Time Synchronization

Time Synchronization
News & Tutorials from Meinberg
  • PTP / IEEE 1588
  • NTP
  • Configuration Guidelines
  • Industry Applications
  • Security
Home » IEEE 1588 » What’s new in IEEE 1588-2019: L1 Sync

What’s new in IEEE 1588-2019: L1 Sync

September 17, 2020 by Douglas Arnold 2 Comments

Five Minute Facts About Packet Timing

Time to play what’s new in IEEE 1588-2019, the game where everyone wins new PTP features.

L1 Sync is short for physical layer frequency synchronization.  Recall that frequency sync is also referred to as syntonization.  So, what is it?  First, two clocks are syntonized if the they have the same frequency.  Consider two clocks emitting a series of pulses, equally spaced in time, as depicted in Figure 1. 

Figure 1. Pulse train outputs from two syntonized clocks

The clocks do not transmit the pulses at the same time, but the time interval from when the clock with the upper waveform transmits a pulse when the clock with the lower waveform transmits a pulse is always the same, because the two clocks have the same frequency.  They are syntonized.

A PTP network with L1 Sync is one in which the clocks of the physical layer of the transport media are locked together.  For example, look at the two Ethernet appliances in Figure 2.

Figure 2. Two syntonized Ethernet appliances

An example of physical layer syntonization that is Synchronous Ethernet, known as SyncE to its close friends. Synchronous Ethernet is a standard of the International Telecommunications Union, and is commonly deployed in telecom networks.

All this begs the question, why do it?  There are several reasons why you might want to syntonize your PTP network:

  1. Your PTP network sits between two networks that require syntonization, such as two legacy SDH networks.
  2. The other devices in the network can use the superior holdover the appliance acting as the frequency reference of the network
  3. The PTP slave servo loop will lock easier with less tracking error when the clock is already syntonized to the PTP grandmaster
  4. Locking the Ethernet clock together allows Ethernet based timing asymmetries to calibrated out.  This is crucial to sub nanosecond performance.
See also  What Makes a Master the Best?

Reasons 2-4 assume that the PTP ports are coherent with respect to L1 Sync.  That means that the clock used for PTP is locked to the clock used for the physical layer.   Reason 4 also assumes that the PTP ports are congruent with respect to L1 Sync.  That means that the PTP grandmaster is also the L1 Sync frequency source.  Figure 3 Shows a network that is congruent.

Figure 3. A congruent PTP L1 Sync network

The source of frequency is a Primary Reference Clock (PRC) to barrow the telecommunications term. Figure 4. shows a network that is not congruent.

Figure 4. An incongruent PTP L1 Sync network

Incongruence is sometimes preferred so that a combined GM/PRC wont be a single point of failure.  If the GM goes into a fault condition, the network can still holdover from the PRC, which generally has a very good oscillator. If the PRC goes into fault then frequency can still be derived from PTP.

The importance of congruence will be made more clear in a future post when we discuss asymmetry corrections for high accuracy PTP. 

If you have any questions about packet timing, don’t hesitate to send me an email at doug.arnold@meinberg-usa.com, or visit our website at www.meinbergglobal.com.

  • share 
  • share  
  • share 

Related posts:

  1. Time of Day, Phase and Frequency – select your source
  2. What’s in the 2019 edition of IEEE 1588: Mixed Multicast Unicast Operation
  3. BMCA deep dive: part 1
  4. What’s in the IEEE 1588 revision: Interdomain interactions
Previous Post - What’s in IEEE 1588-2019: DIY PTP Port States
Next Post - Unicast Master Port Selection in PTP: When should a slave port swipe right

Filed Under: IEEE 1588

Comments

  1. Manik says

    October 20, 2020 at 10:39 am

    I didn’t understand what new is L1 Sync bringing in to IEEE-1588 2019.
    We already have physical-layer-frequency synchronisation required in ITU-T G.8275.1 PTP profile.

    Reply
    • Douglas Arnold says

      January 6, 2021 at 6:57 pm

      IEEE 1588-2019 does not define an L1 Sync protocol, but rather defines a general interface between PTP and any L1 Sync protocol. It also describes how L1 Sync can be used to enhance PTP accuracy.

      Reply

Discussion Area:

If you enjoyed this post, or have any questions left, feel free to leave a comment or question below.

Share Your Comments & Feedback Cancel reply

PTP Software

  • PTP Track HoundPTP / IEEE 1588 Protocol Analyzer

  • NTP / PTP Simulation Software
  • Meinberg Advertisement

    Categories

    • Authors
    • Configuration Guidelines
    • IEEE 1588
    • IEEE 1952
    • Industry Applications
    • NTP
    • Security

    External Links

  • NTP Security
    • Site Notice
    • Privacy Policy

    You may also like

    1. Time of Day, Phase and Frequency – select your source
    2. What’s in the 2019 edition of IEEE 1588: Mixed Multicast Unicast Operation
    3. BMCA deep dive: part 1
    4. What’s in the IEEE 1588 revision: Interdomain interactions

    © 2025 · Time Synchronization Blog

    MENU
    • PTP / IEEE 1588
    • NTP
    • Configuration Guidelines
    • Industry Applications
    • Security