Information and Tools for the Characterisation of Synchronisation Quality

Introduction to Synchronisation

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The goal of synchronisation is to ensure the clocks in all nodes, or network equipment that require time, phase or frequency, are aligned or operating at the same rates, within the budget or error magnitude allowed by the application. Typically applications requiring synchronisation are mission-critical and poor quality synchronisation directly impacts the efficacy of the application or usefulness of the data generated by it.

Synchronisation is usually achieved by having a number of Master Clocks at points in a communications network that get their time or frequency from either their inherent frequency stability or via an external off-air source such as Global Navigation Satellite Systems (GNSS) like GPS or GLONASS. The frequency or time is transported from each Master Clock to multiple end points using either a physical chain of pulses or accurately time-stamped packet data. Timing network engineering of hardware and software configuration of all clocks, nodes and the network path is critical to being able to achieve the required synchronisation quality.

As the physical or logical network distance increases between the Master Clock and an end-point, the quality of the synchronisation that can be achieved degrades. The quality of the clocking algorithms or hardware at the end point can also significantly impact performance and it is not uncommon for different vendors' hardware to perform better or worse than one another even at identical points in the timing network.

As the need for accurate synchronisation grows and timing requirements are becoming ever more stringent, the task of identifying the causes of error and inaccuracy in all elements of the end-to-end timing chain is increasing in both importance and complexity.

Applications for Sync/Time

Sources of Synchronisation Quality Issues

Master Clocks

Synchronisation begins at Master Clocks and is distributed across a network to the end-points or other clocks. Synchronisation quality is measured by comparing the clock at the measurement point against an independent reference signal of known quality.

As timing begins at the Master Clock it is critical that this is installed and configured correctly as any errors here eat into the timing budget and affect all end-points or applications that depend on this clock.

Synchronisation Distribution Network

There are a number of methods of synchronisation distribution; however, providing the synchronisation is within budget at the end-point, the method of distribution is generally not of importance to the application. However, if testing reveals inaccuracies with the synchronisation performance at the endpoint clock it is likely that investigation into the distribution path will be required.

End-Point Clocks

End-point clocks usually require the least configuration of basic parameters in order to optimise the quality of the recovered clock however it is here where the synchronisation is delivered to, and directly impacts, the efficiency of the application. Performing testing at this point will reveal the total of all errors and degradations accumulated in the end-to-end timing chain.

All other factors being equal, it is the stability of the internal oscillator and the clock recovery algorithm that have the greatest bearing on the quality of synchronisation available to the application.

Synchronisation Protocols and Metrics

The table below shows the common synchronisation protocols and metrics, and the type of equipment that is required to measure or calibrate them to ensure optimal performance.

Synchronisation Design, Calibration and Measurement Solutions

Portable source of trusted UTC time and PRC frequencies

Primary use case for this equipment is to provide highly accurate and stable reference signals of known quality to be used as input to other test equipment, resulting in accurate results that can be used to qualify, troubleshoot and optimise timing performance at the measurement point.

Characteristics of such a reference source should include:

  • Ability to lock to GPS for time and frequency accuracy, with the ability to hold 200 nanosecond accuracy over 8 hours if disconnected from GPS and integrated into the test environment.
  • Portable (UTC/time) reference
  • Broad range of output frequencies including 2MHz, 10Mhz, 1PPS and IRIG-B
  • 1PPS measurement function
  • Easy to setup and configure
  • Rechargeable long-life battery for maximum portability
  • Ability to serve PTP/1588v2, NTP and SyncE protocols

Test Equipment that is optimised for testing timing and synchronisation

Primary use case for this equipment is to test all required aspects of the equipment or network, resulting in accurate results that can be used to qualify, troubleshoot and optimise timing performance at the measurement point.

Characteristics of such test equipment should include:

  • Ability to use external timing sources or equipment as measurement references
  • Internal GPS or Rubidium reference
  • Fibre optic and electrical inputs
  • Continuous collection and analysis of measurement data
  • Comparison to standard quality masks such as MTIE
  • Simple presentation and display
  • Broad range of input frequencies including 2MHz, 10MHz, xMHz and 1PPS
  • PTP/1588v2 measurement and PDV analysis
  • SyncE frequency stability testing and ESMC decoding.
  • Time of Day measurement (TOD+1PPS)

Sync & Timing Training

Synchronisation and timing can be complicated subjects to understand, however, specific training courses on these subjects can assist. Important characteristics of such a training course are:

  • Internationally recognised vendor neutral with a non-commercial, pure knowledge bias featuring the latest technologies, procedures and current synchronisation knowledge
  • Covering all aspects of synchronisation and timing systems, technologies and implementation techniques
  • Ability to be tailored to specific requirements and can be held at vendors, or customer premises
  • As well as formal delivery, sometimes it is beneficial to attend synchronisation test and network hardware training and workshops to receive practical hands-on experience of using and measuring synchronisation equipment and signals

Sync & Timing Consultancy

Good synchronisation is vital to ensure an efficient network. Testing of new infrastructure is crucial, and tactical or long-term monitoring of sync health is key to maintaining on-going confidence of network timing quality.

Synchronisation testing and analysis services should recommend the most efficient way to test network sync health by identifying critical points in a network and employing the most appropriate test equipment to maximise the scope and benefits of the testing. Once the data has been collected it should be presented in a comprehensive report along with detailed analysis of the current sync performance and, if required, recommendations on how to improve areas of concern.

Services should include: Sync network data collection, analysis and reporting and, if appropriate, a long term synchronisation monitoring service.

Planning and design services provide should expert advice and documentation for implementation of synchronisation and timing within SDH, PON, Ethernet and IP, using G8.x, DOCSIS, NTP, PTPv2 technologies.

Bespoke synchronisation plans must take into account all network requirements and conform to relevant ITU and IEEE Standards. Using best practices and planning rules, tailored recommendations and should be delivered in a comprehensive report that documents all the key components and technical calculations along with supporting data, network diagrams and rationale.

Sync delivery technologies and products are constantly evolving it is important that a long term strategic view of synchronisation and timing is incorporated into any current requirements.

Acronyms Used

  • SDH Synchronous Digital Hierarchy
  • PON Passive Optical Network
  • IP Internet Protocol
  • DOCSIS Data Over Cable Service Interface Specification
  • NTP Network Time Protocol
  • PTPv2 Precise Time Protocol (IEEE1588) Version 2
  • MTIE Maximum Time Interval Error, a commonly used synchronisation quality metric
  • TIE Time Interval Error, the basic measurement of a timing signal
  • ToD Time of Day
  • 1PPS One Pulse Per Second
  • MIMO Multiple Input/Multiple Output, increases data throughput of radio devices
  • eICIC Enhanced Inter-Cell Interference Coordination, reduces interference and increases data throughput of radio devices.
  • CoMP Co-operative Multipoint, increases data throughput of radio devices
  • PMU Power Measurement Unit
  • GPS Global Positioning System (USA)
  • GLONASS Russian Global Satellite System
  • SyncE Synchronous Ethernet