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Beyond components as Quartz Crystals, Filters and Oscillators we also can offer OEM-Boards and complete equipment / systems for Clocking and Timing. These are used for SYNCHRONIZATION of transmission networks / base stations / server parks and assure the necessary Quality of Service. Synchronization in Telecom Networks – About 20 years ago and in regards to Telecom Networks the world was much simpler as today. The mostly government controlled telecom organisations and owners of these large networks were obliged to respect some basic rules but the situation as a monopolist made it easy. They installed in a central point of the network a so-called Primary Reference Standard which is still by definition a cesium oscillator – the only oscillator without any screw or potentiometer for calibration. This cesium beam standard provides a stable accurate frequency over the lifetime in the range of +-4x10E-12 which can be distributed over the network down to the far end. The classical way for this type of distribution was done in form of a tree divided into different levels. It is easy to understand that for availability and security reasons these Cesium Master-Clocks were installed at different locations, but connected internally to provide under all conditions the recommended availability of better than 99,99 %. The distribution concerning the clocking signals is done by using the HDB3 coded signal 2,048 Mbit/s through all levels and the installed network elements (muxers, switches etc.) are synchronized on that by their internal PLL. If all these elements are well synchronized just down to the end then of course the accuracy give an image of the Master-Clock fulfilling the ITU / ETSI recommendation G.811. It is easy to understand that such a network has to be planned in regards to the synchronization respecting basic rules as e.g. to avoid loops etc. Now we have parallel to these PDH networks more and more applications of the SDH technology / SONET as well as hundreds of base stations (GSM / UMTS / DAB / DVB) and all these are matched. Further we have to consider with the new freedom (no monopoly) the new network providers as the railway organization, power utilities and large companies as ARCOR / Vodafone D2 / O2 / e-plus and their own networks which are more or less interconnected together. Of course in such scenario a clear synchronization path with all the possible handover points needs more and more a very exact sync planning which can’t be guaranteed in all cases. In this situation the upcoming GPS system becomes more and more importance allowing the user by installing a GPS-Receiver to provide on-site punctual the Cesium accuracy from the Cesium oscillators installed in the GPS satellites. With this and the more and more existing SDH technology / rings it is possible to have a flat sync hierarchy instead of the classical tree configuration. Parallel to these transmission networks using the clocking information (2,048 Mbit/s / 2,048 MHz) the real time distribution called synchronization becomes more and more importance. By using the GPS information it is easy to distribute the real time by using the network time protocol NTP to synchronize servers / computers. The typical application for this is today the time stamping of events e.g. in the field of power distribution networks, transfer actions of banks as well as billing systems in mobile networks, labs, air traffic control, assurances, national security services etc. In case of a conflict it is very important if an organisation can show that they have an independent high reliable and absolute time stamping! For both applications there are a couple of high performance and high reliable units / equipment available on the market – most of these are modular and redundant and by using high stable oscillators they provide also in case of a problem regarding the receipt of the GPS signal a good so-called holdover mode. From the practical experience over the last years we have had some so-called GPS anomalies creating some serious problems regarding the clocking and especially the timing information. For this there are so-called Secure Hybrid Systems available, providing two independent receivers (one GPS and one DCF), comparing the result (time) and – if there is an adjustable delta exceeded – squelching the NTP output. Now with the upcoming Galileo system which is not controlled by only one government the whole situation will change. It is thinkable and foreseen to have in the future equipment for clocking and timing by using parallel both information, meaning from GPS and Galileo, providing much more security, availability and precision. |
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