FACEBOOK DEVELOPS GPS CARD

Block diagram of the time card functionality

Facebook has developed a PCIe card designed to improve on time of day for servers.

Many companies rely on public NTP pools such as time.facebook.com to act as their Stratum 1. However, this approach has its drawbacks. These pools add dependency on internet connectivity and can impact overall security and reliability of the system. For instance, if connectivity is lost or an external service is down, it can result in outages or drift in timing for the dependent system.

To remove these dependencies, Facebook built a new dedicated piece of hardware called Time Appliance, which consists of a GNSS receiver and a miniaturized atomic clock (MAC). Users of time appliances can keep accurate time, even in the event of GNSS connectivity loss. While building our Time Appliance, we also invented a Time Card, a PCIe card that can turn any commodity server into a time appliance.

It should be noted that the accuracy of a GNSS receiver is within tens of nanoseconds, while the required ongoing synchronization (calibration) of the MAC is within 10 picoseconds (1,000 times more accurate). 

NTP is intended to synchronize all participating computers to within a few milliseconds of Coordinated Universal Time (UTC). It uses the intersection algorithm, a modified version of Marzullo’s algorithm, to select accurate time servers and is designed to mitigate the effects of variable network latency. NTP can usually maintain time to within tens of milliseconds over the public Internet, and can achieve better than one millisecond accuracy in local area networks under ideal conditions. Asymmetric routes and network congestion can cause errors of 100 ms or more.

The US based NIST has worked with time standards for many years. They have built many ultra accurate clocks. In recent years they have used those advanced clocks as a basis for NTP.

Facebook open soured the card design and support has been integrated into the 5.15 kernel and above. In March Facebook started adopting network time protocol (NTP) which they believe will achieve a 100 fold improvement in precision.

Image of the time card prototype

Probably the best use of the PCIe card would be in outdoor utility cabinets were an antenna can be run to the top of the building. Most existing GPS radios are USB and mobile phones have GPS et al integrated. USB has become the interface of choice. USB cables can extend at least 5 meters which an reach to the top of a utility cabinet.

GPS and other navigation systems all use an atomic clock onboard. A receiver has to consider relativity which was demonstrated long ago which shows Einstein work has practical application.

NTP will continue to be developed and new time standards will find their way into the internet. Most consumer Wifi hardware over the last decade has an option to support NTP. By using it your PC and other devices will have more accurate clocks. Some older gear by show it as SNTP which is the same idea based on an earlier RFC document.

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