The Next Generation of Cable Technology. A technology primer from NORDX/CDT By, Eric d Allmen

Transcription

1 A technology primer from NORDX/CDT By, Eric d Allmen

2 Foreword The Telecommunications Industry Association (TIA) and the International Standards Organization (ISO/IEC) are actively engaged in the development of the next generation cabling standard. The first generation of copper cabling, commonly referred to as Category 5, was introduced in the early 90 s and ushered in the era of multi-megabit data networks. The performance of the first generation cabling system tops out at 1 gigabit per second (or 1000 megabit per second). The next generation cabling system will need to support multi-gigabit data rates. I was not surprised to hear recently, that one of the IEEE committees is considering a data-rate of up to 4 gigabit per second for home networks. Even higher networking speeds are being considered for future applications that will take full advantage of parallel high speed processing and terabyte (1000 gigabyte) storage devices. There is an ongoing debate in the standards committees on what the performance levels should be for next generation cables, links and channels. NORDX/CDT, a subsidiary of Cable Design Technologies, invests significant engineering resources and efforts in cabling standards activities. We take a leading role in the development of cabling standards. We recognize that the standards that we set today will determine the performance of networks 5 to 10 years from now. It is important that we do not set our sights too low to avoid having the cabling become obsolete and need to be replaced within a few years. NORDX/CDT has submitted proposals to the TIA TR 42.7 subcommittee and to the ISO/IEC JTC 1 SC25 WG3 working group responsible for formulating the new Category 6 standard. These proposals specify a cable with a lower attenuation (stronger signal) and much better crosstalk performance. The category 6 cable specifications are still a moving target. The current Category 6* draft specification only takes into account the cable transmission performance at 20 C. For higher temperature installations, either a a lower attenuation cable would be required to achieve the same bandwidth objective of 200 MHz for a Category 6* channel of 100 meters. Alternatively, the maximum distance would need to be shortened considerably, for example, as much as 6 meters when the average cable temperature is 40 C. This is one of the main reasons why a lower attenuation cable needs to be specified. It is the basis of the NORDX/CDT proposals in the standards committees. As we gain further understanding of cabling performance at higher frequencies, the importance of key transmission parameters that determine the signal-to-noise ratio is becoming more and more apparent. The following paper outlines some of our findings with respect to these key transmission parameters. Through the exploration of these parameters and more research into our new cable technology, NORDX/CDT is committed to launching a new performance benchmark in the industry. * TIA/EIA Category 6 working draft 6 (May 2000) Page 2 of 6

3 New Insight into Maximizing Cable Performance There are two key transmission parameters that limit the maximum data-rate capability of copper cable networks. The most important is the signal level, which is primarily determined by the attenuation of the cable. The second is the noise level, which is primarily determined by internally generated crosstalk noise between cable pairs. Examining what influences these two parameters in more detail brings new insight into how the overall cable performance can be improved. A Closer Look at Cable Attenuation Cable attenuation refers to the reduction or loss of signal energy occurring between one end of the cable and the opposite end of the same cable. This attenuation increases as the frequency increases due to a law of physics called the skin effect and the proximity effect. At high frequencies, the magnetic field generated by the flow of electrons within the conductor forces the electrons to crowd to a thin layer at the outer surface of the conductor. The higher the frequency, the thinner the conducting layer. This effect is quite significant and increases as the square root of frequency. For example, if the cable attenuation at 100 MHz is 20 db, the attenuation at 400 MHz will be twice as high or 40 db. Let s look at what this means in terms of the signal strength: If we start with a signal of 1 volt, an attenuation of 20 db corresponds to a voltage of 0.1 volt or 10 percent of the original signal voltage. A signal that is 10 % of the original voltage is only 1% of the signal power since power is related to the square of the voltage. An attenuation of 40 db corresponds to a voltage of 0.01 volts or 1 percent of the original voltage and 0.01% of the signal power. As you can see a high value of attenuation in db has a very large effect on signal power. What does this mean to the performance of data networks? There is a limit to the ability of receivers to restore and detect weak signals over a wide frequency band. It has been mentioned that a practical upper limit for channel attenuation is around 35 db. This parameter alone limits the maximum frequency range for transmission to about 200 MHz for 100 meters of 24 AWG Category 5 cables. As the signal level is decreased, the receiver is more susceptible to external noise interference such as alien crosstalk and impulse noise. This translates to more frequent errors and less reliable transmission. Larger conductor diameter and better material selection improve the attenuation performance of UTP cables. In addition, stability of the twisted pair geometry within the cable core improves attenuation performance by limiting attenuation roughness due to impedance variations at high frequencies. A cross web design is an example of a new cable technology that limits attenuation roughness and enables a higher level of attenuation performance as proposed by NORDX/CDT in the cabling standards committees. The IBDN 4800LX cable is the basis for the alternate cable proposal by NORDX/CDT to the TIA and ISO cabling standards committees. This cable design employs 0.6 mm (23 AWG) copper conductors to achieve a lower attenuation performance as well as a cross-web filler to separate the twisted pairs and to maintain a stable pair orientation * TIA/EIA Category 6 working draft 6 (May 2000) Page 3 of 6

4 within the cable. The result is an exceptional transmission performance that goes well Beyond Cat6. A comparison of the attenuation performance of conventional Category 5 cables, the current Category 6* cable proposal and the IBDN 4800LX cable from NORDX/CDT are illustrated in Figure 1. The IBDN 4800LX cable attenuation is 4 db lower than Category 5 at 100 MHz and 3 db lower than the current draft specification for Category 6* at 200 MHz. Comparison of Cable Attenuation (Category 5 Reference = 0 db) Attenuation Difference (db) -8,0-7,0-6,0-5,0-4,0-3,0-2,0-1,0 0, , Frequency (MHz) Category LX cable Figure 1: Improved Attenuation with 4800LX Cable from NORDX/CDT A Closer Look at Cable Crosstalk Crosstalk is another key transmission parameter. This type of noise causes random variations in the signal amplitude. These variations limit the ability of a receiver to detect changes in the signal waveform thereby affecting the bit error rate (BER) and the reliability of the transmitted signal. Crosstalk is the unwanted interference signal, which comes from coupling between one pair and another. The twist lay and geometry of the pairs within the cable determine the crosstalk performance of cables. The crosstalk performance will be degraded if the separation distance between pairs is reduced or the sequence of pairs changes during manufacturing or installation. Therefore, to achieve very stable crosstalk performance that is more resilient to installation variables, it is necessary to design cables which maintain the separation distance between pairs and the sequence of the pairs within the cable core. In addition to improving cable attenuation, a cross web design enables a higher level of crosstalk performance. A comparison of the near end crosstalk (NEXT) performance for Category 5, the current Category 6* cable proposal and the IBDN 4800LX cable from NORDX/CDT are illustrated in Figure 2. The crosstalk performance of the IBDN 4800LX cable is 18 db higher than Category 5 and 6 db higher than the current Category 6 proposal over the entire frequency range. * TIA/EIA Category 6 working draft 6 (May 2000) Page 4 of 6

5 Comparison of Cable NEXT N E X T d B Frequency (MHz) Category 5 Category LX Figure 2: Improved NEXT Performance with IBDN 4800LX Cable from NORDX/CDT Benefits of Improved Attenuation and Crosstalk As was explained, the two key transmission parameters, namely the signal and noise level, are primarily controlled by the cable attenuation and NEXT performance. The combined improvement of attenuation and NEXT performance parameters will, therefore, provide more headroom and information carrying capacity. This can be measured by examining the attenuation-to-crosstalk ratio (ACR). The ACR is an approximate measure of the Signal-to-Noise ratio. The ACR takes into account the predominant noise source, which is usually, NEXT. The actual Signal-to-Noise ratio also includes other noise sources such as FEXT, impulse noise, alien crosstalk... Figure 3 compares the ACR cable requirements for Category 5, the current Category 6* cable proposal and the 4800LX cable from NORDX/CDT. The frequency at which the cable ACR reaches 10 db is often used as a benchmark number that determines the bandwidth of the cable (note that the corresponding benchmark number that determines the bandwidth of a channel is 3 db ACR). Based on this benchmark, Category 5 cable corresponds to a bandwidth of 100 MHz, the proposed Category 6* cable to a bandwidth of 200 MHz and the 4800LX cable to a bandwidth of 300 MHz at a temperature of 20 C. In considering the information capacity of future networks, the IBDN 4800LX cable provides the greatest capacity to deliver multi-gigabit data rates in the future. * TIA/EIA Category 6 working draft 6 (May 2000) Page 5 of 6

6 Comparison of Cable ACR ACR (db / 100m) 90,0 80,0 70,0 60,0 50,0 40,0 30,0 20,0 10,0 0,0 10,3 24,4 32,3 10,6 19,3 0,5 9, , Frequency (MHz) Category 5 Category LX Figure 3: Improved ACR Ratio with IBDN 4800LX Cable from NORDX/CDT NORDX/CDT Leading the way to the next generation cable technology What is the bottom line? The cabling industry is currently in a state of transition and, as such, making a sound decision on an infrastructure that will meet future requirements can be difficult. Over the next year, the standards for Category 6 will become solidified. By looking ahead, NORDX/CDT is pioneering new territory in cable technology and is breaking new ground for the next generation of cables. We believe that the IBDN 4800LX cable will set a new performance benchmark for the cabling industry. It provides the additional headroom to take into account the effects of temperature, alien crosstalk and installation variables that can degrade performance. By taking advantage of this next generation cable technology, users benefit, in the short term, from the additional headroom for today s most demanding and mission critical applications. More importantly, the users benefit in the long term by investing in a cable infrastructure that will support future higher bandwidth applications. Eric d Allmen Product Manager NORDX/CDT Additional Information The preceding paper outlined the direction of a new cable technology based on some key performance parameters. For a more in depth look at how the new cabling standard proposals will impact our industry, please contact us for a copy of the NORDX/CDT Evolution of Cabling Standards booklet. This document answers some of the most important questions that industry consultants, system designers and installers frequently encounter. For more information on NORDX/CDT and IBDN products, please call: Or visit us on the Net at: * TIA/EIA Category 6 working draft 6 (May 2000) Page 6 of 6

7 * TIA/EIA Category 6 working draft 6 (May 2000) Page 7 of 6