Assuring Symmetric Physical Delay

Prayson Pate
Looking up at two bridges

Assuring today’s networks is a tough job and will become more difficult near term as demand for higher performance, increased volume and quick innovation peaks. The rapid migration to packet-based technologies has created a timing issue that could limit application performance and even jeopardize your business.

Operators of high performance networks require highly accurate timing information and assurance. Precise timing is a vital ingredient for a growing number of applications, such as LTE-A and LTE-TDD wireless backhaul as well as high-speed trading, and one that is often overlooked in a packet-based network. Without it, your network cannot achieve its full potential and you can lose your competitive edge.

Modern packet-based networks have moved from older timing distribution methods such as BITS to standard packet timing protocols such as NTP or IEEE 1588 PTP. These protocols have the advantage of utilizing the same network as the application traffic. However, there are limitations.

In particular, demanding applications which make use of accurate phase and time may require symmetric physical delay, where the physical transport delay is the same in each direction. This requirement cannot be measured or assured using only packet-based methods such as PTP.

One-Way Delay May Not Measure What You Think It Does

Packet-based timing methods can provide an estimate of the wall clock or time-of-day at a remote site, which can then be used for measuring one-way delay. However, the end node must calculate some offset to reflect how far it is from the source of the timing packets. It does so by measuring the round-trip delay from the timing source and then dividing by two based on an assumption of symmetry.

One-way delay measurements can then show the effects of congestion on application traffic but they cannot measure asymmetry in the physical path.

Why Have I Not Heard of This Issue?

I wrote a detailed paper on this topic and it’s posted at the peer-reviewed site Ethernet Academy. The paper goes into the gory details of how the offsets are calculated and how asymmetry in the physical path can’t be detected – again, because symmetry is assumed.

Here are the three key takeaways from the paper related to the use of pure packet-based timing:

  • Asymmetry in the physical data path can’t be detected using one-way frame delay
  • Symmetry in the physical data path must be designed in
  • The clock master should be as close as possible to the UNI

So, do you have a problem in your network? If you are relying on packet-based timing to detect asymmetry in the physical links, then the answer is maybe. You don’t necessarily have a problem. However, you won’t know if you do.

New Solutions Verify Timing

Fortunately, recent innovations using GNSS and other independent timing sources have made it possible to verify symmetry in the access path, as well as to verify the performance of packet-based timing protocols such as NTP and PTP. Using these innovations can ensure that accurate timing and delay measurements are possible — including when the physical media is asymmetric.

At ADVA Optical Networking, our end-to-end timing distribution and assurance solution is based on Oscilloquartz’s long-standing expertise. These technologies and products can assure accurate phase and frequency, automatically and resiliently, including when packet-based methods fail, even when the physical links are asymmetric. We help you make sure everything remains in sync.

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