4.2 Contention-based MAC protocols

Contention-based MAC protocols are crucial for managing wireless sensor network communication. These protocols, like CSMA/CA, use techniques such as carrier sensing and backoff mechanisms to avoid collisions and ensure fair channel access among nodes.

Asynchronous and synchronous MAC protocols offer different approaches to energy conservation. Asynchronous protocols like B-MAC and X-MAC use preamble sampling, while synchronous protocols like S-MAC and T-MAC coordinate sleep schedules. Both types aim to balance energy efficiency and network performance.

CSMA/CA and Common Issues

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

• Wireless nodes sense the channel before transmitting to avoid collisions
• If the channel is busy, the node defers transmission until the channel is free
• After the channel is free, the node waits for a random backoff period before transmitting to minimize the probability of collisions
• Acknowledgment (ACK) packets are used to confirm successful packet reception
• If no ACK is received, the sender assumes a collision occurred and retransmits the packet after another random backoff period

Backoff Mechanism and Hidden/Exposed Terminal Problems

• Backoff mechanism reduces collision probability by spreading out transmission attempts over time
  • Each node selects a random backoff time from a contention window (CW)
  • CW size is adjusted based on the number of failed transmission attempts
• Hidden terminal problem occurs when two nodes that are out of range of each other transmit simultaneously to a common receiver, causing collisions
  • Request to Send (RTS) and Clear to Send (CTS) packets can be used to mitigate this problem
  • The sender transmits an RTS packet to the receiver, and the receiver responds with a CTS packet
  • Nearby nodes overhearing the RTS or CTS packets defer their transmissions
• Exposed terminal problem arises when a node is prevented from transmitting due to a nearby transmitting node, even though the intended receiver is out of range of the interfering node
  • This leads to underutilization of the wireless channel and reduced network throughput

Asynchronous MAC Protocols

Berkeley MAC (B-MAC)

• Asynchronous duty cycling protocol that aims to reduce energy consumption
• Nodes wake up periodically to check for channel activity using preamble sampling
• Long preambles are used to ensure that the intended receiver is awake during the transmission
• Receivers can go back to sleep if the preamble is not intended for them, saving energy
• B-MAC allows for low power operation but suffers from overhearing and long preambles

X-MAC

• Asynchronous protocol that improves upon B-MAC by using shorter, strobed preambles
• Preambles contain the address of the intended receiver, allowing non-target receivers to go back to sleep quickly
• Target receiver sends an early ACK to stop the preamble and start data transmission
• X-MAC reduces energy consumption and latency compared to B-MAC

Preamble Sampling Technique

• Common technique used in asynchronous MAC protocols (B-MAC, X-MAC)
• Nodes periodically wake up and sample the channel for preamble presence
• If a preamble is detected, the node remains awake to receive the incoming packet
• Preamble length must be at least as long as the sampling interval to ensure receiver is awake
• Preamble sampling allows for low duty cycles and energy savings but can lead to increased latency and overhearing

Synchronous MAC Protocols

Sensor-MAC (S-MAC)

• Synchronous duty cycling protocol that aims to reduce energy consumption while maintaining good throughput
• Nodes form virtual clusters and synchronize their sleep/wake schedules
• Each cycle consists of a listen period (for communication) and a sleep period (for energy saving)
• Nodes exchange synchronization and schedule information during the listen period
• S-MAC uses RTS/CTS packets for collision avoidance and hidden terminal problem mitigation
• Adaptive listening is employed to improve throughput by allowing nodes to stay awake for additional transmissions

Timeout-MAC (T-MAC)

• Synchronous protocol that improves upon S-MAC by introducing an adaptive duty cycle
• Nodes dynamically end the listen period if no activation event occurs within a timeout period
• Activation events include the reception of data, RTS, CTS, or ACK packets
• T-MAC adapts to varying traffic loads by adjusting the listen period based on network activity
• Adaptive duty cycling in T-MAC leads to better energy efficiency compared to S-MAC, especially under low traffic conditions
• T-MAC also incorporates the RTS/CTS mechanism and virtual clustering for synchronization and collision avoidance