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Testing for Network PRTC Timing Accuracy in Accordance with G.8272
Times are changing in the world of telecommunication networks. To meet demand for streaming video and other high-speed, high-bandwidth services, mobile operators have been rolling out modern 4G, LTE-TDD and LTE-A networks.

But for these new networks, and in particular small cells, to deliver a quality service with no dropped packets, they need highly precise timing synchronization across the network which has resulted in new methods such as IEEE 1588 Precision Time Protocol (PTP).

For network architects, time synchronization begins with the Primary Reference Time Clock (PRTC) or Grandmaster Clock; whose job is to sync continuously to Universal Coordinated Time (UTC) and distribute precise time to all other devices in the network. This is most commonly obtained from a global navigation satellite system (GNSS), like GPS or BEIDOU. Recently, the International Telecommunications Union (ITU) published a set of recommendations for precision timing in LTE-TDD and LTE-A networks. Those standards include G.8272, which sets out the maximum deviation from UTC that the PRTC’s data output can exhibit without affecting quality of service on the network.

The challenge for next-generation network architects is that this margin is now just 100ns, a huge step-change in precision compared with previous generation (2G, 3G) architectures where µs accuracy was sufficient.

Thorough testing of the PRTC is essential

The level of timing precision required for a good quality of service means network design teams must thoroughly test the PRTC to meet this new standard.

In practice, they must compare the time being output by the PRTC with 'real' reference time as broadcast by navigation satellites. And they must be able to anticipate any issues or conditions that might prevent the PRTC from receiving and outputting precise time.

Test teams may be tempted to simply hook the PRTC up to its regular GNSS antenna pointing upwards and use a timing monitor to compare its output to a time signal received directly from a satellite system. But this method has some serious inadequacies:
  • It only evaluates the accuracy of the time output with the current equipment setup. Any subsequent movement of the antenna, changes to the PRTC or the cable connecting them could have a significant impact on accuracy, which this method of testing would not uncover.
  • It doesn't allow for testing of different conditions that could affect time output in future. For example, if the view of the sky is obscured – perhaps by new tall buildings being constructed nearby – it will affect the ability of the PRTC to obtain precise time data. The same applies to any event that might affect the quality of the phase and time data received from overhead satellites, including atmospheric or man-made interference, multipath effects from the signal bouncing off nearby structures, or even errors within the GNSS itself.
  • It doesn't allow for testing of how the PRTC will handle the insertion of a leap second, an important adjustment to UTC.
  • It doesn't allow for testing how the PRTC will handle phase and time data broadcast by satellite system signals that are not yet fully operational, such as China's BeiDou system, or Europe's Galileo. Any network operator planning to use these systems to obtain precise time in future will want to test how their PRTC handles them before going live – and that can’t be done today without a simulator. Simulation is the answer
All of the above scenarios can be thoroughly and repeatably tested using a combination of GNSS signal simulator, a timing monitor and for some cases, a RF interference simulator may be used too. With these three pieces of equipment, test teams can evaluate the accuracy of the time output of the PRTC in a vast range of possible conditions.

Spirent has partnered with Calnex to offer a complete testbed for recommended G.8272 conformance testing, comprising a Spirent GSS6700 or GSS6300M multi-constellation signal simulator to output realistic satellite signal and time data, a Calnex Paragon-X PTP Timing Monitor to compare the time output by the PRTC with reference time broadcast by GNSS satellites overhead and a Spirent GSS7765 RF interference signal simulator to simulate a wide array of interference sources may also be used.

Spirent/Calnex G.8272 conformance testing solution

Conformance Testing

If required, Spirent can also help you to set up the equipment and design a range of test cases to thoroughly evaluate the PRTC's accuracy.

If you'd like more information on how to use simulation to test PRTC conformance with the ITU's G.8272 recommendation, please contact us.

If you are already using GNSS simulation and PTP timing monitor to characterize the performance of your PRTC, you may like to contact us for assistance with a set of pre-defined test cases.
By: Richard Chan - 1/23/2016 11:21:36 AM
Tags: GNSS Simulation

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