Having multiple schedules is
undesirable in aWSN since border nodes must wake up during the listen times
of all adopted schedules. Consequently, border nodes will die much faster than
other nodes in the network.
SMAC tries to reduce the number of schedules in the network through
singleton schedule elimination technique. After a node broadcasts its schedule,
it will not adopt this schedule until it learns that other nodes have adopted it
102 Ali Abu-el Humos et al.
Chapter 4 Medium Access Control Protocols for Wireless Sensor Networks
Fig. 12. Virtual clusters and border nodes in SMAC.
as well. This will help in eliminating the schedules broadcast by newly added
nodes into an existing network but does not guarantee all nodes to follow the
same schedule.
To solve this problem, the Global Schedule Algorithm technique was proposed
[5]. If a node receives a schedule di?®erent than its own schedule, it will
switch to this schedule if it is older than its own schedule. If the received
schedule has the same age as the node??™s schedule, the one with the lower ID
will be adopted. This node will update its neighboring nodes of any change
on its schedule, so that all nodes in the network will eventually converge to
follow the same schedule. The Fast Path Algorithm was introduced to reduce
the latency in SMAC, provided that all nodes in the network follow the same
schedule and the path from the source node to the sink node is known.
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