Power Saving Modes in NB-IoT

Introduction

NB-IoT (Narrowband IoT) is designed for low-power, long-life IoT devices such as sensors, meters, and trackers that operate on batteries for years.
Because the radio is the main energy consumer, NB-IoT provides mechanisms to reduce how long the radio stays active, without losing the ability to reconnect efficiently.

Keep the radio asleep as much as possible, while maintaining registration with the network.

To achieve this, NB-IoT defines several power-saving features:

• Power Saving Mode (PSM)
• Extended Discontinuous Reception (eDRX)
• Extended Tracking Area Update (TAU)
• Release Assistance Indication (RAI)

Each feature targets a specific part of the power-consumption cycle.

Understanding Device States

Before diving into power-saving modes, it’s important to understand the basic NB-IoT connection states:

1. RRC Connected – Device is actively communicating; radio is on.
2. RRC Idle – Device is not transmitting but still listening periodically for paging messages.
3. PSM (Deep Sleep) – Device is attached to the network but unreachable; radio is completely off.

Power saving modes control how long a device remains in each state and when transitions occur.

Power Saving Mode (PSM)

Overview

PSM allows a device to stay registered with the network while turning off its radio completely.
During PSM, the device cannot send or receive data, but it skips the full reattach process when it wakes up.

How it Works

1. After finishing communication, the device tells the network it wants to enter PSM.
2. The device stays reachable for a short time (defined by T3324, the Active Timer).
3. Once T3324 expires, it enters deep sleep and turns off the radio.
4. After a much longer time (defined by T3412, the TAU Timer), the device wakes up and performs a tracking area update (TAU).

Benefits

• Extremely low current consumption (in microamps).
• Keeps registration context with minimal signaling.

When to Use

Use PSM when:

• The device mainly sends uplink data (e.g., sensor measurements).
• Downlink communication is rare or can wait until the next wake-up.

Key Timers

Extended Discontinuous Reception (eDRX)

Overview

eDRX allows a device in idle mode to check for network paging less frequently than normal LTE devices, reducing how often it wakes its receiver.

This is ideal for devices that need occasional downlink communication but can tolerate latency.

How it Works

• The device sleeps for a period (eDRX cycle) and wakes briefly to listen for paging during the Paging Time Window (PTW).
• If no data is pending, it sleeps again.

Example

If eDRX = 40 seconds and PTW = 2 seconds, the device sleeps for 38 seconds, then listens for paging for 2 seconds.

Benefits

• Lower energy consumption than standard DRX.
• The device remains periodically reachable.

Trade-offs

• Downlink latency increases with longer eDRX cycles.
• Slightly higher power use than full PSM due to periodic wake-ups.

Extended TAU (Tracking Area Update)

Overview

TAU is a procedure used by cellular devices to inform the network that they’re still active and within the same area.

NB-IoT extends the TAU timer (T3412) to allow much longer sleep periods, even several days.

Why It Matters

• Reduces how often the device wakes solely for signaling.
• Saves power for stationary or semi-static devices.

Best Practice

• Use the longest TAU period supported by the network when mobility is low.
• If the device moves frequently, use a shorter TAU to avoid re-registration delays.

Release Assistance Indication (RAI)

Overview

RAI is a signal sent by the device to inform the network that no more uplink or downlink data is expected.
This allows the network to immediately release the RRC connection instead of keeping it open.

Why It Saves Power

When a device finishes data transmission, it normally remains in RRC Connected mode for a short time waiting for possible downlink data.
RAI skips this waiting period, letting the device return to idle or PSM faster.

Use Case

• Ideal for single-shot transmissions (e.g., periodic sensor reports).
• Should not be used if the device expects quick downlink responses.

Combining the Features

These power-saving modes can be combined for optimal performance.

Typical Power Cycle Example

1. Data Transmission: Device wakes, sends sensor data.
2. RAI Used: Informs network to release connection immediately.
3. Idle + eDRX: Device remains reachable for a short time (T3324).
4. Enter PSM: Radio off for long T3412 period.
5. Wake Up: After T3412 expires, device performs TAU and repeats.

Practical Optimization Tips

• Start with defaults and measure actual current consumption per cycle.
• Balance latency and lifetime: longer timers provides better battery life, slower response.
• Use PSM for mostly uplink devices, eDRX for those needing occasional downlink.
• Shorten T3324 if no immediate downlink is expected.
• Implement RAI whenever possible to cut connected time.
• Test across real networks, since timer ranges may differ slightly by operator.

Typical Current Consumption Profile

Summary

By combining PSM, eDRX, extended TAU, and RAI, NB-IoT devices can operate for years on a single battery, while still maintaining reliable, low-bandwidth connectivity.