Choosing a system reference is based on the following considerations:
User-assigned Priority Level
User-assigned Input State
SSM
Active alarms on an input
Priority Level
Telcordia GR-378 and GR-1244 define two reference selection modes:
Revertive
Non-revertive
In the revertive mode, when an input used as the system reference is disqualified (for any reason), that input returns, or reverts, to the system reference when the disqualifying reason is removed. The system reference switches two times: once when the disqualifying event occurs, and again when the input is no longer disqualified.
In the non-revertive mode, the system reference does not revert to the initial input when the reason for disqualification is removed. The system reference switches only once when the disqualifying event occurs.
To provision the TimeProvider as non-revertive, you must provision the user-assigned Priority Level on all inputs to the same value (1, 2, 3, or 4). If you provision any of the inputs to a different Priority Level the TimeProvider is in the revertive mode.
Input State
You can provision each input to one of three states:
Disabled – the input is not used
Monitor – the system monitors the input for signal faults and performance data, but it cannot be selected as the system reference
Enabled – the system monitors the input for signal faults and performance data, and it can be selected as the system reference
SSM
The following table illustrates which input is the active system reference under several different input conditions when the TimeProvider is in the SSM mode.
Sequence |
QLevel on PRC |
QLevel on INP1 |
QLevel on INP2 |
Non-Revertive Active Reference1 |
Revertive Active Reference2 |
1 |
2 |
2 |
2 |
PRC |
INP1 |
2 |
4 |
2 |
2 |
INP1 |
INP1 |
3 |
4 |
4 |
2 |
INP2 |
INP2 |
4 |
4 |
4 |
4 |
INP2 |
INP1 |
5 |
4 |
4 |
2 |
INP2 |
INP2 |
6 |
4 |
2 |
2 |
INP2 |
INP1 |
7 |
2 |
2 |
2 |
INP2 |
INP1 |
8 |
2 |
2 |
LOS |
PRC |
INP1 |
9 |
2 |
LOS |
2 |
PRC |
PRC |
10 |
LOS |
LOS |
2 |
INP2 |
INP2 |
11 |
LOS |
LOS |
LOS |
Holdover |
Holdover |
12 |
LOS |
LOS |
2 |
INP2 |
INP2 |
13 |
2 |
LOS |
2 |
INP2 |
PRC |
14 |
2 |
2 |
2 |
INP2 |
INP1 |
Notes:
1Non-revertive example. Priority Levels set to: PRC = 3, INP1 = 3, and INP2 = 3
2Revertive example. Priority Levels set to: PRC = 2, INP1 = 1, and INP2 = 2
SUB
The following table illustrates which input is the active system reference under several different input conditions when the TimeProvider is in the Subtending mode.
Sequence |
QLevel on INP1 |
QLevel on INP2 |
Non-Revertive Active Reference1 |
Revertive Active Reference2 |
1 |
1 |
1 |
INP1 |
INP1 |
2 |
2 |
1 |
INP2 |
INP2 |
3 |
2 |
2 |
INP2 |
INP1 |
4 |
2 |
1 |
INP2 |
INP2 |
5 |
1 |
1 |
INP2 |
INP1 |
6 |
1 |
LOS |
INP1 |
INP1 |
7 |
LOS |
LOS |
Holdover |
Holdover |
8 |
LOS |
1 |
INP2 |
INP2 |
9 |
1 |
1 |
INP2 |
INP1 |
Notes:
1Non-revertive example. Priority Levels set to: INP1 = 3, and INP2 = 3
2Revertive example. Priority Levels set to: INP1 = 1, and INP2 = 2
PRR Reference Selection
In the PRR mode, the TimeProvider has only one possible reference input: the GPS. If that input is not available, the TimeProvider enters the Bridging mode, which is similar to Holdover mode in SSU or SUB.