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Sample
Sample 8D report: intermittent offline periods with cellular connectivity
This is how a root-cause report in 8D format is structured. Content and data are entirely fictitious.
| Field | Details |
|---|---|
| Report | Sample (8D) |
| Device | Charge point “Model A” from a fictitious manufacturer, cellular connectivity |
| Firmware | Version “n” and predecessor “n−1” |
| Backend | fictitious backend, OCPP 1.6J |
| Status | closed |
D1 – Participants
- Manufacturer: support (second level) and firmware development
- Operator: contact for backend data
- Analysis and report: Sebastian Böttger
D2 – Problem description
| Question | Finding |
|---|---|
| What? | Charge points repeatedly go offline in the backend for a few minutes; charging sessions cannot be started during this time. |
| Where? | Only units with cellular connectivity; units on LAN are not affected. |
| Since when? | Since the rollout of firmware version “n”. |
| How often? | Several times a day, more frequently at night and during low usage. |
| How detected? | Monitoring alerts in the backend, user complaints about sessions not starting. |
D3 – Containment actions
- Keepalive interval shortened on the affected units via configuration key.
- Monitoring alert adjusted so that short interruptions are counted separately from real outages.
D4 – Root cause analysis
| Hypothesis | Check | Result |
|---|---|---|
| Insufficient network coverage at the site | Signal quality from device logs against the times of the gaps | rejected – gaps also occur with good signal quality |
| NAT timeout in the mobile network shorter than the keepalive interval | Time since the last message before each gap analysed | confirmed – gaps only after longer radio silence |
| Firmware “n” sends keepalives less often than “n−1” | Default configuration of both versions compared | confirmed – default value increased in “n” |
| Modem firmware loses the data connection | Modem status messages around the gaps | rejected – connection stable on the modem side |
Root cause: Firmware version “n” increased the keepalive interval. With little traffic, the mobile network drops the connection when the NAT timeout expires. The charger only notices the drop when it next sends data and then re-establishes the connection.
D5 – Permanent corrective actions
- Lower the default keepalive interval for cellular connectivity back below the NAT timeout.
- Detect connection loss faster in the firmware and re-establish the connection immediately.
D6 – Implementation and verification
- Corrected firmware rolled out to a test group first, then to the entire field.
- Effectiveness verified via heartbeat intervals in the backend: the offline periods no longer occurred during the observation period.
D7 – Prevention
- Default configuration documented per connectivity type.
- Test case for long radio silence added to the release process for new firmware versions.
- Error code catalogue and first-level decision tree extended with this failure pattern.
D8 – Closure
Report with evidence handed over to support and development, open questions resolved, case closed.