435  WSN Routing: Introduction

435.1 Learning Objectives

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

  • Understand WSN Routing Fundamentals: Explain why sensor network routing differs from traditional network routing
  • Identify Data-Centric Concepts: Describe how data-centric routing addresses content rather than node identities
  • Recognize Energy Constraints: Understand why energy-aware routing is critical for battery-powered sensor networks

435.2 Prerequisites

Before diving into this chapter, you should be familiar with:

  • Wireless Sensor Networks: Understanding WSN architecture, multi-hop communication, and energy constraints provides the foundation for specialized routing protocols
  • WSN Overview: Fundamentals: Knowledge of sensor network characteristics, data aggregation, and network topologies is essential for understanding routing decisions
  • Routing Fundamentals: Familiarity with basic routing concepts, routing tables, and forwarding mechanisms helps distinguish WSN-specific routing approaches
  • Multi-Hop Ad Hoc: Fundamentals: Understanding self-organizing networks and dynamic routing provides context for data-centric and geographic routing strategies
NoteKey Concepts
  • Routing Protocol: Algorithm determining paths for data packets to travel from source sensors to destination sinks through multi-hop networks
  • Energy-Aware Routing: Protocols that select paths based on node energy levels to balance consumption and extend network lifetime
  • Data-Centric Routing: Routing based on data content rather than node addresses, enabling in-network aggregation and filtering
  • Hierarchical Routing: Organizing network into clusters with cluster heads aggregating data from members before forwarding to sinks
  • Geographic Routing: Protocols using node location information to make forwarding decisions without maintaining routing tables
  • Quality of Service (QoS): Meeting application requirements for delivery reliability, latency, and bandwidth in routing decisions

435.3 Introduction

⏱️ ~12 min | ⭐⭐ Intermediate | 📋 P05.C34.U01

Imagine 1,000 temperature sensors scattered across a farm. Each sensor measures its local area, but data needs to reach a central gateway (barn). How do messages hop from sensor to sensor to get there?

Traditional routing (like internet): Send email to bob@example.com—routers look up address, forward packet hop-by-hop based on routing tables. Works when you know the destination’s address.

WSN routing (different!): Not “send message to sensor #573,” but rather “all sensors detecting temperature > 30°C, report your data.” We care about what data says, not who sent it. This is called data-centric routing.

Why WSN routing is unique:

  1. Energy-aware: Choose paths that balance battery drain across all sensors (don’t overwork nodes near gateway)
  2. Data-centric: Route based on content (“fire detected!”) not addresses
  3. Aggregation: Sensors combine data along the path (100 temperature readings → 1 average)
  4. Many-to-one: Unlike internet (any-to-any), WSNs mostly flow toward one gateway
Term Simple Explanation
Directed Diffusion Routing protocol where “interests” (queries) flood network, data flows back along best paths (like water finding downhill routes)
Data Aggregation Combining multiple sensor readings into summaries (10 nodes report “21°C, 22°C, 21°C…” → one node sends “avg: 21.3°C”)
Energy-Aware Routing Choosing paths that avoid overusing nodes with low batteries (balances energy consumption)
Geographic Routing Using physical location to forward (move packet toward GPS coordinates of gateway)
Link Quality How reliable a wireless connection is (affected by distance, obstacles, interference)

Example: Fire detection network. When smoke sensor detects fire, it doesn’t need to send to “Sensor #573”—it broadcasts “FIRE at location X!” Nearby sensors hear this, aggregate with their data, and forward toward gateway. Path chosen: good battery, strong signal quality, shortest hops. Multiple paths used (redundancy in case one fails).

Key insight: Traditional routing protocols (like those for internet) fail in WSNs because they assume stable topology, ignore energy constraints, and don’t aggregate data. WSN protocols are specialized for battery-powered, dense deployments focused on data collection.

Sensor network routing is like playing a game of telephone across a huge playground where friends pass messages to each other until they reach the teacher!

435.3.1 The Sensor Squad Adventure: The Great Message Relay Race

One sunny day, the Sensor Squad was spread across Greenfield Farm to watch for anything unusual. Sammy the Temperature Sensor was way out by the pond, Lila the Light Sensor was in the orchard, Max the Motion Detector guarded the barn entrance, and Bella the Button sat by the farmhouse where Farmer Jones lived.

“I found something!” shouted Sammy from the pond. “The water is getting too hot for the fish!” But Sammy was too far away for Farmer Jones to hear directly. How could the message get through?

“I’ve got an idea,” said Max. “Let’s play Message Relay! I’m closer to the farmhouse, so Sammy passes to Lila, Lila passes to me, and I’ll tell Bella, who’s right next to Farmer Jones!”

But here’s the tricky part - Lila’s battery was running low from working all day in the sunny orchard. “I’m tired,” Lila yawned. “Maybe find another path?”

So clever Max found that Sammy could pass directly to him instead, skipping tired Lila. The message hopped from Sammy to Max to Bella to Farmer Jones - and the fish were saved!

“That’s called energy-aware routing,” explained Bella proudly. “We pick paths where friends aren’t too tired, so everyone’s batteries last longer!”

435.3.2 Key Words for Kids

Word What It Means
Routing Finding the best path for a message to travel, like choosing which friends to pass a note through
Multi-hop When a message jumps from friend to friend to friend (like stepping stones across a creek)
Energy-aware Being smart about who does work - don’t always ask the tired friend to help!
Data aggregation Combining messages together - instead of 5 friends each saying “it’s hot,” one friend says “everyone says it’s hot!”

435.3.3 Try This at Home!

The Telephone Game Challenge: Gather 4-6 friends or family members and spread out across your yard or living room. One person whispers a message, and everyone passes it along until it reaches the last person. Now try it again, but this time skip the person in the middle - did the message travel faster? Did it stay accurate? This is how sensors choose different paths to send their messages! Try having one person pretend to be “tired” (they speak very quietly) and see if the message still gets through - that’s why sensors need backup routes!

Routing in Wireless Sensor Networks (WSNs) differs fundamentally from traditional network routing due to the unique characteristics and constraints of sensor networks. Unlike conventional networks where routing focuses on establishing end-to-end communication between arbitrary nodes, WSN routing is typically data-centric, application-specific, and energy-aware.

This chapter introduces the specialized routing protocols designed for WSNs, including Directed Diffusion, link quality-based routing, data aggregation techniques, and the Trickle algorithm for network reprogramming.

%% fig-alt: "WSN routing characteristics comparing traditional address-centric routing with WSN-specific data-centric, energy-aware approaches"
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graph TD
    subgraph "WSN Routing Characteristics"
        TRAD["Traditional<br/>Routing"]
        WSN["WSN<br/>Routing"]

        TRAD_FEAT["Address-centric<br/>Peer-to-peer<br/>Minimize latency"]
        WSN_FEAT["Data-centric<br/>Many-to-one<br/>Minimize energy"]

        CHALLENGES["WSN Challenges:<br/>Energy constraints<br/>High node density<br/>Frequent failures<br/>Limited resources"]
    end

    TRAD --> TRAD_FEAT
    WSN --> WSN_FEAT

    WSN_FEAT --> CHALLENGES

    CHALLENGES -.->|"Specialized protocols"| PROTOCOLS["Directed Diffusion<br/>Energy-aware routing<br/>Data aggregation<br/>Link quality metrics"]

    style TRAD fill:#95A5A6,stroke:#2C3E50,stroke-width:3px,color:#fff
    style WSN fill:#2C3E50,stroke:#16A085,stroke-width:3px,color:#fff
    style TRAD_FEAT fill:#ECF0F1,stroke:#2C3E50,stroke-width:2px
    style WSN_FEAT fill:#D5F4E6,stroke:#16A085,stroke-width:2px
    style CHALLENGES fill:#E67E22,stroke:#2C3E50,stroke-width:3px,color:#fff
    style PROTOCOLS fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff

Figure 435.1: WSN routing characteristics comparing traditional address-centric routing with WSN-specific data-centric, energy-aware approaches 

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flowchart TD
    START(["Choose WSN<br/>Routing Protocol"]) --> Q1{"Network<br/>Size?"}

    Q1 -->|"< 100 nodes"| FLAT["Flat Routing<br/>(All nodes equal)"]
    Q1 -->|"100-1000 nodes"| Q2{"Node<br/>Locations?"}
    Q1 -->|"> 1000 nodes"| HIER["Hierarchical<br/>(LEACH, clusters)"]

    Q2 -->|"GPS/Known"| GEO["Geographic Routing<br/>(GPSR, greedy forward)"]
    Q2 -->|"Unknown"| Q3{"Data<br/>Pattern?"}

    Q3 -->|"Event-driven<br/>(fire, intrusion)"| DD["Directed Diffusion<br/>(Data-centric)"]
    Q3 -->|"Continuous<br/>(temperature)"| TREE["Tree-based<br/>(CTP, aggregation)"]

    FLAT --> F_USE["Use: Small labs,<br/>simple deployments"]
    HIER --> H_USE["Use: Large farms,<br/>city-scale WSN"]
    GEO --> G_USE["Use: Mobile sensors,<br/>outdoor tracking"]
    DD --> DD_USE["Use: Fire detection,<br/>security alerts"]
    TREE --> T_USE["Use: Environmental<br/>monitoring, smart buildings"]

    style START fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style Q1 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style Q2 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style Q3 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style FLAT fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style HIER fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style GEO fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style DD fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style TREE fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff

Figure 435.2: Alternative View: Protocol Selection Decision Tree - This decision tree guides practitioners in selecting the appropriate WSN routing protocol. Start with network size: small networks (< 100 nodes) can use flat routing where all nodes are equal. Large networks (> 1000 nodes) require hierarchical clustering (LEACH) to manage scale. For medium networks, consider whether GPS locations are available (use geographic routing) or focus on data patterns: event-driven applications (fire detection) suit Directed Diffusion while continuous monitoring suits tree-based aggregation (CTP). Each leaf node shows the recommended use case. {fig-alt=“Decision tree flowchart for selecting WSN routing protocols starting with network size question: less than 100 nodes leads to flat routing for small labs; greater than 1000 nodes leads to hierarchical LEACH for large farms; 100-1000 nodes asks about node locations - if GPS known use geographic routing GPSR for mobile sensors, if unknown check data pattern - event-driven leads to Directed Diffusion for fire/security detection, continuous monitoring leads to tree-based CTP for environmental and smart building applications”}
WarningCross-Hub Connections

Interactive Learning Resources:

  • Simulations Hub: Network Topology Visualizer helps understand WSN routing topologies and compare star, mesh, and tree structures. Try the routing simulator to visualize Directed Diffusion interest propagation and gradient formation.

  • Quizzes Hub: Test your understanding of WSN routing with protocol comparison questions, link quality estimation exercises, and data aggregation calculations. Focus on LEACH cluster formation and Trickle algorithm operation.

  • Videos Hub: Watch animated explanations of Directed Diffusion phases, WMEWMA link estimation visualizations, and Trickle polite gossip demonstrations. Video walkthroughs show real WSN deployments using CTP and RPL protocols.

Hands-On Practice: Use network simulators (NS-3, TOSSIM) to compare routing protocols. Implement simple Directed Diffusion on Arduino-based WSNs. Measure link quality with RSSI on ESP32 networks.

CautionCommon Misconception: “Shortest Path = Most Energy Efficient”

Misconception: “In WSNs, the shortest path (fewest hops) is always the most energy-efficient route.”

Reality: Shortest paths often waste energy due to poor link quality requiring retransmissions.

Quantified Example: - Path A (Shortest): 2 hops, each link has 50% delivery rate - Expected transmissions: 1/0.5 + 1/0.5 = 4 total transmissions - Energy: 4 transmissions × 20 mJ = 80 mJ

  • Path B (Longer): 3 hops, each link has 90% delivery rate
    • Expected transmissions: 1/0.9 + 1/0.9 + 1/0.9 = 3.33 total transmissions
    • Energy: 3.33 transmissions × 20 mJ = 66.6 mJ
  • Energy savings: Path B saves 17% energy despite being longer!

Why This Matters: Hop-count routing (traditional) would choose Path A, wasting 13.4 mJ per packet. In a network sending 10,000 packets/day, that’s 134 Joules wasted = 3-4 weeks shorter network lifetime.

Correct Approach: Use ETX (Expected Transmission Count) or MIN-T metrics that account for link quality. Route selection should minimize total expected transmissions, not just hop count.

Real Impact: Urban WSN deployment switched from hop-count to ETX routing. Result: Average transmissions per packet decreased from 4.2 to 2.1 (50% reduction), network lifetime extended from 8 months to 18 months.

435.4 Summary

This chapter introduced the fundamental differences between traditional network routing and WSN routing:

Key Takeaways:

  1. Data-Centric Approach: WSN routing focuses on data content rather than node addresses
  2. Energy Awareness: Routing decisions must balance energy consumption across all nodes
  3. Many-to-One Pattern: Unlike internet routing, WSN traffic flows primarily toward sink nodes
  4. Specialized Protocols: Traditional protocols fail in WSNs due to energy, scale, and topology constraints

435.5 What’s Next?

Continue to the next chapter to understand the specific challenges that make traditional routing unsuitable for wireless sensor networks.

Continue to WSN Routing Challenges →