Dimensiones de la información financiera 1 Confiabilidad

At each hop in geographical routing, messages are sent to nodes that are geographically closest to the destination. It is theorised that by routing in this manner, messages will be sent along the most direct path from a source to the sink. Since the energy expended in transmitting is a function of the transmission distance, the energy consumed by nodes in the network should therefore be minimised. It is common for no path discovery to occur in a geographical routing protocol, since nodes need only be aware of the location of their neighbours and the location of the end destination. Typical module selections for geographical routing protocols are shown in Figure 4.4.

Location Aided Routing (LAR)

LAR-1 and LAR-2 [63] are two protocols for routing to a mobile node using location information. Each protocol operates by estimating the location of a destination node and

Search Method Table Entries Local Link/Node Costs Distance Path Costs Min element Discovery Costing Next Hop

Figure 4.4: Typical module options for geographical routing protocols

routing data towards that location. In LAR-1, messages are forwarded towards a node’s last known location and then flooded throughout the area where the node might be, based on its speed and the time since the node’s position was last known. If the node’s last position is unknown, the message is simply flooded throughout the network. In LAR-2, each node forwards data towards a node’s last known position if they are closer to the destination or if they are not much further from the destination than their predecessor. Thus, the message is not routed along the most direct route but is slightly spread out. In both of these protocols, nodes must know their precise location. The authors suggest the use of GPS. However, this may be too imprecise and too energy intensive to be practical. A further problem with LAR is that there is no solution to the local minima problem [60] in which a message is routed to the edge of avoidcontaining no nodes. The node at which the message arrives may have no neighbours that are closer to the destination. If a local minima is present, messages may be lost in the network as they cannot be forwarded further.

Greedy Perimeter Stateless Routing (GPSR)

GPSR [60] also uses geographic location information as a basis for routing. Unlike LAR, GPSR makes the assumption that nodes are stationary and only considers the use ofgreedy

forwarding, i.e. routing a message to the neighbour that is geographically closest to the destination. A node periodically sends out a beacon message indicating its ID and position. Thus, all of the node’s neighbours know where it is. GPSR is able to resolve the local minima problem. When a message is routed to a node on the edge of avoidin which no neighbour is geographically closer to the destination node, GPSR uses aright-hand ruleto route around the perimeter of the void. Once the void has been negotiated, geographical

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routing can resume. GPSR requires that node connectivity is bidirectional, thus allowing a node to determine the geographic locations of its neighbours.

Power-Efficient Gathering in Sensor Information Systems (PEGASIS)

PEGASIS [78] uses the geographical positions of nodes to form a chain. Whenever a source receives any incoming data, it is aggregated with the source’s own data and forwarded to the next source in the chain, i.e. whichever source is geographically closest. One source in the chain is designated as the leader and forwards any incoming data to a sink, which may be a long distance away. The leader is rotated in order to balance the energy expended from long distance communications. The chain is formed by linking nearby nodes together so that transmission powers may be dropped and energy expenditure lowered. However, since data may be forwarded through many sources before it arrives at a sink, the latency between data being generated and data arriving at the sink may be high.

Geographical and Energy Aware Routing (GEAR)

GEAR [126] is a geographical routing protocol that also considers the energy reserves at each neighbouring node. Nodes estimate their neighbours’ energy reserves by tracking the data sent to each neighbour since that neighbour last announced its energy reserves. The suitability of a neighbour for the next hop for a message is based on a weighting of two factors:

• the distance of the neighbour to the destination, and

• the estimate of the remaining energy of that neighbour compared to other neighbour- ing nodes.

The weighting is such that when all nodes have an equal estimated remaining energy, the next hop is the node that is nearer to the destination. Conversely when all nodes are equidistant from the destination, the next hop is the node with the greatest estimated re- maining energy. When the message reaches the target geographic region, the message is disseminated across all the nodes in that area. One difficulty with this protocol is in knowing or estimating the remaining energy on each neighbour node. Activities such as

processing or sensing may affect the energy reserves of a neighbour. Furthermore, as the number of sources is increased, estimates become harder to make. For example, if sources A and B each use a node C they are only aware of their own contributions to the loss of C’s energy unless they are both involved in routing for each other. It is implied that bidirection- ality is required for GEAR to work, as nodes must update their neighbours regarding how much energy they have remaining.

Weighted Energy Aware Routing Protocol (WEAR)

WEAR [102] is a proposed improvement of the GEAR protocol. In WEAR, each node is assigned a cost. The cost is weighted on four different factors:

• the proximity of the node to the sink, with closer nodes having a higher cost;

• the proximity of the node to a hole (void), i.e. a region with no nodes, with a node’s cost being increased by being near a void;

• the node’s energy reserves; and

• the distance of the node from the target, with nearer nodes having a lower cost. These four factors are weighted and combined to provide a node cost. In considering all these factors, WEAR aims to improve two perceived flaws of GEAR. Firstly, the overuse of nodes nearer the sink and secondly the expansion of routing voids by continually taking the same path to avoid a void. Factors 1 and 4 appear to be contradictory, in that a node near the sink is both encouraged and discouraged. However, the former is used to discourage the unnecessary use of nodes near the sink in order to achieve load balancing while the latter is used to encourage direct paths between a source and sink. WEAR assumes the presence of bidirectional links.

Geographic routing causes messages to be routed along the most direct path to a destination. At each hop, the node nearest the destination is selected to be the next hop. However, this behaviour would appear to encourage the use of long hops. As shown in Section 2.5.2.1, there is conflicting evidence as to whether this is a sensible approach or not.

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