With an ever-increasing number of NTP servers becoming available with even more options, choosing the correct one for your organization can become bewildering. However, with a little thought about your requirements, it can become a very straightforward decision.
Enclosure Format – Rack-Mountable, Desktop or DIN rail Mounting
The first and possibly easiest choice to make is the type of enclosure that you would like to have for your network time server. By far the most common enclosure type is 19 inch rack-mountable. A rack-mountable enclosure can be easily fitted into existing cabinets in your computer room. A typical appliance may have a large LCD display on the front panel, for status and configuration information. Various connectors are generally located at the rear of the unit for connecting antennas and network cables.
Rack-mountable NTP servers are generally 1U high, however, many vendors provide 2U or even 3U high appliance, which use more of your precious rack-space.
A desktop enclosure is a small compact unit that can easily be located on a desk or in a cupboard. Often, desktop-type appliances are lower-cost and may not have a LCD display – to reduce costs. DIN rail mounted units are also available, which are useful in manufacturing and industrial environments for mounting inside cabinets on the shop-floor.
Single or Multi-LAN
Most NTP time servers are available with a single Ethernet network port. This will allow synchronization of clients on a single network. However, if multiple isolated networks are to be synchronized, multiple appliances will be required. Alternatively, some NTP servers are available with multiple network ports, which allow a single appliance to synchronize a number of isolated networks. If you have a requirement to synchronize multiple independent networks, a multi-LAN unit can provide quite a cost saving.
Accuracy and Throughput
Both accuracy and throughput of a time server is usually dependent on the speed and power of the processor used in the device. The faster and more powerful the processor, the more accurate and the greater throughput the device will have. Typically, a low-end processor can synchronize to within a few milliseconds of a hardware reference clock and serve a few thousand clients. A higher-end processor will synchronize to within a few microseconds of a reference clock and serve tens or even hundreds of thousands of network time clients.
Decide what sort of accuracy your application needs along with the maximum number of clients you are likely to have. It is always a good idea to add a margin to allow for network expansion.
Hardware Clock Reference
All stratum one NTP time servers need to synchronize to an external hardware clock. The most popular time reference for NTP servers is GPS. GPS is a global navigation and positioning system, which can also provide very accurate time. It has a number of advantages over other sources of time. Mainly, that it is available worldwide and is highly accurate. Galileo will also supplement the GPS system, when it comes online. However, a GPS antenna needs a good view of the sky, ideally, on a rooftop, which can increase installation costs.
Radio time references can often be received by indoor-located antennas, which lower installation costs. However, radio broadcasts have a finite range and are not available in all areas. Radio time references include; MSF, DCF-77 and WWVB.
Backup Hardware Clocks
A backup hardware clock is an accurate hardware time reference inside the network time server. It is used to maintain an accurate time in the event that the primary reference clock (GPS, Radio, CDMA) is temporarily lost. Many lower-cost NTP servers do not have a backup clock and will not provide a holdover period in the event of reference clock loss. There are a number of backup hardware clock solutions available – TCXO, OCXO and Rubidium, which vary is cost and accuracy. TCXO offers a good cost/performance compromise, while OCXO and particularly Rubidium are more accurate, but also much more expensive.
For many applications, a simple web-based monitoring solution, showing synchronization and reference clock information, is sufficient. However, more complex applications may require additional monitoring. Generally, SNMP traps generated by a NTP server are used to monitor alarm conditions. Remote sys-logging can also be used to keep a log of events.
To summarize, for most general-purpose applications, a GPS based, rack-mountable NTP time server with a TCXO based backup hardware clock will suffice. Additional options, including multiple LAN ports also add flexibility.
About the Author.
Andy Shinton has spent his entire career within the IT industry, mainly in the Time and Frequency sector. Since 2002, he has headed TimeTools Research and Development Division. Andy regularly writes white-papers and articles about NTP and Network Timing Solutions.