The PSM/eDRX Timer Ledger

Ada audits the PSM/eDRX timer ledger — one time conversion, two measured-energy equations

ADA · CALCULATION AUDIT

Foundations · optional mathematics and physics

The PSM/eDRX Timer Ledger

One time conversion, two measured-energy equations, ~4 minutes

A water meter that reports once a day spends almost all of its 86,400-second cycle asleep, waking only for a short post-uplink window to catch queued settings — and a maintenance campaign adds a temporary window where it listens more often before returning to that low-power policy. The chapter is deliberate that this is not a battery-life button: requested PSM and eDRX timers are only proposals until the network grant and a measured current trace confirm them. So this audit rebuilds the timer ledger to ask what it can honestly claim — the structure of the lifetime calculation, or the lifetime itself?

Companion to the chapter NB-IoT Power Saving (PSM/eDRX) — every number here comes from that chapter.

Convert the daily cycle

The normal meter example reports once per day, so the cycle length is fixed before any modem grant is inserted.

daily_cycle_seconds = 24 h x 60 min/h x 60 s/min = 86400 s

normal_reachable_seconds = granted_active_timer_s + granted_eDRX_paging_seconds

normal_unreachable_seconds = 86400 - normal_reachable_seconds

For PSM-only normal operation, the eDRX paging term is zero rather than guessed.

Keep current as measured physics

Every current claim stays tied to a measured state and duration.

state_mAh = measured_current_mA x measured_duration_s / 3600

normal_day_mAh = sleep_mAh + wake_send_mAh + active_timer_mAh + paging_mAh + retry_mAh + host_overhead_mAh

maintenance_day_mAh = normal_day_mAh + maintenance_eDRX_mAh + command_transfer_mAh + exit_validation_mAh

The monthly maintenance example changes the mode ledger; it does not license a battery-life claim without a return-to-PSM trace.

This audit proves the calculation structure only. Because the chapter has no measured current table, the release gate must still demand real trace values before claiming lifetime.

Every symbol above is taken from the chapter's own PSM/eDRX timer example and re-derived step by step.