The 4th generation mobile technique LTE(Long Term Evolution) is demanding much higher requirements on clocking andsynchronisation as with former technologies. Even more, the back-hauling of theantennas for LTE (eNodeB) is purely based on Ethernet, which did not offerspecial features for clock and synchrony up to now. Ethernet is normally anasynchronous and packet-oriented transmission technology mainly used in LAN.Due to that there was no need to fulfil any demands on these quality featuresand in regular products nothing was prepared to support them.
Understanding the newdemands of back-hauling antennas via Ethernet, arcutronix implements in itsaccess devices synchronous Ethernet (Sync-E) and PTP.
Sync-E works onphysical layer similar to SDH. The Precision-Time Protocol, defined by IEEErecommendation 1588v2-2008, is a packet-based method to synchronise clocks andis independent to the physical transport, which is a big difference to Sync-E.Using Master-Slave architecture, the timing source (Grandmaster) sendsperiodically packets, which are carrying the actual time/clock. In amulti-stage procedure, each attached slave determines the packet-delay betweenitself and the Grandmaster. Knowing this individual packet-delay the slave canadjust its clock correctly. The algorithm used to calculate the delay of thetiming-packets requires precise timestamps for the sending and receiving time.Depending on the quality of the timestamps, the slave’s established clock canbe the same as on the Grandmaster, varying only a few Nano-seconds.
As for all packet-oriented procedures, thereare variations in the packet-delay for PTP packets and so the value is notconstant, but slightly fluctuating. The smaller the packet-delay variation, thebetter is the result for the calculated clock at the PTP slave. In addition,any change in the topology of the network, caused by failure of lines or nodes,will lead to massive change in the mean-packet-delay between Grandmaster andslave.
The calculation of the packet-delay betweenslave and Grandmaster plus the optimisation of the network to smallpacket-delay variations is one of the big challenges for operators andproviders. The used network models are never matching to existing lines. Theycan only be seen as a generalisation of common known networks.
arcutronix offers in its products the optionto log the calculated packet-delays and to store this over a long period oftime (up to 3 years). Using these loggings, conclusions about the network canbe done and ways for corrections can be found. Any changes in the topology canbe back-tracked and the influence on the quality-level of the PTP. So this newfeature is not only a mighty tool for operators but also very well suited intest cases and to improve further models of existing topologies.
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