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5.1 Multi-sources & Resiliency
Figure 5-2:
Another key aspect of FOCA is how to determine when there is a "failure" on a
timing source. Some cases are obvious such as the link is down, no packets are
exchange but other cases can be more complex to identify: all these cases are
detailed in the appendix VCS code tables
page 161).
For a deeper understanding of the behavior of the FOCA algorithm it is recom-
mend reading the section
arios are detailed.
56
FOCA algorithm under scenario 1
In t
, the active reference (solid green line) is WR1 because the primary ref-
1
erence has reached a CRITICAL state (dashed red line).
In t
, the primary reference WR0 becomes available again (dashed green
2
line) but the device keeps using WR1 as the active reference as no failure
has been detected on this timing source.
In t
, an error is detected on WR1 and the FOCA algorithm will act dif-
3
ferently according to the configuration of its strategy.
A. If the strategy is to re-evaluate all timing sources when a failure
occurs, and the primary reference is eligible, the WR0 will be selected
as the active reference.
B. If the strategy is to only fall-down, the FOCA algorithm will select the
next available timing source in the list and will thus lock on the
external GNSS reference. With this strategy the only way to use back
WR0 as the active reference is to restart the devices' synchronization
daemon (/etc/init.d/ppsi restart) or to reach the last timing source
and wait for a critical error.
("Grand Master (GM VCS Code)" on
"Others" on page 168
CHAPTER
in the appendix where more scen-
•
5
WR-Z16 User Manual Rev. v5.0
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