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In [ 10 ] GPSR was experimented and compared with non-position-based protocol;
Dynamic Source Routing (DSR) [ 26 ]. GPSR protocol consistently delivered over
94% data packets successfully; it is competitive with DSR in this respect on 50 node
networks, and increasingly more successful than DSR as the number of nodes
increases. The routing protocol traffic generated by GPSR was constant as mobility
increased, while DSR must query longer routes with longer diameter and do so more
often as mobility increases. Thus, DSR generates drastically more routing protocol
traffic in simulations with over 100 nodes [ 10 ]. Therefore, the scalability seems to be
the major advantage of this class of algorithms over source-based protocols. However,
these simulations did not include the traffic and time required to look up the position
of the destination. It was also assumed that the position of the destination is accurately
known by the sender [ 6 ].
Nearly stateless schemes are likely to fail if there is some instability in the trans-
mission ranges of the mobile host, when the network graph includes nodes with
irregular transmission ranges [ 17, 23 ]. Instability in the transmission range means
that the area a mobile host can reach is not necessarily a disk. This unstable situation
occurs if there are obstacles (e.g., buildings, bad weather) that disrupt the radio
transmission [ 24 ]. In GPSR, as other greedy forwarding protocols, periodic beaconing
creates lot of congestion in the network and consumes nodes' energy. In addition,
GPSR uses link-layer feedback from Media Access Control (MAC) layer to route
packets; such feedbacks are not available in most of the MAC layer protocols [ 1 ].
Finally, planarizing the underlying graph (network) is computationally expensive
and requires up-to-date neighborhood information [ 1 ].
Another scalable position-based routing protocol is Angular Routing protocol
(ARP) [ 1 ]. In ARP, nodes emit a hello packet on need-basis (non-periodic) at a rate
proportional to their speeds. These hello packets enable that each node maintains a
one-hop neighbor table. ARP uses geographic forwarding to route packets to the
destination. If the geographic forwarding fails, it uses an angle-based forwarding
scheme to circumvent voids in sparse networks. ARP does not need any link-layer
feedbacks like GPSR.
If a source wants to send a packet to a specific destination, it selects the geographi-
cally closest node towards to the destination among its neighbors as the next hop.
Each intermediate node follows this next hop selection criterion. Thus, at each hop
the packet progresses toward the destination by a distance £0.9R, where R is the
radio range of the node. This is done to avoid the problem of leaving the next hop
node out from the transmission range of the current node.
If no node is closer to the destination than the source node, or any intermediate
node, then the node selects a neighboring node that makes minimum angle, among
available neighbors. Figure 4.5 shows the angle-based forwarding to circumvent
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