Overview

An essential characteristic of wireless communication is that it provides an inherently shared medium. All medium-access control (MAC) protocols for wireless networks manage the usage of the radio interface to ensure efficient utilization of the shared bandwidth. MAC protocols designed for wireless sensor networks have an additional goal of managing radio activity to conserve energy. Thus, while traditional MAC protocols must balance throughput, delay, and fairness concerns, WSN MAC protocols place an emphasis on energy efficiency as well.

We shall discuss in this chapter a number of contention-based as well as schedule-based MAC protocols that have been proposed for WSN. A common theme through all these protocols is putting radios to a low-power “sleep mode” either periodically or whenever possible when a node is neither receiving nor transmitting.

Traditional MAC protocols

We begin with a focus on contention-based MAC protocols. Contention-based MAC protocols have an advantage over contention-free scheduled MAC protocols in low data rate scenarios, where they offer lower latency characteristics and better adaptation to rapid traffic variations.

Aloha and CSMA

The simplest forms of medium-access are unslotted Aloha and slotted Aloha. In unslotted Aloha, each node behaves independently and simply transmits a packet whenever it arrives; if a collision occurs, the packet is retransmitted after a random waiting period. The slotted version of Aloha works in a similar manner, but allows transmissions only in specified synchronized slots. Another classic MAC protocol is the carrier sense medium-access (CSMA) protocol.