1. A survey of geographic routing protocols for
Vehicular Ad Hoc Networks (VANETs)
Jesus Gabriel Balderas Lopez
New Mexico State University.
Fall 2010
CS 479/579
Problem definition:
Routing has been a challenge in Vehicular Ad Hoc Networks
(VANETs) because of the features of this kind of networks. We know
that in VANETs exist a frequent change in the topology of the network
and this is due to the fast mobility of the nodes/vehicles. There
also exist the problem of connectivity between the nodes for the same
reason.
VANETs have many applications; we can use them to improve road
traffic safety and efficiency with real time information about the
status of the road, if there is an accident we can know it and look
for a new route to go to your destination; we can also use VANETs for
media sharing between two vehicles (like the example given in class
where the driver of one of the vehicles is able to know the song that
is playing is the neighbor vehicle and sends a request to download
the song and play it on his vehicle). There are many applications
of VANETs and in order to accomplish these things we need some new
routing protocols that consider the challenges mentioned above.
In this survey we are going to analyze only routing strategies that
use geographical location information obtained from street maps,
traffic models or even more prevalent navigational systems on-board
the vehicles. The reason of this is that geographic routing has been
identified as a more promising routing paradigm for VANETs; therefore,
most of the routing protocols available for VANETs use some kind of

2. geographic information.
In the following sections, I classify the geographic routing
protocols for VANETs according to the routing type; I describe each
protocol and describe how the protocol uses the location information
for routing. After this classification I present my insights of the
problem, and finally I present some complexities that I ran into while
trying to solve this problem.
Classification:
The easiest way to classify the geographic routing protocols
is by type of routing (Unicast, Broadcast or Geocast). Other way
to classify them is by the use that the protocol gives to the
position information (Packet forwarding, Route Selection, Cluster
formation, Formation of cells, Classify Forwarding Group, or Route
Request Forwarding). For this survey I will use the type of routing
classification and in each protocol I’ll talk about how the protocol
uses the geographic information.
Unicast:
GPSR (Greedy Perimeter Stateless Routing) [2] is probably
the best known geographic routing protocol for VANETs. GPSR uses
the positions of routers and a packet’s destination to make packet
forwarding decisions. The position of a packet’s destination and
positions of the candidate next hops are sufficient to make correct
forwarding decisions, without any other topological information. In
this protocol the authors assume that all wireless routers know their
own position, either from a GPS device, if outdoors, or through other
means. They also assume bidirectional radio reachability. Finally,
they assume that packet sources can determine the locations of packet
destinations, to mark packets they originate with their destination’s
location.
GPSR consists of two methods for forwarding packets: greedy
forwarding, which is used wherever possible, and perimeter forwarding,
which is used in the regions greedy forwarding cannot be.
● Greedy forwarding A forwarding node can make a locally optimal,
greedy choice in choosing a packet’s next hop. The locally
optimal choice of next hop is the neighbor graphically closest to
the packet’s destination. This scheme is followed successively

3. until the destination is reached.
The figure above represents an example of greedy next hop choice.
Here, the x receives a packet destined for D. x’s radio range is
denoted by the dotted circle about x, and the arc with radius is
equal to the distance between y and D is shown as the dashed arc
about D. x forwards the packet to y, as the distance between y
and D is less than that between D and any of x’s other neighbors.
This greedy forwarding process repeats until the packet reaches
D.
Periodically, each node transmits a beacon to the broadcast MAC
address, containing only its own identifier and position. This
process provides all nodes with their neighbors’ positions.
There are topologies in which the only route to a destination
requires a packet move temporarily farther in geometric distance
from the destination. An example of such topology is shown in the
next figure.
In this figure, x is closer to D than its neighbors w and y.
Although two paths, (x -> y -> z -> D) and (x -> w -> v -> D),
exists to D, x will not choose to forward to w or y using greedy
forwarding because x is the local maximum in its proximity to D.
Some other mechanism must be used to forward the packets in these
situations.
● The Right-Hand Rule: Perimeters: This rule states that when

4. arriving at node x from node y, the next edge traversed is the
next one sequentially counterclockwise about x from edge (x, y).
The figure to the right illustrates this rule. It is known that
the right-hand rule traverses the interior of a closed polygonal
region (a face) in clockwise edge order- In this case, the
triangle bounded by the edges between nodes x, y, z, in the order
(y -> x -> z -> y). They call the sequence of edges traversed by
the right-hand rule a perimeter.
It is important to recall that all nodes maintain a neighbor
table, which stores the addresses and locations of their single-hop
radio neighbors.
All packet data packets are marked initially at their
originators as greedy mode. Packet sources also include the geographic
location of the destination in packets.
When a forwarding node receives a packet in greedy mode, it
searches its neighbor table for the neighbor geographically closest to
the packet destination. If this neighbor is closer to the destination,
the node forwards the packet to that neighbor. When no neighbor is
closer, the node marks the packet into perimeter mode.
GPSR forwards perimeter mode packets using a simple planar graph
traversal.
GPSR works best in a free open space scenario with evenly
distributed nodes. It also suffers from several problems. First, in
city scenarios, greedy forwarding is often restricted because direct
communications between nodes may not exist due to obstacles such as
buildings and trees. Second, if apply first planarized graph to build
the routing topology and then run greedy forwarding or face routing on
it, the routing performance will degrade. Third, mobility can also
induce routing loops for face routing, and last, sometimes packets may
get forwarded to the wrong direction leading higher delays or even

5. network partitions.
Geographic Source Routing (GSR) [3] is other position-based
routing protocol for VANETs. GSR assumes the aid of a street map in
city environments. This street map is used to know the city topology.
GSR uses something called Reactive Location Service (RLS) to get the
destination position. GSR combines geographic routing and topological
knowledge from street maps; the sender determines the junctions that
have to be traversed by the packet using the Dijkstra’s shortest path
algorithm and then forward the packet in a position-based fashion
between the junctions. This protocol was designed for city
environments.
GPCR (Greedy Perimeter Coordinator Routing) [4] is other unicast
geographic routing protocol for VANETs. The main idea of GPCR is to
take advantage of the fact that streets and junctions form a natural
planar graph, without using any global or external information such as
a static street map. It consists of two parts: A restricted greedy
forwarding procedure and a repair strategy which is based on the
topology of real-world streets and junctions and hence does not
require a graph planarization algorithm.
● Restricted Greedy Routing: A special form of greedy forwarding
is used to forward a data packet towards the destination. Since
obstacles block radio signal, data packets should be routed along
streets. Junctions are the only places where actual routing
decisions are taken. Therefore packets should always be forwarded
to a node on a junction rather than being forwarded across a
junction. This is illustrated in the figure below where node
u would forward the packet beyond the junction to node 1a if
regular greedy forwarding is used. By forwarding the packet
to node 2a an alternative path to the destination node can be
found without getting stuck in a local optimum. They call a node
located in the area of a junction a coordinator.

6. ● Repair Strategy: The repair strategy of GPCR avoids using graph
planarization by making routing decision on the basis of street
and junctions instead of individual nodes and their connectivity.
As a consequence the repair strategy of GPCR consist of two
parts:
○ On each junction it has to be decided which street the
packet should follow next.
○ In between junctions greedy routing to the next junction can
be used.
If the forwarding node for a packet in repair mode is a
coordinator then the node needs to determine which street the
packet should follow next. To this end the topology of the city
is regarded as a planar graph and the well known right-hand rule
is applied.
The next image is taken from [1] This image compares Greedy
forwarding (used in GPSR) vs Restricted greedy routing in the area of
junctions (used in GPCR) in (a), and (b) illustrates the right hand
rule used in the repair strategy of GPCR.

7. Anchor-based Street and Traffic Aware Routing (A-STAR) [5] was
proposed for city environments. A-STAR is similar to GSR; it uses the
street map to compute the sequence of junctions (anchors) through
which a packet must pass to reach its destination. Bur unlike GSR, A-
STAR computes the anchor paths with traffic awareness.
A-STAR is also different from other protocols because it employs
a new local recovery strategy for packets routed to a local minimum
that is more suitable for a city environment than the greedy approach
of GSR and the perimeter-mode of GPSR. In the local recovery state,
the packet is salvaged by traversing the new anchor path. To prevent
other packets from traversing through the same void area, the street
at which local minimum occurred is marked as “out of service”
temporarily and these streets are not used for anchor computation or
re-computation during the “out of service” duration and they
resume “operational” after the time out duration.

8. Clustering for Open Inter-vehicular communication (IVC) Networks
(COIN) [6] is a cluster-based protocol. In cluster-based routing, a
virtual network infrastructure must be created through the clustering
nodes in order to provide scalability. Each cluster can have a cluster
head, which is responsible for intra-and inter-cluster coordination in
the network management functions. Nodes inside a cluster communicate
via direct links. Inter-cluster communication is performed via the
cluster-heads. The image below illustrates clustering in VANETs. COIN
uses the location information for cluster formation. Cluster head
election is based on vehicular dynamics and driver intentions, instead
of ID or relative mobility as in classical clustering methods. COIN
produces much more stable structures in VANETs while introducing
little additional overhead.
LORA_CBF [7] is other location based routing algorithm that uses
cluster-based flooding for VANETs. Each node can me the cluster-head,
gateway or cluster member. If a node is connected to more than one
cluster, it is called gateway. The cluster-head maintains information
about its members and gateways. Packets are forwarded from a source to
the destination by protocol similar to greedy routing. If the location
of the destination is not available, the source will send out the
location request (LREQ) packets. This phase is similar to the route
discovery phase of AODV, but only the cluster-heads and gateways will
disseminate the LREQ and LREP (Location Reply) messages.
Broadcast:
The simplest way to implement a broadcast service is flooding in
which each node re-broadcast messages to all of its neighbors except
the one it got this message from. Flooding guarantees the message
will eventually reach all nodes in the network. Flooding performs
relatively well for a limited small number of nodes and is easy to be
implemented. But when the number of nodes in the network increases,
the performance drops quickly. Flooding may have a very significant
overhead and selective forwarding can be used to avoid network
congestion.

9. BROADCOMM [8] is an emergency broadcast protocol based on a
hierarchical structure for a highway network. In BROADCOMM, the
highway is divided into virtual cells, which moves as the vehicle
moves. The nodes in the highway are organized into two level of
hierarchy: the first level includes all the nodes in a cell; the
second level is represented by the cell reflectors, which are a few
nodes usually located closed to the geographical center of the cell.
Cell reflectors behaves for a certain time interval as a base station
or cluster head that will handle the emergency messages coming from
members of the same cell, or close members from neighbors cells.
BROADCOMM outperforms similar flooding based routing protocols
in the message broadcasting delay and routing overhead. However, it is
very simple and only works with simple highways networks.
UMB (Urban Multi-Hop Broadcast) [9] is designed to address the
broadcast storm, hidden node, and reliability problems of multi-hop
broadcast in urban areas. This protocol assigns the duty of forwarding
and acknowledging broadcast packet to only one vehicle by dividing the
road portion inside the transmission range into segments and choosing
the vehicle in the furthest non-empty segment without apriori topology
information. When there is an intersection in the path of the message
dissemination, new directional broadcast are initiated by repeaters
located at the intersections.
The most important goals of UMB, according to the authors, are
as follows:
1. Avoiding collisions due to hidden nodes: In order to decrease
the effect of hidden nodes, a mechanism similar to RTS/CTS
handshake in point-to-point communication is employed by their
UMB protocol. They refer to RTS and CTS as Request To Broadcast
(RTB) and Crear To Broadcast (CTB), respectively.
2. Using the channel efficiently: Forwarding duty is assigned to
only the furthest vehicle in the transmission range without using
the network topology information.
3.Making the broadcast communication as reliable as possible:
To achieve the reliability goal, an ACK packet is sent by the
vehicle which was selected to forward the packet.
4. Disseminating messages in all directions at an intersection:
New directional broadcast are initiated by the simple repeaters
installed at the Intersection Broadcast mechanism.
The next image illustrates the sequence of packets in UMB to
avoid collisions due to hidden nodes.

10. The image below illustrates the intersection broadcast in UMB
for disseminating messages in all directions at an intersection.
Vector-based TRAcking DEtection (V-TRADE) and History-enhanced V-
TRADE (HV-TRADE) [10] are GPS based message broadcasting protocols.
The basic idea is similar to the unicast routing protocol Zone Routing
Protocol (ZRP). Based on position and movement information, their
methods classify the neighbors into different forwarding groups. For
each group only a small subset of vehicles (called border vehicles) is
selected to rebroadcast the message. They show significant improvement
of bandwidth utilization with slightly loss of reachability, because
the new protocols pick fewer vehicles to rebroadcast the messages. But
they still have routing overhead as long as the forwarding nodes are
selected in every hop.
Geocast:
Geocast routing is basically a location-based multicast routing.
The objective of a geocast routing is to deliver the packer from a

11. source node to all the other nodes with a specified geographical
region (Zone of Relevance, ZOR). Vehicles outside the ZOR are not
alerted to avoid unnecessary and hasty reactions. The source node is
usually inside the ZOR
Most geocast routing methods are based on directed flooding,
which tries to limit the message overhead and network congestion of
simple flooding by defining a forwarding zone and restricting the
flooding inside it. Non-flooding approaches (based on unicast routing)
are also proposed, but inside the destination region, regional
flooding may still be used even for protocols characterized as non-
flooding.
Inter-Vehicles Geocast protocol (IVG) [11] consists in informing
all the vehicles of a highway about any danger such as an accident or
any other obstacle. In this case, risk areas are determined according
to the driving direction and the position of the vehicles. The node
which receives an alarm message should not rebroadcast it immediately
but has to wait some time, called defer time, to take a decision about
rebroadcast. When this defer time expires and if it does not receive
the same alarm message from another node behind it, it deducts that
there is no relay node behind it. Thus it has to designate itself as a
relay and starts broadcasting alarm messages to inform the vehicles
which might be behind it. The defer time of node (x) receiving a
message from another node (s) is inversely proportional to the
distance separating them that is to favorite the farthest node to wait
less time and to rebroadcast faster
Cached Geocast [12] is other geocast protocol. The main idea of
their cached greedy geocast inside the ZOR is to add a small cache to
the routing layer that holds those packets that a node cannot forward
instantly due to a local minimum. When a new neighbor comes into a
reach or known neighbors change their positions, the cached message
can be possibly forwarded to the newly discovered node. Their distance
aware neighborhood strategy takes frequent neighborhood changes into
account. It chooses the closest node to destination which is inside

12. the range r (smaller than the transmission range) instead of the node
transmission range in the general greedy routing mode. The improved
neighborhood selection taking frequent neighborhood changes into
account significantly decreases network load and decreases end-to-end
delivery delay.
Beside of the classical geocast routing, there is a special
geocast, called Abiding Geocast [13], where the packets need to
delivered to all nodes that are sometime during the geocast lifetime
(a certain period of time) inside the geocast destination region.
Services like position-based advertising, position-based publish-and-
subscribe, and many other location-based services profit from abiding
geocast. For VANETs, abiding geocast allows realization of information
and safety applications like virtual warning signs. Similar to real
traffic of warning signs, they are attached to a certain geographical
position or area. When a vehicle enters such an area, the virtual
warning sign is displayed for the driver. The authors provided three
solutions:
1. A server is used to store the geocast messages.
2. An elected node inside the geocast region stores the messages.
3. Each node stores all geocast packets destined for its location
and keeps the neighbor information.
My insights
In this work I only present a few position-based routing
protocols for Vehicular Ad hoc Networks, there are so many and more to
come because this is a field of wireless networks that is increasing
so fast. We can see in this paper that have different protocols
depending the environment, if this is for a city of highway, and the
routing type.
When I started to work on this project I wanted to be able to
mention the best protocol for VANETs but this is a hard job. If we
want to implement one of these protocols we need to think about the
requirements. If it is a city scenario maybe I would choose a unicast
protocol like A-STAR because it is also consider traffic load in the
streets. Another option is a broadcast protocol like UMB, I think this
is a good option because this protocol tries to avoid collisions due
to hidden nodes, makes broadcast a reliable way of communication
because implements ACK packets in the process of communications
between nodes; it also adds the idea of repeater in the intersections
to rebroadcast the packets and this is useful because you don’t
necessarily need other vehicles in the streets to forward the packet

13. to the destination.
I think there is so much work to do in the area of routing in
VANETs because in our day to day life we are not always in the city
nor in the highways; it’s necessary to create a new protocol able to
work in both city environments and highways but this is a big
challenge.
I never thought to find a protocol like Abiding Geocast that is
designed for advertising and publishing depending on the region the
vehicle is located and this is an amazing application for VANETs, not
only for real time traffic information or emergency information.
I am impressed of all these protocols and applications and I
think there is more to come.
Complexities
I think that the complexities of solving this problem of routing
in VANETs are related in the implementation and experimentation.
This is because in order to have better results we need no implement
the protocol in a real-life scenario and not just simulate it with
some kind of traffic simulator; also we could need to implement the
protocol not just in one city or highway, maybe 2 or more is better to
obtain results not just in a particular place. I think this part is
the hardest one.
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