8 NB-IoT Power Saving (PSM/eDRX)
Reachability Timers, Current Traces, and Network Grants
Overview: Power Saving Is a Reachability Contract
NB-IoT power saving is not a battery-life button. Power Saving Mode (PSM) and extended discontinuous reception (eDRX) decide when a device can be reached after it sends or receives traffic. A useful design starts by writing the product's downlink requirement, then chooses the least reachable mode that still lets operations do the work they actually need.
PSM keeps the device registered from the network's point of view, but the sleeping device is not listening for arbitrary downlink. eDRX keeps a device periodically reachable by waking it for paging opportunities. Both modes are negotiated with the network, so requested timer values are only proposals until the field logs show what was granted.
Write the reachability promise before picking timers. A daily meter may only need a short post-uplink window for queued settings. A leak alarm may need a bounded acknowledgement path but not arbitrary cloud commands. A maintenance campaign may need a temporary window where devices listen more often, followed by proof that they return to the normal low-power policy. Those promises decide whether PSM, eDRX, a scheduled hybrid, or another technology is acceptable.
The approval evidence should include the granted timer values, the whole-board current trace, server receipts, missed-command behavior, and recovery from weak coverage. If any of those are missing, the power mode is still an assumption rather than a release decision. Record the approved timer policy with firmware version and operator profile, because both can change the reachable state later.
Use PSM when the device can lead
Fixed meters and simple sensors often fit PSM because the device can wake, report, listen briefly for acknowledgements or queued changes, and then sleep until the next event or periodic update.
Use eDRX when the cloud needs a window
eDRX fits delayed command paths where the operator can wait for paging windows but cannot wait for the next device-originated report.
Reject mode labels without evidence
A rollout gate needs granted timer values, current traces, payload receipts, downlink timing, retry behavior, and recovery evidence from representative sites.
If the service promise says "the cloud can command the device at any time," PSM-only behavior is not the right interpretation. Either change the service promise, use a reachable mode, or schedule explicit maintenance windows.
Practitioner: Build the Gate From Granted Timers and Traces
The practical workflow is a ledger. Record the requirement, request candidate PSM or eDRX behavior, read the granted values, and test the same run through modem logs, server receipts, and whole-board current measurements. The battery model should use measured phase durations rather than a single sleep-current number.
Do not measure the modem alone. Host processors, sensors, regulators, debug interfaces, flash writes, and retry logic can dominate the budget after the radio appears to be asleep. The proof is a wake-to-sleep trace from the final firmware on representative hardware.
Approval Sequence
- Write the maximum acceptable downlink delay in operational language.
- Choose PSM, eDRX, or a scheduled hybrid mode from that delay.
- Request candidate behavior and record the modem configuration.
- Read granted values after attach and update the application schedule from the grant.
- Capture a whole-board current trace and matching server receipt.
- Repeat in weak coverage and recovery scenarios before pilot approval.
A clean lab trace is a baseline, not a fleet decision. The review should include at least one representative marginal site and one recovery path, because retry time can cost more energy than the intended sleep policy saves.
Under the Hood: Timers Shape the State Machine
The underlying state machine has three reviewable boundaries. After a device-originated transaction, an active timer can keep the device reachable for pending downlink. eDRX can add periodic paging opportunities while reducing continuous listening. PSM can remove paging reachability until the device wakes by application event or periodic update. The exact values and transitions depend on the network grant and the modem implementation, so the state machine must be observed rather than assumed.
The state machine also includes the host application. The host decides when sensors run, when flash is written, when the modem is allowed to wake, how retries are bounded, and when queued commands are safe to apply. A modem may enter PSM correctly while a regulator, debug interface, sensor heater, or retry loop keeps the board above the expected current. Under-the-hood review therefore pairs modem state with board-level measurement.
Use fault cases to test the transitions. Failed attach, refused timer grant, missed acknowledgement, cloud outage, profile error, and weak coverage can all keep the device active longer than the clean trace. The approved timer model should state the retry limit, backoff behavior, event log, and support action for each case. That model is what lets a reviewer distinguish expected delayed reachability from a defect.
Reason About the Energy Budget by Phase
A practical battery model sums measured phases rather than trusting a single nominal state. The model should include registration or resume behavior, uplink and receive time, active timer duration, eDRX paging windows where used, PSM sleep, host overhead, retry allowance, and maintenance windows.
Active timer
The active timer is useful for acknowledgements and queued configuration after an uplink. If it is too long, it can dominate an otherwise quiet daily-report design.
Paging windows
eDRX paging windows make delayed downlink possible, but every paging opportunity has a measurable energy cost and an operational latency consequence.
Recovery paths
Attach failure, poor coverage, application retry, SIM or profile faults, and cloud outage can keep the device active long enough to invalidate the clean model.
Worked Review: Scheduled Maintenance
For a meter that reports once per day, normal operation can use PSM after the daily report if commands can wait. A monthly maintenance window can temporarily use eDRX or another more reachable policy for devices in the campaign. The approval record should prove three things: commands arrive during the maintenance window, firmware exits the higher-reachability mode afterward, and the post-window current trace returns to the normal sleep profile.
8.1 Start With the Story
A battery-powered device spends most of its life doing nothing visible. PSM and eDRX are the tools that decide whether it sleeps completely, listens occasionally, or stays reachable enough for the job.
Start simple: write the wake-up, listen, send, and sleep story before calculating battery life.
8.2 Summary
- PSM is a good default when the device can initiate useful communication and cloud actions can wait for a device-originated window.
- eDRX provides bounded but delayed reachability; it should be approved only when the paging cost and latency fit the workflow.
- Requested timers are not enough. The battery model and application schedule must use network-granted behavior.
- Field validation needs modem logs, whole-board current traces, cloud receipts, weak-coverage cases, and recovery paths.
- Maintenance modes need explicit exit conditions so a fleet does not remain more reachable, and less power efficient, after service work ends.
8.3 Key Takeaway
NB-IoT PSM and eDRX are reachability contracts. Choose them from the service delay requirement, then prove the granted timers and whole-device current behavior before treating the battery model as rollout evidence.
8.4 See Also
- NB-IoT Fundamentals - service model, narrowband tradeoffs, and NB-IoT fit evidence.
- NB-IoT Architecture - device, access network, core, and application boundaries.
- NB-IoT vs LTE-M Comparison - when power behavior, mobility, payload, and reachability point to different cellular IoT options.
- Cellular IoT Deployment Planning - field gates, rollout records, and operational ownership.