Wireless Sensor Networks : Technology, Protocols, and Applications

178 MEDIUM ACCESS CONTROL PROTOCOLS FOR WIRELESS SENSOR NETWORKS
5.6.2 MAC Layer
The IEEE 802.15.4 MAC-layer specification is designed to support a vast number
of industrial and home applications for control and monitoring. These applications
typically require low to medium data rates and moderate average delay requirements
with flexible delay guarantees. Furthermore, the complexity and implementation
cost of the IEEE 802.15.4 standard compliant devices must be low to
minimize energy consumption and enable the deployment of these devices on a
large scale.
To address the needs of its intended applications while enabling the deployment
of a large number of monitoring and control devices at a reduced implementation
cost, the IEEE 802.15.4 MAC-layer specification embeds in its design several
unique features for flexible network configurations and low-power operations.
These features include:
Support for various network topologies and network devices
The availability of an optional superframe structure to control the network
devices’ duty cycle
Support for direct and indirect data transmissions
Contention- and schedule-based media access control methods
Beaconed and nonbeaconed modes of operation (In the beacon mode, the
protocol uses a superframe structure to coordinate access to the medium—
both contention-based access and guaranteed time slots allocation are supported;
in the nonbeaconed mode, the protocol uses an unslotted CSMA/CAbased
access scheme.)
Efficient energy management schemes for an extended battery life, including
adaptive sleep for extended period of time over multiple beacons
Flexible addressing scheme to support the deployment of large-scale networks,
theoretically over 65,000 nodes per network
In the following, the classes of network devices supported by the IEEE 802.15.4
MAC standard and the network topologies that can be achieved using these devices
are discussed. The optional superframe structure is then described and the two
modes of operations, beaconed and nonbeaconed modes, are discussed. Depending
on the mode of operations used, two MAC layer protocols are specified. The basic
operations of these two MAC layer protocols, including the procedures that govern
the data transfer between the network devices in each mode of operations, are
described.
Device Types and Network Topologies To accommodate the MAC protocol, the
IEEE 802.15.4 standard distinguishes devices based on their hardware complexity
and capability. Accordingly, the standard defines two classes of physical devices: a
full-function device (FFD) and a reduced-function device (RFD). These device
types differ in their use and how much of the standard they implement. An FFD