As network speeds increase and standards shift, installers must find solutions that will meet present-day certification requirements and will not force them to over-invest in the future.
May 1, 2000
The evolution of higher speed networks is driving the cabling industry to grapple with a steadily shifting set of standards for cable testing and certification. Consequently, installation contractors face a myriad of test equipment choices and vendor claims regarding changing specifications for test accuracy levels.
With future cabling test standards not yet finalized, predictions are circulating about which accuracy levels will be required for tomorrow. In this environment, contractors must cost-effectively test to current requirements without over-investing in an undefined future. At the same time, contractors must be careful not to under-invest in technology and face near-term obsolescence.
Regardless of which types of test methodologies have been used over the years, the overriding issue has been whether the tester can meet the specified Radio Frequency (RF) parameters for certifying the cable installation to the required accuracy levels.
Originally, Level II accuracy requirements for testers came about with the adoption of TSB-67, which defined four required test measurements: Line Map, Link Length, Attenuation and Cross-talk (defined as Near-end Cross-talk or NEXT). It also specified the definitions for basic link and channel test configurations, in addition to the test methodologies for certifying cabling installations. TSB-95 augments TSB-67 by defining the additional parameters of Return Loss and ELFEXT (Equal Level Far End Cross-talk).
Enhanced Category 5 and Level II-E now allow existing Category 5 cabling to reliably carry 100 MHz traffic. From a test parameter standpoint, Category 5e and Level II-E accuracy essentially pushed each measurement category up by 3 dB over existing Category 5 requirements. As a next step in the speed evolution, Category 6 defines cabling certified to carry 200 MHz traffic, which will require testing to a new Level III accuracy at swept frequencies to 250 MHz. However, the movement from 100 MHz to 200 MHz traffic levels requires a significant improvement by as much as 10 dB in each of the critical RF parameters that characterize the test device’s accuracy.
The evolution to Level III accuracy represents a monumental challenge for test equipment designers, as a doubling of frequency can be equivalent to quadrupling the difficulty of maintaining test parameters. For example, because attenuation is a function of frequency, it will continue to increase predictably as frequency increases. However, as attenuation increases, the maintenance of a positive Attenuation to Crosstalk Ratio (ACR) means that the cross-talk level has to shift downward by the amount the attenuation has increased. At higher frequencies, the tester must also measure deeper into its dynamic range. For any cable test device, the heart of test accuracy lies in its ability to meet strictly defined RF parameters, including Dynamic Accuracy, Residual Noise, Residual NEXT, Return Loss, Output Signal Balance and Common-mode Rejection, across the entire applicable frequency range.
Recently, there has been a significant amount of industry discussion regarding the relative merits of “scalar” versus “vector” architectures for meeting tighter testing and certification standards. While both architectures have their benefits, the bottom line is that either one must accurately measure the magnitude of each RF parameter.
This objective can only be accomplished through stringent hardware design methodologies and component selection. Conformance with the basic RF parameters cannot be “added in” through software features or compensated for by adding vector capabilities. Accuracy is either designed in at the hardware level or it simply is not there.
MEETING THE NEED
As higher network speeds drastically reduce the margin of error between data transmission bit rates and the maximum physical capacity of cabling, the need for comprehensive testing has become more important to ensure sufficient “headroom” for real-world network demands. For example, the potentially negative effects of elements such as attenuation and cross-talk have become critical factors in network integrity. In addition, capabilities such as accurate distance to fault measurements have become necessary to assist in rapid diagnosis and resolution of problems to certify cabling for higher speed networks.
Traditionally, a simple “pass/fail” on the summary of numeric worst case values of the different test parameters could adequately satisfy the requirements for certification to pertinent standards. A “pass” result essentially determined that all selected test parameters did not exceed the defined limits. With higher speed networks, it has also become very important to clearly relate the numeric values to specified limits throughout the frequency range. This enables an assessment of the available margin or headroom between the test profile and a potential failure under actual operating conditions.
Clear visualization of available margins can best be accomplished by incorporating improved graphical display capabilities directly into the test devices. By plotting frequency steps across the whole frequency range, new-generation test devices can produce graphical readouts and printouts of each test parameter’s measured curve in relation to specified limit lines over the entire selected frequency range. Field operators can use graphical displays to get instant visual overviews on the real performance capacity of the installed cabling. These can provide valuable information about the remaining margin to the limit lines, the quality of the mounting and installation practice, and the actual performance of the cabling material.
In the graph above, actual ACR data is visually displayed in clear relationship to the specified control limits. This provides an immediate and intuitive analysis of both the acceptability of the test and the amount of margin available throughout the entire frequency range, from one to 100 MHz.
In addition to tracking the improvements in cable tester capabilities and the changes to cabling test standards and specifications, professional installers and corporate IT staff must also stay abreast of the real-world issues associated with migration to higher speed cabling.
For example, installers face other important considerations when determining if they should even invest in Category 6 cabling. To achieve 200 MHz performance (tested to 250 MHz) within the RJ-45 form factor, manufacturers are “electrically tuning” their connector designs to the point where they are losing the mix-and-match flexibility that previously enabled any combination of connectors, cabling and patch cables to perform reliably. If cable installers and system administrators ultimately have to maintain matched sets of cabling, connectors and other components specifically for Category 6 implementations, the economics and management hassles may cause some users to simply opt for Category 5e until a better solution comes along.
Furthermore, as the carrying capacities of copper links approach their theoretical limits with Category 6 and a significant cost increment looms ahead for converting to Category 7, many companies are also seriously looking toward fiber optic cabling as an alternative for the bulk of their local area network (LAN) premises wiring applications. Fiber-based LANs are already viable and many industry observers predict that fiber-to-the-desktop is not far over the horizon.
Several factors are currently holding the full-scale implementation of fiber optics at bay. According to the IEEE standard, Gigabit Ethernet links may be implemented as either 1000Base-SX links using less expensive short wavelength technology (850nm) over multi-mode fiber, or as 1000Base-LX links using more expensive long wavelength laser technology (1300nm, 1310nm or 1550nm) over single-mode fiber.
Should companies install the less expensive multi-mode fiber to meet their immediate needs or should they invest in the more costly single-mode fiber, whi
ch will most likely be the preferred medium in the next few years? In addition, when the costs of single mode fiber eventually come down, will “fork-lift” changes to replace multi-mode fiber be more expensive than installing single-mode fiber today? These questions are making the decision to move to fiber optics more difficult.
For the next few years, installers are likely to see a steady increase in the use of fiber, along with the continued use of Category 5e copper and the emergence of Category 6 copper wiring. Installation contractors will find it necessary to balance their investments in both fiber test and copper test equipment. Lower cost options for quick-test verification of fiber cabling before it is installed, and for checking out raw unterminated cabling after installation, will be as important as power loss testers and the high level OTDR test units.
For optimal value, many installers are also looking toward investing in high frequency copper test devices with options for adding fiber testing kits. This not only amortizes their capital investment over a broader base of installations, it also reduces the amount of training required for technicians to conduct and interpret fiber tests in the field.
THE BOTTOM LINE
Field installation contractors continue to require rigorously designed and cost-effective solutions that fully meet their present-day certification requirements and do not force them to over-invest. Whether the near-term market demand moves aggressively from Category 5e to Category 6 copper cabling or diverges more toward the use of fiber optics, professional installers will need a full spectrum of test capabilities, tailored to their real-world field testing requirements.
Ultimately, installers need to look to their relationships with test equipment suppliers to ensure required accuracy levels for today’s needs and clear upgrade and migration paths that can handle tomorrow’s demands.CS
G.W. (Jerry) Renken is a Senior RF Design Engineer, specializing in transmission lines, with Wavetek Wandel Goltermann’s LAN division in San Diego, California. Mr. Renken has been involved with transmission line applications, varying from cardiac pacemakers to guidance systems, for more than 25 years. He is a Professional Engineer and holds three patents.