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Consultant Programme Article 25 / 2008 April 2008 The Myth of Zero Bit Error Rate Transmission A powerful way of measuring the performance of a digital communications channel is to measure its bit error rate. This is a ratio of how many digital ‘bits’ of information have been sent compared to those successfully and correctly received at the other end. It is termed BER (for Bit Error Rate) and expressed as ten to the power of a number where that number expresses how many bits of data can be sent before one error is allowed, e.g. a BER of 10-9 means that for every thousand million bits sent (i.e. one with nine noughts after it) only one will be received in error. Ethernet protocols, as described in the IEEE 802.3 series of Standards, call for performance ranges typically in the order Fluke DTX-1800 testing TIA Cat 6 Perminant Link of 10-10 to 10-12. A gigabit Ethernet signal, transmitting a thousand million bits of data determines a number which is then added per second at a bit error rate of 10-12 to the end of the frame. The receiving would therefore expect to see an error not circuitry does the same process and more than once every thousand seconds, checks to see if the number it has or about four per hour, and still be within calculated is the same as the number specification. Ten gigabit Ethernet, also appended to the transmitted frame. If it is expecting 10-12, would expect an error not then it is most likely that the frame has more often than once every 100 seconds. arrived uncorrupted. If the number is different there is a very high likelihood that What happens if there is an error? the frame has been corrupted en route. Modern communications systems usually The receiving station can then choose to contain some form of error correcting ignore it or ask the transmitting station to protocol whereby a mathematical process retransmit that packet. on the number of bits sent in each frame The end user of the data would probably © 2008 Connectix Limited Consultant Programme Article 25 / 2008 each layer depends upon correct operation of the parameters below, all the way down to the fundamental layer of the channel’s electrical performance. The electrical parameters are those measured when a cable tester is connected to each end of the link and automatically cycles through the following measurements, across the bandwidth of interest, e.g. 250 MHz for Category 6: properly received frames. • • • • • • • • • • • • There are a number of ways of measuring Ultimately every communications channel the performance of a communications is defined by its Signal to Noise Ratio. channel. This is as it sounds: a measure of the ratio Connectix 10G patch panels and cables notice nothing but if the number of lost packets or retransmits was high then the system would appear to slow down. A real-time digital video signal would freeze-frame and then jump on to the next Attenuation (insertion loss) NEXT Return Loss Power Sum Next (PSNEXT) Far End Cross Talk (FEXT) PSFEXT PSELFEXT Resistance Delay Differential delay Attenuation to Crosstalk Ratio (ACR) Power Sum ACR (PSACR) of the wanted signal compared to the • The electrical (or optical) parameters, unwanted noise trying to drown it out. e.g. attenuation, Near End Cross Talk There will come a point where the etc (NEXT) receiving circuitry can no longer • The signal to Noise Ratio (SNR) differentiate between the legitimate signal • The Bit Error Rate and the background noise and no • Packet transmission checking using the transmission is possible regardless of the LAN protocol required, e.g. Ethernet error correcting protocols in place. Digital signals do not degrade gracefully, We can consider this to be a hierarchy as © 2008 Connectix Limited Consultant Programme Article 25 / 2008 unlike analogue signals, they tend to work corrupted at gigabit transmission speeds. perfectly then reach a point where the SNR is so poor that the BER starts It is worth noting here that the higher the to speed of transmission then the more decline rapidly, and then they fail completely. sensitive to noise (and therefore leading to poor SNR and BER) they become. A lowly The Attenuation to Crosstalk (ACR) test of LAN protocol, like 10BASE-T transmits a the cable tester is analogous to the SNR data stream of ‘ones’ and ‘zeroes’ as a measurement, though not exactly the series of positive and negative voltage same thing. ACR is literally a measure of levels, starting out at about 1V for a ‘one’ the ratio of the Attenuation of the link to and minus one volt for a ‘zero.’ the Near End Cross Talk, which is usually the largest and dominant source of a This noise in a cable link. modulating information onto an electrical simple amplitude method of signal is not the only way. Information may For the BER to be adequate the SNR be encoded by using many different levels must be within specified limits. This of voltages, or using phase or frequency electrical components, or any mixture of these. So within whereas 10BASE-T uses simple two level sometimes coding, 100BASE-T uses three voltage overlooked though is that the external levels and 1000BASE-T and 10GBASE-T electrical noise environment must also be use many voltage levels and a phase below certain limits or else electrical noise component. The downside of this extra will get into the cabling and degrade data the SNR. bandwidth of the cable is that the required means that parameters specification. the must cable all What be is being sent down the limited SNR must go up to compensate as the Is a zero Bit Error Rate possible? The higher speed protocols are much more answer is yes, but only in a laboratory sensitive to interference than slow ones. environment enough So whereas a certain SNR may result in a external noise even the most heavily low BER and zero errors for 100BASE-T, screened cable will finally succumb. the same circumstances may lead to an Experiments have shown that when the unacceptable level of SNR and Bit external electrical field strength exceeds Error Rate in a 1000BASE-T, gigabit 3V/m around the cable then high speed Ethernet link. data within the cable starts to get This argument becomes even more because given © 2008 Connectix Limited Consultant Programme Article 25 / 2008 extreme when we get to 10GBASE-T (ten gigabit Ethernet). The SNR, and hence the BER will be optimised by the following: • Making all the cable components so that they meet or exceed the individual electrical specifications as laid down in the Standards. • Match the components together so that they perform as ‘System’. This is especially true of the characteristic impedance. For structured cabling components this must be as close to 100 ohms as possible. Mismatched Connectix cable, IDCs, PCB and sockets, engineered to precisely match 100 ohms. components result in Return Loss. This is a reflection of the signal back down the most demanding requirements of all line which will of course corrupt the data. Ethernet protocols. • Ensure the ambient electrical noise noise does not become a negative factor environment is within the normal ‘benign’ Connectix limits. This is usually taken as lower than following: also To ensure external recommends the 3V/m which is the performance level electrical equipment on sale in the EU • Use unscreened cable up to 1000Mb/s must be able to demonstrate in order to and when the ambient field strength is display the CE mark of compliance. less than 3V/m from zero to 250MHz. Ambient levels of 3V/m or more definitely This is the general expectation of the place us in the realm of screened cable or ordinary office environment. even optical fibre. • Use screened cable up to 10,000Mb/s Connectix has engineered the and when the ambient field strength is components of their cabling systems to between 3 and 10V/m over 500MHz. This precisely match each other and can thus would be considered as an electrically guarantee a bit error performance rate noisy industrial environment. better than 10-12 and thus exceeds the • Use optical fibre if the ambient field © 2008 Connectix Limited Consultant Programme Article 25 / 2008 strength exceeds 10V/m, e.g. close to airport radars, radio transmitters and high voltage equipment such as x-ray sources. Note that the field strength values quoted here can be measured approximately by meters available to the general public or accurately by specialist EMC consultants with very sophisticated measuring equipment. Is zero bit error rate possible in the real world? No. Can a superior and more than adequate BER be achieved with well engineered products like Connectix? Yes. Connectix pre-term, tested before leaving factory If a manufacturer claims a zero bit error rate performance under all circumstances ask to see the evidence and remind them of a quote from the late Arthur C Clarke, “The laws of physics are not subject to human appeal.” UK Head Office: Connectix Limited 33 Broomhills Industrial Estate Braintree, Essex CM7 2RW © 2008 Connectix Limited