Wireless Sensor Networks : Technology, Protocols, and Applications

FUNDAMENTALS OF MAC PROTOCOLS 147
queue. With respect to throughput, a MAC protocol is stable if the throughput does
not collapse as the load offered increases. Accommodating load fluctuations while
maintaining system stability is difficult to achieve in time-varying large-scale
WSNs. One possible approach is for the MAC protocol to adapt to high fluctuations
in the traffic load through careful scheduling of bursty traffic.
Fairness A MAC protocol is considered to be fair if it allocates channel capacity
evenly among the competing communicating nodes without unduly reducing the
network throughput. Achieving fairness among competing nodes is desirable to
achieve equitable QoS and avoid situations where some nodes fare better than
other nodes. As a result, no application is starved or penalized excessively. It is
worth noting that the definition of fairness above assumes that the demands of all
communicating nodes, expressed in terms of channel capacity, are equivalent. It
could be the case, however, that the network must accommodate various traffic
sources with different traffic generation patterns and a wide range of QoS requirements.
To accommodate heterogeneous resource demands, communicating nodes
are assigned different weights to reflect their relative resource share. Proportional
fairness is then achieved based on the weights assigned. A MAC protocol is considered
to be proportionally fair if it is not possible to increase the allocation of any
competing node without reducing the service rate of another node below its proportional
fair share.
Fair resource allocation in wireless networks is difficult to achieve, as global
information may be required to coordinate access to the communication medium
among all contending stations. The time-varying characteristics of the wireless
links makes it difficult to compute the fair share of each contending node, even
if a centralized resource allocation approach is used.
Energy Efficiency A sensor node is equipped with one or more integrated
sensors, embedded processors with limited capability, and short-range radio
communication ability as discussed in Chapter 3. These sensor nodes are powered
using batteries with small capacity. Unlike in standard wireless networks, wireless
sensor nodes are often deployed in unattended environments, making it difficult to
change their batteries. Furthermore, recharging sensor batteries by energy scavenging
is complicated and volatile. These severe constraints have a direct impact on
the lifetime of a sensor node. As a result, energy conservation becomes of paramount
importance in WSNs to prolong the lifetime of sensor nodes. One possible
approach to reducing energy consumption at a sensor node is to use low-power
electronics. The integration of low-power chips in the design of sensor nodes is a
necessary step toward achieving high levels of power efficiency. Energy gains
resulting from energy-efficient chip design, however, can easily be squandered if
the processing and communication capabilities of the sensor node are not operated
efficiently. Achieving this goal requires the design of energy-aware communication
protocols.
Energy efficiency is one of the most important issues in the design of MAC protocol
for wireless sensor nodes. Several sources contribute to energy inefficiency in