Image Processing Reference
In-Depth Information
In the sensing state, the sensing circuitry is active and a node periodically sends data to the gateway
at a constant rate. In the relaying state, the sensing circuitry is off, but the transceiver is active to
perform data forwarding. In the sensing-relaying state, a node performs both sensing and data for-
warding so both the sensing circuitry and the transceiver must be active, while in the inactive state
a node turns of both its sensing and communication circuitry. he state of each node is decided by
the gateway depending on the node residual energy and the required performance. Gateway nodes
make an estimation of the residual energy for each node in their cluster based on the number of
transmissions. his is possible as the MAC protocol is a centralized TDMA, controlled by the gate-
way. However, to compensate for model inaccuracy, low-frequency periodical updates on the energy
status are sent by sensor nodes.
Typical network functioning consists of two alternating phases: a data cycle, in which sensing
nodes send data to their gateway, and a routing cycle, in which gateway nodes decide the optimal
status and route assignments for each node and then directly transmit this information to nodes. It
should be noted that while sensor nodes can forward data through multiple hops, gateways always
perform direct transmissions. During the routing cycle, all the routing tables are computed by the
gateway and then transmitted to each node. This is modelled as a path-optimization routing prob-
lem, where a least-cost path from each node to the gateway has to be found. The cost of a path is
the sum of the costs of all the links traversed. A metric is provided that takes into account and bal-
ances several different parameters, including distance, residual energy, expected lifetime, overhead
to switch a node from inactive to enabled status, sensing costs, maximum connection per relay, error
rate, etc. In this way, the optimal route from each node to the gateway can be calculated using the
Dijkstra algorithm [Dij]. Besides routing tables, TDMA slot assignment also has to be scheduled
by gateways and sent to nodes. hen the data cycle can start: nodes set their state and active ones start
transmitting or forwarding during their time slots. At predefined times, all nodes have to switch on
their receivers in order to receive new decisions from their gateway node. Rerouting may be triggered
by application events or by the low battery charge of a node.
Energy efficiency in this protocol is achieved by shutting down sensing or communication circuitry
when it is not needed. However, routing decisions are static for each data cycle, and may not change
until a node runs out of charge. Even if the least-cost algorithm considers energy and the expected
lifetime of networks, the choice is only performed once at the start of a routing cycle. The limit of
static centralized routing is that, if no other changes occur in the network, rerouting will only be
performed when some node has depleted its energy. Dynamically balancing network traffic could
lead to a higher lifetime for the whole network.
Scalability is not affected by centralized routing, as several gateways can be deployed when the
number of nodes per cluster increases. So scalability here could be better than in LEACH. However,
this approach cannot be used where it is difficult to place multiple mains-powered gateway nodes.
This protocol was extended in [Akk] with QoS management, in terms of end-to-end delay, which
can efficiently handle best-effort traffic. This protocol will be dealt with in Section ., where QoS-
based EAR is addressed.
7.6 Location-Based Energy-Aware Routing Protocols
7.6.1 Minimum Energy Communication Network (MECN) Protocol
The MECN protocol [Rod] is not specifically designed for sensor networks. However, the net-
work model it proposes well suits WSNs. In fact, this is a distributed routing protocol where nodes
are location-aware, i.e., equipped with a low-power global positioning system (GPS) [Sha], and
periodically transmit to a master-site node (the sink). Synchronous communications are used, so
that nodes can sleep between subsequent communications, thus lowering their duty cycle and
power consumption. MECN aims to be a self-configuring routing protocol that minimizes energy
