Five Minute Facts About Packet Timing
By Doug Arnold.
This is a very brief introduction to the Best Master Clock Algorithm. For a more detailed description refer to the more detailed post: BMCA Deep Dive.
A key to the resiliency of the Precise Time Protocol is the Best Master Clock Algorithm, or BMCA. The BMCA allows a clock to automatically take over the duties of Grandmaster when the previous Grandmaster loses its GPS, gets disconnected due to a switch fault, or for what ever reason is unable to continue as Grandmaster.
To understand how this works consider a day in the life of an Ordinary Clock. Recall from a previous post that an Ordinary Clock can be designed such that it is capable of acting as either a master or a slave. The states that this clock can be in are shown in the state diagram below:
After power up the first thing the clock does is “listen”, in other words it look for Announce messages from the PTP general multicast address. An Announce message contains the properties of the clock which sent it. If the Ordinary Clock sees an Announce message from a better clock it goes into a slave state, or passive if it is not slave capable. If the Ordinary Clock does not see an Announce message from a better clock within the Announce Time Out Interval, then it takes over the role of Grandmaster. This runs continuously so master capable devices are constantly on the look out for the possible loss of the current master clock. For this reason it is critical that the Announce Time Out Interval be set longer than the Announce Interval in your network. If you don’t then master capable devices will keep jumping to the conclusion that the master has gone away and they need to take over. Its like a bunch of political pundits on a talk show who never listen and keep talking over each other.
OK. So I have already used up two of my five minutes and I still haven’t told you what makes one master better than another. Let’s get right to it. The list below shows the criteria in order of precedence.
One last complication is Steps Removed. If two boundary clocks are getting their time from the same Grandmaster, then the one which is connected to the Grandmaster through fewer Boundary Clocks is better. Transparent clocks don’t contribute to steps removed because they are, … well transparent.
Modern packet networks face a wide variety of new challenges when confronting the rapidly growing demand for data communication. One of the nontrivial emerging requirements is the vital need of tight synchronization between network elements within the network.
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Unlike legacy synchronous networks (SONET/SDH), packet networks are asynchronous by design, a fact that increases the magnitude of the new challenge. Furthermore, the required timing and synchronization aspects are now much greater than those delivered by the legacy synchronous networks, which dictated frequency sync only. Modern packet networks such as LTE and especially 5G mobile networks require, in addition to Frequency, also tight Phase and Time (ToD – Time of Day) synchronization at unimaginable resolutions of several nano seconds.
To resolve these strict timing requirements IEEE has published the Precision Time Protocol (PTP) standard, also known as IEEE. This is a master-slave, packet-based protocol that allows to accurately propagate the Frequency, Phase and ToD over packet networks by exchanging accurately time-stamped Sync messages between network elements. When implemented properly coupled with adequate hardware, PTP can provide astounding accuracies in the nanosecond range.
In order to assure the required accuracy level is achieved the timing transport network must also consist of proper network elements, all the way from the PTP Grandmaster, the source of the signal, throughout the distribution network and down to the edge where the slave is located.
The timing source in the network is generated by a PTP Grandmaster, a network element with an integrated GNSS receiver that sends PTP timing packets throughout the network. There are several key elements one needs consider when selecting a PTP Grandmaster.
Accuracy – an overall system characteristic, which is influenced by system architecture, timestamping accuracy, performance of the GNSS receiver and the different algorithms and processes executed in the system (e.g. filtering, servo, etc).
Stability – mostly derived from the type and quality of the Grandmaster’s internal oscillator, which varies from low level OCXO up to highly stable Rubidium clocks. The Grandmaster internal oscillator also plays a dominant role in its holdover function whenever all clock inputs fail.
Scalability – the Grandmaster ability to support small sized networks that can scale up to vast number of slave elements. Scalability is also measured by the number of physical interfaces the Grandmaster possess and the number of PTP clock instances it can support.
Resiliency – ability to support multiple timing inputs that serve as alternative time sources in case the GNSS input signal is lost.
Contact us to discuss your requirements of PTP Grandmaster Clock. Our experienced sales team can help you identify the options that best suit your needs.