Can accurate delivery of time over optical transport answer data center synchronization challenges?

Synchronization networks need new levels of reliability and precision. Let’s see if new technology, which uses the DWDM transport layer to carry highly accurate PRTC-grade timing, can offer the answer.
Ulrich Kohn
Light streaks

With compute power increasing in central and edge data centers, more and more data is being processed in a highly distributed way. This calls for compute processes to be precisely synchronized and transaction records chronologically ordered with new levels accuracy. Today’s timing networks simply cannot deliver the levels of precision required. 

Improving accuracy and reliability

The synchronization network technologies we currently rely on are increasingly not up to the job. While satellite-based synchronization is sufficiently accurate, it suffers from various vulnerabilities. Packet transport networks can be designed in a highly resilient way, but they produce packet delay variations and asymmetry, negatively impacting the quality of timing signals. An underlying OTN network adds additional delay variations. What’s more, while atomic cesium clock technology is highly reliable and well established, it faces a challenge from ever increasing stability requirements. In short, technology innovation in synchronization is urgently needed to enhance accuracy and resilience.

Challenges for accurate data center synchronization

More resilient and precise satellite-delivered timing

GNSS-delivered synchronization is subject to various vulnerabilities. Malicious jamming and spoofing attacks compromise timing availability and accuracy, and several outages have been reported in recent years. In addition, interference from high-power RF transmitters or ionospheric disturbances caused by strong sun activity can negatively impact satellite-delivered synchronization.

To tackle these challenges, we need a new generation of GNSS receivers that utilize a combination of different methods. Using multiple frequency bands and multiple constellations improves resilience and accuracy and reduces the impact of malicious attacks. AI/ML-powered management systems analyze the performance of many GNSS receivers, detect anomalies and initiate countermeasures even before services are affected.

Enhanced synchronization over transport networks

Packet networks cause asymmetric packet delay and delay variations, impacting the quality of network-delivered timing. Packet network devices need to actively compensate these characteristics by measuring delay in real time. This can be provided by transparent/boundary clocks as a capability of the packet forwarding device or, better still, as a functionality of a co-located synchronization device building an overlay synchronization network. 

The image below shows such a synchronization overlay network interconnecting highly accurate boundary class D clocks over a bidirectional optical timing channel. (With 5ns accuracy, class D is the highest quality level defined by ITU-T G.8275.1.)

Optical timing channel delivers ultra-accurate time

The overlay approach avoids delay variations related to OTN technology as the optical timing channel operates out-of-band and directly interconnects boundary clocks with native Ethernet-carrying PTP traffic. Bidirectional optical transport over a single fiber eliminates delay differences from dual fiber approaches.

Optical cesium atomic clocks for higher accuracy

Recently, the first commercial optical cesium atomic clock was launched. This technology is proving to achieve higher accuracy and stability compared to legacy magnetic cesium technology with extended lifetime. Initial deployments confirm that optical cesium technology delivers significantly better stability even over extended periods of time. With improved holdover characteristics, those ultra-stable clocks can be used to deliver precise synchronization while other sources are disturbed or not available. What’s more, this new commercial optical cesium atomic clock also provides modern and secured remote management.

Towards ultra-precise data center timing for metrology and scientific research

Increased timing accuracy is needed in metrology and scientific research. This requires a co-evolution of atomic clock technology, timing-transport networks and satellite-delivered synchronization technologies. ADVA is the only company providing this unique combination of advanced synchronization technologies. 

GNSS

For more info, here’s the slide deck on our optical timing channel (OTC). It provides accurate synchronization from the core of the network all the way to the edge by combining ePRTC core clocks and ultra-precise boundary clocks to ensure nanosecond timing.

 

 

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