Unit 3: Ad-Hoc Networks and Wireless Sensor Networks (WSNs)

Introduction to Infrastructure and Infrastructure-less Wireless Networks

Wireless networks are broadly classified into infrastructure networks and infrastructure-less networks based on their topology and reliance on central network support. Infrastructure networks use fixed network infrastructure, such as base stations and access points, for connectivity and data routing, whereas infrastructure-less networks (also known as ad-hoc networks) operate without any fixed support, relying on the nodes themselves for establishing connections and transferring data.

Infrastructure Network

An infrastructure network is a wireless network that depends on fixed infrastructure components like base stations, access points, and routers to manage communication between devices. This structure provides a stable and reliable communication environment, typically within a controlled setting such as an office, building, or city-wide network. Infrastructure networks have several advantages, such as centralized control, efficient bandwidth management, and the ability to support a large number of devices. However, they may have limitations in flexibility and mobility, as communication heavily depends on the available infrastructure.

Infrastructure-less Wireless Networks

Infrastructure-less wireless networks, also known as ad-hoc networks, lack a centralized authority or fixed infrastructure. In these networks, nodes are self-organizing and dynamically form communication paths as required. Ad-hoc networks offer flexibility and can be rapidly deployed, making them suitable for applications where a traditional network infrastructure is unavailable or impractical, such as in disaster relief, military operations, and remote areas.

Issues in Ad-Hoc Wireless Networks

Due to the absence of fixed infrastructure, ad-hoc networks encounter unique challenges that need to be addressed for efficient communication. Some of the main issues in ad-hoc wireless networks include:

1. Dynamic Topology: Nodes in ad-hoc networks are often mobile, which results in frequent changes in network topology. This dynamic behavior necessitates constant route discovery and management to maintain reliable connections.
2. Limited Bandwidth: Wireless networks have limited bandwidth compared to wired networks. In ad-hoc networks, this limitation can result in congestion and reduced data transfer rates, especially as the number of nodes increases.
3. Energy Constraints: Devices in ad-hoc networks typically rely on battery power, making energy efficiency critical. Routing protocols and network protocols must be optimized to reduce energy consumption.
4. Security Vulnerabilities: Without centralized control, ad-hoc networks are more susceptible to security threats such as eavesdropping, denial-of-service attacks, and data tampering. Implementing effective security mechanisms is challenging but essential.
5. Quality of Service (QoS): Ensuring QoS for applications with specific bandwidth, delay, and jitter requirements is difficult in ad-hoc networks due to dynamic topology, limited resources, and frequent route changes.

Ad-Hoc Network MAC Layer

The Medium Access Control (MAC) layer is responsible for coordinating access to the shared communication channel in wireless networks. In ad-hoc networks, where no centralized controller exists, designing an efficient MAC layer is essential for ensuring smooth communication.

MAC Layer Design Issues in Ad-Hoc Networks

1. Channel Utilization: In an ad-hoc environment, efficiently utilizing the available bandwidth is essential to avoid interference and packet collisions. Proper channel utilization helps maintain data integrity and minimizes packet loss.
2. Contention and Collision: Without a fixed controller, nodes independently contend for channel access, increasing the likelihood of packet collisions. Mechanisms are required to minimize these collisions.
3. Power Consumption: Since devices are usually battery-operated, minimizing power consumption at the MAC layer is crucial for prolonging network lifetime.
4. Scalability: The MAC protocol should perform efficiently as the network size grows, ensuring performance is not compromised with an increase in the number of nodes.

MAC Layer Design Goals

The primary goals for MAC protocols in ad-hoc networks include:

• Fairness: Provide fair channel access to all nodes.
• Efficiency: Maximize data throughput by reducing collisions and contention.
• Adaptability: Dynamically adjust to network changes and varying conditions.
• Low Latency: Reduce delays to ensure timely data transmission.
• Energy Efficiency: Conserve energy to extend the network's operational time.

Classification of MAC Protocols in Ad-Hoc Networks

MAC protocols in ad-hoc networks can be classified based on their approach to handling channel access. Major categories include:

1. Contention-Based Protocols: Use random access methods to compete for channel access, such as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).
2. Contention-Free Protocols: Utilize scheduling techniques to reserve channel access, such as Time Division Multiple Access (TDMA).
3. Hybrid Protocols: Combine features of both contention-based and contention-free protocols to achieve a balance between efficiency and reliability.

MACAW (MACA for Wireless)

MACAW (MACA for Wireless) is a refinement of the MACA protocol, designed to address several issues in wireless networks. MACAW introduces Request to Send (RTS), Clear to Send (CTS), Data (DATA), and Acknowledgement (ACK) frames to manage channel access and reduce collisions. By employing these frames, MACAW improves reliability and reduces interference in ad-hoc networks, making it suitable for scenarios where network topology is frequently changing.

Ad-Hoc Network Routing Layer

Routing in ad-hoc networks is challenging due to the lack of centralized control and dynamic network topology. Routing protocols must efficiently establish and maintain routes despite node mobility, limited bandwidth, and energy constraints.

Issues in Designing a Routing Protocol for Ad-Hoc Networks

1. Dynamic Topology: The routing protocol must quickly adapt to frequent changes in network topology, ensuring that routes are updated as nodes move or connections change.
2. Limited Bandwidth: Efficiently utilizing bandwidth is essential, especially as routing control packets can consume a significant portion of the available bandwidth.
3. Energy Constraints: Routing algorithms should minimize power usage to conserve battery life in devices.
4. Scalability: The protocol should scale to accommodate large networks without degrading performance.
5. Security: With the lack of a fixed infrastructure, routing protocols must be robust against various security threats.

Classification of Ad-Hoc Network Routing Protocols

Routing protocols in ad-hoc networks are classified into three main categories based on their approach to route discovery and maintenance:

1. Proactive (Table-Driven) Routing Protocols: Maintain up-to-date routing information to all nodes. Examples include Destination-Sequenced Distance-Vector (DSDV).
2. Reactive (On-Demand) Routing Protocols: Discover routes only when needed, reducing the overhead associated with maintaining up-to-date routes. Examples include Ad-Hoc On-Demand Distance Vector (AODV) and Dynamic Source Routing (DSR).
3. Hybrid Routing Protocols: Combine features of both proactive and reactive approaches to optimize performance based on network size and traffic patterns.

Specific Routing Protocols

Destination-Sequenced Distance-Vector (DSDV)

DSDV is a proactive routing protocol that uses a table-driven approach to maintain routes between nodes. Each node periodically broadcasts its routing table to its neighbors, ensuring that all nodes have consistent route information. DSDV utilizes sequence numbers to prevent routing loops, providing a stable and loop-free routing structure.

Ad-Hoc On-Demand Distance Vector (AODV)

AODV is a reactive protocol that discovers routes only when needed. When a source node needs to communicate with a destination node, it initiates a route discovery process by broadcasting a Route Request (RREQ) message. If the destination or an intermediate node with a valid route receives the RREQ, it replies with a Route Reply (RREP) message. AODV also uses sequence numbers to ensure the freshness of routes, and it maintains only the most recent path to minimize control overhead.

Dynamic Source Routing (DSR)

DSR is another on-demand protocol that maintains source routes for each data packet. When a node needs to communicate with a destination, it initiates a route discovery process and stores the route in the packet header. DSR is advantageous for networks with low mobility and is well-suited for scenarios where communication occurs between frequently changing pairs of nodes.

Wireless Sensor Networks (WSNs)

Wireless Sensor Networks (WSNs) consist of distributed sensor nodes that monitor physical or environmental conditions, such as temperature, humidity, and motion, and communicate this data to a central station. WSNs have applications in environmental monitoring, industrial automation, healthcare, and military surveillance.

Applications of Sensor Networks

1. Environmental Monitoring: WSNs are used for monitoring ecosystems, weather, and natural disasters.
2. Industrial Automation: Sensor networks are employed in industrial settings to monitor equipment, detect faults, and manage resources.
3. Healthcare: WSNs enable remote health monitoring for patients, tracking vital signs and alerting healthcare providers to any abnormalities.
4. Military Surveillance: Sensor networks can be deployed in strategic areas to monitor movement and gather intelligence in military operations.

Comparison with Ad-Hoc Wireless Networks

Sensor Node Architecture

A sensor node typically comprises the following components:

1. Sensing Unit: Includes sensors to monitor environmental parameters and convert them into electrical

signals. 2. Processing Unit: Manages data processing and communication tasks, often using low-power microcontrollers. 3. Transceiver: Responsible for wireless communication between nodes. 4. Power Unit: Supplies energy to the sensor node, generally through batteries or solar cells.

Issues and Challenges in Designing a Sensor Network

1. Energy Efficiency: Since sensor nodes are usually battery-powered, protocols must minimize energy usage.
2. Scalability: The network should support a large number of sensor nodes without performance degradation.
3. Data Aggregation: Efficiently collecting and processing data from multiple nodes to reduce redundancy and transmission overhead.
4. Fault Tolerance: The network should remain operational even if individual nodes fail.

Classification of Sensor Network Protocols

Sensor network protocols can be classified based on their communication model and data dissemination strategy:

1. Flat-Based Protocols: All nodes perform similar tasks, such as Flooding and Gossiping.
2. Hierarchical Protocols: Nodes are organized into clusters, with specific roles assigned to each node, such as Low-Energy Adaptive Clustering Hierarchy (LEACH).
3. Location-Based Protocols: Use node location information to optimize communication, like Geographic Adaptive Fidelity (GAF).

Sensor Network Architecture

Layered Architecture

In the layered architecture, communication is divided into layers, with each layer performing specific functions. This model simplifies protocol design and enhances modularity, supporting efficient data flow management.

Clustered Architecture

The clustered architecture organizes nodes into clusters, with each cluster having a cluster head that collects and transmits data from sensor nodes to a central station. This setup minimizes energy consumption, reduces data redundancy, and allows efficient management of large-scale sensor networks.