Wireless Sensor Networks : Technology, Protocols, and Applications

160 MEDIUM ACCESS CONTROL PROTOCOLS FOR WIRELESS SENSOR NETWORKS
its lifetime. The retransmission of colliding packets is yet another source of significant
energy waste. A high number of these collisions may lead to severe performance
degradation of the MAC-layer protocol. Similarly, excessive overhearing,
which causes a node to receive and decode packets intended for other sensor
nodes, unnecessarily increases energy consumption and can severely degrade the
network throughput. These packets are eventually dropped after the node realizes
that the destination address is different from its own address.
The main objective of most MAC-layer protocols is to reduce energy waste
caused by collisions, idle listening, overhearing, and excessive overhead. These
protocols can be categorized into two main groups: schedule- and contention-based
MAC-layer protocols. Schedule-based protocols are a class of deterministic MAClayer
protocols in which access to the channel is based on a schedule. Channel
access is limited to one sensor node at a time. This is achieved based on preallocation
of resources to individual sensor nodes. Contention-based MAC-layer protocols
avoid preallocation of resources to individual sensors. Instead, a single radio
channel is shared by all nodes and allocated ondemand. Simultaneous attempts to
access the communications medium, however, results in collision. The main objective
of contention-based MAC layer protocols is to minimize, rather than completely
avoid, the occurrence of collisions. To reduce energy consumption, these
protocols differ in the mechanisms used to reduce the likelihood of a collision while
minimizing overhearing and control traffic overhead.
Resolving collisions is typically achieved using distributed, randomized algorithms
to reschedule channel access among competing sensor nodes. The basic
approach used to reduce overhearing is to force nodes into a sleep state when
they become inactive. Un-coordinating sleeping, however, can make communications
among neighboring nodes difficult. To address this shortcoming, a variety
of less restrictive schedules have been proposed by different MAC-layer protocols
to coordinate the activity of the network sensors.
In the following section we first discuss schedule-based MAC-layer protocols for
WSNs. We then briefly review a variety of contention-based MAC-layer protocols.
We conclude the section with two cases studies. The first study focuses on S-MAC,
a low-duty-cycle contention-based MAC-layer protocol specifically designed for
WSNs. S-MAC strives to retain the flexibility of contention-based MAC-layer protocols,
such as IEEE 802.11, while reducing energy waste caused by idle listening,
collisions, overhearing, and excessive control overhear. S-MAC uses the concepts
of low-duty-cycle coordinated sleep and wakeup time periods to reduce power consumption
while achieving high throughput.
The second case study focuses on the IEEE MAC-layer protocol specification
for a low-data-rate wireless personal area network standard: IEEE 802.15.4,
Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications
for Low Rate Wireless Personal Area Networks (LR-WPANs). The IEEE 802.15.4
specification supports three traffic types: periodic, intermittent, and repetitive.
Furthermore, the protocol specification supports fixed, portable, and moving
devices operating at data rates ranging from 20 to 250 kbps. When lines of communication
exceed 30ft, the standard allows for the creation of self-configuring