942 IEEE 802.15.4 Review: Beacon-Enabled Networks
Beacon-enabled networks provide synchronized operation with guaranteed time slots. This review covers:
- Superframe Structure: Understanding SO (Superframe Order) and BO (Beacon Order)
- GTS Allocation: Guaranteed Time Slots for time-critical traffic
- Duty Cycle Control: Managing active/inactive periods for power efficiency
Master these concepts for time-critical IoT applications.
942.1 Learning Objectives
By the end of this review, you will be able to:
- Calculate Superframe Timing: Determine beacon intervals and active periods from SO/BO
- Allocate GTS Resources: Plan Guaranteed Time Slot allocation for time-critical traffic
- Optimize Duty Cycle: Balance capacity against power consumption
- Analyze Network Modes: Choose between beacon-enabled and non-beacon operation
942.2 Prerequisites
Required Chapters: - 802.15.4 Review: Architecture - Foundational concepts - 802.15.4 Review: Power Management - Duty cycle concepts - 802.15.4 Fundamentals - Core standard
Estimated Time: 30 minutes
942.3 Superframe Structure Overview
In beacon-enabled mode, the coordinator broadcasts periodic beacons that define the network timing structure.
Key Parameters: - aBaseSuperframeDuration: 15.36 ms (base timing unit) - Superframe Order (SO): Controls active period duration - Beacon Order (BO): Controls beacon interval (total cycle) - Constraint: BO >= SO (ensures inactive period exists)
Timing Formulas:
Active Superframe Duration = aBaseSuperframeDuration x 2^SO
Beacon Interval = aBaseSuperframeDuration x 2^BO
Inactive Period = Beacon Interval - Active Duration
Duty Cycle = Active Duration / Beacon Interval = 2^SO / 2^BO = 2^(SO-BO)942.4 Superframe Slot Structure
The active superframe is divided into 16 equal slots:
| Segment | Slots | Description |
|---|---|---|
| Beacon | 0 | Coordinator beacon for synchronization |
| CAP | 1-13 | Contention Access Period (CSMA/CA) |
| GTS (CFP) | 14-15 | Guaranteed Time Slots (Contention-Free Period) |
Example Configuration (SO=4, BO=6):
Base duration: 15.36 ms
Active superframe: 15.36 ms x 2^4 = 245.76 ms
Beacon interval: 15.36 ms x 2^6 = 983.04 ms
Slot duration: 245.76 ms / 16 = 15.36 ms per slot
Duty cycle: 245.76 / 983.04 = 25%
Inactive period: 983.04 - 245.76 = 737.28 ms942.5 GTS Allocation for Time-Critical Traffic
942.5.1 HVAC Control Example
A smart building uses IEEE 802.15.4 in beacon-enabled mode for HVAC control: - SO=4, BO=6 (25% duty cycle) - HVAC commands require guaranteed delivery within 50 ms - 2 GTS slots allocated (1 slot each)
CAP Availability Calculation:
Active superframe: 16 slots
Beacon: slot 0 (implicit)
GTS allocation: 2 slots (slots 14-15)
CAP: slots 0-13 = 14 slots
Available for contention-based access:
14 slots / 16 total = 87.5%GTS Timing:
Slot duration: 15.36 ms
GTS duration: 2 x 15.36 ms = 30.72 ms
HVAC command delivery: < 50 ms requirement met (15.36 ms per slot)942.6 Why Increase Superframe Order (SO)?
Current Configuration (SO=4):
Active period: 245.76 ms (16 slots x 15.36 ms)
Each slot: 15.36 ms
CAP traffic: 13 slots x 15.36 ms = 199.68 ms per superframeIncreased Configuration (SO=5):
Active period: 15.36 ms x 2^5 = 491.52 ms
Each slot: 491.52 / 16 = 30.72 ms
Benefits:
+ More time per slot for longer packets
+ More time for CSMA/CA backoff within single superframe
+ Better accommodation of mixed traffic
+ Reduced probability of CAP contention
Drawback:
- Higher duty cycle = more power consumption
If BO stays same: duty cycle = 491.52 / 983.04 = 50% (2x increase)
Need to increase BO proportionally to maintain 25% duty cycleTo maintain 25% duty cycle with SO=5:
SO=5, BO=7
Active: 491.52 ms
Beacon interval: 15.36 ms x 2^7 = 1966.08 ms
Duty cycle: 491.52 / 1966.08 = 25%942.7 Configuration Comparison
| Parameter | SO=4, BO=6 | SO=5, BO=7 |
|---|---|---|
| Active superframe | 245.76 ms | 491.52 ms |
| Beacon interval | 983.04 ms | 1966.08 ms |
| Slot duration | 15.36 ms | 30.72 ms |
| CAP slots | 14 (87.5%) | 14 (87.5%) |
| GTS slots | 2 (12.5%) | 2 (12.5%) |
| CAP duration | 215.04 ms | 430.08 ms |
| GTS duration | 30.72 ms | 61.44 ms |
| Duty cycle | 25% | 25% |
Trade-off: Increasing SO doubles slot duration (more capacity per slot) but doubles beacon interval (higher latency to first transmission opportunity).
942.8 Knowledge Check: Beacon Networks
942.9 Non-Beacon vs Beacon-Enabled Mode
| Aspect | Non-Beacon Mode | Beacon-Enabled Mode |
|---|---|---|
| Synchronization | None | Beacon-based |
| Channel Access | Unslotted CSMA/CA | Slotted CSMA/CA + GTS |
| Power Management | Device-controlled sleep | Superframe-synchronized sleep |
| Time-Critical Traffic | Best effort | Guaranteed (GTS) |
| Complexity | Lower | Higher |
| Best For | Event-driven, sparse traffic | Periodic, time-sensitive |
When to Use Beacon Mode: - Time-critical applications (HVAC control, industrial automation) - Synchronized networks with predictable traffic - Need for guaranteed bandwidth allocation
When to Use Non-Beacon Mode: - Event-driven sensors (motion, alarms) - Sparse, unpredictable traffic - Simpler deployments, lower coordinator requirements
942.10 Summary
This beacon networks review demonstrated:
- Superframe Timing: SO controls active duration, BO controls beacon interval
- GTS Allocation: 87.5% CAP available with 2 GTS slots allocated
- Duty Cycle Control: BO-SO difference determines duty cycle (25% for SO=4, BO=6)
- Design Trade-offs: Higher SO increases capacity but requires proportional BO increase to maintain duty cycle
942.11 Whatβs Next
Continue to 802.15.4 Review: Security and Channel Management to explore AES-128 security overhead, channel hopping for interference avoidance, and the adaptive mechanisms that enable self-healing mesh networks.