DICOM PS3.17 2021e - Explanatory Information

R.6.5 Implementation Considerations

There are several public reference implementations of NTP server and client software available. These are in widespread use and have been ported to many platforms (including Unix, Windows, and Macintosh). There are also proprietary and built-in NTP services for some platforms (e.g., Windows 2000). The public reference implementations include sample interfaces to many kinds of external clock sources.

There are significant performance considerations in the selection of locations for servers and clients. Devices that need high accuracy synchronization should probably be all on the same LAN together with an NTP server on that LAN.

Real time operating system (RTOS) implementations may have greater difficulties. The reference NTP implementations have been ported to several RTOSs. There were difficulties with the implementations of the internal system clock on the RTOS. The dual frequency/phase adjustment requirements may require the clock functions to be rewritten. The reference implementations also require access to a separate high resolution interval timer (with sub microsecond accuracy and precision). This is a standard CPU feature for modern workstation processors, but may be missing on low end processors.

An RTOS implementation with only ordinary synchronization requirements might choose to write their own SNTP only implementation rather than use the reference NTP implementation. The SNTP client is very simple. It may be based on the reference implementation or written from scratch. The operating system support needed for accurate adjustment is optional for SNTP clients. The only requirement is the time base stability requirement, which usually implies the ability to specify fractional seconds when setting the time.

The conflict between the user desire to use local time and the NTP use of UTC must be resolved in the device. DHCP offers the ability to obtain the offset between local time and UTC dynamically, provided the DHCP server supports this option. There remain issues such as service procedures, start up in the absence of DHCP, etc.

The differences between local time, UTC, summer time, etc. are a common source of confusion and errors setting the battery clock. The NTP algorithms will eventually resolve these errors, but the final convergence on correct time may be significantly delayed. The device might be ready for medical use before these errors are resolved.

DICOM PS3.17 2021e - Explanatory Information