Synchronization of the system time with external timers or in a network

BS2000 provides a privileged and a POSIX “ADJUST TIME” interface for the adaptation of the system time (not the hardware TODR!) to external clocks. This interface is used for:

• synchronization in an NTP network

• synchronization in an XCS network

• Synchronization with the SVP time (SU /390)

• Synchronization with the carrier system time (SU x86)

This type of synchronization job causes the system time to be clocked more slowly or more quickly for a period which is four times as long as the required adaptation value. Here, it should also be noted that two successive GTIME calls for a task continue to return time values that increase in unbroken sequence.

During the adaptation phase, the system time that is being calculated is also corrected by the synchronization calculation that has already been performed:

(-t - t_a) / (t_e - t_a) *d

where:

t	current time
ta	time of start of synchronization
te	time of end of synchronization
d	adaptation value

Only when synchronization is terminated is the TODR correction value corrected by the required adaptation value.

There is a defined sequence of priorities, which corresponds to the list below, between the individual instances which may request synchronization jobs. If two or more instances are active, then only jobs requested by the instance with the highest priority are performed.

The priorities have been defined in the following ascending order (1. = highest priority, 2. = second-highest priority, etc.):

1. BS2000 is a participant in an NTP network which has been configured with a trustworthy external timer (e.g. NTP server whose stratum value is not greater than 4).

2. The Management Unit, which synchronizes the SVP/carrier system time of the Server Units, is connected to a trustworthy external timer (NTP server whose stratum value is not greater than 4).

3. BS2000 is a participant in an XCS network.

4. BS2000 is a participant in an NTP network which has been configured without an external timer.

5. The Management Unit, which synchronizes the SVP/carrier system time of the Server Units, is either not connected to an external timer, or it is connected to an "untrusted" timer (stratum value of the NTP server is greater than 4).

When time is synchronized on the basis of the SVP clock (items 2 and 5), it must be borne in mind that the SVP clock only has a resolution in the seconds range. As a result, deviations of ± 1 second can occur between the SVP clock and the BS2000 time, and on SU /390 also between the SVP clock and the time of the Management Unit. In total this results in a maximum deviation of ± 2 seconds from the time source for the BS2000 system time on SU /390, and of ± 1 second on SU x86.

In an NTP or XCS network (items 1, 3 and 4) the transfer and synchronization of the time takes place with a considerably higher resolution. The deviation of the BS2000 system time from the authoritative time source is in the millisecond range.