Cryptography Reference
In-Depth Information
Advertisement phase:
In the beginning, when clusters are being created, the decision about whether a node
should be a cluster head is dependent on the percentage of already-existing cluster heads
and the number of times an ordinary node has accepted the responsibility of a cluster
head. In this model, the assumption is that all nodes in the network begin with the same
amount of energy, and that, on taking on the role of a cluster head, almost an equal
amount of energy is consumed by each cluster head. Any node
n
in the cluster would
choose a random number between 0 and 1. If this number is less than the threshold
T
(
n
), this node would step up to be the cluster head for the current round. Therefore,
ì
ï
ï
P
if
nG
Î
ï
æ
ö
ï
ï
1
÷
ç
1
-
Pr
od
=
í ç
÷
Tn
()
(7.1)
÷
ç
è
ø
ï
ï
ï
ï
ï
î
P
0
otherwise
where
P
is the desired percentage of cluster heads,
r
is the current round, and
G
is the
set of nodes that have not been cluster heads in the last 1/
P
rounds. Considering
T
(
n
),
each node will take on the role of a cluster head at some point within 1/
P
rounds. Let
P
be the probability that each node will be a cluster head at round 0. Subsequently, these
nodes cannot be cluster heads until 1/
P
rounds. Now that we are left with fewer nodes
that can take the role of a cluster head, the value of
P
must be increased. After (1/
P
) - 1
rounds, the threshold values
T
(
n
) = 1 for any nodes that have not yet assumed the role
of a cluster head. In addition, after 1/
P
rounds, all nodes again become eligible to take
on the responsibility of a cluster head.
At each round, the selected cluster heads broadcast an advertisement to all the
other ordinary nodes in the cluster using the CSMA-MAC protocol and use the same
amount of energy for transmission. For this reason, all ordinary nodes should keep
their radio in the listening mode to hear the advertisement from the concerned cluster
heads. Upon receiving the advertisement message, each ordinary node inspects the
received signal strength of the message and finalizes the cluster to which it belongs.
Assuming that there is a symmetric propagation channel between the communicating
nodes, advertisement messages received with the largest signal strength would indicate
that it is closest to that cluster head and would thereby take less transmission energy.
In the case of an ordinary node receiving messages with equal signal strength, the node
can pick any one of the cluster heads in a randomized manner.
Cluster set-up phase:
Upon selecting the cluster heads, each node in the network responds back to its respec-
tive cluster head using the CSMA-MAC protocol.
Schedule-creation phase:
Upon receiving all the messages from the ordinary nodes, the cluster heads create a
TDMA schedule informing each node, by broadcasting the time slots in which they
should transmit messages.
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