Synchronous Contention-Based MAC Protocols for Delay-Sensitive Wireless Sensor Networks: A Review and Taxonomy

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Duty cycling allows obtaining significant energy saving compared to full duty cycle (sleepless) random access MAC protocols. However, it may result in significant latency. In slotted duty-cycled medium access control (MAC) protocols, sensor nodes periodically and synchronously alternate their operations between active and sleep modes. The sleep mode allows a sensor node to completely turn off its radio and save energy. In order to transmit data from one node to another, both nodes must be in active mode. The synchronous feature makes the protocols more appropriate for delay-sensitive applications compared to asynchronous protocols. The latter involve additional delay for the sender to meet the receiver's active period, which is eliminated with synchronous approach where nodes sleep and wake up all together. Despite the possible increase of contention by grouping active periods, the delay due to packets retransmissions after collisions is less significant compared to the waiting time of asynchronous protocols. Furthermore, contention-based feature makes the protocol conceptually distributed and more dynamic compared to TDMA-based. This manuscript deals with timeliness issues of slotted contention-based WSN MAC protocols. It provides a comprehensive review and taxonomy of state-of-the-art synchronous MAC protocols. The performance objective considered in the proposed taxonomy is the latency, in the context of energy-limited WSN, where energy is considered as a constraint for the MAC protocol that yields the need of duty-cycling the radio. The main contribution is to study and classify these protocols from the delay efficiency perspective. The protocols are divided into two main categories: static schedule and adaptive schedule. Adaptive schedule are split up into four subclasses: adaptive grouped schedule, adaptive repeated schedule, staggered schedule, and reservation schedule. Several state-of-the-art protocols are described following the proposed classification, with comprehensive discussions and comparisons with respect to their latency.
Wireless Sensor Networks, MAC protocols,, Synchronous Protocols, Delay, QoS