Position based routing uses
information about the geographic coordinates and positions of nodes to establish
a route throughout the network. The protocol allows delivery of a message to
all nodes in a geographic area instead of the entire nodes in the network (James Bernsen, 2009). The protocols under
this category provides route creation free benefits sine no route from source
node to destination node need to be created and maintains, almost the protocols
in this category presume a positioning system of some kind like GPS.

2.7.1      
Position-based greedy (PBG)

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Position-based greedy V2V protocol is an example of
this protocol.  In the forwarding
approach of this protocol, an intermediate node in a route forwards messages to
its neighbour in the direction of the next destination. This approach requires
the intermediate node to have the position of itself, the position of its
neighbours, and the position of the destination. The node initial position is
obtained through GPS while the neighbours’ positions are obtained through
message exchanges and the position of the destination node is usually
determined by a location service (James Bernsen, 2009).

2.7.2      
Connectivity Aware Routing Protocols (CAR)

CAR protocols predicts the position of destination
vehicle maintain the route by repairing route as the position changes and find
a route to a destination; through cache maintain by nodes for successful
transmissions between various source and destination pairs.  Nodes using CAR protocols send periodic Hello
beacons that contain information on their velocity. On the received of the Hello
beacons, the node will record recipient of the message in its neighbour table
and calculate its own velocity vector and velocity vector of its neighbour. Record
entries expire from the neighbour table when the distance between nodes exceeds
a threshold or after two HELLO intervals. The beaconing interval adapts to
traffic density by increasing in frequency when there are few neighbours and
decreasing in frequency when neighbours are abundant. Beacons can also be backtrack
on forwarded data packets. These measures help to reduce wasted bandwidth and
network congestion (James Bernsen, 2009).

2.7.3      
Spatially aware packet routing (SAR)

In this protocol, a node determines its location on
the spatial manner and uses the information obtained from street to calculate a
shortest path to a target destination. The geographic location paths are
embedded into the header of the packet when the path is available. The intermediate
nodes are included in the source route and its position-anchored. When a node
needs to forward a packet, it inspects the packet header for the next
geographic location in the route. When a forwarding node cannot find a node
along the predetermined routing path, then, the forwarding node can choose to
place the packet in a Suspension Buffer (SB) where it will remain until a
suitable node is located along the routing path. Alternatively, the node can re-compute
a route path and store in the packet header
(James Bernsen, 2009).

2.8       Broadcast Routing Protocol

Broadcast Routing Protocol
is for safety related applications such as collision warnings and emergency situations,
when the need arises, a call is made for the nodes to send packets to all the
nodes in the network situated close to the sender with high delivery rate and
short delay, this protocol allows information sharing among vehicles
participating in the network, due to the flooding of the network with broadcast   messages , this can lead to increase in  bandwidth usage (Yue Liu, 2009 ).

2.8.1               
Distributed Vehicular Broadcast Protocol (DV-CAST)

The DV-CAST protocol is
designed to deal with extreme conditions such as heavy traffic during rush
hours and light traffic during certain hours of the day. The protocol works by
sending regular hello messages for information broadcast and a flag variable
for nodes to check the packet status. According to (Wisitponghan,
2010),
This protocol is robust against different types of vehicular traffic conditions
and depends only on the local one-hop neighbour information observed by each
node via the use of periodic Hello messages. The robustness of the protocol
allow it to handle broadcast storm or disconnected network problems
simultaneously while incurring only a small amount of additional overhead.

2.8.2         
Urban MultiHop Broadcast (UMB)

According to (G¨okhan Korkmaz, 2004) Urban MultiHop
Broadcast is designed issues associated with broadcast storm, hidden node, and
reliability problems of multiHop broadcast in urban areas. The protocol assigns
the responsibility of forwarding and acknowledging broadcast packet to only one
vehicle by dividing the road part inside the transmission ranges into a segment
and choosing the vehicle in the furthest non-empty segment without a priority
topology information. When there is a junction in the path of the message
dissemination, new directional broadcasts are initiated by the repeaters
located at the junction, with this; the protocol is helpful in overcoming the interference,
packet collision and hidden node problems during packet delivery from source to
destination.

2.9         
Geocast Routing Protocol

As stated by (Maihofer, 2004) the goals of all the
protocols were to enable the transmission of a packet to all nodes within a
geographic region. For instance, multicast enables a packet to be sent to an
arbitrary group of nodes, and geocast is a subclass of multicast and can be
implemented with a multicast service by defining the multicast group to be a
certain geographic region. The nodes use the principle of routing data packet from
one source to all nodes belonging to the destination. The objective of geo-cast
routing is to deliver packets from source node to all other nodes within a
specified geographical region, also called Zone of Relevance (Chawla, 2014). The reference zone of
forwarding is used to hold the message forwarding until it reaches the  zone of relevance through flooding. Few of the
protocols are DG-CASTOR and DRG.

2.9.1      
Direction-based geocast routing protocol (DG-CASTOR)

In Vehicular
Ad-hoc Network, DG-CASTOR works based on the principles of linkage availability
and the node inference. The node communicates amongst  them self  provided they have the same status to communicate
with the source. The Rendezvous boundary represents the Geocast routing area
and the Rendezvous node communicates between the source and the neighbours’
nodes in which the link availability was estimated (Chawla, 2014). The goal of the
protocol is to route data packets through the nearest nodes from the  source node that have the greatest chance to
communicate with the source considering their communication ability and the
distance between their locations (Gulliver, 2015).

2.9.2      
Distributed robust geocast routing protocol
(DRG)

The DRG routing protocol works based on
the forwarding algorithm on the principles of selective and relay scheme. Each
node receiving a geocast message to check the message relevance based on its
location. Two basic approaches were used as the zone of relevance (ZOR) to
represent the set of geographic criteria a node must satisfy principle criteria
in order for the geocast message to be relevant to that node, while the zone of
forwarding (ZOF) is the set of geographic criteria a node must satisfy in order
to forward a geocast message (Harshvardhan P. Joshi, 2006). If the node that
belongs to ZOR reads the message then it will forwards the message that belongs
to ZOF range else the packet will not be sent to the destination. DRG protocol
is based on flooding technique, hence, this can produce significant network
overhead (Gulliver, 2015).

2.10    
Cluster Based Routing Protocols

Cluster Based Routing Protocols divides
the network into small groups called clusters, the nodes having the same
characteristics such as moving with same velocity in the network can form a
cluster and share information.  The
communication among the clusters occurs via pre-selected nodes called Cluster
Heads (CHs). CHs are responsible for coordinating the cluster members; the CH
finds the destination route; by propagating routing overhead, which is
proportional to the number of the clusters instead of the number of nodes. The
objectives of using clusters are to minimize the control overhead, and increase
the network scalability (Ahmad Abuashour, 2016).  Few routing protocols are CBDRP and CBLTR.

2.10.1    
Cluster Based directional Routing protocol (CBDRP)

In this protocol, vehicles with the same direction of
movement are divided into some clusters and a head of the cluster is chosen in
each cluster,   the source vehicle sends request
to the head of the its cluster during request broadcast, the cluster head,
forwards the request to the next cluster head with the same destination, at the
end of the broadcast, the destination head hands the request to the destination
vehicle. The channel is maintained in the process when there is  head in an intermediate cluster. Moreover,
only the head in each cluster is responsible  to find the destination channel (Tao Song, 2010).

2.10.2    
Cluster-Based Life-Time Routing (CBLTR)

Cluster-Based Life-Time Routing (CBLTR) protocol as
described by (Ahmad Abuashour, 2016) divides the network
segment into multiple stationary clusters. In each cluster, the most suitable
CH candidate is chosen based on CHs neighbours and destination location. This
will eliminate the route discovery process by dividing the network into
multiple clusters. The aims are to increase the route stability and average
throughput, as well as reduce the number of re-selection process for new CHs.

2.11  Comparisons of routings in
vehicular networks

Table 2.1 shows comparisons of the vehicular touring
protocols discuses above with their metrics.

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