Gugrajah_Yuvaan_ Ramesh_2003.pdf

Routing Protocols for Ad Hoc Networks Chapter 2
other nodes in the network. The transmission period of the LPs is detennined by the
mobility of a node. The faster a node moves, the more often it must communicate its
location. Dream also uses "distance effect" which is the fact that the greater the
distance separating two nodes, the slower they appear to be moving with respect to
each other. Nodes that are far apart from each other therefore need to update each
other's locations less frequently than nodes closer together. Each LP has a certain
"lifetime" in terms of geographical distance which detennines how far an LP travels
before being discarded. Faster moving nodes transmit shorter lived LPs more
frequently while slow moving nodes transmit longer lived LPs less frequently. By
differentiating between nearby and faraway nodes, DREAM attempts to limit the
overhead of LPs.
LPs are used to update the location tables of the receiving nodes and contain the
source identification, coordinates of the source, and the time the LP originated. When
a source node needs to transmit data to a destination node, it calculates a circle
around the most recent location information for the destination nodes, much like the
expected zone in LAR. The source then defines the forwarding zone to be a cone
whose vertex is at the source node and whose sides are tangent to the circle
calculated for the expected location of the destination node. The authors of DREAM
[Basagni98] use a minimum cone angle of 30° in their simulation. The source node
then forwards the data packet to the neighbours in the forwarding zone. This is unlike
LAR and other on-demand routing protocols where a route request is forwarded first.
The neighbours receiving the data packet then compute their own forwarding zones
based on their own location tables and forward the data packet accordingly. When
the destination receives the data packet an acknowledgement (ACK) is returned to
the source along the reverse of the path that was used for the data. If the source does
. .
not receive the ACK within a timeout period, the source resorts to the DREAM
recovery procedure, which involves flooding the data packet through the network. A
destination node receiving a flooded data packet does not return an ACK. DREAM
also defines a timeout value on location information. A source node resorts to
flooding if the location information is older than the specified limit.
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