported by this robust cabling type today.

By 1990, network engineers were beginning to recognise the performance and reliability that switched Ethernet provided over Token Ring. Concurrently, twisted–pair cable design and manufacturing capabilities had progressed to the point where individual foils were no longer required to provide internal crosstalk isolation, and overall shields were not necessary to provide immunity against outside noise sources in the 10Base–T and 100Base–T bands of operation. The publication of both the 10Base–T application in 1990 and the first edition ANSI/EIA/TIA generic cabling standard in 1991, in conjunction with the lower
cost associated with unshielded twisted–pair (UTP) cabling, firmly established UTP cabling as the media of choice for new LAN network designs at that time.

A couple of decades later, as Ethernet application technology evolved to 10Gbps transmit rates, a marked resurgence in the specification of screened and fully–shielded twisted–pair cabling systems has occurred. The practical benefits of screens and shields can enhance the performance of traditional UTP cabling designs to support high bandwidth transmission. But it is also necessary to dispel a few common myths and misconceptions regarding the behaviour of screens and shields.

Balanced transmission

The benefit of specifying balanced twisted–pair cabling for data transmission is clearly demonstrated by examining the types of signals that are present in factory and building environments. Electrical signals can propagate in either common mode or differential (i.e. balanced) mode. Common mode describes a signal scheme between two conductors where the voltage propagates in phase and is referenced to
ground. For instance, electromagnetic noise induced from disturbers such as motors, transformers, fluorescent lights, and RF sources, also propagates in common mode. Indeed most signal, power and control voltage conduits around the factory environment deliver their power, signals or interference referenced to ground – except where [usually twisted–pair] cabling operates in balanced or differential mode transmission.

Differential mode transmission refers to a signal which is split into two halves of equal magnitude, but with each half–signal 180o out of phase to the other half, and that both halfsignals propagate over two conductors of a single twisted–pair. In a balanced circuit, two signals are referenced to each other rather than one signal being referenced to ground. There is no ground connection in a balanced circuit and, as a result, these types of circuits are inherently immune to interference from most common mode noise disturbers.

In theory, common mode noise couples onto each conductor of a perfectly balanced twistedpair equally. Differential mode transceivers detect the difference in peak–to–peak magnitude between the two signals on a twisted–pair by performing a subtraction operation. In a perfectly balanced cabling system, the induced common mode signal would appear as two equal voltages that are simply subtracted out by the transceiver, thereby resulting in perfect noise immunity.

As an aside, recording and broadcast studios connect individual microphones to the mixing desk using balanced cable to eliminate common mode mains hum and other sources of electrical interference.

Back in the real world of industrial networks, twisted–pair cables are not perfectly balanced and their limitations must be understood by application developers and system specifiers alike. TIA and ISO/IEC committees take extreme care in specifying balance parameters such as TCL (Transverse Conversion Loss), TCTL (Transverse Converse Transfer Loss) and ELTCTL (Equal Level Transverse Converse Transfer Loss) in their standards for higher grade (i.e., Category 6 and above) structured cabling. By examining the performance limits for these parameters, and by noting when they start to approach the noise isolation tolerance required by various Ethernet applications, it becomes clear that the practical operating bandwidth defined by acceptable levels of common mode noise immunity due to balance [or rather the
limits set by imbalance] is approximately 30MHz. While this provides more than sufficient noise immunity for applications such as 100Base–T and 1000Base–T, Shannon capacity modelling demonstrates that this level provides no headroom over the minimum 10GBase–T noise immunity requirements. Fortunately, the use of shielding significantly improves noise immunity, doubles the available Shannon capacity, and substantially increases practical operating bandwidth for future applications.

One effect of degraded twisted–pair signal balance above 30MHz is modal conversion, which occurs when differential mode signals convert to common mode signals and vice versa. The conversion – caused by a local imbalance within an essentially balanced system – can adversely affect noise immunity from the environment, as well as contribute to crosstalk between pairs and balanced cables. It must be minimised whenever possible. Shielding can decrease the potential for modal conversion by limiting noise coupled onto the twisted–pair from the environment.

Fundamentals of interference

All applications require positive signal–to–noise (SNR) margins to transmit within allocated bit error rate (BER) levels. This means that the data signal being transmitted must be of greater magnitude than all of the combined noise disturbers coupled onto the transmission line (i.e., the structured cabling). As shown in Fig. 2, noise can be coupled onto twisted–pair cabling by any or all of three ways:


Fig. 2. LAN noise sources

• Differential noise (Vd): Noise induced from an adjacent twisted–pair or balanced cable;

• Environmental noise (Ve): Noise induced by an external electromagnetic field;

• Ground loop noise (Vg): Noise induced by a difference in potential between conductor ends.

Different applications have varying sensitivity to interference from these noise sources depending upon their capabilities. For example, the 10GBase–T application is commonly recognised as being extremely sensitive to alien crosstalk (differential mode cable–to–cable coupling) because its digital signal processing capability electronically cancels internal pairto– pair crosstalk within each channel. Unlike pair–to–pair crosstalk, alien crosstalk cannot be cancelled by DSP. Conversely, since the magnitude of alien crosstalk is very small compared to the magnitude of pair–to–pair crosstalk, the presence of alien crosstalk only
minimally affects the performance of other applications, such as 100Base–T and 1000Base– T that employ partial or no crosstalk cancelling algorithms.

Electromagnetic compatibility (EMC) describes both a system's susceptibility (immunity) to interference from outside sources, and the system's potential to disturb outside sources by radiating interference. Together both the received and radiated interference levels describe an important indicator of a system's ability to co–exist with other electronic/ electrical devices. Noise immunity and emissions performance is reciprocal, meaning that a cabling system's ability to maintain immunity to interference is inversely proportional to its potential to radiate. Interestingly, while much unnecessary emphasis is placed on immunity
considerations, it is an understood fact that structured cabling systems do not radiate or interfere with other equipment or systems in the telecommunications environment!

Differential noise disturbers: Alien and internal pair–to–pair crosstalk are examples of differential mode noise disturbers that must be minimised through proper cabling system design. Susceptibility to interference from differential mode sources is dependent upon system balance, and may be improved by isolating or separating conductors that are interfering with each other. Cabling with improved balance (i.e., Category 6 and above) exhibits better internal crosstalk and alien crosstalk performance. Since no cable is perfectly balanced, strategies such as using dielectric material to separate conductors or using metal foil to isolate conductors are used to further improve crosstalk performance.

For example, Cat. 6A F/UTP cabling is proven to have substantially superior alien crosstalk performance than category 6A UTP cabling because its overall foil construction reduces alien crosstalk coupling to virtually zero.Category 7 S/FTP (Screened Foiled Twisted Pair) is proven to have substantially superior pairto– pair and alien crosstalk performance than any category 6A cabling design because its individual foiled twisted–pair construction reduces pair–to–pair and alien crosstalk coupling to virtually zero. These superior crosstalk levels could not be achieved solely through compliant balance performance.

Environmental noise disturbers: Environmental noise is electromagnetic noise that is comprised of magnetic fields (H) generated by inductive coupling (expressed in A/m) and electric fields (E) generated by
capacitive coupling (expressed in V/m). Magnetic field coupling occurs at low frequencies (i.e., powerline 50 or 60Hz) where the balance of the cabling system is more than sufficient to ensure immunity, which means that its impact may be ignored for all types of balanced cabling. Electric fields, however, can produce common mode voltages on balanced cables depending on their frequency. The magnitude of the voltage induced can be modelled assuming that the cabling system is susceptible to interference in the same manner as a loop antenna [1]. For ease of analysis, equation (1) represents a simplified loop
antenna model that is appropriate for evaluating the impact on the electric field generated due to various interfering noise source bandwidths, as well as the distance relationship of the twisted–pairs to the ground
plane. Note that a more detailed model, which specially includes the incidence angle of the electric fields, is required to calculate accurately actual coupled noise voltage.

Where λ is the wavelength of the interfering noise source;

A = the area of the loop formed by the disturbed length of the cabling conductor (l) suspended an average height (h) above the ground plane;

E = the electric field intensity of the interfering source.

The wavelength, λ , of the interfering source can range anywhere from 5000km for a 60Hz signal to shorter than 1m for RF signals in the 100MHz and higher band. The electric field strength density varies depending upon the disturber, is dependent upon proximity to the source, and is normally reduced to null levels at a distance of .3m from the source. The equation demonstrates that a 60Hz signal results in an electric field disturbance that can only be measured in ¦ÌV, while interference sources in the MHz frequency range can generate a fairly large electric field disturbance. For reference, 3V/m is considered to be a reasonable approximation of the average electric field present in a light industrial/commercial environment and 10V/m is considered to be a reasonable approximation of the average electric field present in an industrial environment.

The one variable that impacts the magnitude of the voltage coupled by the electric field is the loop area, A, which is derived by multiplying the disturbed length of the cabling (l) by the average height (h) from the ground plane. The cross–sectional view in Fig. 3 depicts the common mode currents that are generated by an electric field. It is these currents that induce unwanted signals on the outermost conductive element of the cabling (i.e., the conductors themselves in a UTP set–up or the overall screen/shield in a screened/fullyshielded environment). What becomes readily apparent is that the common mode impedance, as determined by the distance (h) to the ground plane, is not very well controlled in UTP environments. This impedance is dependent upon factors such as distance from metallic raceways, metallic structures surrounding the pairs, the use of non–metallic raceways, and termination location. Conversely, this common mode impedance is well defined and controlled in screened/fully–shielded cabling environments since the screen and/or shield acts as the ground plane.


Fig.3:CommonModeCurrents

Average approximations for (h) can range anywhere from tens of centimetres for UTP cabling, but are significantly more constrained (i.e. less than a millimetre) for screened and fully–shielded cabling. This means that screened and fully–shielded cabling theoretically offers 100 to 1000 times the immunity protection from electric field disturbances compared to UTP cabling!

It is important to remember that the overall susceptibility of twisted–pair cables to electric field disturbance is dependent upon both the balance performance of the cabling and the presence of a screen or shield. Well balanced (i.e., Category 6 and above) cables should be immune to electromagnetic interference up to 30MHz. The presence of a shield or screen is necessary to avoid electromagnetic interference at higher frequencies, an especially critical consideration for future–proofing network infrastructure. For example, it is reasonable to model that an emerging application using DSP techniques will require a minimum SNR of 20dB at 100MHz. Since the minimum isolation yielded by balance alone is also 20dB at 100MHz, the addition of a screen or shield will be necessary to ensure sufficient noise immunity headroom for the application.

Ground Loops

Ground loops develop when there is more than one ground connection and the difference in common mode voltage potential at these ground connections introduces (generates) noise on the cabling. The mechanism is shown in Fig. 4. That common mode noise from ground loops can only appear on screens and shields is a misconception; this noise regularly appears on the twisted–pairs as well. Most of the ground loop–induced EMF comes from the building AC powerline. In the US, the primary noise frequency is 60Hz and its related harmonics, often referred to as AC hum. In other regions of the world, the primary noise frequency is of course 50Hz and harmonics.

Since each twisted–pair is connected to a balun (balance to unbalance) transformer which forms the common mode noise rejection circuitry at both the NIC and network equipment ends, differences in the turns ratios and common mode ground impedances can result in common mode noise. The magnitude of the induced noise on the twisted–pairs can be reduced, but not eliminated, through the use of common mode terminations, chokes, and filters within the equipment.

Ground loops induced on the screen/shield typically occur because of a difference in potential between the ground connection at the telecommunications grounding busbar (TGB) and the building ground connection provided through the network equipment chassis at the work area–end of the cabling. One should note that it is not mandatory for equipment manufacturers to provide a low impedance building ground path from the shielded RJ45 jack through the equipment chassis. Sometimes the chassis is isolated from the building ground with a protective RC circuit and, in other cases, the shielded RJ45 jack is completely isolated from the chassis ground.

TIA and ISO standards identify the threshold when an excessive ground loop develops: when the difference in potential between the voltage measured at the shield at the work area–end of the cabling and the voltage measured at the ground wire of the electrical outlet used to supply power to the workstation exceeds 1.0Vrms. This difference in potential should be measured and corrected in the field to ensure proper network equipment operation, but values of this magnitude are very rarely found where building regulations specify compliant grounding systems. Furthermore, since the common mode voltage induced by ground loops is low frequency, the balance performance of the cabling infrastructure by itself is sufficient to ensure immunity regardless of the actual voltage magnitude.

Design of screens and shields

Shielding offers the benefits of significantly improved pair–to–pair crosstalk performance, alien crosstalk performance, and noise immunity that cannot be matched by any other cabling design strategy. Category 6A and lower rated F/UTP cables are constructed with an overall foil surrounding four twisted–pairs as shown in Fig. 5.


Fig. 5. F/UTP construction

Category 7 and higher rated S/FTP cables are constructed with an overall braid surrounding four individually foil shielded pairs. (Figure 6). Optional drain wires are sometimes provided.


Fig.6.S/FTPconstruction

Shielding materials are selected for their ability to reflect the incoming wave, their absorption properties, and their ability to provide a low impedance signal path. As a rule, more conductive shielding materials yield greater amounts of incoming signal reflection. Solid aluminium foil is the preferred shielding media for telecommunications cabling because it provides 100% coverage against high frequency (i.e., greater than 100MHz) leakage, as well as low electrical resistance when properly connected to ground. The thickness of the foil shield is influenced by the skin effect of the interfering noise currents. Skin effect is the phenomenon where the depth of penetration of the noise current decreases as frequency increases. Typical foil thicknesses might be from 0.04 to 0.05mm to match the maximum penetration depth of a 30MHz signal. This design approach ensures that higher frequency signals will not be able to pass through the foil shield. Lower frequency signals will not interfere with the twisted–pairs because of the inherent performance of a balanced system. Braids and drain wires add strength to cable assemblies and further decrease the end–toend electrical resistance of the shield when the cabling system is properly connected to ground.

Grounding and cabling systems

When it comes to applicable standards, ANSIJ– STD–A defines the building telecommunications grounding and bonding infrastructure. The remit begins at the service equipment (power) ground and extends throughout the building. It is important to recognise that the infrastructure applies to both UTP and screened/fully–shielded cabling systems. The standard mandates that:

• The telecommunications main grounding busbar (TMGB) is bonded to the main building service ground. Actual methods, materials and appropriate specifications for each of the components in the telecommunications grounding and bonding system vary according to system and network size, capacity and local codes;

• If used, telecommunications grounding busbars (TGB's) are bonded to the TMGB via the telecommunications bonding backbone;

• All racks and metallic pathways are connected to the TMGB or TGB;

• The cabling plant and telecommunications equipment are grounded to equipment racks or adjacent metallic pathways.

Guide to alien crosstalk

Alien crosstalk is the most significant transmission parameter affecting 10GBase–T performance and should be carefully evaluated by end–users and installers during the cabling specification process.
It is defined as:

Unwanted signal coupling from one balanced twistedpair component, channel, or permanent link to another twisted–pair component, channel, or permanent link.

Since alien crosstalk is only caused by differential (or balanced) signal coupling,it is not affected one or the other by common mode noise such as radiation from motors or fluorescent lights, etc.

Alien crosstalk is only specified by the current standards as a power sum parameter for components and cabling to approximate the energy present when all cabling pairs are energised. High power sum alien crosstalk levels can compromise the operation of a 10GBase–T application by significantly reducing expected signal–to–noise margins, potentially causing retransmission or even switch auto–negotiation to a lower Ethernet speed.

Power sum alien crosstalk measured at the near–end of the transmitter is called power sum alien near–end crosstalk loss (PSANEXT loss).

Power sum alien crosstalk measured at the far–end of the transmitter is called power sum alien attenuation to crosstalk ratio, far–end (PSAACRF).

Alien crosstalk in 10Gbpsready systems

Category 6A/class EA and class F/FA cabling are specified to support the 10GBase–T application over worstcase 100m, 4connector channel topologies.

UTP (Category 6A/class EA) has increased cable diameter up to 9.0 mm (0.354 in.) and separation between connectors to reduce alien crosstalk.

F/UTP (Category 6A/class EA and class F/FA) includes a foil screen to virtually eliminate the effect. S/FTP (Class F/FA) with full shielding eliminates alien crosstalk.

The main difference between Cat6A/class EA and Cat6/class E UTP cable is the greatly increased outside jacket wall thickness.Design strategies use thicker jackets to separate the copper cores from each other to ensure compliant alien crosstalk performance.Installation practices that deform the jacket (e.g.excessive pathway fill,overcinched tie wraps, etc.) can compromise the margins.

The transmission specifications of Cat6A/class EA cabling are significantly more stringent than those specified for Cat6/class E cabling. For example, Cat6A/class EA cabling induces 80% less alien crosstalk voltage than that produced by a straight Cat6 channel. Furthermore, Cat6A/class EA systems are also specified to have more stringent insertion loss requirements to maintain a sufficient signaltoalien crosstalk margin up to 500MHz required by a 10GBase–T application.

 TIA and ISO standards provide one additional step for the grounding of screened and shield cabling systems. Specifically, clause 4.6 of ANSI/TIA–B.1 and clause 11.3 of ISO/IEC 11801:2002 state that the cable shield shall be bonded to the TGB in the telecommunications room, and that grounding at the work area may be accomplished through the equipment power connection. This procedure is intended to support the optimum configuration of one ground connection to minimise the appearance of ground loops, but recognises that multiple ground connections may be present along the cabling. Since the possibility that grounding at the work area through the equipment may occur was allowed for within ANSI–J–STD–A, there is no need to avoid specifically grounding the screened/shielded system at the end user's PC or device.

 It is important to note the difference between a ground connection and a screen/shield connection. A ground connection bonds the screened/shielded cabling system to the TGB or TMGB, while a screened/shield connection maintains electrical continuation of the cable screen/shield through the screened/shielded telecommunication connectors along the full length of cabling. Part of the function of the screen or shield is to provide a low impedance ground path for noise currents that are induced on the shielding material. Compliance with TIA and ISO specifications ensures that a low impedance path is maintained through all screened/shielded connection points in the cabling system. For optimum alien crosstalk and noise immunity performance, shield continuity should be maintained throughout the cabling system end–to–end. The use of UTP patch cords in screened/shielded cabling systems should be avoided.

 It is suggested that building end–users perform a validation to ensure that screened and shielded cabling systems are properly ground to the TGB or TMGB. A recommended inspection plan involves:

• Visually inspection to verify that all equipment racks, cabinets and metallic conduits are bonded to the TGB or TGMB using a 6AWG conductor;

• Visually inspection to verify that all screened/shielded patch panels are bonded to the TGB or TGMB likewise;

• Perform a DC resistance test to ensure that each panel and rack/cabinet grounding connection exhibits a DC resistance measurement of <1Ω between the bonding point of the panel/rack and the TGB or TMGB. (Note: some local/regional standards specify a maximum DC resistance of <5Ω at this location.);

The antenna myth

It is a common myth that screens and shields can behave as antennas because they are long lengths of metal. There is a fear that screens and shields can ¡®attract' signals that are in the environment, or radiate signals that appear on the twisted–pairs. The truth is that both screens, shields and the copper balanced twisted–pairs in a UTP cable will behave as an antenna to some degree. The difference is that, as demonstrated by the simplified loop antenna model, the noise that couples onto the screen or shield is actually 100 to 1000 times smaller in magnitude than the noise that is coupled onto an unshielded twisted–pair in the same environment. This is due to the internal pairs' well–defined and controlled common mode impedance to the ground plane that is provided by the screen/shield.

To illustrate the point here follows an analysis of the two types of signal disturbers that can affect the noise immunity performance of balanced twisted–pair cabling: those below 30MHz and those above 30MHz.

At frequencies below 30MHz, noise currents from the environment may penetrate the screen/shield and affect the twisted–pairs. However, the simplified loop antenna model shows that the magnitude of these signals is substantially smaller (and mostly attenuated due to the absorption loss of the aluminium foil), meaning that unshielded twisted–pairs in the same environment are actually subjected to much a higher electric field strength. The good news is that the balance performance of the cable itself is sufficient up to 30MHz to ensure minimum susceptibility to disturbance from these noise sources regardless of the presence of an overall screen/shield.

At frequencies above 30MHz, noise currents from the environment cannot penetrate the screen/shield due to skin effects and the internal twisted–pairs are fully immune to interference. Unfortunately, balance performance is no longer sufficient to ensure adequate noise immunity for UTP cabling at these higher frequencies. This can have an adverse impact on the cabling system's ability to maintain the signal–to–noise levels required by applications employing DSP technology.

The potential for a cable to behave as an antenna can be experimentally verified by arranging two balanced cables in series, injecting a signal into one cable to emulate a transmit antenna across a swept frequency range, and measuring the interference on an adjacent cable to emulate a receiving antenna[2]. As a rule of thumb: the higher the frequency of the noise source, the greater the potential for interference. As recorded in Fig. 7, the coupling between two UTP cables (shown in black) is a minimum of 40dB worse than the interaction between two properly grounded F/UTP cables (shown in blue). It should be noted that the 40dB margin corresponds to a voltage coupling 100 times less, thus confirming the modelled predictions. Clearly, the UTP cable is radiating and receiving (i.e. behaving like an antenna) substantially more than the F/UTP cable¡­


Fig.7.UTPVS.F/UTPsusceptibility(DataprovidedcourtesyofNEXANS/BerkTek)

A second antenna myth is related to the inaccurate belief that common mode signals appearing on a screen or shield can only be dissipated through a low impedance ground path. The fear is that an ungrounded screen will radiate signals that are ¡®bouncing back and forth' and ¡®building up' over the screen/shield. The fact is that, left ungrounded, a screen/shield will still substantially attenuate higher frequency signals because of the lowpass filter formed by its resistance, distributed shunt capacitance, and series inductance. The effects of leaving both ends of a foil twistedpair cable ungrounded can also be verified using the previous experimental method. With the results presented in Fig. 8, the coupling between two UTP cables (shown in black) is still a minimum of 20dB worse than the interaction between two ungrounded F/UTP cables (shown in blue). It should be noted that 20dB of margin corresponds to 10 times less voltage coupling. Even under worst–case, ungrounded conditions, the UTP cable behaves more like an antenna than the F/UTP cable.


Fig.8.UTPvs.Ungrounded F/UTP Susceptibility (Dataprovided courtesy of NEXANS/Berk–Tek)

Modelled and experimental results clearly dispel the antenna myth. Screens and shields offer substantially improved noise immunity compared to unshielded constructions above 30MHz... even when improperly grounded.

The ground loop myth

Ground loops only appear on screened and shielded cabling systems – another common myth! The fear is that ground loops resulting from a difference in voltage potential between a screen/shielded cabling system's ground connections cause excessive common mode currents¡­ It is said that these currents may adversely affect data transmission. In truth, both screens, shields and the balanced twisted–pairs in a UTP cable are affected by differences in voltage potential at the ends of the channel.

The difference in the transformer common mode termination impedance – at the NIC and the network equipment – naturally results in common mode noise current being induced on each twisted–pair. Grounding of the screened/shielded system in multiple locations can also result in common mode noise current induced on the screen/shield. However, these currents do not affect data transmission because, regardless of their voltage magnitude, their waveform is always associated with the profile of the building AC power (i.e. 50 Hz or 60 Hz). Due to the excellent balance of the cabling at low frequencies, common mode currents induced onto the twistedpair, either directly from equipment impedance differentials or coupled from a screen/shield, are simply subtracted out by the transceiver as part of the differential transmission algorithm.

Why screened/shielded cabling?

Achievable SNR margin is dependent upon the combined properties of cabling balance, and the common mode and differential mode noise immunity provided by screens and shields. Applications rely on a decent SNR margin to ensure proper signal transmission and minimum BER. With the emergence of 10GBase–T, it is now quite clear that the noise isolation provided by good balance alone is just barely sufficient to support transmission objectives. The alien crosstalk and noise immunity benefits provided by F/UTP and S/FTP cabling designs have been demonstrated to offer almost double the Shannon capacity, and this performance advantage has caught the attention of application developers and system specifiers. It is often said that the telecoms industry has come full circle in the specification of its preferred media type. In actuality, the present generation of screened and fullyshielded cabling systems represent a fusion of the best features from the last two generations of LAN cabling: excellent balance to protect against low frequency interference and shielding to protect against high frequency interference.

[1] B. Lord, P. Kish, and J.Walling, Nordx/CDT, 'Balance Measurements of UTP Connecting Hardware', 1996

[2] M. Pelt, Alcatel Cabling Systems, 'Cable to Cable Coupling', 1997

Valerie Maguire is a senior engineer at the Siemon Company

www.siemon.com


Source: Industrial Ethernet Book Issue 72 / 37
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