extra information in the beacon packets about their speed, direction, and total
- The Greedy Routing with Abstract Neighbor Table (GRANT) protocol [ 64 ]
depends on the idea of extended neighborhood knowledge where each node
knows about its x-neighborhood. To avoid the overhead of exchanging x-hop
neighbor information, GRANT divides the plane into areas and assigns only
one representative for each area.
- The GpsrJ+ protocol [ 65 ] uses two-hop neighbor beaconing to provide a
broader view for the nodes making decisions.
- The Connectivity-Aware Routing (CAR) protocol [ 66 ] follows the DSR
approach for route discovery. It only discovers the path as a list of anchor
points (nodes at junctions or road curves) and this list of anchor points is
stored in the packet header.
- The Contention-Based Forwarding (CBF) protocol [ 67 ] does not depend on
exchanging periodic beacon packets among the neighboring nodes. It utilizes
the concept of contention among the nodes and gives a priority of forwarding
for only one node. In CBF, a node holding a packet broadcasts it to all its
direct neighbors. Based on its distance to the destination, each node that
receives the packet sets a timer for rebroadcasting the packet with the nearest
node having the shortest timer. The actual forwarder is the nearest neighbor
and the other potential forwarders are suppressed [ 7 ].
(2) Information-Assisted Protocols
As mentioned above, these protocols import street or traffic information from
external sources to help in forming more efficient routes. All the surveyed infor-
mation-assisted protocols are also beacon-based ones. Examples are:
- The Geographic Source Routing (GSR) protocol [ 68 ] assumes the availability of
city maps for its operation. It runs the selection algorithm on the map-based
graph (i.e., the set of available junctions).
- In addition to utilizing static maps, the Anchor-based Street and Traffic Aware
Routing (A-STAR) protocol [ 69 ] depends on the use of real-time traffic infor-
mation. A-STAR utilizes two types of maps: a statically rated map (one based
on stable bus routes) and a dynamically rated map (one based on real-time traffic
conditions retrieved from monitoring RSUs) [ 7 ].
(b) Route Selection
Most position-based routing protocols are based on the concept of greedy
routing; a node holding a packet forwards it to the neighbor closest to the desti-
nation. Since the decision is done at the node getting the packet, route selection in
the position-based greedy routing protocols is considered intermediate-based.