% $Id: intro.tex 20822 2010-01-03 14:28:25Z gokhale $
\section{Introduction}
\label{sec:intro}

Car drivers often experience getting stuck in a traffic jam on their
way to work despite having checked the traffic reports prior to
leaving their homes.  These unanticipated events can have an adverse
societal impact, {\em e.g.}, students are marked as tardy in school
and business professionals may miss important meetings or flights.
Unanticipated events can also adversely impact mission-critical
operations, {\em e.g.}, medical doctors who may be scheduled to
perform a surgery may be delayed or first responders may find it hard
to reach the scene of an accident.

% This para may not be needed
\iffalse
Traffic congestion has adverse consequences on the safety, economy and
environment.  The loss of productivity due to delays stemming from
traffic jams, and increased fuel consumption leads to adverse economic
consequences~\cite{Meyer_AASHTO:07}.  Different
studies~\cite{Brodrick_Emissions:02,Chapman_Transport:07} have also
shown an increase in greenhouse emissions due to vehicle idling
during traffic jams.
\fi

Intelligent Transportation Systems (ITS)~\cite{Weiland_ITS:00} are
envisioned to address the numerous challenges faced by the
transportation sector.  One category of solutions envisioned in ITS
pertains to the real-time and reliable delivery of traffic-related
information to drivers both for safety-critical applications (such as
blind spot warnings during lane changing) and for applications that
improve driving experience and help the environment (such as
notification of congestion and rerouting advise that can help to
alleviate traffic congestion and lost productivity).

Supporting these applications requires a thorough understanding of the
ITS problem space.  An ITS comprises three types of communication
networks: (1) a wireless network formed among the vehicles for
vehicle-to-vehicle (V2V) communication, (2) a wireless network that
involves the vehicles communicating with the the road-side
infrastructure (V2I), and (3) a predominantly wireline network that
connects the multiple infrastructure elements.

Real-timeliness and reliability of information dissemination via V2V
and V2I communication is a hard problem due to multiple challenges.
Some challenges are imposed by the physics of the system including the
wireless radio transceiver power, shared nature of the wireless
channel, mobility of the vehicles, and density of the vehicles.  Other
challenges arise from the vagaries of the cyber infrastructure
including behavior of protocols like 802.11 media access layer (MAC),
address resolution protocol (ARP), IP addressing and routing, and TCP
retransimission and congestion control.

% Andy, please add a good citation for CPS.
% DONE
Cyber Physical Systems (CPS)~\cite{Wolf_CPS:09} provide a natural
framework to address these challenges since they integrate the
physical, cyber, and human factors in a single framework.  This paper
makes the following contributions to CPS-based solutions for real-time
and reliable dissemination of ITS information:

\begin{BulletList}
\item It surveys the literature to highlight key insights gained by
      prior efforts that either pinpoint the impact on timeliness and
      reliability made by the behavior of the physical medium or
      illustrate software-driven solutions that attempt to maintain
      the quality of service (QoS).

\item It highlights an integrated perspective of the physics- and
      cyber-imposed impediments to realizing the ITS property of
      real-time and reliable information dissemination based on
      additional evaluations we conducted.

\item It presents a CPS-based solution to overcome some of these
      impediments.  The evaluations presented in the paper are
      conducted using the OMNeT++ simulator and the INETMANET
      framework developed for OMNeT++ (\url{www.omnetpp.org}).
\end{BulletList}

The rest of the paper is organized as follows:
Section~\ref{sec:relwork} discusses related research that provide key
insights into the problem space; Section~\ref{sec:impediments}
discusses the impediments in realizing the ITS vision;
Section~\ref{sec:soln} demonstrates how our CPS solution approach
overcomes these impediments; and finally Section~\ref{sec:conclusions}
presents concluding remarks.

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