Abstract
Vehicular networks have recently attracted great interest
in the research community, and multihop data dissemination
has become an important issue. To improve data-delivery performance,
we propose deploying static nodes at road intersections to
help relay data. In this paper, we present SADV, which is a staticnode
assisted adaptive data-dissemination protocol for vehicular
networks. With the assistance of static nodes at intersections, a
packet is forwarded to the static node when there are no vehicles
available to deliver the packets along the optimal path. The static
node is able to store the packet and transmit it when the optimal
delivery path becomes available. In addition, we let adjacent static
nodes measure the delay of forwarding data between each other in
real time so that the routing decision that is made at static nodes
can adapt to the changing vehicle densities. Moreover, a multipath
routing mechanism is also adopted in SADV, which is effective in
reducing the data-delivery delay. Our simulation results show that
SADV outperforms other multihop data dissemination protocols,
particularly under median or low vehicle density where the network
is frequently partitioned. In this paper, we also present some
heuristic deployment strategies to maximize SADV performance
under partial deployment of static nodes and analyze them by
simulations.
Index Terms—Geographic routing, mobile ad hoc networks
(MANETs), multihop routing, vehicular networks.
I. INTRODUCTION
VEHICULAR networks have recently attracted great interest
in the research community. Many potentially useful
applications have been envisioned in vehicular networks
[1]–[3]. These range from safety applications, such as vehicle
collision avoidance, to other valuable applications, such as realtime
traffic estimation for trip planning, information retrieval,
and media content sharing. Moreover, by embedding sensors in
vehicles, a mobile sensor network can be established to monitor
roads and other environmental conditions. The vehicular
networks can act as “delivery networks” to transfer data from
remote sensor nets to Internet servers.
Most of the previous research on intervehicle communication
is limited to vehicles within one hop or a few hops away [1], [2]
[4]–[6], such as communicating with nearby upstream traffic vehicles to avoid collisions. However, it is also important to
send data from a vehicle to a destination that is several miles
away through multihop relay by a number of intermediate
vehicles. For example, the sensing data from a vehicle may
need to be sent to a sink that is deployed miles away, or a
vehicle may want to send queries to a remote site such as a
gas station. Thus, a multihop routing algorithm is needed in
a large vehicular network for these applications.
A vehicular network can be regarded as a special type of
mobile ad hoc networks (MANETs) with some unique features.
First, as vehicles move at high speed, the topology of the
vehicular network changes rapidly. Second, unlike MANETs
where an end-to-end connection is usually assumed, vehicular
networks are frequently disconnected, depending on the vehicle
density. In addition, the movement of vehicles is constrained by
the roads, which renders many topology holes in the network.
These characteristics make the classical MANET routing algorithms
such as Ad Hoc On-Demand Distance Vector [7] and
Greedy Perimeter Stateless Routing [8] inefficient in vehicular
network, and significantly influence the design of alternative
routing protocols.
Due to the rapidly changing topology in vehicular networks,
geographic routing mechanisms become more preferable than
topology-based routing mechanisms because of the latter’s high
maintenance overhead of routing tables. “Opportunistic forwarding”
[9]–[11] has been proposed, targeting networks where
the existence of an end-to-end path cannot be assumed. Specifically,
a vehicle will carry the packet and forward it to a new
vehicle when they meet. “Trajectory-based forwarding” [12],
[13] is an extension to geographic routing mechanism, which
tries to deliver packets along a sequence of predefined roads
or a trajectory. It is particularly suitable for vehicular networks
where vehicles are restricted on roads with static structures.
Mobility-centric data dissemination (MDDV) [14] and
vehicle-assisted data delivery (VADD) [15] are two multihop
routing protocols in vehicular networks, which combine geographic
forwarding, opportunistic forwarding, and trajectorybased
forwarding to deliver data from a mobile vehicle to a
static position beside the road. They abstract each road as a link
where the packet-delivery delay depends on the vehicle density
on the road.

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