947 IEEE 802.15.4 Review: Protocol Stack and Specifications

947.1 Learning Objectives

By the end of this chapter, you will be able to:

Understand Protocol Stack Architecture: Explain how IEEE 802.15.4 layers interact with higher-layer protocols
Navigate Band Selection: Choose appropriate frequency bands based on range, data rate, and regional requirements
Apply Technical Specifications: Use key parameters for network capacity planning and design
Make Configuration Decisions: Select bands and protocols based on application requirements

947.2 Prerequisites

Required Chapters:

802.15.4 Fundamentals - Core standard introduction
Basic understanding of wireless networking concepts

Related Chapters

Deep Dives:

802.15.4 Topic Review - Complete review hub
802.15.4 Comprehensive Review - Detailed specification

Other Review Topics:

Frame Structure and Security - MAC frames and security
Network Operations - Device types and CSMA-CA
Protocols and Performance - Higher-layer protocols

Higher-Layer Protocols:

Zigbee Fundamentals - Mesh networking
Thread Fundamentals - IPv6 mesh
6LoWPAN Fundamentals - IPv6 compression

Estimated Time: 15 minutes

947.3 Protocol Stack Architecture

Understanding how 802.15.4 fits into the broader networking stack is essential for IoT development. IEEE 802.15.4 defines only the PHY and MAC layers, leaving network and application layers to higher-layer protocols.

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graph TB
    subgraph App["Application Layer"]
        A1[Home Automation]
        A2[Industrial Control]
        A3[Sensor Networks]
    end

    subgraph NWK["Network Layer (Not in 802.15.4)"]
        N1[Zigbee NWK]
        N2[Thread Routing]
        N3[6LoWPAN Adaptation]
    end

    subgraph MAC["MAC Layer - 802.15.4"]
        M1[CSMA-CA Channel Access]
        M2[Frame Types: Beacon/Data/ACK/Cmd]
        M3[16-bit & 64-bit Addressing]
        M4[Guaranteed Time Slots]
    end

    subgraph PHY["PHY Layer - 802.15.4"]
        P1[2.4 GHz: 250 kbps]
        P2[915 MHz: 40 kbps]
        P3[868 MHz: 20 kbps]
    end

    App --> NWK
    NWK --> MAC
    MAC --> PHY

    style MAC fill:#16A085,stroke:#2C3E50,color:#fff
    style PHY fill:#16A085,stroke:#2C3E50,color:#fff
    style NWK fill:#E67E22,stroke:#2C3E50,color:#fff
    style App fill:#7F8C8D,stroke:#2C3E50,color:#fff

Figure 947.1: IEEE 802.15.4 protocol stack with MAC, PHY, and higher-layer protocols

947.3.1 Layer Responsibilities

The IEEE 802.15.4 standard explicitly defines the PHY and MAC layers, while network and application layers are implemented by protocols built on top of 802.15.4:

Layer	IEEE 802.15.4 Scope	Higher Layer Protocols
Application	Not specified	Application-specific logic
Network	Not specified	Zigbee, Thread, 6LoWPAN routing
MAC	Defined	CSMA-CA, beacons, GTS, addressing
PHY	Defined	Modulation, channels, power

The MAC layer handles:

Channel Access: CSMA-CA collision avoidance mechanism
Frame Types: Beacon, Data, ACK, and MAC Command frames
Addressing: Both 16-bit short and 64-bit extended addresses
Optional Features: Guaranteed Time Slots (GTS) for deterministic access

The PHY layer handles:

Modulation: O-QPSK for 2.4 GHz, BPSK for sub-GHz bands
Spreading: Direct Sequence Spread Spectrum (DSSS)
Channel Selection: Multiple channels per frequency band
Energy Detection: For Clear Channel Assessment (CCA)

947.4 Band and Protocol Selection Decision Tree

When designing an 802.15.4-based system, the first decisions involve frequency band and protocol selection:

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flowchart TD
    START["Need LR-WPAN?"] --> Q1{"Range<br/>requirement?"}

    Q1 -->|"<100m indoor"| BAND24["2.4 GHz Band"]
    Q1 -->|"100-300m"| BAND_SUB["Sub-GHz Band"]

    BAND24 --> Q2{"Data rate<br/>needs?"}
    Q2 -->|"250 kbps OK"| USE24["Use 2.4 GHz<br/>16 channels<br/>Global use"]

    BAND_SUB --> Q3{"Region?"}
    Q3 -->|"Americas"| USE915["Use 915 MHz<br/>40 kbps<br/>10 channels"]
    Q3 -->|"Europe"| USE868["Use 868 MHz<br/>20 kbps<br/>1 channel"]

    USE24 --> STACK["Select Network Stack"]
    USE915 --> STACK
    USE868 --> STACK

    STACK --> Q4{"Protocol<br/>needed?"}
    Q4 -->|"Home automation"| ZIGBEE["Zigbee NWK"]
    Q4 -->|"IPv6 native"| THREAD["Thread/6LoWPAN"]
    Q4 -->|"Custom"| CUSTOM["Custom on MAC"]

    style START fill:#2C3E50,stroke:#16A085,color:#fff
    style BAND24 fill:#16A085,stroke:#2C3E50,color:#fff
    style BAND_SUB fill:#16A085,stroke:#2C3E50,color:#fff
    style USE24 fill:#E67E22,stroke:#2C3E50,color:#fff
    style USE915 fill:#E67E22,stroke:#2C3E50,color:#fff
    style USE868 fill:#E67E22,stroke:#2C3E50,color:#fff
    style ZIGBEE fill:#7F8C8D,stroke:#2C3E50,color:#fff
    style THREAD fill:#7F8C8D,stroke:#2C3E50,color:#fff
    style CUSTOM fill:#7F8C8D,stroke:#2C3E50,color:#fff

Figure 947.2: Band and Protocol Selection Decision Tree - Start with range requirements to select between 2.4 GHz (shorter range, higher throughput) and sub-GHz bands (longer range, better penetration). Then select the specific band based on your deployment region. Finally, choose the appropriate network layer protocol based on your application needs. {fig-alt=“Decision tree flowchart for selecting IEEE 802.15.4 band and protocol. Starts with range requirement decision, branches to 2.4 GHz band for under 100m indoor or sub-GHz for 100-300m range. Sub-GHz further branches by region: Americas uses 915 MHz with 40 kbps and 10 channels, Europe uses 868 MHz with 20 kbps and 1 channel. After band selection, shows network stack choices: Zigbee NWK for home automation, Thread/6LoWPAN for IPv6 native, or custom protocols on MAC layer.”}

947.4.1 Decision Rationale

When to choose 2.4 GHz:

Global deployment (single hardware SKU)
Higher data rates required (250 kbps)
Dense deployment with many channels needed (16 channels)
Cost optimization (most common, cheapest components)

When to choose sub-GHz:

Extended range required (100-300m without mesh)
Better building penetration needed
Less interference (fewer devices in sub-GHz bands)
Lower data rate acceptable

947.5 Technical Specifications Quick Reference

947.5.1 Key Operating Parameters

The three frequency bands offer different trade-offs:

Parameter	2.4 GHz Band	915 MHz Band	868 MHz Band
Frequency Range	2400-2483.5 MHz	902-928 MHz	868-868.6 MHz
Channels	16 (Ch 11-26)	10 (Ch 1-10)	1 (Ch 0)
Channel Spacing	5 MHz	2 MHz	-
Data Rate	250 kbps	40 kbps	20 kbps
Modulation	O-QPSK	BPSK	BPSK
Chip Rate	2 Mcps	600 kcps	300 kcps
Spreading	DSSS (32:1)	DSSS (15:1)	DSSS (15:1)
Typical Range	10-100 m	100-300 m	100-300 m
Global Availability	Yes	Americas	Europe

947.5.2 Understanding the Data Rate Differences

The significant data rate difference (250 kbps vs 20-40 kbps) comes from:

Modulation Scheme: O-QPSK carries 2 bits per symbol vs BPSK’s 1 bit
Chip Rate: Higher chip rates enable higher symbol rates
Spreading Factor: 32:1 for 2.4 GHz vs 15:1 for sub-GHz

Despite lower data rates, sub-GHz bands are often preferred for:

Battery Life: Lower frequencies propagate further, requiring less transmit power
Range: Better diffraction and penetration through obstacles
Interference: Less crowded than 2.4 GHz (no Wi-Fi, Bluetooth, microwave ovens)

947.5.3 Network Capacity Limits

Understanding capacity limits helps with network planning:

Parameter	Maximum Value	Notes
Devices per PAN	65,535 (16-bit addresses)	Plus coordinator
PANs per Channel	65,535 (16-bit PAN ID)	Collision domain
Payload Size	127 bytes (total frame)	102 bytes after MAC overhead
Superframe Slots	16 (beacon-enabled)	For GTS allocation
Beacon Order (BO)	0-15	Beacon interval = 15.36ms x 2^BO
Superframe Order (SO)	0-15 (<=BO)	Active period = 15.36ms x 2^SO

947.5.4 Practical Network Sizing

While the standard allows 65,535 devices per PAN, practical limits are much lower:

Network Type	Practical Limit	Limiting Factor
Star (no mesh)	100-200 devices	Coordinator capacity
Tree mesh	500-1000 devices	Routing table size
Full mesh	200-300 devices	Broadcast storms
Zigbee mesh	250 devices	Profile recommendation
Thread mesh	250 devices	Partition limits

947.6 Channel Planning

947.6.1 2.4 GHz Channel Map

The 2.4 GHz band provides 16 channels:

Channel	Center Frequency	Wi-Fi Overlap	Recommendation
11	2405 MHz	Channel 1	Avoid
12	2410 MHz	Channel 1	Avoid
13	2415 MHz	Channel 1	Avoid
14	2420 MHz	Channel 1-6 gap	Marginal
15	2425 MHz	Channel 6 edge	Good
16	2430 MHz	Channel 6	Avoid
17	2435 MHz	Channel 6	Avoid
18	2440 MHz	Channel 6	Avoid
19	2445 MHz	Channel 6-11 gap	Marginal
20	2450 MHz	Channel 11 edge	Good
21	2455 MHz	Channel 11	Avoid
22	2460 MHz	Channel 11	Avoid
23	2465 MHz	Channel 11	Avoid
24	2470 MHz	Above channel 11	Marginal
25	2475 MHz	Clear	Best
26	2480 MHz	Clear	Best

Best practice: Use channels 25, 26 (no Wi-Fi overlap), or 15, 20 (minimal overlap) in Wi-Fi-dense environments.

947.6.2 Sub-GHz Channel Maps

915 MHz Band (Americas):

10 channels (Ch 1-10)
Center frequencies: 906 + 2(k-1) MHz for channel k
Less interference than 2.4 GHz
Longer range, better penetration

868 MHz Band (Europe):

Single channel (Ch 0)
Center frequency: 868.3 MHz
Very limited spectrum, but also less interference
Duty cycle restrictions apply (1% in some regions)

947.7 802.15.4 Variants for Specialized Applications

The base 802.15.4 standard has been extended for specific use cases:

Variant	Year	Key Feature	Target Application
802.15.4-2003	2003	Original standard	General LR-WPAN
802.15.4-2006	2006	Clarifications, GTS improvements	General use
802.15.4a	2007	UWB (Ultra-Wideband)	Precise positioning
802.15.4c	2009	China PHY (780 MHz)	Chinese market
802.15.4d	2009	Japan PHY	Japanese market
802.15.4e	2012	TSCH, DSME, LLDN	Industrial automation
802.15.4f	2012	Active RFID	Asset tracking
802.15.4g	2012	SUN PHY (long range)	Smart grid utilities
802.15.4j	2013	Medical BAN	Healthcare
802.15.4k	2013	LECIM	Critical infrastructure
802.15.4m	2014	TV white space	Rural connectivity
802.15.4n	2016	China 314-316 MHz	Chinese market
802.15.4q	2016	Ultra-low power	Wearables

947.7.1 Most Important Variants

802.15.4e (Industrial):

Time-Slotted Channel Hopping (TSCH) for deterministic latency
Up to 99.999% reliability in industrial environments
Used by WirelessHART and 6TiSCH
Channel hopping mitigates interference

802.15.4g (Smart Grid):

Extended range: 2-5 km
Multiple PHY options (FSK, OFDM, O-QPSK)
Used by Wi-SUN for utility networks
Sub-GHz bands for better penetration

802.15.4a (UWB):

Precise ranging (10 cm accuracy)
High data rate option (27 Mbps)
Low power
Used for indoor positioning and asset tracking

947.8 Summary

This chapter covered the foundational architecture and specifications of IEEE 802.15.4:

Protocol Stack: 802.15.4 defines only PHY and MAC layers, with higher-layer protocols (Zigbee, Thread, 6LoWPAN) providing network and application functionality
Frequency Bands: Three bands (2.4 GHz, 915 MHz, 868 MHz) offer trade-offs between data rate, range, and regional availability
Channel Planning: In 2.4 GHz environments with Wi-Fi, use channels 25-26 or 15, 20 to minimize interference
Network Capacity: While theoretically 65,535 devices, practical limits are 100-1000 depending on topology
Variants: Extensions like 802.15.4e (industrial), 802.15.4g (smart grid), and 802.15.4a (UWB) address specialized requirements

947.9 Knowledge Check

Quick Check

Question: What is the maximum data rate for IEEE 802.15.4 in the 2.4 GHz band?

Explanation: C. The 2.4 GHz band operates at 250 kbps using O-QPSK modulation. The 915 MHz band operates at 40 kbps and the 868 MHz band at 20 kbps.

Question: When would you choose 915 MHz over 2.4 GHz for an 802.15.4 deployment?

Explanation: B. Sub-GHz frequencies (915 MHz, 868 MHz) provide better penetration through obstacles and longer range (100-300m typical) compared to 2.4 GHz (10-100m). However, 915 MHz only works in the Americas and has lower data rates (40 kbps vs 250 kbps).

What’s Next

Continue to Frame Structure and Security to understand MAC frame formats and security features in IEEE 802.15.4.