IEEE1588 clarified by sync experts

Over the last few years, telecom operators have started to introduce packet-based networks and thus two letters became the most pronounced ones: ‘I’ and ‘P’. This being said, the migration to a packed based technology invariable comes with a lot of interrogations and changes. Our company Oscilloquartz, global designer and manufacturer of Cesium Clocks, GNSS Clocks, OCXO, SSU and IEEE 1588v2 (PTP) solutions for telecom networks, had to adapt our offering and services to propose new tailored solutions for our customers.

From our customers, we can say that there is a general consensus about concerns and questions on packet network and how to care for synchronisation. There are a lot of questions and sometimes some glaring misconceptions concerning precise time protocol (PTP) IEEE1588v2.  The below is a summary which we hope will help you clarify your thinking on the subject and give you some tips on how to prepare to improve your synchronisation performances in the network of the future.

What you should understand …

IEEE 1588v2 is one of the possible solutions for packet-based network. But there are at least three others: GPS or GLONASS, SSU, SyncE and NTP. Before deployment, you need to consider all the available possibilities so as to avoid going the wrong way. Not all networks can accommodate PTP. We offer our services to determine and evaluate your network.

In theory, one IEEE1588v2 Grandmaster is enough for hundreds of IEEE1588v2 slaves and even possible to serve thousands. But in practice, a good preparation and concept planning of Grandmaster location is always the best way to use a minimum number of sync clock.

A PTP slave embedded in NodeB is always good enough to keep a good synchronisation performance. In many cases it is justified. However, some applications, like TDD, require a better filtering and sometimes the capability to keep accuracy under 10 µs for some hours, or even days. Without using an ovenised quartz or rubidium oscillator, this performance cannot be reached. Oscilloquartz proposes clocks with quartz or rubidium oscillators. Thus, your network is guaranteed to always get the required and accurate time source.

Unlike TDM, in packet networks there are more chances that the ethernet link is congested by traffic, or even lost than having trouble in sync clocks. Therefore, using a single time source is not the ideal solution. We recommend to have at least two distinct Grandmaster placed at different places for each connected slave and create redundancy at the network level rather than at the equipment only. Your network will then benefit from the increased MTBF and reliability.

It is often thought that the higher the packet exchange rate, the better is the PTP quality. Again, it might be the case in theory. ITU-T G.8265.1, the profile for frequency synchronisation says that the rate can be up to 128 packets per second. At Oscilloquartz, we always test the standard and do simulations to find the optimum solution for our customers. After calculation, field testing and lab testing we found that the optimal packet rate should always be between 16 and 32 pkt/s in a telecom network. Although the precision is increased when going to higher rate, the difference between 32 and 64 is very small compared to 8 and 16 pkt/s. Moreover, higher is the rate; smaller is the network bandwidth. Thus, going to 128 is useless in practice and needlessly resource consuming.