Wireless Sensor Networks : Technology, Protocols, and Applications

146 MEDIUM ACCESS CONTROL PROTOCOLS FOR WIRELESS SENSOR NETWORKS
of state transitions between message arrival and message transmission. Therefore,
deterministic MAC schemes guarantee an upper bound for the access time. Determinism
is a crucial requirement in a real-time environment, where the correctness
of the application depends on the adherence of its underlying tasks to their specified
execution deadline.
Throughput Throughput is typically defined as the rate at which messages are
serviced by a communication system. It is usually measured either in messages
per second or bits per second. In wireless environments it represents the fraction
of the channel capacity used for data transmission. Throughput increases as the
load on the communication system increases initially. After the load reaches a certain
threshold, the throughput ceases to increase, and in some cases, it may start to
decrease. An important objective of a MAC protocol is to maximize the channel
throughput while minimizing message delay.
Robustness Robustness, defined as a combination of reliability, availability, and
dependability requirements, reflects the degree of the protocol insensitivity to errors
and misinformation. Robustness is a multidimensional activity that must simultaneously
address issues such as error confinement, error detection and masking,
reconfiguration, and restart. Achieving robustness in a time-varying network such
as a WSN is difficult, as it depends strongly on the failure models of both the links
and the communicating nodes.
Scalability Scalability refers to the ability of a communications system to meet its
performance characteristics regardless of the size of the network or the number of
competing nodes. In WSNs, the number of sensor nodes may be very large, exceeding
thousands and in some cases millions of nodes. In these networks, scalability
becomes a critical factor. Achieving scalability is challenging, especially in timevarying
environments such as wireless networks. A common approach to achieve
scalability is to avoid relying on globally consistent network states. Another
approach is to localize interactions among the communicating nodes, through the
development of hierarchical structures and information aggregation strategies.
Grouping sensor nodes into clusters, for example, allows the design of shared medium
access protocols which are highly scalable. Similarly, aggregating information
from different sensors allows the development of traffic patterns which can be
exploited efficiently to scale the MAC protocol to a large number of sensor nodes.
Stability Stability refers to the ability of a communications system to handle fluctuations
of the traffic load over sustained periods of time. A stable MAC protocol,
for example, must be able to handle instantaneous loads which exceed the maximum
sustained load as long as the long-term load offered does not exceed the maximum
capacity of the channel. Typically, the scalability of a MAC protocol is
studied with respect to either delay or throughput. A MAC protocol is considered
to be stable, with respect to delay, if the message waiting time is bounded. These
systems can be characterized by a bounded backlog of messages in the transmission