952 IEEE 802.15.4 Quiz: Addressing and Network Structure

952.1 Learning Objectives

After completing this quiz section, you should be able to:

Explain IEEE 802.15.4 addressing modes and when to use each for optimal overhead
Calculate address space allocation using the Cskip tree addressing algorithm
Compute superframe timing parameters from SO (Superframe Order) and BO (Beacon Order)
Distinguish between FFD and RFD device types and their memory/capability differences

Navigation

Return to: Quiz Bank Part 1 Overview

Other Quiz Sections: - Addressing and Network Structure (Current) - Power and Performance Calculations - Device Types and Security

Study Materials: - 802.15.4 Fundamentals - Core concepts - 802.15.4 Topic Review - Quick reference

952.2 Quiz: Addressing Modes

Question 11 (Single-Answer MCQ): An IEEE 802.15.4 coordinator has 64-bit extended address 0x123456789ABCDEF and 16-bit short address 0x0000. A new device with extended address 0xFEDCBA9876543210 sends an Association Request. The coordinator assigns it short address 0x0001. What addressing mode should the device use for subsequent data frames to the coordinator to minimize overhead?

Explanation: 802.15.4 addressing modes optimize overhead: After association, both coordinator (short 0x0000) and device (short 0x0001) have 16-bit addresses within the same PAN. Best practice: 16-bit short addressing (C) minimizes overhead: source address (2 bytes) + destination address (2 bytes) + 2-byte PAN ID = 6 bytes total addressing overhead out of 127-byte frame. Why not others? (A) 64-bit addressing wastes 18 bytes (8+8+2 PAN) - only needed for initial association or device without short address. (B) Mixed mode wastes 12 bytes (8+2+2) - only used during association response. (D) Implicit addressing is WRONG: intra-PAN compression (PAN ID Present only in source) saves 2 bytes (4 bytes total), but REQUIRES both addresses to be present; you can’t have implicit dest address to coordinator. Typical sensor frame: 6-byte addressing + 9-byte PHY/MAC header + 2-byte FCS + 102-byte max payload = 119 bytes. Using 64-bit addressing would reduce payload to 93 bytes (11% capacity loss).

952.3 Quiz: Tree Addressing (Cskip Algorithm)

Question 15 (Single-Answer MCQ): A smart home deploys 30 802.15.4 sensors (RFDs) and 5 routers (FFDs) in a mesh network. The PAN coordinator assigns 16-bit short addresses sequentially starting from 0x0001. If the network uses TreeAddrMode with Lm (max depth) = 3, Cm (max children per parent) = 10, and Rm (max routers per parent) = 5, how does the coordinator calculate the address space block for each router?

Explanation: 802.15.4 Tree Addressing (Cskip algorithm) enables distributed address allocation: Each router receives a contiguous address block to assign to its children without querying the coordinator. Cskip calculation (Address Skip): Cskip = 1 + Cm x (Cskip_next) for depth < Lm-1, where Cskip_next recursively computes addresses for child routers. Example (Lm=3, Cm=10, Rm=5): Depth 0 (coordinator): Cskip(0) = 1 + 10 x Cskip(1). Depth 1 (routers): Cskip(1) = 1 + 10 x Cskip(2). Depth 2 (leaf routers): Cskip(2) = Cm = 10. Working backwards: Cskip(1) = 1 + 10 x 10 = 101; Cskip(0) = 1 + 10 x 101 = 1011. So each depth-1 router gets 101 addresses; coordinator allocates 0x0001-0x0065 to Router 1, 0x0066-0x00CA to Router 2, etc. Benefits: Routers autonomously assign addresses (child 1 = parent_addr + 1, child 2 = parent_addr + 1 + Cskip, etc.), enabling scalability without centralized coordination. Trade-off: Tree topology wastes address space if Rm < actual routers (address gaps), motivating stochastic addressing in modern 802.15.4 deployments. Zigbee uses Cskip for address assignment but allows mesh routing beyond tree structure.

952.4 Quiz: Superframe Timing Calculation

Question 12 (Single-Answer MCQ): An industrial 802.15.4 network uses beacon-enabled mode with Superframe Order (SO) = 3 and Beacon Order (BO) = 5. If the base superframe duration (aBaseSuperframeDuration) is 15.36 ms, how often does the coordinator transmit beacons, and what percentage of time is the network active?

Explanation: Superframe calculation: Beacon Interval (BI) = aBaseSuperframeDuration x 2^BO = 15.36 ms x 2^5 = 15.36 x 32 = 491.52 ms (coordinator beacons every 491.52 ms). Superframe Duration (SD) = aBaseSuperframeDuration x 2^SO = 15.36 ms x 2^3 = 15.36 x 8 = 122.88 ms (active period). Active percentage = SD / BI = 122.88 / 491.52 = 25%. This means devices wake during the 122.88 ms active period (contention access + GTS), then sleep for 368.64 ms inactive period, repeating every 491.52 ms. Why 25% duty cycle? Industrial sensors report every few seconds; spending 25% awake provides enough time for data transmission while achieving 3-5 year battery life. Relationship: BO >= SO (Beacon Order >= Superframe Order) ensures inactive period exists; if BO = SO, network is 100% active (no sleep). Trade-offs: Higher SO = longer active period (more capacity, higher power), Higher BO = longer beacon interval (better battery, higher latency).

952.5 Quiz: FFD vs RFD Memory Requirements

Question 13 (Single-Answer MCQ): Why do IEEE 802.15.4 RFDs (Reduced Function Devices) typically have significantly lower RAM requirements (8-32 KB) compared to FFDs (64-256 KB), and what network capability does this prevent?

Explanation: RFD vs FFD memory architecture: RFDs are leaf nodes (end devices) implementing only device functionality: transmit to parent, receive from parent, no routing table. This saves RAM because: (1) No neighbor table - FFDs maintain neighbor discovery table (10-50 entries x 20 bytes = 200-1000 bytes), RFDs only store parent address (10 bytes). (2) No routing table - FFDs implement routing protocol (RPL, AODV) requiring route cache (20-100 entries x 30 bytes = 600-3000 bytes), RFDs have no routing logic. (3) No forwarding buffers - FFDs buffer packets for forwarding (5-10 packets x 127 bytes = 635-1270 bytes), RFDs only buffer own transmissions. Result: RFD firmware ~8-16 KB RAM (sensor application + 802.15.4 MAC + buffer), FFD firmware ~64-128 KB RAM (routing + neighbor management + multi-packet buffering). Trade-off: RFDs achieve 10x lower cost and longer battery life, but cannot participate in mesh routing, requiring strategic FFD placement for network coverage. Typical network: 80-90% RFDs (sensors), 10-20% FFDs (routers/coordinators).

952.6 Summary

This quiz section covered the fundamental aspects of IEEE 802.15.4 addressing and network structure:

Topic	Key Concept	Practical Impact
Addressing Modes	16-bit short vs 64-bit extended	6 vs 18 bytes overhead (12-byte savings)
Tree Addressing (Cskip)	Hierarchical address allocation	Distributed assignment without coordinator
Superframe Timing	SO/BO determine active duty cycle	25% active = 3-5 year battery life
FFD vs RFD	Routing capability vs simplicity	10x cost/power savings for RFDs

952.6.1 Key Formulas

Superframe Timing: - Beacon Interval (BI) = aBaseSuperframeDuration x 2^BO - Superframe Duration (SD) = aBaseSuperframeDuration x 2^SO - Duty Cycle = SD / BI = 2^(SO-BO)

Addressing Overhead: - Short addressing: 6 bytes (2+2+2 PAN) - Extended addressing: 18 bytes (8+8+2 PAN) - Mixed mode: 12 bytes (8+2+2 PAN)

952.7 What’s Next

Continue to: - Power and Performance Calculations - Battery life, GTS, and variant selection questions - Device Types and Security - FFD/RFD details, AES-128, channel hopping - Quiz Bank Part 1 Overview - Return to main navigation