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Power Management

Summary
  • A cellular modem's radio is by far the biggest power draw on a battery-powered device — not just while transmitting, but every time it wakes up just to listen for the network.
  • 3GPP defines a stack of increasingly aggressive sleep mechanisms that trade reachability for battery life: DRX (baseline), eDRX (extended, Release 13), and PSM (deepest, Release 12).
  • These aren't alternatives — a real device typically layers all three, choosing how deep to sleep based on how quickly it needs to be reachable from the network.

Think of these modes as:

"Layers of increasingly deep sleep, each with a longer alarm clock before the modem stirs to check if anyone's calling — DRX naps between heartbeats, eDRX naps between meetings, and PSM naps between billing cycles."

The Power Problem: RRC States & Paging

A modem isn't just "on" or "off" from the network's point of view — it moves between RRC (Radio Resource Control) states, and its power draw differs by orders of magnitude between them:1

RRC State What It Means Relative Power
RRC_CONNECTED Actively transmitting/receiving data Highest
RRC_IDLE Registered, but no active data session Low, but periodic
RRC_INACTIVE (5G only) A middle state — context kept, but radio mostly off Very low
PSM Deregistered from the radio's perspective, still registered on the core network Lowest

Table 1. Cellular Modem Power States

Even in RRC_IDLE, the modem can't just switch off — the network needs a way to reach it for an incoming call, SMS, or downlink data. That mechanism is paging: the network broadcasts a paging message on a schedule, and the device wakes briefly to check if its own ID is in it.1 Every power-saving mode below is really just a different answer to "how long can the modem sleep between paging checks?"

DRX — Discontinuous Reception

DRX is the baseline mechanism, present since 2G/3G and mandatory in LTE. The modem and network agree on a paging cycle: the modem wakes for a short On Duration window, checks the Paging Occasion, and sleeps again if nothing's addressed to it.2

Typical LTE Idle-mode DRX parameters
defaultPagingCycle:  1.28 s                  # (1)!
onDurationTimer:     few ms                  # (2)!
UE-specific cycle:   optional, up to 2.56 s  # (3)!
  1. defaultPagingCycle — The network-broadcast default paging cycle length; LTE's standard default is 1.28 seconds.
  2. onDurationTimer — How long the modem actually keeps its receiver active during each wake-up to check for a page — typically just a few milliseconds.
  3. UE-specific cycle — The network can assign a device a longer-than-default cycle (up to 2.56 s in LTE) if it doesn't need to be reached as urgently.

There's a second, separate flavor: Connected-mode DRX (C-DRX), used while the modem is in RRC_CONNECTED but idle between bursts of data (e.g. between web page loads) — much shorter cycles (tens to hundreds of milliseconds), aimed at saving power without dropping out of the connected state entirely.2

DRX alone isn't enough for IoT

A 1.28-second paging cycle means the modem's receiver wakes roughly 67,000 times a day. For a device meant to run for years on a coin cell, that's still far too frequent — which is exactly the gap eDRX and PSM were designed to close.

eDRX — Extended DRX

eDRX, introduced in 3GPP Release 13, stretches the idle-mode paging cycle far beyond legacy DRX limits — from tens of seconds up to tens of minutes (LTE-M) or hours (NB-IoT).3

eDRX cycle structure
eDRX_cycle:               up to 43.69 min (LTE-M)  # (1)!
                          up to 2.91 hr (NB-IoT)   # (2)!
Paging Time Window:       1.28 s – 40.96 s         # (3)!
Legacy DRX (within PTW):  still active             # (4)!
  1. eDRX_cycle (LTE-M) — Maximum extended cycle length for LTE-M is 2,621.44 seconds — about 43.7 minutes.
  2. eDRX_cycle (NB-IoT) — NB-IoT supports much longer cycles, up to 10,485.76 seconds — roughly 2.9 hours.
  3. Paging Time Window (PTW) — When the eDRX cycle ends, the modem doesn't just check once — it opens a window of this length and listens for paging occasions throughout it, to reduce the chance of a missed page due to timing drift.
  4. Legacy DRX (within PTW) — Standard short-cycle DRX still runs inside the Paging Time Window — eDRX doesn't replace DRX, it just adds a much longer sleep period around it.

PSM — Power Saving Mode

PSM, from 3GPP Release 12, is the deepest state: the modem stops monitoring paging entirely and effectively powers down its radio, while remaining registered on the core network — no re-attach is needed when it wakes.4 Two timers control the cycle:

Timer Full Name Role
T3324 Active Timer How long the modem stays reachable (paging-monitoring) after the last data activity, before entering PSM
T3412 (extended) Periodic TAU Timer The maximum duration of the PSM sleep itself, before the modem must wake and re-register

Table 2. PSM Timers

PSM timer example (AT+CPSMS= style values)
T3324 (Active Timer):       2 min     # (1)!
T3412-ext (TAU Timer):      ~24 hr    # (2)!
Max theoretical T3412-ext:  413 days  # (3)!
  1. T3324 — The device stays awake and reachable for 2 minutes after its last activity — enough time for the network to deliver a delayed downlink message before the device disappears into PSM.
  2. T3412-ext — Once the Active Timer expires, the modem sleeps for up to this long before it's required to wake and perform a Tracking Area Update — here, once a day.
  3. Max theoretical T3412-ext — The extended TAU timer's encoding allows values up to roughly 413 days — the absolute outer bound the standard permits, rarely used in practice but often cited as PSM's headline number.

PSM devices are unreachable while asleep

During PSM, the network genuinely cannot page the device — there's no mechanism to interrupt the sleep early. Anything mobile-terminated (an incoming command, an OTA update) has to wait until the device wakes on its own, either at the end of T3412 or because it initiated its own uplink transmission. This is the trade-off that makes PSM unsuitable for anything that needs to be reachable on demand.

Modes Comparison

Mode Typical Sleep Depth Wake Latency (network → device) Best For
DRX Shallow ~1.28 s (idle) / tens of ms (connected) General cellular devices, phones
eDRX Medium Up to ~44 min (LTE-M) / ~3 hr (NB-IoT) Sensors reporting periodically, tolerant of delayed downlink
PSM Deepest Up to ~413 days (until next scheduled wake or uplink) Devices that are almost always mobile-originated only (e.g. meters that just report on their own schedule)

Table 3. DRX vs. eDRX vs. PSM

Picking a mode for a real device
  • Asset tracker reporting hourly, occasionally needs a remote "locate now" command → eDRX, so it stays reachable within a bounded window between reports.
  • Water meter reporting once a day, never needs to be pinged externally → PSM, maximizing battery life since nothing needs to reach it between reports.
  • Smartphone → DRX / C-DRX only — it needs to receive calls, messages, and app pushes at any moment, so it can never sleep deeply enough for eDRX or PSM to make sense as the primary mode.

Useful Resources

  1. 3GPP. (n.d.). TS 36.304 — User Equipment (UE) procedures in idle mode. https://www.3gpp.org/DynaReport/36304.htm ↩↩

  2. 3GPP. (n.d.). TS 36.321 — Medium Access Control (MAC) protocol specification. https://www.3gpp.org/DynaReport/36321.htm ↩↩

  3. 3GPP. (n.d.). TS 23.682 — Architecture enhancements for facilitating communications with packet data networks and applications. https://www.3gpp.org/DynaReport/23682.htm ↩

  4. 3GPP. (n.d.). TS 23.401 — GPRS enhancements for E-UTRAN access. https://www.3gpp.org/DynaReport/23401.htm ↩