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flowchart TD
Q["Does cloud need to send<br/>commands to device?<br/>(firmware updates, remote config)"]
Q -->|NO| PSM["Use PSM<br/>(Power Saving Mode)"]
Q -->|YES| EDRX["Use eDRX or<br/>PSM+eDRX Hybrid"]
PSM --> PSM_EX["Examples:<br/>Water meter, soil moisture,<br/>asset tracker"]
PSM_EX --> PSM_LIFE["Battery life: 15+ years"]
EDRX --> EDRX_EX["Examples:<br/>Smart lock, parking sensor,<br/>valve actuator"]
EDRX_EX --> EDRX_LIFE["Battery life: 5-10 years"]
subgraph HYBRID["Hybrid Approach (Best of Both)"]
H1["Default: PSM<br/>(deep sleep, no downlink)"]
H2["When update needed:<br/>Enable eDRX temporarily"]
H3["After update:<br/>Return to PSM"]
H1 --> H2 --> H3
end
PSM_LIFE --> HYBRID
EDRX_LIFE --> HYBRID
HYBRID --> RESULT["Smart meter example:<br/>364 days PSM + 1 day eDRX<br/>Battery life: 12-15 years"]
style Q fill:#2C3E50,stroke:#16A085,color:#fff
style PSM fill:#16A085,stroke:#2C3E50,color:#fff
style EDRX fill:#E67E22,stroke:#2C3E50,color:#fff
style PSM_LIFE fill:#16A085,stroke:#2C3E50,color:#fff
style EDRX_LIFE fill:#E67E22,stroke:#2C3E50,color:#fff
style RESULT fill:#7F8C8D,stroke:#2C3E50,color:#fff
1135 NB-IoT Power Saving Modes: PSM and eDRX
1135.1 Learning Objectives
By the end of this chapter, you will be able to:
- Understand PSM and eDRX: Explain Power Saving Mode and Extended Discontinuous Reception mechanisms
- Calculate Battery Life: Compute device lifetime using active, idle, and sleep current profiles
- Configure Power Modes: Set appropriate T3412, T3324 timers for different application requirements
- Choose Between Modes: Select optimal power mode based on downlink requirements and battery targets
1135.2 Prerequisites
Before diving into this chapter, you should be familiar with:
- NB-IoT Fundamentals: Understanding basic NB-IoT concepts, deployment modes, and system architecture provides the foundation for advanced power management topics
- Cellular IoT Fundamentals: Knowledge of cellular network protocols and 3GPP standards helps you understand power-saving mechanisms in the context of LTE architecture
- LPWAN Fundamentals: Understanding LPWAN power optimization strategies provides context for comparing NB-IoT’s PSM/eDRX with other technologies
NoteRelated Chapters
Deep Dives: - NB-IoT Labs and Implementation - Configure PSM/eDRX with AT commands - NB-IoT Channel Access - Uplink/downlink structure and tone configurations - NB-IoT Coverage Enhancement - Repetition mechanisms for deep coverage
Comparisons: - Cellular IoT Comprehensive Review - NB-IoT vs LTE-M power modes
Related Topics: - LoRaWAN Architecture - Alternative LPWAN power strategies - Energy-Aware Considerations - Battery life design
NoteQuick Reference: Acronyms and Timers
- PSM (Power Saving Mode): Deep sleep with the radio off; device stays registered but is unreachable for downlink.
- eDRX (extended Discontinuous Reception): Periodic paging windows; device is reachable for downlink, but with higher idle power than PSM.
- TAU (Tracking Area Update): Brief wake-up to refresh network registration and location context.
- T3412 (Periodic TAU timer): How long the device can stay in PSM before performing a TAU (hours to days).
- T3324 (Active timer): How long the device stays reachable after an uplink (seconds to minutes) before entering PSM.
1135.3 Getting Started (For Beginners)
If you’re new to NB-IoT power management, this section will help you understand how NB-IoT devices achieve 10+ year battery life.
1135.3.1 What is NB-IoT Power Management? (Simple Explanation)
Analogy: Think of NB-IoT power modes like your smartphone’s power settings, but WAY more extreme:
- Your smartphone: “Sleep mode” saves battery, but the phone still checks for notifications every few seconds - battery lasts 1-2 days
- NB-IoT PSM mode: “Deep sleep” - the device completely turns off the radio and sleeps for hours or days - battery lasts 10-15 years!
The key difference: NB-IoT devices don’t need to be instantly reachable (unlike your phone). They wake up, send data, and go back to deep sleep.
1135.3.2 Real-World Examples
Example 1: Smart Water Meter (PSM Mode)
Scenario: Municipal water meter in basement
- Reports water usage once per day (midnight)
- No downlink needed (utility company doesn't send commands)
- Must last 15 years (replacing battery costs $50 labor + truck roll)
Power profile (24 hours):
├─ Sleep (PSM): 23 hours 59 minutes (radio OFF, 5 μA)
├─ Wake up at midnight: 1 second
├─ Send water reading (50 bytes): 5 seconds (200 mA)
├─ Receive ACK: 1 second (50 mA)
└─ Return to PSM: immediate
Daily energy consumption:
- Sleep: 23.98 hours × 5 μA = 120 μAh = 0.12 mAh
- Transmit: 7 seconds × 150 mA (average) = 0.29 mAh
Total per day: 0.41 mAh
Battery life with 6,000 mAh battery (2× AA lithium):
6,000 mAh ÷ 0.41 mAh/day = 14,634 days = **40 years**!
(In practice: 15 years due to battery self-discharge)Why PSM works here: - Device doesn’t need to receive commands (uplink-only) - Reporting schedule is predictable (midnight daily) - Deep sleep maximizes battery life
Example 2: Smart Parking Sensor (eDRX Mode)
Scenario: Street parking sensor
- Reports occupancy changes immediately (car arrives/leaves)
- Must receive commands from cloud (e.g., "enable maintenance mode")
- Battery replacement every 5 years is acceptable
Power profile (typical day):
├─ Sleep with eDRX listening: 23 hours (15 μA average)
│ └─ Wake every 2.91 hours to check for downlink (100 ms)
├─ Occupancy change events: 4× per day (car in/out)
│ └─ Each transmission: 5 seconds (200 mA)
└─ Total active time: 20 seconds/day
Daily energy consumption:
- eDRX sleep: 23 hours × 15 μA = 345 μAh = 0.345 mAh
- Transmissions: 4 × 5s × 200mA = 1.11 mAh
Total per day: 1.46 mAh
Battery life with 6,000 mAh battery:
6,000 ÷ 1.46 = 4,110 days = **11.3 years** ✅Why eDRX is needed here: - Device must be reachable for cloud commands (downlink required) - Still achieves 10+ year battery life - Slightly higher power consumption than PSM (15 μA vs 5 μA)
1135.3.3 The Two Essential Power Saving Modes
NB-IoT offers two main power-saving modes. Here’s how to choose:
| Power Mode | Sleep Current | Can Receive Messages? | Wake-up Time | Battery Life | Use Case |
|---|---|---|---|---|---|
| PSM (Power Saving Mode) | 3-5 μA | No (radio OFF) | Hours to days | 15+ years | Uplink-only sensors (meters, trackers) |
| eDRX (Extended DRX) | 15 μA | Yes (wakes periodically) | Seconds to minutes | 5-10 years | Devices needing downlink (actuators, remote config) |
Key insight: The difference between 5 μA (PSM) and 15 μA (eDRX) seems tiny, but over 10 years: - PSM: 5 μA × 87,600 hours = 438 mAh - eDRX: 15 μA × 87,600 hours = 1,314 mAh - Difference: 876 mAh (can double battery size requirements!)
1135.3.4 Common Beginner Questions
Q1: What’s the difference between PSM and “turning the device off”?
A: PSM keeps network registration, “off” loses it:
PSM (Power Saving Mode):
1. Device tells network: "I'm going to sleep for 24 hours"
2. Network says: "OK, I won't page you until then"
3. Device sleeps (5 μA)
4. Wake up → immediate connection (already registered)
└─ No handshake needed, just send data (fast!)
Fully OFF (not registered):
1. Device powers off completely (0 μA)
2. Wake up → must re-register with network
├─ Cell search: 5-10 seconds
├─ Attach procedure: 10-20 seconds
└─ Finally send data: 5 seconds
Total: 20-35 seconds active (vs 5 seconds with PSM)
Battery impact:
- PSM: 5 seconds active (0.28 mAh)
- Full OFF/ON: 25 seconds active (1.39 mAh)
→ PSM uses 80% less energy per transmission!Q2: Can I use Wi-Fi-style “sleep mode” with NB-IoT to save power?
A: No, NB-IoT requires PSM or eDRX for long battery life:
Wi-Fi sleep (like ESP32 light sleep):
- Radio stays synchronized with access point
- Must wake every 100 ms to receive beacon
- Sleep current: 800 μA - 15 mA (depending on implementation)
- Battery life: weeks to months
NB-IoT PSM (deep sleep):
- Radio completely OFF (loses synchronization)
- No periodic wake-ups required
- Sleep current: 3-5 μA
- Battery life: 10-15 years
Why 100× difference?
- Wi-Fi: Designed for high-speed, always-on communication
- NB-IoT: Designed for infrequent, scheduled communicationQ3: How do I choose between PSM and eDRX?
A: Ask: “Does the cloud need to send commands to the device?”
1135.4 Power Saving Mode (PSM)
PSM allows devices to enter deep sleep while remaining registered to the network:
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sequenceDiagram
participant Device as NB-IoT Device
participant Network as Network
Device->>Network: Send uplink data<br/>(100 bytes)
Note over Device: Active TX: 200mA<br/>Duration: 5 seconds
Network->>Device: ACK
Note over Device: Enter Idle mode<br/>T3324 Active Timer starts
Note over Device: Idle RX: 50mA<br/>Duration: 30 sec (T3324)
Note over Device,Network: Can receive downlink during T3324
Note over Device: T3324 expires
Device->>Network: Enter PSM
Note over Device: Deep Sleep<br/>Current: 3-5 µA<br/>Duration: Hours to days (T3412)
Note over Device,Network: Device unreachable<br/>Radio completely OFF
Note over Device: T3412 TAU timer expires
Device->>Network: Tracking Area Update
Note over Device: Brief connection<br/>Re-register with network
Device->>Network: Enter PSM again
NoteAlternative View: Power State Machine
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stateDiagram-v2
[*] --> Connected: Power on
Connected --> Active_TX: Send data
Active_TX --> Active_RX: TX complete
Active_RX --> Idle: RX done
Idle --> PSM_Sleep: T3324 expires
Idle --> Active_TX: Send data
Idle --> Active_RX: Downlink
PSM_Sleep --> Connected: T3412 expires
PSM_Sleep --> Active_TX: App wake
state "Active TX (200mA)" as Active_TX
state "Active RX (50mA)" as Active_RX
state "Idle (10mA)" as Idle
state "PSM Sleep (3uA)" as PSM_Sleep
This state machine diagram shows NB-IoT power states: devices cycle through Connected, Active (TX/RX), Idle, and deep PSM Sleep modes based on timers and events.
{fig-alt=“NB-IoT Power Saving Mode sequence diagram showing device sending uplink data using 200mA for 5 seconds, entering idle mode for T3324 duration (30 seconds at 50mA) when downlink can be received, then entering deep PSM sleep at 3-5 µA for hours to days until T3412 periodic TAU timer expires, when device briefly wakes to re-register with network before returning to PSM.”}
PSM characteristics: - Device remains registered but unreachable - No paging monitoring (radio off) - Wakes up periodically or on application trigger - Current consumption: < 5 µA (comparable to powered off) - Wake-up triggers: Timer expiry, application event
Configurable timers: - T3324 (Active Timer): Time in idle mode after data transfer before entering PSM - T3412 (Periodic TAU): Time between tracking area updates (can be hours to days)
1135.5 Extended Discontinuous Reception (eDRX)
eDRX extends the sleep period while remaining reachable for mobile-terminated data:
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sequenceDiagram
participant Device as NB-IoT Device
participant Network as Network
Device->>Network: Send uplink
Note over Device: Enter eDRX sleep<br/>Current: 15 µA
Note over Device,Network: eDRX cycle: 2.91 hours max
Note over Device: Sleep... (2.5 hours)
Note over Device: Wake for paging window
Device->>Network: Check for paging
Network-->>Device: No downlink
Note over Device: Return to eDRX sleep<br/>Current: 15 µA
Note over Device: Sleep... (2.5 hours)
Note over Device: Wake for paging window
Device->>Network: Check for paging
Network->>Device: Downlink available!
Network->>Device: Send command (100 bytes)
Note over Device: Process command<br/>Active: 200mA, 10 sec
Device->>Network: ACK
Note over Device: Return to eDRX sleep
{fig-alt=“NB-IoT Extended Discontinuous Reception sequence diagram showing device in eDRX sleep mode at 15 µA, periodically waking (up to 2.91 hour cycles) to check for paging. First wake finds no downlink and device returns to sleep. Second wake receives downlink command, device processes it at 200mA for 10 seconds, acknowledges, then returns to eDRX sleep maintaining reachability while conserving power.”}
eDRX characteristics: - Device remains reachable (can receive downlink) - Periodically wakes to listen for paging - eDRX cycle: up to 2.91 hours (10,485 seconds) in NB-IoT - Sleep current: ~15 µA (slightly higher than PSM) - Trade-off: Reachability vs power consumption
1135.6 Power Mode Comparison
| Mode | Current | Reachable | Latency | Use Case |
|---|---|---|---|---|
| Active/Connected | 50-300 mA | Yes | < 1s | Data transfer |
| Idle (normal DRX) | 1-5 mA | Yes | < 10s | Frequent communication |
| Idle (eDRX) | 15 µA | Yes | seconds-minutes | Occasional downlink |
| PSM | <5 µA | No | hours-days | Uplink-only, scheduled |
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graph LR
ACTIVE["Active/Connected<br/>50-300 mA<br/>< 1s latency"]
IDLE["Idle DRX<br/>1-5 mA<br/>< 10s latency"]
EDRX["eDRX<br/>15 µA<br/>seconds-minutes"]
PSM["PSM<br/>< 5 µA<br/>hours-days"]
ACTIVE -->|Data transfer complete| IDLE
IDLE -->|No activity<br/>Enter light sleep| EDRX
EDRX -->|Extended inactivity<br/>Enter deep sleep| PSM
PSM -->|TAU timer<br/>or uplink trigger| ACTIVE
EDRX -.->|Downlink arrives<br/>during paging| ACTIVE
PSM -.->|Cannot receive<br/>Radio OFF| PSM
style ACTIVE fill:#E74C3C,color:#fff
style IDLE fill:#E67E22,color:#fff
style EDRX fill:#F39C12,color:#fff
style PSM fill:#27AE60,color:#fff
{fig-alt=“NB-IoT power mode state machine showing transitions between Active/Connected (50-300mA, <1s latency), Idle DRX (1-5mA, <10s), eDRX (15µA, seconds-minutes), and PSM (< 5µA, hours-days). After data transfer, device enters Idle, then eDRX light sleep, then PSM deep sleep for maximum power savings. PSM can only wake on TAU timer or uplink trigger (radio OFF, cannot receive downlink). eDRX can wake on paging for downlink reception while maintaining low power.”}
CautionPitfall: Ignoring Initial Attach Power Cost in Battery Calculations
The Mistake: Developers calculate battery life based only on daily transmission energy (e.g., “50 bytes × 4 times/day = 0.5 mAh/day”), then are shocked when field devices die 40% faster than predicted.
Why It Happens: NB-IoT initial attach after PSM wake-up requires a complete RRC connection establishment, authentication, and security mode command exchange. This signaling overhead consumes 220-300 mA for 5-10 seconds BEFORE any user data is transmitted, adding 0.3-0.8 mAh per attach cycle that’s often omitted from calculations.
The Fix: Include full attach sequence in power budget: 1. RACH preamble: 50ms @ 200mA = 0.003 mAh 2. RRC setup + NAS signaling: 3-5 seconds @ 220mA = 0.18-0.30 mAh 3. Authentication: 1-2 seconds @ 150mA = 0.04-0.08 mAh 4. Security mode: 0.5-1 second @ 150mA = 0.02-0.04 mAh 5. Bearer setup: 0.5-1 second @ 150mA = 0.02-0.04 mAh Total attach overhead: 0.27-0.48 mAh per wake cycle (typically exceeds user data transmission cost for small payloads). For devices waking 4 times/day: attach overhead = 1.1-1.9 mAh/day. Compare to 50-byte TX: only 0.07 mAh. Attach is 15-27x more expensive than data!
1135.7 Knowledge Check
Test your understanding of NB-IoT power modes: