wireless ad hoc networking

Communication Protocols 41
Destination
(X D ,Y D )
Flooding
region
Source
(X S , Y S )
Figure 2.8
Sector shape of the flooding region in DREAM.
methods to determine a route: greedy forwarding when it is possible;
and perimeter forwarding (also referred to face routing) when
greedy forwarding fails. In GPSR, packets carry the source and
destination locations. In the normal greedy mode, the forwarder
selects the neighbor which is geometrically closest to the destination
as the next hop. Accordingly, the destination is greedily approached
hop by hop until it is reached. It is possible for an intermediate node
to encounter voids where all its direct neighbors are farther from the
destination than itself. Then the packet is transferred to the perimeter
mode to route beyond the void. GPSR applies the right-hand
rule to traverse a void (as shown in Figure 2.9). A perimeter-mode
packet is marked with the location of the node from which the
packet enters the perimeter mode. It can go further than the current
forwarding node to traverse the perimeter of a void. The right-hand
rule is followed until the packet arrives at a node that is closer
to the destination than the node that makes the packet enter the
perimeter mode. Perimeter forwarding is also referred to as face
routing.
Geographical and Energy Aware Routing (GEAR) 40 is motivated to
solve the problem of void in geography-based routing, inspired by
the traditional distance-vector algorithms. The basic idea is that, for
each transmitted packet, the forwarder will build a learned cost H
as the estimated transmission cost to the destination. The learned
cost H has an initial value and is adjusted adaptively in communications.
Finally it reaches the optimal value which can reflect the
real cost of transmissions to a particular destination. The detailed
protocol is as follows. Given the destination location, the H of a