4.3 Schedule-based MAC protocols

Schedule-based MAC protocols in wireless sensor networks aim to minimize energy consumption and collisions. These protocols divide channel access into time slots, assigning them to specific nodes for transmission. This approach ensures efficient use of network resources and extends battery life.

TDMA, FDMA, and CDMA are key techniques used in schedule-based MAC protocols. They organize transmissions by time, frequency, or code, respectively. Proper synchronization and clustering strategies are crucial for these protocols to work effectively in resource-constrained sensor networks.

Time Division Techniques

TDMA-based Protocols

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• Time Division Multiple Access (TDMA) divides the channel into time slots, each allocated to a specific node for transmission
• Nodes transmit data only during their assigned time slots, avoiding collisions and improving channel utilization
• TDMA requires precise time synchronization among nodes to ensure proper slot alignment and prevent overlapping transmissions
• Lightweight Medium Access Control (LMAC) is a TDMA-based protocol designed for energy efficiency in WSNs
  • LMAC uses a distributed slot selection algorithm to assign unique time slots to nodes
  • Nodes exchange control messages to inform neighbors about their slot selections and resolve conflicts

Traffic-Adaptive Slot Assignment

• Traffic-Adaptive Medium Access (TRAMA) protocol adapts slot assignments based on traffic patterns and node schedules
  • TRAMA consists of a random access period for slot contention and a scheduled access period for data transmission
  • Nodes exchange traffic information and schedules to determine the most suitable slots for transmission
• Slot assignment techniques aim to allocate time slots efficiently based on network topology, traffic demands, and energy constraints
  • Centralized slot assignment relies on a central coordinator to allocate slots to nodes based on global network information
  • Distributed slot assignment allows nodes to select their own slots through local coordination and negotiation with neighbors

Frame Structure and Synchronization

• TDMA-based protocols organize time into frames, which are further divided into time slots
• Each frame consists of a fixed number of slots, and the frame structure repeats periodically
• Nodes are assigned specific slots within the frame for transmission and reception
• Frame synchronization ensures that all nodes have a common understanding of the frame boundaries and slot timings
  • Beacon messages are commonly used for frame synchronization, where a designated node broadcasts beacons to align the frames across the network
  • Guard times are added between slots to accommodate clock drift and prevent overlapping transmissions

Frequency and Code Division Techniques

FDMA and CDMA

• Frequency Division Multiple Access (FDMA) divides the available frequency spectrum into distinct channels, each assigned to a specific node or group of nodes
  • Nodes transmit on their allocated frequency channels, allowing simultaneous transmissions without interference
  • FDMA requires precise frequency synchronization and stable oscillators to maintain channel separation
• Code Division Multiple Access (CDMA) assigns unique spreading codes to each node, enabling multiple nodes to transmit simultaneously on the same frequency band
  • Each node's data is spread across a wide bandwidth using its assigned code, making it resistant to interference and providing a certain level of security
  • CDMA relies on the orthogonality of spreading codes to minimize interference among concurrent transmissions

Advantages and Challenges

• FDMA and CDMA offer the advantage of allowing simultaneous transmissions, potentially increasing network capacity and reducing latency
• However, these techniques face challenges in WSNs due to the limited frequency spectrum, energy constraints, and complexity of code management
• Implementing FDMA and CDMA in WSNs requires careful frequency planning, code assignment, and synchronization mechanisms to ensure efficient and interference-free operations
• The limited resources and low-power nature of sensor nodes make it challenging to incorporate complex frequency and code division techniques in WSNs

Synchronization and Clustering

Time Synchronization

• Time synchronization is crucial in WSNs to ensure coordinated operations, data fusion, and power management
• Nodes need to maintain a common notion of time to perform time-sensitive tasks, such as scheduling, data aggregation, and duty cycling
• Time synchronization protocols aim to minimize clock drift and establish a global time reference across the network
  • Sender-receiver synchronization involves exchanging timestamped messages between nodes to estimate clock offsets and skews
  • Reference broadcast synchronization uses a common reference message to synchronize nodes within a broadcast domain

LEACH Protocol and Clustering

• Low-Energy Adaptive Clustering Hierarchy (LEACH) is a clustering-based protocol that organizes nodes into clusters to improve energy efficiency and network lifetime
• In LEACH, nodes self-organize into clusters, with each cluster having a designated cluster head
  • Cluster heads are responsible for aggregating data from member nodes and transmitting it to the base station
  • Cluster head role is rotated among nodes to balance energy consumption and prevent premature node failures
• Clustering reduces the communication overhead and energy expenditure by localizing data transmission within clusters
  • Nodes communicate with their cluster heads, which in turn communicate with the base station, minimizing long-distance transmissions
  • Clustering enables scalability, load balancing, and efficient data aggregation in large-scale WSNs