screened cable

Communications cable has always come in unscreened and screened versions, orshielded, to use the American terminology.

Screening means putting aluminium foil and/or copper braid around the conductors within the cable and/or around the whole cable as well.

The purpose of screening is to keep external electromagnetic energy from gettinginto the cable and corrupting the data travelling down it. It also keeps electromagnetic energy inside the cable and thus stops the cable from being asource of interference.

Screening therefore seems like a good idea but the commonly perceived downsideis larger and more expensive cables, compared to unscreened, and more complexinstallation techniques, as screens have to be correctly bonded to earth to workeffectively.

The traditional Anglo-American approach over the last twenty years or so has beento ask Why use the more expensive (screened) approach if it works just as wellwith the cheaper (unscreened) components? and this is why most of the worldsstructured cabling market tends to reflect a 90% unscreened/10% screened model. There are a few notable exceptions, particularly Germany, where the market has traditionally opted for more than 90% screened. The French market has been abit more ambivalent with an approximate 50:50 share.

With several decades experience behind us now of LANs and structured cabling it is fair to say that we havent seen masscollapses of communications infrastructure due to noise on unscreened cable.

However, the increase in data rates required by future generations of Local Area Networks puts more and more pressure onunscreened technology. Faster data rates need a higher bandwidth and a better signal-to-noise ratio to make them work reliably.

The maximum amount of data sent down any communication channel is defined very simply by the available bandwidth andthe prevailing signal to noise ratio, and was elegantly proven by Claude Shannon in the 1940s.

Data rate, measured in Mb/s, is not directly synonymous with bandwidth, measured in MHz, (although more bandwidth meansmore potential data transmission) as the way the information is coded onto the cable depends upon multiple voltage levels andmultiple changes of phases. We tend to think of information going down a wire as a series of Ones and Zeros which in turnare usually represented as simple square waves with a positive voltage meaning a One and negative voltage depicting a Zero.It isnt quite as simple as this in reality and for example 10BASE-T (10 Mb/s) specifies a 16 MHz channel. Fast Ethernet (100Mb/s) and gigabit Ethernet, 1000BASE-T, both require 100 MHz bandwidth: the jump to ten gigabit Ethernet, 10GBASE-T,however will require a 500 MHz bandwidth.

The diagram shows the actual power distribution versus frequency forboth Fast Ethernet, 100BASE-TX and gigabit Ethernet, 1000BASE-T.

We can see that the scrambling technique used spreads the energymostly across zero to 100 MHz, although there is still significant energyup to 200 MHz, implying that the extra bandwidth afforded by Cat 6cabling (although 100BASE-TX and 1000BASE-T are specified for Cat 5)will improve the signal to noise ratio and the bit error rate of the link.Ten gigabit Ethernet, 10GBASE-T, has its power spectral density spreadover a 500 MHz bandwidth.

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Screened cable Time to take another look?

To take a very simplified view: if the difference between a One and Zero on 10BASE-T is two volts at the transmitter, then thedifference for 1000BASE-T, with a five voltage level code, is going to be half a volt. Gigabit Ethernet therefore must be at leastfour times more sensitive to noise than 10BASE-T. By the time we get to the other end of the cable, the attenuation of thecable has reduced the voltage levels to hundreds of millivolts and a significant noise level then becomes of the order of justtens of millivolts. 10GBASE-T is complicated even more by numerous voltage levels and changes of phases. The rate ofchange of amplitude or phase of the signal dictates the bandwidth required and the proximity of the voltage levels to eachother is limited by the ambient signal-to-noise ratio.

An Ethernet interface is totally unconcerned about the cabling it is connected to and how well it performs. Auto-negotiationexists to help ensure communication partners are similarly configured to avoid mismatches in their mode of operation.However, there is no mechanism to measure and configure the mode based on the quality of the communication channel.

The auto-negotiation mechanism does not have the same transmission requirements as the communications technology ithelps to support. It is quite possible for autonegotiation to facilitate establishment of a link connected across a channel whichcan not support transmission of data at the negotiated mode of operation. Thus it is conceivable that a pair of 100Mbpsdevices could negotiate 100Mbps operation on Cat 3 cabling which is not capable of supporting actual transmission at100Mbps.

Sight of an illuminated LINK indicator has always been used to indicate the physical channel is good it shows the link is established however it does not mean it will work!

Where link performance is concerned there are two issues to contend with, impaired link performance through defects andcorruption due to induced noise. The first point is generally as a result of poor installation practice and should be identified/rectified as part of the certification process. The second point is the hard one to deal with if the properties of unshielded twisted pair do not go far enough to provide immunity to noise in a particular environment.

But what is the effect of all of this? A link will work, fail completely or operate with reduced performance. It is this last situationthat is the problem. Generally degraded end user experience is the main result the cabling is usually the last aspect to beconsidered while trying to rectify the problem.

In the pursuit of simplicity Ethernet has no form of error correction. Re-transmission is not automatic; it has to be instigated bysomething other than the transmitting interface - usually the protocol software or application that originally generated the needto transmit the frame. There is no feedback in Ethernet to allow a receiver to signal the fact that an error occurred. The needfor re-transmission is usually identified as a result of a lack of positive acknowledgement at the sender. This situation can havea very severe performance impact on application response as experienced by an end user. In some cases this retransmissiononly occurs after a timeout of some seconds has elapsed.

It is possible to experience noise issues with symptoms of reduced service and poor response and see server/application inefficiency get the blame. Network errors due to noise are rarely identified even though they are a real problem. Managed LANinfrastructure equipment and modern operating systems keep statistics relating to LAN frame errors however this informationis not reported proactively and often overlooked.

As LAN infrastructures get larger with greater distances and more links in the path between client / server pairs the chance offame loss due to induced noise increases. If the highest level of performance is required and high EMI is expected then all precautions should be considered one of which will be the use of a shielded cabling system combined with proper earthbonding.

Lets consider why simple twisted pair cable even works at all at higher frequencies. Well for a start, it is twisted.

A twisted pair cable gives much better transmission performance than individual laid conductors. This is because it is a



balanced cable. This means that an equal and opposite current flows down each of the two conductors of the twisted pair.There is no common earth return for all the signals as for example in an RS232 multicore.

As current flows down any conductor it creates an electromagnetic field around it. This field can be a cause of pickup, orcrosstalk, into adjacent conductors, and can be a major source of interference. The higher the frequency of operation, theworse the crosstalk phenomenon becomes. By having an equal but opposite current flowing in each conductor, a large partof the external magnetic field is cancelled out, greatly improving the crosstalk performance of the cable. The tighter the twistin the pair of conductors then the closer to the surface will be the cancellation effect.

The twisted pair cable is also more immune to external noise and crosstalk from other conductors. Induced noise tends to becommon mode, i.e. the noise current flows down each conductor, in the same direction and of the same magnitude. Severaltricks in the receiver design can cancel out this common mode noise as the receiver is only looking for differential signals onthe two conductors of the pair, i.e. equal and opposite.

Cable manufacturers improve their products by tightly twisting the pairs and making the physical and electrical characteristicsof each conductor identical, so they are indeed balanced, plus making the rate of twist different for each pair to again minimisecrosstalk between pairs.

The electronic chip manufacturers have also done their bit by using complex digital signal processing to cancel out much ofthe internal crosstalk of a cable and also to overcome return loss problems to a certain amount as well.

Return Loss is energy reflected back down the cable due to a mismatch in the characteristic impedance of the load comparedto the cable. This reflected energy will interfere with the data flowing down the cable. Characteristic impedance is a ratherpeculiar concept in being the impedance (i.e. the complex addition of the cables resistance, capacitance and inductance) ofan infinitely long length of that cable. In the real world it means you must match all the components in a structured cabling system or else signal reflections will severely degrade the capacity of that system. Structured cabling is specified with a characteristic impedance of 100 ohms, but the specification allows for plus or minus 15 ohms, so some signal reflections willalways be present.

As frequencies rise the physical dimensions of the cable path start to have an impact on how the overall system works andwe enter the world of transmission line theory. Effects start happening when the physical size of a structure gets to be aboutone tenth of the wavelength travelling down that structure. If we take a 10GBASE-T signal with a maximum frequency of 500MHz, the wavelength is the velocity divided by the frequency or 300,000,000 m/s divided by 500,000,000 Hz, which is 0.6 m.One tenth of this is 6 cm. To be accurate we should really use a velocity of 210,000,000 m/s because electrical signals travel more slowly in a cable than free space, hence the NVP, or Nominal Velocity of Propagation is about 0.7. Thus any structure of around 4 cm will start to act like a transmitter, or a receiver, at these frequencies.

In radio terms the frequency beyond 300 MHz is where the VHF band starts to become the UHF (Ultra High Frequency) bandand antenna structures take on a more human dimension and scale. For example the TV antenna on your roof (512-806 MHz)contains structures from a few centimetres to a few tens of centimetres in length. Whereas the mobile telephone in your pocket (900 MHz) has an antenna small enough to fit completely within the structure.

The planned introduction of 10GBASE-T has reopened the debate about screened cable because of the high frequencies ituses, the difficulty in restraining UHF signals, and the lack of any practical experience! You still cant buy a 10GBASE-T chip!

In Europe all electronic equipment must meet the EU EMC Directive. Although this doesnt apply to individual passive components like cables and connectors, it does to an entire system when it is connected together and activated. The Directivedetails amounts of electromagnetic energy an item may radiate and also how much it can withstand and still work normally. A



typical figure quoted is a field strength of 3 volts per metre. Experiments havealready shown that 3 V/m fields, at these kinds of frequencies, can induce signallevels of tens of millivolts within unscreened cables.

The forthcoming Augmented Category 6 standards, designed to work hand-in-hand with 10GBASE-T operation, give all the necessary electrical parameters toensure success in this venture and specify the cabling plant up to 500 MHz, butwhether these cables can withstand external noise, now called Alien Crosstalk, andstill meet the EC EMC Directive, has yet to be proven in the field. Many American-based cable manufactures want to prove that unscreened cable will work at these frequencies for no other reason than to continue market expectations thatunscreened cable will always work for everything.

At these higher frequencies there are only two certain ways to ensure immunityfrom outside noise, and to keep your own cable generated noise in check; theseare physical distance from the sources of noise and earthed metallic screensbetween the sources of noise. Screened cabling takes the second path.Unscreened cabling is forced to take the former.

This means a jump in cable diameters for unscreened cables from around 6 mmdiameter to around 9 mm. This is a 225% increase in cross sectional area! Theeffect on cable containment plant in dense cabling installations will be dramatic.

On the other hand, if we take a screened Cat6A cable, at just 7.5 mm in diameter,the extra space consumed is only around 50%, and with none of the concernsabout Alien Crosstalk and EMC Directive compliance that unscreened cable maybe subjected to.

At a New York conference last year (Data Centre Decisions) 80% of respondents were expecting to implement 10GBASE-T intheir data centre with around 50% now considering screened cable in preference to unscreened; a marked turnaround from previous attitudes. Maybe it is time to take another look at screened cable.

Conclusions Screened cable is very effective at keeping electrical noise out of a cable. Screened cable is very effective at keeping electromagnetic energy confined within the cable. Screened cable is only slightly larger than Cat 6 UTP. Screened cable takes up roughly two thirds of the space consumed by unscreened Cat6A. Screened cable must be effectively bonded to earth at the patch panel. Screened cable can be laid parallel to other data cables for 90 m without interference. Screened cable can be used in preconnectorised cabling solutions without Alien Crosstalk issues. Screened cable does not need difficult to manage mitigation techniques such as patchcord separators and cable

unbundling.



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Unsheilded 10Gb Cable

Sheilded 10Gb Cable

Cat 5e UTP Cable

screened cable

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