Criteria for Selecting a Time Source
Before you select a time source for your network, ask the following questions:
· How accurate is the time that is provided?
Accuracy is affected by the time source itself, as well as the transmission media. As long as the inaccuracy is known, it can be compensated for.
· How reliable is the time source?
The time source must be available. If the time source is not available, the server connected to the time-provider uses times from other servers and compensates for any time difference when the source again becomes available.
· What is the extent of coverage?
The time source must be available in the geographical area where the time-provider server is located.
· What is the level of known inaccuracy?
If this is known, DTS can compensate for it. Most sources have known inaccuracy levels.
· What is the cost?
· Does the source conform to the operating environment?
The available power supply and physical conditions must be compatible with the source; consult the supplier for the specifications.
The table below summarizes the selection criteria for each type of time source.
Time-Provider Selection Criteria
| Type | Coverage | Inaccuracy | Cost |
| Telephone |
| NIST | Regional | 10 msec. | Variable fee per call |
| Radio |
| MSF | Europe | 10 msec. | $1K to 2K |
| WWV | North America | 100 msec. | $1K to 2K |
| WWVB | North America & Europe | 10 msec. | $1K to 2K |
| WWVH | Eastern & Central North Pacific | 100 msec. | $1K to 2K |
| Satellite |
| GOES | Worldwide | 1 msec. corrected | $2K to $20K |
| GPS | Worldwide | < 100 nsec. | $15K to $20K |