What is a PTP Clock?

Following on from my post “What is Time?”, a clock is simply a device that counts regular events from a common starting point. That applies to all clocks and calendars, with the possible exception of a sundial! The regular events might be days, months and years, or they might be pendulum swings, quartz vibrations, or atomic transitions.

For example, my watch counts the resonant vibrations of a quartz crystal. I manually set the time to a known starting point, usually by comparing to a clock that I assume has a more accurate version of the time in my time-zone, for example my cell phone. In turn, my cell phone gets its time from the cellular network, which gets its time from a time server, which gets its time from a national time server, which gets its time from UTC. In that way, there is a loose traceability of my watch right back to UTC time, even if it is somewhat inaccurate when it reaches my watch. The crystal frequency of my watch doesn’t exactly match UTC frequency, so the time gradually deviates from the correct value, and I periodically adjust my watch to keep it reading close to the correct time.

A PTP clock is no different. A PTP slave clock receives messages from a PTP master clock several times a second, and adjusts its time to match the incoming messages. In between these messages, it relies on a counting “ticks” of a local oscillator – typically quartz crystal vibrations – to keep the time advancing, just like my watch also relies on its internal crystal. PTP boundary clocks work on the same principle, they receive time messages from a grandmaster clock, adjust their own time, and then pass that time along to subsequent boundary clocks or slave clocks.

Clearly, at the end of the chain, the time is not as accurate as the start of the chain. Each clock introduces a certain amount of inaccuracy. For example, these errors might be caused by inaccuracy in the time adjustments, or by instability in the crystal, causing the time to deviate between adjustments. The performance of a clock can be defined in terms of four key parameters: noise generation, noise tolerance, noise transfer, and transient response or holdover. Noise generation is the amount of time error a clock introduces, measured with an “ideal” timing signal at the input. Noise tolerance is the amount of time error a clock can tolerate at its input and still function correctly. Noise transfer is how much noise a clock passes from its input to its output, or in other words, how much does the clock filter a noisy incoming signal. Holdover (sometimes called “long-term transient response” defines how long a clock will maintain accurate time if the input timing signal is lost. In subsequent posts will look in more detail at each of these parameters.

Tim Frost
Strategic Technology Manager, Calnex Solutions.

Recent Blogs

Related Blogs

Four Boardroom Members

How to Optimise Your IT Network and Spend

Feb 06, 2019
Network emulation can be a key tool to overcome barriers in getting the most out of your…
3890 Read more

Responding to IT Network Issues

Jan 22, 2019
If simple remedial scripts are not enough to fix an IT network issue, a more…
4544 Read more

Keeping Audiences 'In the Moment'

Nov 16, 2018
Today’s broadcast audiences are seeking more. They want user defined, on demand content.…
1442 Read more

Archived Blogs

1189 More
1434 More

Timing not Telecoms

Nov 08, 2016
934 More

5G Coming Soon

Aug 22, 2016
959 More

What is 1588 PTP?

Aug 04, 2016
1183 More

5G on the Horizon

Aug 01, 2016
955 More
977 More

What is Time Error?

Oct 21, 2015
959 More

LTE-A & VoLTE Rollout

Sep 22, 2015
931 More

LTE Picks Up Speed

Aug 22, 2015
907 More

What is the Time?

Aug 22, 2015
936 More

Mobile and Sync

Aug 22, 2015
896 More

What is SyncE?

Aug 22, 2015
1813 More
1013 More

Microwave Update

Aug 22, 2015
1004 More

Unravelling Standards

Aug 22, 2015
969 More

Partial Progress?

Aug 22, 2015
913 More

Interpreting ITU

Aug 22, 2015
905 More

Confusion Rules!

Aug 22, 2015
955 More

Basestations Need Sync

Aug 22, 2015
948 More

ITSF 2015 Edinburgh

Aug 22, 2015
896 More

India to Follow China?

Aug 07, 2015
912 More
HOW CAN CALNEX HELP YOU FURTHER?

Click your area of interest below for more tutorials and real-world case studies.