Networking Reference
In-Depth Information
In making routing decisions, protocols can be either non-overlay or overlay
based:
(1) Non-Overlay Based Protocols
In this category of routing, all nodes can be involved in the decision making
process with equal roles and functionalities. Some of the protocols based on non-
overlay routing are:
- The GPSR and GPSR ? AGF are examples of protocols utilizing the classic
greedy routing approach for selecting the next forwarding node (i.e., picking the
neighbor closest to the destination).
- The GRANT protocol depends on the use of extended greedy routing. As each
node keeps information about its x-hop neighborhood, it has a new metric for
selecting the next forwarding node.
(2) Overlay Based Protocols
Overlay-based protocols depend on the use of representative nodes for the
routing operation overlaid on top of the real network. These nodes have special
roles and, in most cases, they are responsible for the routing decisions. In VANETs,
these nodes are those at the junctions as junctions are the best places for making
routing decisions as there are many options to follow there. Following are some of
the functionalities utilized by the overlay-based protocols:
- The Greedy Perimeter Coordinator Routing (GPCR) protocol [ 70 ] utilizes the
idea of greedy routing for forwarding packets along a road segment. When a
packet reaches a junction, it stops there for deciding which road segment is best
to follow. The reason behind this is preventing the packet from going in a wrong
direction that will add extra unfavourable delay. It gives priorities to the nodes
at the junctions—coordinators—as they have more available options and a
better view.
- The GpsrJ+ protocol does not restrict packets to stop at junctions. A node
holding a packet may bypass the junction if it finds, by prediction, that nodes at
the junction will forward the packet along the same direction.
- Some protocols depend on applying Dijkstra's algorithm for calculating the
shortest path composed of a set of junctions from a source to a destination. An
example is the GSR protocol. In GSR, the algorithm can be run only once with
the list of selected junction points included in the packet header or the algorithm
can be rerun at each forwarding node. Another example is the A-STAR protocol
that, in calculating the shortest path, also considers the traffic density of the road
segments.
- The Vehicle-Assisted Data Delivery (VADD) protocol [ 71 ] is designed for
delay-tolerant VANETs. It is also based on using the junction points as decision
making points. At each junction, vehicles choose the outgoing road with the
lowest delay. Delay can be computed using a set of linear equations based on
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