Network Synchronization Questions
- This FAQ
- Access to our NTP Client Software Resource page.
In summary, follow these steps:
- Break your organization down into Timing Dependent Entities (TDEs): departments, groups, subnets, and/or functions having discernibly unique synchronization requirements.
- Determine the timing requirements for each TDE: legal, operational, or mandated.
- Select the worst case scenario.
- Select Reference Clock Sources: i.e. UTC(NIST), UTC(USNO), GPS, and synchronization methodology to satisfy the worst case scenario.
- Establish hierarchical network timing topology including segmenting the network to take advantage of the multiple NTP ports on the SyncServers:
Stratum 0 Reference Clock Sources (like GPS) Stratum 1 Primary Time Servers – the SyncServers. Stratum 2 Secondary Time Servers; generally application servers, NOS servers, and/or routers. Stratum 3 Workstations, servers, Controlled Timed Devices (CTDs).
- Install NTP client/server application software:
- Develop a methodology for synchronizing non-NTP synchronized CTDs.
NTP uses a hierarchical structure in which Stratum 0 is the USNO clock. Stratum 1 is a radio receiver that receives the time from Stratum 0 (via GPS). Stratum 2 is a client that receives the time over a network connection from a Stratum 1 clock. Stratum 3 is a client that gets the time from Stratum 2… and so on to a theoretical Stratum 15.
In the telecommunications world, stratum refers to the holdover performance of an oscillator in the event of loss of synchronization. Stratum 1, Stratum 2, Stratum 3, and Stratum 4 are the most typical.
I thought GPS was for position. Does it also supply time?
Most definitely. Since the first days of longitude determination in navigation, time is the REQUIRED 4th dimension. Although GPS was originally developed for military applications, it is now the #1 source of precision time in the world. Each GPS satellite has on board multiple atomic clocks. Virtually all of our power, telecommunication, and networking infrastructure depends on accurate GPS timing for synchronization purposes.
How accurate is GPS for network timing?
When tracking GPS, the SyncServers are accurate to less than 50 nanoseconds to UTC (USNO). Time out of these units via NTP delivers less than 14 microsecond timestamp accuracy. Network factors reduce client synchronization accuracy to 0.5 to 3 ms typically. You can not get any better time synchronization in a local network than you can from a GPS synchronized local time server.
What do I need to track GPS satellites?
Symmetricom NTP time servers are supplied with a GPS antenna (roof or window mounted), mounting kit, and 50 ft. (15 meters) of standard coaxial cable. The roof antenna (all weather) is mounted such that it has a view of most of the sky. The GPS satellite constellation consists of a minimum of 24 satellites with 12 hour inclined orbits. That means that there should always be plenty of satellites within the hemisphere to satisfactorily operate the system. Mount the antenna, string the cable, connect the cable to the back of the Network Time Server and wait several minutes for it to track satellites and obtain a time solution (note: if only part of the sky is in view, it may take longer to get the time).
For window mounted antenna installations it is best to use a window that faces the equator or has the best view of the sky. Generally more satellites will be in view toward the equator than away from it, east or west facing windows will also work. Polar facing windows will also work but in general are not preferred. Windows that have the best view of the sky are preferred over those that don’t, regardless of orientation. Attach the antenna above the window sill versus at the top of the window. This will improve the upward visibility from the antenna to the sky. Note that some window glazing treatments may reduce or block the GPS signals preventing the time server from acquiring the time.
Will GPS work inside a building?
The GPS signal is a relatively low strength signal (~ -132dBm) at an extremely high frequency of 1,575 Megahertz. In order for the antenna to receive such a signal, it is necessary to have a relatively unobstructed view of the sky to view satellites within the GPS constellation (clouds are not a concern). You can track satellites with a window mounted GPS antenna.
The primary concern with reception in a building window is that you effectively block off at least a 180° azimuth viewing angle for available satellites. The time server can accommodate this by deriving the time from as little as one intermittent satellite signal. However, the solar reflective coating found on some buildings today effectively reflects the GPS signal and may reduce the signal strength to a level the time server cannot track.