Merge pull request #88670 from sr1kar99/2019-diagram

DIAGRAMS#521: Added a diagram to illustrate holdover in a T-GM clock

Author: stevsmit

images/holdover_in_t_gm.png

@@ -6,7 +6,7 @@
 [id="holdover-in-a-grandmaster-clock_{context}"]
 = Holdover in a grandmaster clock with GNSS as the source
 
-Holdover allows the grandmaster (T-GM) clock to maintain synchronization performance when the GNSS source is unavailable. During this period, the T-GM clock relies on its internal oscillator and holdover parameters to reduce timing disruptions.
+Holdover allows the grandmaster (T-GM) clock to maintain synchronization performance when the global navigation satellite system (GNSS) source is unavailable. During this period, the T-GM clock relies on its internal oscillator and holdover parameters to reduce timing disruptions.
 
 You can define the holdover behavior by configuring the following holdover parameters in the `PTPConfig` custom resource (CR):
 
@@ -37,5 +37,22 @@ The T-GM clock reaches the maximum offset in 60 seconds.
 
 [NOTE]
 ====
-The phase offset is converted from picoseconds to nanoseconds. As a result, the calculated phase offset during holdover is expressed in nanoseconds per second, and the resulting slope is measured in nanoseconds per second.
+The phase offset is converted from picoseconds to nanoseconds. As a result, the calculated phase offset during holdover is expressed in nanoseconds, and the resulting slope is expressed in nanoseconds per second.
 ====
+
+The following figure illustrates the holdover behavior in a T-GM clock with GNSS as the source:
+
+.Holdover in a T-GM clock with GNSS as the source
+image::holdover_in_t_gm.png[Holdover in a T-GM clock with GNSS as the source]
+
+image:darkcircle-1.png[20,20] The GNSS signal is lost, causing the T-GM clock to enter the `HOLDOVER` mode. The T-GM clock maintains time accuracy using its internal clock.
+
+image:darkcircle-2.png[20,20] The GNSS signal is restored and the T-GM clock re-enters the `LOCKED` mode. When the GNSS signal is restored, the T-GM clock re-enters the `LOCKED` mode only after all dependent components in the synchronization chain, such as `ts2phc` offset, digital phase-locked loop (DPLL) phase offset, and GNSS offset, reach a stable `LOCKED` mode.
+
+image:darkcircle-3.png[20,20] The GNSS signal is lost again, and the T-GM clock re-enters the `HOLDOVER` mode. The time error begins to increase.
+
+image:darkcircle-4.png[20,20] The time error exceeds the `MaxInSpecOffset` threshold due to prolonged loss of traceability.
+
+image:darkcircle-5.png[20,20] The GNSS signal is restored, and the T-GM clock resumes synchronization. The time error starts to decrease.
+
+image:darkcircle-6.png[20,20] The time error decreases and falls back within the `MaxInSpecOffset` threshold.