NTP is one of the oldest protocols still in wide-spread use today. It was originally developed by Dr David Mills of the University of Delaware as a means of synchronizing time critical processes on the Internet. Originally developed on the Linux operating system, the NTP distribution has since been ported to the Microsoft Windows operating system. However, Linux remains the primary platform for NTP development.
NTP basically allows network time clients to synchronize time with accurate time servers and hardware reference clocks. Using NTP a client machine can typically synchronize to within a few tens on milliseconds of a NTP server across a network. Using NTP allows all clients and network infrastructure in an Enterprise to synchronize to the same precise time.
NTP servers can utilize a number of hardware clock references such as GPS and national time and frequency radio services. These hardware clocks provide an accurate time reference that be used as a timing resource for clients. Typically, hardware reference clocks connect to a Linux-based NTP server using a standard RS232 serial or USB port to provide precision timing.
NTP Server Reference Clocks.
A number of external hardware reference clocks are available to synchronize NTP servers. One of the most common is the Global Positioning System (GPS). GPS provides a highly accurate source of time, typically to a few hundred nanoseconds. GPS has the advantage that a good signal can be obtained anywhere in the world. All that is required is a GPS receiver and antenna. Provided that a GPS antenna has a good view of the sky, a good consistent signal lock can be guaranteed. The one major drawback to GPS is that the antenna requires line of site to the GPS satellites. The ideal location for a GPS antenna is therefore on a roof-top with a full 360 degree view of the sky. This can be inconvenient and even expensive for many installations.
An alternative is to utilize a radio time and frequency broadcast such as MSF, DCF-77 and WWVB. Radio signals generally penetrate normal brick-built buildings. A good radio signal can therefore often be received in-doors close to the NTP server itself. Radio broadcasts, however, have a finite range and can only be received within a few-thousand kilometer radius of the transmitter.
NTP Server Backup Clock References.
NTP Servers often contain backup clock references that can be utilised in the event of main reference signal lock. Many sources of backup references are available but generally take the form of oscillators of various expense and price. Lower end crystal oscillators are very low-cost devices but offer no more accuracy that a standard watch or clock. Higher-end TCXO or OCXO oscillators utilize a precisely calibrated crystal that is disciplined by the GPS or radio reference clock. This allows for quite a high degree of accuracy, at quite a reasonable cost. Typically, a GPS disciplined TCXO crystal oscillator will provide an accuracy of about 1 second per week, far better than any standard crystal oscillator. At the high-end of the spectrum are rubidium oscillators which operate on rubidium oscillations with are highly precise. Rubidium oscillators can operate at an error of only a few seconds per year. However, a rubidium oscillator is very expensive and may only be considered for very high-end timing applications.
Provided an NTP servers primary time source is reliable, then a standard crystal oscillator may very well fit the bill. However, to provide a degree of holdover protection from system outages, a disciplined TCXO is good compromise, giving good accuracy at an affordable price.