236  Multi-Hop Ad Hoc: Core Concepts

236.1 Learning Objectives

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

  • Define Multi-Hop Networking: Explain how messages travel through intermediate nodes to extend network range
  • Understand Ad Hoc Principles: Describe self-organizing networks that form without fixed infrastructure
  • Analyze Energy Trade-offs: Compare single-hop vs multi-hop transmission costs
  • Apply Hop Count Optimization: Determine the optimal number of hops for coverage vs latency balance
  • Identify Routing Approaches: Distinguish between proactive, reactive, and hybrid routing strategies

236.2 Prerequisites

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

  • Wireless Sensor Networks: Understanding WSN architectures, topologies, and multi-hop communication patterns provides the foundation for ad hoc network concepts
  • Networking Basics for IoT: Knowledge of network layers, routing concepts, and MAC protocols is essential for understanding ad hoc routing algorithms

236.3 What is Multi-Hop Networking?

TipSimple Explanation

Analogy: Multi-hop networking is like passing a note in class—when you can’t reach the person you want to talk to directly, you ask people in between to pass the message along.

Simple explanation:

  • In a normal network, devices talk directly to a central router (like Wi-Fi)
  • In multi-hop, devices pass messages through each other
  • If device A can’t reach device C directly, it sends through device B
  • Each “pass” is called a “hop”

236.3.1 Why Do We Need Multi-Hop?

NoteThe Range Problem

Multi-hop routing path diagram showing how data packets travel from source node through multiple intermediate relay nodes to reach the destination base station. Each hop represents a wireless transmission between adjacent nodes, extending network range beyond single-hop radio coverage. Benefits include extended range, lower power per hop, and obstacle avoidance. Challenges include increased latency, energy consumption, and routing complexity.

Multi-hop routing path showing data packets traveling through intermediate relay nodes from source to destination
Figure 236.1: Multi-hop routing path: Packets traverse intermediate relay nodes to extend coverage and reduce per-hop power requirements.

Benefits:

  • Extended range beyond single-hop limits
  • Lower transmit power per hop (energy efficiency)
  • Obstacle avoidance through alternate paths

Challenges:

  • Latency increases with hop count
  • Cumulative energy consumption across relays
  • Routing complexity and overhead

236.3.2 Real-World Examples

Example 1: Forest Fire Detection

Problem: Monitor 100 km² of forest for fires

  • Cell towers? No - Too expensive
  • Wi-Fi? No - Range only 100m
  • Satellite? No - Too slow and expensive

Solution: Multi-hop sensor network

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graph LR
    Source["Source<br/>Node"]
    Hop1["Hop 1<br/>Relay"]
    Hop2["Hop 2<br/>Relay"]
    Hop3["Hop 3<br/>Relay"]
    Dest["Destination<br/>Base Station"]

    Source --> |Packet| Hop1
    Hop1 --> |Forward| Hop2
    Hop2 --> |Forward| Hop3
    Hop3 --> |Deliver| Dest

    Note["Benefits:<br/>Extended range<br/>Lower power per hop<br/>Obstacle avoidance<br/><br/>Challenges:<br/>Latency increases<br/>Energy consumption<br/>Routing complexity"]

    style Source fill:#16A085,stroke:#2C3E50,color:#fff
    style Dest fill:#2C3E50,stroke:#16A085,color:#fff
    style Hop1 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Hop2 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Hop3 fill:#E67E22,stroke:#2C3E50,color:#fff

Figure 236.2: Forest Fire Detection: Multi-Hop Sensor Mesh for Remote Monitoring

Each sensor monitors temperature, humidity, smoke. Passes data through neighbors. Solar powered, $20 each. Range: 500m per hop, unlimited with mesh.

Example 2: Smart Agriculture

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graph LR
    Source["Source<br/>Node"]
    Hop1["Hop 1<br/>Relay"]
    Hop2["Hop 2<br/>Relay"]
    Hop3["Hop 3<br/>Relay"]
    Dest["Destination<br/>Base Station"]

    Source --> |Packet| Hop1
    Hop1 --> |Forward| Hop2
    Hop2 --> |Forward| Hop3
    Hop3 --> |Deliver| Dest

    style Source fill:#16A085,stroke:#2C3E50,color:#fff
    style Dest fill:#2C3E50,stroke:#16A085,color:#fff
    style Hop1 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Hop2 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Hop3 fill:#E67E22,stroke:#2C3E50,color:#fff

Figure 236.3: Smart Agriculture: Multi-Hop Irrigation Control Network

Benefit: Sensor detects dry soil -> Message hops to gateway -> Command hops back to pump -> Irrigation starts automatically!

236.4 Single-Hop vs Multi-Hop Trade-offs

TipTradeoff: Single-Hop vs Multi-Hop Networks

Decision context: When deploying a wireless sensor or IoT network, choosing between direct single-hop communication to a gateway versus multi-hop relay through intermediate nodes affects range, power consumption, reliability, and complexity.

Factor Single-Hop Multi-Hop
Range Limited by radio range (typically 100-500m) Extended indefinitely via relays
Transmit Power High (reach distant gateway directly) Low per hop (short-range links)
Latency Lower (direct path) Higher (accumulates per hop)
Reliability Single link failure = lost data Multiple paths enable redundancy
Complexity Simple (direct addressing) Complex (routing protocols needed)
Energy Distribution Uniform across devices Uneven (nodes near sink drain faster)
Scalability Limited by gateway capacity Scales organically with relays
Infrastructure Requires gateway within range of all Only edge nodes need gateway access

Choose Single-Hop when:

  • All devices are within radio range of the gateway (small deployments)
  • Low latency is critical and cannot tolerate multi-hop delays
  • Simplicity is paramount (no routing protocol complexity)
  • Devices have sufficient power budget for long-range transmission
  • Network density is low (few devices, widely spaced)

Choose Multi-Hop when:

  • Deployment area exceeds single-hop radio range (large farms, forests)
  • Infrastructure deployment is costly or impossible (remote areas)
  • Energy efficiency per transmission matters (battery-constrained devices)
  • Path redundancy is needed for reliability (mission-critical data)
  • Organic network growth is expected (add nodes anywhere)

Default recommendation: Use single-hop for deployments under 200 devices in a compact area with reliable power. Use multi-hop when coverage area exceeds 1km diameter or when devices are severely power-constrained. For multi-hop, limit to 3-5 hops maximum - beyond that, latency and reliability degrade faster than range benefits grow.

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quadrantChart
    title Multi-Hop Trade-offs: Hops vs Performance
    x-axis Few Hops (1-2) --> Many Hops (5+)
    y-axis Poor Performance --> Good Performance
    quadrant-1 Diminishing returns
    quadrant-2 Sweet spot
    quadrant-3 Single hop limitations
    quadrant-4 Over-engineered

    Range Extension: [0.85, 0.75]
    Power per Hop: [0.78, 0.82]
    Path Redundancy: [0.70, 0.68]
    End-to-End Latency: [0.80, 0.25]
    Reliability: [0.72, 0.35]
    Routing Complexity: [0.88, 0.20]

Figure 236.4: Alternative View: Multi-Hop Trade-offs Analysis - This quadrant chart reveals the hidden trade-offs of multi-hop networking. Range extension and power per hop improve with more hops (upper-right), making them compelling reasons to use multi-hop. However, end-to-end latency, reliability, and routing complexity degrade significantly (lower-right). The “sweet spot” (upper-left) shows path redundancy and obstacle avoidance benefits are achievable with moderate hop counts. Key insight: 3-5 hops typically offers the best balance; beyond that, diminishing returns set in as complexity outweighs benefits. {fig-alt=“Quadrant chart analyzing multi-hop trade-offs: x-axis from Few Hops to Many Hops, y-axis from Poor to Good Performance. Upper-right shows Range Extension and Power per Hop improving with more hops. Upper-left sweet spot contains Path Redundancy. Lower-right diminishing returns zone shows End-to-End Latency, Reliability, and Routing Complexity degrading with more hops. Demonstrates that 3-5 hops is optimal before complexity outweighs benefits.”}

236.5 Types of Routing Protocols

Type How It Works Analogy Best For
Proactive Every node knows all paths upfront GPS with pre-downloaded maps Stable networks, low delay needed
Reactive Find path only when needed Ask for directions each time Mobile nodes, save memory
Hybrid Mix of both approaches GPS + ask locals for shortcuts Large networks

236.6 Ad Hoc Network Key Characteristics

An ad hoc network is a decentralized wireless network where nodes communicate without fixed infrastructure. Each node acts as both an endpoint and a router, forwarding packets for other nodes to extend network coverage.

TipAd Hoc Network Properties
Infrastructure-less:
No access points, base stations, or central routers required
Self-organizing:
Network topology forms automatically based on node proximity
Multi-hop communication:
Nodes relay messages through intermediaries to reach distant destinations
Dynamic topology:
Network structure changes as nodes move, join, or leave
Peer-to-peer:
All nodes have equal status; no hierarchy required
Autonomous:
Each node makes independent routing decisions 

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graph LR
    Source["Source<br/>Node"]
    Hop1["Hop 1<br/>Relay"]
    Hop2["Hop 2<br/>Relay"]
    Hop3["Hop 3<br/>Relay"]
    Dest["Destination<br/>Base Station"]

    Source --> |Packet| Hop1
    Hop1 --> |Forward| Hop2
    Hop2 --> |Forward| Hop3
    Hop3 --> |Deliver| Dest

    style Source fill:#16A085,stroke:#2C3E50,color:#fff
    style Dest fill:#2C3E50,stroke:#16A085,color:#fff
    style Hop1 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Hop2 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Hop3 fill:#E67E22,stroke:#2C3E50,color:#fff

Figure 236.5: Ad Hoc Network Properties: Multi-Hop Self-Organizing Mesh Communication

Why Multi-Hop?

Wireless communication has limited range due to:

  • Path loss proportional to distance squared: \(P_r \propto \frac{1}{d^2}\)
  • Environmental obstacles (buildings, terrain)
  • Power constraints (battery-operated devices)
  • Regulatory limits on transmission power

Multi-hop extends effective network coverage far beyond single-hop radio range.

236.7 Energy Distribution: The Funnel Effect

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graph LR
    subgraph Edge["Edge Nodes (Low Load)"]
        E1[Sensor A<br/>Own data only<br/>Battery: 95%]
        E2[Sensor B<br/>Own data only<br/>Battery: 92%]
        E3[Sensor C<br/>Own data only<br/>Battery: 94%]
    end

    subgraph Middle["Mid-Tier (Medium Load)"]
        M1[Relay 1<br/>3 sources<br/>Battery: 70%]
        M2[Relay 2<br/>4 sources<br/>Battery: 65%]
    end

    subgraph Near["Near Sink (High Load)"]
        N1[Relay 3<br/>8 sources<br/>Battery: 35%]
    end

    subgraph Sink["Gateway"]
        GW[Base Station<br/>All traffic<br/>Mains powered]
    end

    E1 --> M1
    E2 --> M1
    E3 --> M2

    M1 --> N1
    M2 --> N1

    N1 --> GW

    style E1 fill:#16A085,color:#fff
    style E2 fill:#16A085,color:#fff
    style E3 fill:#16A085,color:#fff
    style M1 fill:#E67E22,color:#fff
    style M2 fill:#E67E22,color:#fff
    style N1 fill:#E74C3C,color:#fff
    style GW fill:#2C3E50,color:#fff

Figure 236.6: Energy distribution showing the “funnel effect” in multi-hop networks. Edge nodes only transmit their own data (high battery). Nodes near the sink relay traffic from many sources (depleted battery). This uneven energy consumption is a critical design consideration. 

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graph TB
    subgraph SingleHop["SINGLE-HOP (Star Topology)"]
        SH_GW[Gateway]
        SH_S1[Sensor 1<br/>100mW TX]
        SH_S2[Sensor 2<br/>100mW TX]
        SH_S3[Sensor 3<br/>500mW TX<br/>Far away]

        SH_S1 -->|Direct| SH_GW
        SH_S2 -->|Direct| SH_GW
        SH_S3 -->|Direct| SH_GW

        SH_PROS[Pros: Simple, Low latency]
        SH_CONS[Cons: High power for distant nodes<br/>Limited range]
    end

    subgraph MultiHop["MULTI-HOP (Mesh Topology)"]
        MH_GW[Gateway]
        MH_S1[Sensor 1<br/>10mW TX]
        MH_S2[Sensor 2<br/>10mW TX]
        MH_S3[Sensor 3<br/>10mW TX<br/>Far away]
        MH_R[Relay<br/>10mW TX]

        MH_S1 -->|1 hop| MH_GW
        MH_S2 -->|1 hop| MH_GW
        MH_S3 -->|Hop 1| MH_R
        MH_R -->|Hop 2| MH_GW

        MH_PROS[Pros: Extended range, Lower TX power]
        MH_CONS[Cons: Added latency, Relay energy]
    end

    style SH_GW fill:#2C3E50,color:#fff
    style SH_S3 fill:#E74C3C,color:#fff
    style SH_PROS fill:#16A085,color:#fff
    style SH_CONS fill:#E74C3C,color:#fff
    style MH_GW fill:#2C3E50,color:#fff
    style MH_S1 fill:#16A085,color:#fff
    style MH_S2 fill:#16A085,color:#fff
    style MH_S3 fill:#16A085,color:#fff
    style MH_R fill:#E67E22,color:#fff
    style MH_PROS fill:#16A085,color:#fff
    style MH_CONS fill:#E67E22,color:#fff

Figure 236.7: Comparison of single-hop vs multi-hop approaches. Single-hop requires high TX power for distant nodes and has limited range. Multi-hop enables lower per-node TX power and extended range but adds latency and relay overhead. Choice depends on node distances and latency requirements.

236.8 Self-Check Questions

Before diving into the technical details, test your understanding:

  1. Basic Concept: If sensor A is 2 hops from the gateway, what does that mean?
    • Answer: Sensor A’s messages pass through 2 other devices before reaching the gateway
  2. Why Multi-Hop?: Why not just use a powerful radio that can reach everywhere?
    • Answer: Long-range radios use much more power (battery dies fast) and are more expensive
  3. Trade-off: What’s the downside of more hops?
    • Answer: More delay, more chances for message loss, more devices using battery
NoteKey Takeaway

In one sentence: Multi-hop networks extend range by passing messages through intermediate nodes, trading latency and complexity for coverage in areas where infrastructure is unavailable or uneconomical.

Remember this: 3-5 hops is usually the sweet spot - beyond that, latency, reliability, and routing complexity degrade faster than range benefits grow.

Multi-hop networking is like playing telephone with your friends - passing a message from person to person until it reaches someone far away!

236.8.1 The Sensor Squad Adventure: The Great Forest Relay

The Sensor Squad was on a mission deep in Whispering Woods. Sammy the Sensor spotted smoke rising from a distant hilltop! “Fire! We need to tell the Forest Ranger at the station, but it’s too far away for my radio to reach!”

Lila the LED looked around. “I can’t flash bright enough for the ranger to see me from here either. We’re five whole hills away!”

Max the Microcontroller had a brilliant idea. “What if we don’t try to reach the ranger directly? Look - there are other sensors on each hilltop! We can send the message to our friend on the next hill, and THEY send it to the next hill, and so on!”

The team got to work. Sammy whispered “FIRE AT HILL 5” to Sensor Sally on Hill 4. Sally passed it to Sensor Sam on Hill 3. Sam told Sensor Sue on Hill 2. Sue shared with Sensor Steve on Hill 1. And Steve - who was close enough to the ranger station - finally delivered the message: “FIRE AT HILL 5!”

“It worked!” cheered Bella the Battery. “And the best part? Each of us only had to send a SHORT message to our neighbor. That used way less energy than if Sammy tried to SHOUT all the way to the ranger station!”

The ranger rushed to put out the fire, and the forest was saved - all because the Sensor Squad worked together, hop by hop!

236.8.2 Key Words for Kids

Word What It Means
Hop One jump of a message from one device to the next closest one
Multi-Hop When a message takes several jumps to reach its destination
Relay A helper device that receives a message and passes it along
Range How far a device can send its message in one hop
Mesh Network Many devices connected together, each helping pass messages
Gateway The final destination where all messages need to arrive

236.8.3 Try This at Home!

The Flashlight Relay Game:

  1. Gather 4-5 family members and spread out across different rooms (or across a yard)
  2. The first person has a simple message (like “PIZZA TIME!”)
  3. Use flashlights to signal: turn on = start of message, blink pattern = the message
  4. Each person must “relay” (pass along) the blinks to the next person
  5. The last person says the message out loud!

Try two ways:

  • Single-hop attempt: Can the first person’s flashlight be seen by the LAST person directly? (Probably not through walls!)
  • Multi-hop success: Pass the signal room to room - now it works!

What you learned:

  • Each “hop” (room-to-room) is short and easy
  • The message travels farther than any single flashlight could reach
  • If one person misses the signal, the message stops (like a broken relay node!)

236.9 Summary

This chapter covered the fundamental concepts of multi-hop networking:

  • Multi-Hop Definition: Messages travel through intermediate relay nodes to reach destinations beyond direct radio range
  • Ad Hoc Characteristics: Self-organizing, infrastructure-less networks with dynamic topology and peer-to-peer communication
  • Trade-off Analysis: Single-hop offers simplicity and low latency; multi-hop provides extended range and energy efficiency per hop
  • Optimal Hop Count: 3-5 hops typically balances coverage with reliability and latency requirements
  • Energy Distribution: The “funnel effect” causes nodes near the gateway to consume more energy due to relay duties
  • Routing Protocol Classes: Proactive (maintain routes), reactive (discover on-demand), and hybrid approaches

236.10 What’s Next

Continue to Multi-Hop Applications to explore real-world IoT deployments including disaster rescue, agricultural monitoring, and archaeological site networks. Then complete your understanding with Multi-Hop Assessment for worked examples and knowledge checks.