Image Processing Reference
In-Depth Information
to the discovery state. In order to balance energy consumption and increase network lifetime, node
ranking needs to be conveniently set. he value depends on the state and the residual energy of the
nodes. Nodes in the active state always have a higher rank than those in the discovery state, while
between nodes in the same state those with a longer expected lifetime have a higher rank.
GAF can run over any ad-hoc routing protocol, such as AODV [Perk], dynamic source
routing (DSR) [Joh], TORA [Park], destination-sequenced distance-vector (DSDV) [Perk],
andmaybeusedforWSNsaswell.SimulationsshowthatwithGAFthereisnodelayincrease,while
theconsumedenergyishighlyreduced.hedownsideisthatpacketlossmayslightlyincrease,aseach
timeapreviouslyactivenodegoestosleepthereisatopologychangetheaboveroutingprotocolhas
to react to.
7.8.2 Span Protocol
In [Che], another distributed coordination protocol for wireless ad-hoc networks, called Span, is
presented. he objective of the Span protocol is to reduce energy consumption without significantly
reducing network capacity or the connectivity of a multi-hop network. To achieve this, Span elects in
rotation some coordinators that stay awake and actively perform multi-hop data forwarding, while
the other nodes remain in power-saving mode and check whether they should become coordinators
at regular intervals. Coordinators form a forwarding backbone that should provide as much capacity
as the original network.
Each node makes periodic local decisions on whether to be a coordinator or not. Such decisions
are based on a coordinator eligibility rule. If a node has two noncoordinator neighbors that cannot
communicate with each other either directly or through other coordinators, then it will become a
coordinator. In order to avoid contention in elections and to keep the number of coordinators small,
nodes wait for a random delay period before sending their announcement message. hen they elect
themselves as coordinators only if the eligibility rule still holds after the wait. A convenient selection
scheme for the random period is proposed to keep the number of coordinators low and to achieve
rotation. his takes two different factors into account, i.e., the number of additional pairs that will be
connected if the node becomes a coordinator and its residual energy.
This protocol achieves significant energy saving while maintaining the performance of the upper-
layer routing protocol almost unaltered. As compared to GAF [Xu], it has an adaptive but less
predictablenumberofforwardingnodes,asinGAFthegridisixedandnodesinsidethesamecellare
considered to be equivalent. Besides, Span does not require nodes to know their location. However,
Span requires a modification to the lookup mechanism of the routing protocol, as only nodes in the
active state have to be considered in choosing the next hop. Finally, the adoption of Span for WSNs
could be constrained by the fact that it is designed for the IEEE . PHY and MAC protocol, e.g.,
it relies upon its ad-hoc power saving functions to buffer packets for sleeping nodes.
7.8.3 Sparse Topology and Energy Management (STEM) Protocol
The STEM protocol presented in [Sch] is a topology control protocol specifically designed for
WSNs. The assumption of STEM is that nodes in a WSN may spend most of the time only sens-
ing the surrounding environment waiting for a target event to happen. hus, unlike other topology
management schemes that coordinate the activation of nodes during the transmission phase, STEM
optimizes the energy efficiency of nodes during the monitoring state, i.e., when no one is sending data.
STEM exploits the fact that, while waiting for events, the network capacity can be heavily reduced,
thus resulting in energy savings.
So all nodes can be asleep when no data transmission is needed. On the other hand, it is important
for nodes to be able to wake up neighbors when transmissions occur. he solution proposed by STEM
is to keep the duty cycle of nodes very low unless data transmission starts. Nodes keep their radios
