A. CASTEIGTS, A. NAYAK AND I. STOJMENOVIC
More practically, the standard assumes periodic
exchange of beacons (or hello messages) allowing cars
to discover their neighborhood, and a non slotted com-
munication with no delay or bandwidth guarantees (as a
counterpart of the small latency). Also, the bit error rate
can be substantial, as high mobility causes fast fading
conditions. Reliability management and acknowledg-
ments are thus expected to be handled at the upper
network layer.
Note that some research works propose implicit
extensions of the DSRC standard, such as in
Reference [14] where the authors suggest to vary
the rate of the hello beacons (adaptive beaconing)
according to the density and speed of vehicles. Some
issues related to the beaconing scheme, such as the
possible gap observed between the perceived position
of surrounding cars and their real positions, are
discussed in Reference [15].
The physical equipment needed for these specifi-
cations consists of one or two wireless transceivers
(802.11p mandatory, and 802.11a/b/g optional) and
a GPS receiver, all connected to a central processing
unit called the On-Board Unit (OBU), which will host
the network stack and run most of the communication
protocols. This unit will also be connected to a variety
of in-car sensors to acquire detailed state of the car,
and to input/output devices to interact with the driver
(or passengers).
3. Applications
The potential set of new applications is multifold, and
generally classified as traffic safety, traffic efficiency,
and value-added applications. Applications in each
category may lie at different places of the architecture
previously presented.
3.1. Applications for Traffic Safety
Safety is the primary purpose of vehicular networks.
Most critical safety applications are concentrated on
the road ad hoc domain, since they are time constrained
and cannot afford the delay induced by routing opera-
tions throughout the infrastructure. Examples of delay
critical safety applications are cooperative collision
avoidance, pre- or post-crash warning, rollover
warning, abrupt obstacle avoidance (e.g., animal or
tree) or other hazard detection (e.g., icing, surface
water, pool of oil, pothole, etc.) that can be directly
broadcast among neighboring cars. Less critical, but
still related to safety in the ad hoc domain, are speed
management (for example speed limit warning or
control,orcurve optimal speed announcement) and
preventive coordination among cars (assisted lateral
control, lane departure warnings, ghost driver detec-
tion, etc. The reader interested in specific time and
bandwidth requirements of such applications can find a
classification in this regard in References [16] or [17].
The information related to these applications can be
collected by vehicles and road-side units and delivered
to incoming traffic. Additional issues to resolve
include dissemination and aggregation choices, such
as deciding which information is relevant in which
area, or what is the delay after which an information is
outdated. For example if an accident occurs on a given
lane of a two direction road with hard separation in the
middle, the information of this event is not directly rel-
evant to the vehicles arriving on the other side, nor for
vehicles crossing a bridge over this road. The relevance
of possible notifications is indeed highly variable over
time and space, and identifying these limitations is a
difficult task. An example of dissemination framework
(
NOTICE) was proposed in Reference [18].
Once collected, a piece of information can be
reported to the ITS central server, with slightly lower
time constraints, to be processed for statistics or mid-
term to long-term decisions (e.g., sending a rescue
team, closing a road segment, setting up alternative
paths, etc.). Safety applications motivate the design
of fast broadcasting and geocasting protocols for
immediate warning delivery in the ad hoc domain.
3.2. Applications for Traffic Efficiency
Traffic efficiency is the next priority for vehicular
networks and ITS. The number of vehicles on the road
normally keeps increasing while the construction of
new roads is costly and not always physically feasible.
The integration of vehicles with the traffic management
system offers new opportunities of optimizing the traf-
fic flow, comprising better route selections, better traffic
balance, shorter travel time and, accordingly to most
of these aspects, lower emissions of greenhouse gases.
The traffic efficiency can be improved in several ways,
the most important of which is to help drivers select
the best route between two given points. Nowadays,
drivers have the possibility of being assisted in this
task by a GPS devices equipped with road maps and a
software to chose a proper route. These devices already
improved the driving experience by assisting the driver
in studying the road network and generally shortening
the resulting journey. However, the information it cur-
rently uses is still of a static nature (without real time
Copyright © 2009 John Wiley & Sons, Ltd. Wirel. Commun. Mob. Comput. (2009)
DOI: 10.1002/wcm