This White Paper discusses the subject of 10 Gigabit/second data transmission over balanced twisted-pair cabling. Balanced twisted-pair cabling refers to either unshielded twisted pair (UTP) or screened twisted pair (ScTP) cabling. The term screened or shielded twisted pair generally refers to a TIA recognised cable construction with an overall metallic screen over a 4-pair core, also designated as F/UTP in the International Standards. Arguments can be made in support of unshielded twisted-pair (UTP) or screened twisted-pair (ScTP) cabling for 10 Gb/s applications over copper.
It is important for the end user to understand that shielded cabling is not a magic cure to solve the problem of noise. It can often make the situation worse. When considering all the issues, advanced UTP solutions such as Belden’s IBDN System 10GX provide not only superior noise performance, but do so without the additional bonding and grounding requirements of ScTP cabling.
Unshielded vs shielded cabling
There is a perception that screened cable provides better noise immunity because the screen acts as a defensive barrier against electromagnetic interference. The effectiveness of shielding as an electromagnetic barrier depends on many things, including;
1. The quality and the reliability of the shield terminations.
2. The balance of the twisted pairs.
3. The impedance of the ground connection.
4. The ground potential difference between local and remote grounds.
All these factors can have a significant impact on system performance and noise immunity. Noise immunity performance depends very much on the pair balance for both unshielded and shielded cabling. It is important to understand the sources of external noise and the mechanisms for counteracting external noise.
Shielding works, right?
Here is a quote from a noted expert on shielding: “In high-speed digital applications, a low impedance connection between the shield and the equipment chassis at both ends is required in order for the shield to do its job.”
Two conditions are necessary to ensure that shielding works at high frequencies.
• First, the screen needs to be grounded at both ends.
• Second, the screen needs to be terminated with a low impedance connection to ground. A low impedance ground connection requires a 360 degree contact between the foil and the shielded connector housing. A short length of ground wire (sometimes called a pigtail) is not a low impedance connection.
What is a low impedance connection?
In the standards, an impedance of 1 Ohm or less is considered to be a low impedance connection. #6 AWG ground wire is typically used for grounding connections for equipment, racks and patch panels. It is generally assumed that it provides a low impedance path to ground.
At power line and audio frequencies the impedance of a grounding conductor is approximately equal to the DC resistance. Above audio frequencies inductance begins to dominate and at radio frequencies the inductive impedance of even a short wire or circuit-board trace can be quite high. For example, at 100 MHz, the impedance of 50 mm of a 6 AWG ground conductor is 19 Ohms and is nowhere close to the 1 Ohm impedance that is required. 10GBase-T transmission bandwidth goes up to 400 MHz.
A ground conductor (pigtail) is not a low impedance ground at high frequencies. One approach to achieve lower impedance at high frequencies is to use a flat strap instead.
To ensure that the inductance of a ground strap is sufficiently low, its width must be at least one-fifth or, better yet, one-third of its length. If a designer cannot achieve this ratio, there will not be a satisfactory high-frequency current return path.
It is very difficult to achieve a low impedance ground at high frequencies. Distances to the ground reference plane must be kept very close. That is why most circuit board designs have a ground substrate very close to the circuit traces. Also, shielded connectors need to be 360 degree bonded to shielded equipment enclosures and to the shield of the cable to ensure a low impedance path to ground.
Noise coupling mechanism
Noise is induced into a balanced twisted pair cable due to the presence of an external electromagnetic field. The noise voltage that is induced into each conductor of a pair respectively and this voltage is called the common-mode voltage because each wire of a pair is exposed to approximately the same field at any distance along the length of cable. The magnitude of this common mode voltage is proportional to the field strength (usually specified in amperes/metre for magnetic field or volts/metre for electric field), the rate of change of field (frequency), and the loop area, which depends on the distance of the cable above the ground plane.
The whole concept of balanced pair transmission is that the signal is applied across the two conductors of a pair in differential-mode, also called transverse mode, where equal and opposite voltages (+V/-V) are applied on each conductor of a pair. If the twisted pair is well balanced with respect to ground, the difference in the noise voltage between Vcm1 and Vcm² (Vdm) is approximately zero. The pair balance is expressed as the ratio between the common-mode voltage (Vcm) and the differential-mode voltage (Vdm) in dB. TIA Category 6 and 6A standards define this parameter as TCL or Transverse Conversion Loss.
A tightly twisted well-balanced pair, such as a bonded pair, with equal conductor diameter, equal spacing and equal conductor length achieves the best TCL performance.
The purpose of the screen is to counteract the effect to the external field. The external field causes a current to flow in the shield, which induces an opposing voltage on each conductor of a pair. The net effect is to reduce the common-mode and the extent the common-mode noise voltage is reduced depends on the design of the screen (screening effectiveness) and on the impedance to ground – a high impedance to ground at either end defeats the purpose of the screen.
From this one may conclude that the presence of the screen would help to reduce external noise coupling, but this is only part of the story. The other part has to do with the balance of a pair. Bringing the screen in close proximity to the pair, increases the capacitive and inductive coupling to ground. The unbalance to ground is amplified due to the presence of a screen. Also any asymmetry or displacement of the screen due to manufacturing or installation can worsen the pair balance.
The noise immunity performance of both UTP and ScTP cables is directly related to the pair balance. The TIA 568 B.2-9 and TIA 568 B.2-10 standard specifies pair balance requirements for Category 6 and Category 6A cables, connectors and channels. The specified parameter for pair balance is called TCL. TCL is a test that is currently performed by manufacturers of cable and connecting hardware.
The pair balance of cables is affected by the manufacturing process and design. The pair balance of connecting hardware is primarily affected by design. TCL is not a parameter that can be measured in the field at this time. This might change in the future.
The TCL requirements for Category 6A UTP and ScTP cables are shown in the table. The Cable TCL is 40 dB at 10 MHz, 30 dB at 100 MHz and 23 dB at 500 MHz. The slope is 10 dB / decade. (See Table 1 below.)
In order to understand the meaning of TCL, an electrical transient on a power line that is running in close proximity to a telecommunications cable induces a common mode voltage on each conductor of a pair relative to ground. A TCL balance of 40 dB means that 1% of this common-mode voltage is converted into a differential mode voltage between the two conductors of a pair. It is this differential-mode voltage that appears as noise at the input of a transceiver. It is desirable to achieve a TCL balance of 40 dB or better to mitigate the noise coupling from power line transients and other sources of external noise.
You may have the best balanced cable in the world, but the pair balance performance of the channel can be dragged down by the performance of the connectivity (the mated plug/jack connection). The native RJ-45 plug and jack are inherently unbalanced because of the capacitive and inductive unbalance between the contacts in the plug that is not fully compensated in the jack. The method of compensation needs to compensate not only for pair-to-pair crosstalk but also the balance within and between pairs.
Ground loops – grounding at both ends
Last but not least is the problem of ground loops. In consulting the literature, it is reported that a vast majority of all EMC problems have inadequate grounding as the main culprit. Having a low DC resistance #6 AWG conductor does not ensure a low impedance path to ground at high frequencies.
Ground loop is a problem when a cable shield is terminated at both ends to a local ground and a remote ground. A ground potential difference between these ground points can cause stray currents to flow in the shield and couple noise into the signal conductors.
What size of ground potential difference problems are we talking about? TIA and ISO standards are speaking about Common Mode Noise of a maximum of 1 Volt in ‘well grounded’ plants. Literature is also speaking of the current measured on a main service grounding (in a large building) in terms of Amps. These currents are an accumulation of all the stray ground currents throughout the building.
What can those grounding currents and voltage differences do? Small voltage differences just cause noise to be added to the signals. This can cause humming noise to audio, interference bars to video signals and transmission errors to computer networks. Higher currents can cause more serious problems like sparking in connections, damages to equipment, heating cables and burned signal cables.
One way to avoid the problem of ground loops noise currents flowing in the shield is to ground the shield at one end only. However this solves one problem but creates another. A single ended ground does not work at high frequencies because of signal propagation and standing wave effects at the end of the cable when the screen is floating.
The input impedance of a screen that is grounded at one end looks like an open (infinite impedance) at the opposite end of the cable at a quarter wavelength and at every odd multiple of a quarter wavelength. At 100 MHz, a quarter wavelength corresponds to ½ a metre of cable, which is quite short indeed. At those resonant frequencies a maximum voltage (standing wave) is induced on the shield in the presence of EM interference. Grounding the screen at one end, works well for audio signals (less than 20 kHz) because these standing wave effects do not occur for cable lengths that are shorter than 1 km.
All that being said, it does not mean that a floating shield cable cannot be effective to reduce external noise coupling. It all depends on the pair balance of the cable conductors. It is possible to design and manufacture screened twisted pair cables with much better pair balance. For example, Belden uses a patented floating screen design for its 10GX patch cords. The essential elements of the design include a circumferential screen that is interposed between the two jackets. The stable position of the screen and the bonded-pair construction ensures an electrically-balanced design that is the key to ensuring exceptional EMC performance.
Summary
Unshielded cabling solutions
LAN cabling is predominantly UTP, and the design and installation practices for UTP are much simpler, better known and more widely used by technicians and throughout the cabling contractor community. There is much less experience with shielded cabling in North America, with less than 3% of network cabling installations utilising shielded products and techniques in 2007.
IEEE’s 10GBase-T Ethernet standard does not require screened cabling. In fact, one of the development criteria for this standard was operability over UTP cabling in order to leverage and protect both the installed-base and the knowledge-base in the market. Category 6A UTP complies fully with 10GBase-T requirements for worst case channels up to 100 metres and for 30 metre short reach mode operation.
Category 6A UTP cabling can offer a very significant improvement in noise immunity, because of better pair balance characteristics and the elimination of problems due to ground loops. Pair balance performance is about 10 to 15 dB better than what is currently specified in the TIA Category 6A standard.
Shielded cabling solutions
Screened cabling relies heavily on the reliability of three subsystems:
1. Signal transmission integrity.
2. Shield integrity and quality of shield terminations.
3. Building bonding and grounding system.
Proper shielding terminations are critical to ensure good electromagnetic shielding performance, including alien crosstalk performance. In one example, a shielded system failed the Category 6A alien crosstalk requirements because the foil connection did not make good contact with connector housing. Although shield continuity was present because of the drain wire, leakage around the connector housing was significant enough to cause alien crosstalk failures.
Shielded cables are more susceptible to performance degradation because of the deformation of the screen during installation. There are currently three different types of shielded systems: F/UTP, U/FTP, S/FTP. Not all are compatible with each other; and there are trade-offs in performance and shielding effectiveness.
Generally speaking, network installation time and labour costs are higher for shielded systems due to the additional time required to terminate each shielded connector.
The shield needs to be grounded at both ends through a low impedance ground to work effectively at high frequencies. A ground at one end only looks like an open circuit at a ¼ wavelength and at odd multiples of ¼ wavelength. At those frequencies, the screen acts as an antenna, not a barrier, leaving only pair balance as an effective countermeasure against EMI and network performance degradation.
For more information contact Greg Pokroy, Jaycor International, +27 (0)21 447 4247, [email protected], www.jaycor.co.za
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