R-APS is defined in ITU-T Y.1731 and G.8032.
| Term | Definition |
|---|
| RPL | Ring Protection Link — the link that is blocked during normal operation |
| RPL Owner | The node responsible for blocking/unblocking the RPL |
| APS Channel | Used exclusively for OAM and R-APS messages |
| WTR | Wait-to-Restore — RPL owner waits for ring to stabilize before re-blocking the RPL after SF recovery (default: 5 min, min: 1 min) |
| WTB | Wait-to-Block — dampens transitions to prevent rapid flapping |
| Hold-off Timer | A fault is only reported to the ring after this timer expires |
| SF | Signal Fail |
| DNF | Do Not Flush — a filtering database flush is not necessary |
| RB | RPL Blocked |
| State | Meaning |
|---|
| Idle | RPL is blocked — normal operation |
| Pending | WTR or WTB timer is running — transitional state |
| Protected | RPL is unblocked — a fault is present somewhere in the ring |
| Message | Meaning |
|---|
| R-APS(SF) | Signal Fail — node is reporting a fault |
| R-APS(NR) | No Request — ring is stable, no fault |
| R-APS(NR,RB) | No Request, RPL Blocked — sent by RPL owner confirming RPL is blocked and ring is healthy |
A protection switch can be triggered by:
- Detection of a Signal Fail (SF)
- A remote request received over R-APS
- Expiry of a G.8032 timer
- Nodes may oscillate between Idle and Pending as they discover each other
- Not all nodes are visible to each other yet
- All nodes settle into Pending state
- The RPL owner’s WTR timer is running (default 5 min; configurable to 1 min)
- No RPL blocking occurs until WTR expires
- WTR expires on the RPL owner
- All nodes simultaneously transition to Idle
- This confirms all nodes received R-APS(NR,RB) from the RPL owner
- RPL owner continues sending R-APS(NR,RB) every 5 seconds as a keepalive
| Indicator | Healthy | Unhealthy |
|---|
| Node states | All nodes in the same state | Mixed states → APS channel is partitioned |
| APS message rate | One R-APS every 5 seconds from the same Node ID (usually RPL owner) | Faster or slower rate → possible partitioning or cross-talk between APS channels |
| State transitions | Stable; no rapid flapping (WTR/WTB dampen transitions) | Rapid flapping < 5 second intervals → likely APS channel cross-talk |