SOCIEDAD Y POLÍTICA: UNA COMUNICACIÓN INTERESADA

In this category of protocols nodes are addressed by their location. The distance between nodes is measured on the basis of incoming signal strengths and used to estimate energy consumption. By estimating signals strengths, nodes can calculate relative coordinates of neighbouring nodes that can be utilized in routing data efficiently especially in the absence of standard addressing scheme[36].

Furthermore, nodes equipped with a low power GPS receiver can obtain their location directly by communicating with a satellite [37]. The location information could be used to efficiently diffuse quires to a certain region of a sensor network rather than flooding the entire networks. To save more energy, some location based schemes demand that nodes should go to sleep if there is no activity. Some protocols that was originally developed for mobile ad hoc networks are also applicable to sensor networks while others like Cartesian and trajectory-based routing [38] [39] are not. In this section we review the most prominent energy aware location-based protocols.

3.4.1 GAF

Geographic Adaptive Fidelity (GAF) [37] is an energy-aware location-based routing algorithm developed for mobile ad hoc networks but can be used in sensor networks. It can also be considered as a hierarchical protocol where the clusters are based on geographic location. The network area is divided into fixed zones to form a virtual grid. Each node associate itself with a zone in the grid based on its GPS-indicated location. Nodes within the same zone collaborate to save energy by turning off unnecessary nodes without affecting the level of routing

fidelity. Nodes that are located with the same zone on the grid are considered equivalent in terms of packet routing. Thus, GAF can substantially increase the lifetime as the number of nodes increase. GAF nodes can be in one of three states: discovery, for determining the neighbouring nodes in the grid, active reflect participation in routing and sleep when radio is turned off. Mobile nodes estimates its leaving time of grid and forward it to its neighbours so that sleeping neighbours adjust their sleeping time accordingly to keep high level of routing fidelity.

Before the leaving time of the active nodes expires, sleeping nodes wakeup and one of them becomes active. In [37] the fixed zones are chosen to be square and equal. GAF strives to keep the network connected by keeping representative nodes always in active node for each region on its virtual grid. Simulation results have shown that GAF performs as well as a normal ad hoc routing protocol. It was proved that GAF saved energy which result an increase of network lifetime and decrease in delays and packet rates.

3.4.2 MECN

Small Minimum Energy Communication Network (MECN) was proposed in [40], it use low power GPS to compute an-energy efficient sub-networks. The main objective of MECN is to form a sub-network such that the number of nodes and the transmission power is minimized.

This allows finding global minimum power paths without considering the entire network. This is achieved by utilizing a localized search for each node considering its relay region. MECN identifies a relay region for every node. The relay region consists of nodes in a surrounding area where transmitting through those nodes is more energy efficient than direct transmission. The relay region for node pair (1, r) is depicted in Figure 7 redrawn from [41].

Figure 7: Relay region of transmit-relay node pair (i, r) in MECN.

The enclosure of a node I is then created by the union of all nodes reachable by i. the protocol can be viewed as two stages:

1. Construct the enclosure graph which contains globally optimal links in terms of energy consumptions. This phase requires local computations in nodes.

2. Identify optimal links on the enclosure graph using distributed Belman-Ford shortest path algorithm with power consumption as the cost metric.

MECN support fault tolerance because they are self-reconfiguring and thus can dynamically adapt to nodes failure or deploying/ leaving sensors.

The small minimum energy communication network (SMECN) [40] is an extension to MECN which consider obstacles between any pair of nodes.

In MECN, it is assumed that every node can transmit to every other node which is not possible every time. But the network is assumed to be fully connected as in MECN. In terms of the number of edges, the sub-network constructed by SMECN is smaller than the one constructed by MECN. As a result, the sub-network constructed by MECN is smaller than that constructed by SMCN if the broadcast region is circular around the broadcasting node for a given power setting. The energy consumption needed to transmit data from a node to all its neighbours in

SMECN sub-graph is less than that needed in MECN sub-graph.

Moreover, in SMECN maintenance cost is less than that in MECN since the former makes it more likely that the path used requires less energy consumption. Finally, a sub-network with smaller number of edges introduces more overhead in the algorithm.

3.4.3 GEAR

Geographic Energy Aware Routing (GEAR) [42] exploit the fact that data queries often contain geographic attributes to send queries only to a particular area which reduce the number of messages significantly which can conserve more energy. GEAR uses energy aware and geographically informed neighbour selection heuristics to route a packet to destination region. In GEAR environments, nodes keep and estimated cost (a combination of residual energy and distance to destination) and a learning cost of reaching the destination through its neighbours (a refinement of the estimated cost that accounts for routing around holes in the network).

When a node does not have any closer neighbour to the target region than itself, hole occurs. In the absence of any holes, the estimated cost is equal to learned cost. The route setup for next packet is adjusted by propagating the learned cost on hop back every time a packet reaches the target. There are two phases in the algorithm:

1. Forwarding packets towards the target region: When a node has data to send it checks its neighbours to find closer neighbour to the target region than itself. In case of more than one node, the nearest neighbour to the target region is selected as next hop. When there is a hole, one of the neighbours is selected to forward the packet based on the learning cost function.

2. Forwarding the packets within the region: As soon as the packet reaches the target region, it can be diffused in that region by either

recursive geographic forwarding or restricted flooding. The latter is more energy efficient in high density networks.

When compared to GPSR [43] which is a geographic routing protocol that uses planar graphs to solve the holes problem. GEAR was found to reduce energy consumption for route setup and perform better than GPSR in terms of packet delivery. The simulation results have also revealed that for an uneven traffic distribution, GEAR delivers 70% to 80% more than (GPSR). For uniform traffic pairs GEAR delivers 25% - 35% more packets than GPSR.