Tutorial 11: Wireless Sensor Networks and the Internet of Things

Sensor(s), processor, radio transceiver, and power supply (battery or energy harvester).

WSN focuses on sensing and monitoring with low‑power, resource‑constrained nodes. IoT encompasses a wider ecosystem including actuators, smart devices, cloud services, and diverse communication models.

Device-to-Device (D2D), Device-to-Cloud, Device-to-Gateway, and Back‑end Data Sharing.

LoRa (Long Range).

250 kbps.

NB‑IoT uses licensed cellular spectrum and provides better reliability, deeper indoor coverage, lower latency, and integration with existing LTE/5G infrastructure, but requires subscription.

To reduce energy consumption by putting the radio to sleep for most of the time, waking periodically to listen for transmissions.

By using a periodic listen/sleep schedule (duty cycling) where nodes synchronise to a common schedule and turn off their radios during sleep periods.

LEACH forms clusters with a cluster head (CH) that aggregates data from cluster members and transmits to the base station. The CH role rotates among nodes to balance energy consumption.

RPL (Routing Protocol for Low‑Power and Lossy Networks) is an IETF standard routing protocol for IPv6‑based IoT networks (6LoWPAN). It builds a Destination‑Oriented DAG (DODAG) for efficient routing.

Energy harvesting captures ambient energy to power devices. Sources: solar (photovoltaic), thermal (thermoelectric), vibrational (piezoelectric), and RF energy.

Edge computing brings computation close to the devices (at the edge of the network). Fog computing extends this with a hierarchical architecture that includes gateways and intermediate nodes, providing more processing and storage layers between edge and cloud.

To reduce the amount of data transmitted by combining data from multiple nodes, saving energy and bandwidth.

A gateway acts as a bridge between local devices (e.g., ZigBee, BLE) and the Internet (Wi‑Fi, Ethernet, cellular), performing protocol translation, data aggregation, and sometimes edge processing.

DTLS (Datagram Transport Layer Security) for UDP‑based IoT protocols, and TLS for TCP‑based communication. Lightweight cryptography (AES‑CCM) is also used.

Mirai was a malware that infected IoT devices (cameras, DVRs) with default credentials, using them as a botnet to launch large‑scale DDoS attacks. It highlighted the security weaknesses of IoT devices.

A secure element provides hardware‑based security for storing cryptographic keys and performing secure operations (e.g., authentication, encryption), protecting against physical and software attacks.

Star topology: nodes communicate directly with a central coordinator (gateway). Simple but limited range. Mesh topology: nodes can relay packets for each other, providing multi‑hop communication and better coverage and resilience.

2‑15 km (up to 50 km in ideal line‑of‑sight conditions).

6LoWPAN (IPv6 over Low‑Power Wireless Personal Area Networks) allows IPv6 packets to be transmitted over IEEE 802.15.4 networks by using header compression and fragmentation, enabling IP connectivity for sensor nodes.

Adaptive sampling adjusts the sensing rate based on the variability of the measured data. When the environment is stable, the sampling rate is reduced to save energy.

BLE is typically used for wearables, health monitors, and beacons (short‑range, low‑latency). ZigBee is used for home automation, industrial control, and mesh networks (more devices, lower data rate).

A digital twin is a virtual representation of a physical object or system that is updated with real‑time data from sensors. It is used for simulation, analysis, and predictive maintenance.

Limited memory, CPU, and battery power make it difficult to implement strong encryption, secure boot, or regular firmware updates, leading to vulnerabilities.

CoAP (Constrained Application Protocol) is a lightweight RESTful protocol for IoT devices, replacing HTTP for resource‑constrained environments. It runs over UDP and supports multicast, low overhead, and simple request/response interactions.

Use LoRa for long‑range, low‑power communication. Deploy 20‑30 sensor nodes with soil moisture sensors, placed in a grid. Each node is solar‑powered with a battery backup. A LoRa gateway is placed on a high point (e.g., a building or pole) to receive data. Nodes send data every hour (or on demand). The gateway transmits data via satellite or fixed Ethernet (if available) to the cloud for analytics.

Assume 1% transmit, 9% receive, 90% sleep for S‑MAC: I = 0.01×30 + 0.09×20 + 0.9×0.01 = 0.3 + 1.8 + 0.009 = 2.109 mA. Without duty cycling: 100% receive (if always listening) = 20 mA. S‑MAC saves ~90% energy.

ZigBee light switch → ZigBee Gateway (hub) → Wi‑Fi router → Internet → Cloud server (back‑end) → Wi‑Fi router → Smartphone app. The gateway performs protocol translation (ZigBee to IP).

LEACH operates in rounds. Each round has a setup phase (cluster formation and CH selection based on a random threshold) and a steady‑state phase (data transmission). The threshold is T(n) = p/(1 - p*(r mod 1/p)) where p is the desired percentage of CH nodes. Nodes that have been CH recently have a lower probability, ensuring rotation.

Transmission duty cycle = 10 ms / 60 s = 0.000167. Average current = 0.000167 × 100 mA + (1 - 0.000167) × 0.005 mA ≈ 0.0167 + 0.005 = 0.0217 mA. Battery life = 1000 mAh / 0.0217 mA ≈ 46,000 hours ≈ 5.25 years.

Edge computing processes video frames locally (at the camera or gateway), reducing the amount of data sent to the cloud, lowering bandwidth costs and latency. It enables real‑time detection of events (e.g., traffic violations, accidents) and can operate with intermittent cloud connectivity.

RPL is suitable for large, low‑power networks where IPv6 connectivity is needed and nodes have some processing capability. LEACH is good for homogeneous networks with a single sink. GPSR works well if nodes have GPS (location awareness). For a large area with many nodes, RPL provides flexibility and supports scalable IPv6 routing.

CoAP is a request/response protocol for constrained devices, similar to HTTP but over UDP. MQTT is a publish/subscribe protocol that uses a broker. CoAP is lightweight and supports multicast; MQTT is more suitable for many‑to‑one data collection with QoS levels.

BLE beacons broadcast advertising packets periodically (e.g., every 100 ms). The packet contains the beacon's UUID, major/minor numbers, and measured transmit power (for ranging). The smartphone scans and uses the signal strength to estimate distance for navigation.

Secure boot ensures that the device only loads authenticated firmware. It uses cryptographic signatures to verify the bootloader and OS before execution. This prevents attackers from loading malicious firmware, providing a root of trust for the device.

NB‑IoT is most suitable because it offers deep indoor/sub‑surface penetration (due to lower frequency and higher power), works on licensed cellular bands with good reliability, and supports battery‑powered devices. LoRa could also work but may be less reliable in dense urban environments with underground pipes.

In a mesh network, if one node fails, the network can dynamically reroute packets through other nodes. The routing protocol (e.g., RPL) detects the failure via link‑layer notifications or lack of acknowledgments and finds an alternative path. This provides fault tolerance and resilience.

Energy consumption per round = E_sense + E_process + E_tx(d). E_tx ∝ d^n (n is path loss exponent). Using Friis, transmit power ∝ d² (free‑space) or d⁴ (two‑ray). Thus, increasing distance significantly increases energy consumption, making multi‑hop communication more energy‑efficient than direct transmission.

802.15.4 is designed for low‑rate, low‑power, low‑cost WPANs. It supports star and peer‑to‑peer topologies. Beacon‑enabled mode uses a superframe structure with active and inactive periods. The coordinator sends beacons for synchronisation, and nodes use CSMA/CA during the active period.

S‑MAC uses fixed duty cycle, good for periodic traffic but wastes energy in low load. T‑MAC adapts the active time based on traffic, better for variable load. X‑MAC uses low‑power listening with short preambles, good for low‑duty‑cycle networks. Each has trade‑offs; the choice depends on application requirements.

Data fusion combines data from multiple sensors to produce a more accurate or comprehensive result (e.g., Kalman filtering). Data aggregation is a simpler form that reduces data volume (e.g., averaging). Fusion improves accuracy and decision‑making but requires more processing.

ZigBee uses AES‑128 encryption at the MAC and NWK layers. Security services include: MAC layer encryption (for data frames), NWK layer encryption and integrity, and APS layer encryption for application data. Key management: trust centre distributes keys (network key, link key).

MQTT: publish/subscribe, broker‑based, supports 3 QoS levels (0‑2). Overhead: 2‑4 bytes header, TCP. Use cases: telemetry, sensor data. CoAP: request/response, UDP, similar to HTTP, supports confirmable/non‑confirmable messages. Overhead: 4 bytes header. Use cases: RESTful APIs for constrained devices.

TSCH synchronises nodes to a schedule (timeslots) and uses channel hopping to avoid interference and fading. It provides deterministic communication, high reliability, and low power by allowing nodes to sleep between slots. Used in industrial IoT (e.g., WirelessHART).

A digital twin is a virtual replica of physical assets, updated with real‑time sensor data. It uses simulation and machine learning to predict failures, optimise maintenance schedules, and improve asset performance. Benefits: reduced downtime, lower costs, and enhanced safety.

5G mMTC supports 1 million devices/km², low power, deep coverage, and low cost. Challenges: signalling congestion, battery life, and coverage. 5G uses NB‑IoT and LTE‑M integrated with 5GC, flexible numerology, and power‑saving features (eDRX, PSM).

Edge computing offers low latency, privacy, and bandwidth savings, but it has limited storage and processing power. Cloud computing provides scalability, analytics, and centralised management. A hybrid approach (fog/edge + cloud) is more realistic; edge pre‑processes data, and cloud handles long‑term storage and complex analytics.

LoRaWAN uses a star‑of‑stars topology: end‑devices (sensors) communicate with gateways (multiple) that forward to a network server. Security: AES‑128 encryption with a session key (AppSKey for application, NwkSKey for network). Authentication uses a device‑specific AppKey and join procedure (OTAA or ABP).

Sensors: ultrasonic or magnetic sensors in parking spaces. Technology: LoRa (for range) or NB‑IoT (for reliability). Architecture: sensors → LoRa gateways → network server → cloud (with database and analytics). Mobile app: shows real‑time availability, navigation. Security: DTLS for cloud communication, secure provisioning of sensors, and encrypted data storage.