Image Processing Reference
In-Depth Information
recognize the need to lower the duty cycle of the nodes by dynamically powering the radio on and of.
Forthisreason,inthesamepapertheyalsoproposetheuseofself-organizingmediumaccesscontrol
for sensor networks (SMACS), which is a MAC protocol that allows nodes to discover neighbors and
create a transmission schedule to communicate in a time division multiple access (TDMA) fashion.
Each link is assigned a time slot and a random frequency band. In this way SMACS can decrease the
duty cycle of the nodes as well as the likelihood of collisions.
7.4.3 Energy-Aware Routing Protocol
The idea behind the energy-aware routing (EAR) protocol [Sha] is that always using the lowest-
energy path to route data may not be the optimal choice to optimize the WSN lifetime in the long
term. In fact, a routing protocol that finds an optimal path and uses only that will quickly drain the
energy of the nodes on that path. This would lead to a large disparity in the energy levels of the
nodes, with the risk of network partitioning. In order to enhance network survivability, that is, to
maintain network connectivity as long as possible, the EAR protocol does not find a single optimal
route but a set of good routes, and probabilistically chooses one of them. In this way, a different path
is used at different times, so there is a balance in the energy of different nodes and the whole network
lifetime increases. his is similar to Directed Difusion, as data flows once again along different paths.
However, while Directed Diffusion constantly sends data along each path with different data rates,
here only one path at a time is used.
The protocol comprises three different phases: setup, data communication, and route maintenance.
During the setup phase the routing tables are created, with all the routes from sources to destina-
tions and the relative energy costs. Connections are initiated by the destination nodes by localized
looding,i.e.,requestpacketsareforwardedtoallneighborsthatareclosertothesourcenodethanthe
node itself. his mechanism requires a location subsystem on each node. When a request is received
(at each hop), the total cost of the path is updated by adding the energy metric for the neighbor that
senttherequest.hatis,iftherequestissentfromnode
N
i
to node
N
j
,
N
j
calculates the cost of the
path as
C
N
j
,
N
i
=
Cost
(
N
i
)+
Metric
(
N
j
,
N
i
)
.
(.)
e
ij
R
i
,where
e
ij
is the
energy used for transmitting and receiving on the link,
R
i
is the normalized value for the residual
energy, and α and β are two weighting factors.
Paths having a very high cost are discarded, while low-cost neighbors are added to the forwarding
table on
N
j
.Intheforwardingtable,FT
j
, each neighbor is assigned a probability that is inversely
proportional to the cost, that is:
The energy metric proposed in [Sha] from node
N
i
to node
N
j
is
C
ij
=
/
C
N
j
,
N
i
P
N
j
,
N
i
=
.
(.)
∑
FT
j
/
C
N
j
,
N
k
k
∈
hen
N
j
can calculate the average cost to reach the destination by probabilistically choosing the
neighbors in the forwarding table FT
j
,thatis:
Cost
(
N
j
)=
∑
i
P
N
j
,
N
i
C
N
j
,
N
i
.
(.)
∈
FT
j
heaveragecostjustcalculatedissetinthecostieldoftherequestandforwardedtowardthesource
node.
In the Data Communication phase of the protocol, each node simply forwards data packets to
a random node in its forwarding table according to the stored probabilities, until the data packet
