% $Id: solution.tex 20825 2010-01-03 15:17:20Z gokhale $
\section{A CPS Solution for V2I Communications}
\label{sec:soln}

This section describes our approach to address the impediments to
real-time and reliable information dissemination in ITS V2I networks.
We consider our approach a CPS solution since cyber artifacts (such as
middleware) are responsible for the runtime adaptations that are
effected based on sensing of physical parameters (such as speed, radio
power, and signal-to-noise ratio).  The components of our CPS solution
include the following:

\noindent\textbf{(1) Overcoming RSU association overhead.} Our
solution involves a web service that provides a map of RSUs, which a
vehicle is expected to encounter along the route it is going to
travel.  The client for such a service could very well be an add-on to
an existing navigation system which not only computes the effective
route but also retrieves a map of RSUs.  RSUs can be categorized as
points-of-interest.

For a 802.11x based network, the retrieved map of RSUs comprise
their MAC addresses and GPS locations.  The MAC addresses are
necessary so that the vehicle can eliminate the process of RSU
association.  For improving reliability, alternate RSUs can be
simultaneously used through a technique known as beacon
stuffing~\cite{Chandra_HOTMOBILE:07} but our solution currently does
not utilize this method.  The GPS coordinates provide the RSU
location, which can be used by a vehicle to determine when to
proactively switch from one RSU to another as the vehicle moves out of
range of one RSU. 

In our simulation-based solution, such a map is provided to a vehicle
as part of the configuration file used at simulation
initialization-time. 

\noindent\textbf{(2) Overcoming speed-imposed impediments.}  Having an
RSU map does not mean that a vehicle will automatically be in range
with one of the RSUs.  The speed of the vehicle is one factor that
determines the time window available for communication.  Scheduling
data communications only during this window is necessary since
otherwise mechanisms for flow control and congestion control may
significantly impact performance.

To enable a vehicle to determine when to schedule data communications,
cyber artifacts (such as middleware) must be aware of when the vehicle
gets in the range of the RSU and how much time window is available for
the current vehicle speed.  When the signal-to-noise ratio increases
beyond a threshold, the wireless radio in a vehicle gets in range of a
RSU.  Data from our experiments for signal-to-noise ratio for
different speeds but the same radio power is shown in
Figure~\ref{fig:speed_vs_snir}.
\begin{figure}[htb]
  \centering
  \includegraphics[width=0.6\columnwidth]{Speed_1_Veh_wActScan_SNIR}
  \bfcaption{Variation in Signal to Noise Ratio with Changing Speeds}
  \label{fig:speed_vs_snir}
\end{figure}
The figure reveals that a ratio above 25 made the vehicle get in
range.

Note how the signal-to-noise ratio spectrum shifts in time for
different speeds. Although for this paper we did not consider
vehicle-to-vehicle (V2V) communications, it is possible to leverage
the phase shifts to switch between V2V and V2I communications to
improve reliability of communication.  For example, a slow moving
vehicle that may lose connectivity with a RSU may offload some
communications to a faster moving vehicle that may have acquired
connectivity with the next RSU.  This dimension of work will be the
focus of our future work.

Next, the vehicle must determine the time window it has to
communicate.  A mathematical relationship between the vehicle speed
and the time window can be determined from experimental data shown in
Figure~\ref{fig:Speed_1_Veh_wActScan_STATE} and/or mathematical
analysis~\cite{Rudack_IVC:02}.  This relation can be used not only for
discrete speed values but even when the vehicle is accelerating or
decelerating. 

\iffalse
%%% No space
A relation we derived for our
experimental data is shown in Figure~\ref{fig:TimeWindow4Comm}. 

\begin{figure}[htb]
  \centering
  \includegraphics[width=0.6\columnwidth]{TimeWindow4Comm}
  \bfcaption{Relation between Speed and Time Window for Communication}
  \label{fig:TimeWindow4Comm}
\end{figure}
\fi

Finally, if the vehicle is hosting multiple applications, the cyber
artifacts must be aware of the total data that can be received within
the time window, which can be obtained from
Figure~\ref{fig:speed_vs_packets} so that the cyber artifacts can
prioritize and control the flow of traffic from different
applications~\cite{Burgess_MaxProp:06}.

Sensing of physical factors and relaying them to higher level
middleware artifacts will require cross-layer communications and
optimizations.  Our simulation leverages a module called the
NotificationBoard that serves as an event-channel to relay events of
interest to subscribers.

\noindent\textbf{(3) Overcoming impediments due to radio power.}
Deployment of RSUs is optimized for the speed limit for that road.
When vehicles move faster than their speed limits but maintain high 
radio power levels, there is every possibility that their radio range
will overlap multiple RSUs.  On the other hand, slow moving vehicles
will encounter regions of no connectivity.  To handle this problem,
higher layer cyber artifacts within a vehicle can dynamically adjust
their radio power levels so that connectivity with a RSU can be
maximally maintained without overlaps.

\iffalse
%%% No space
The observed behavior and performance for RSU discovery based on power
levels helps us to derive a function between the radio power and
available time window for data communication as depicted in
Figure~\ref{fig:TimeWindow4Comm_Power}.

\begin{figure}[htb]
  \centering
  \includegraphics[width=0.6\columnwidth]{TimeWindow4Comm_Power}
  \bfcaption{Relation between Speed and Time Window for Communication}
  \label{fig:TimeWindow4Comm_Power}
\end{figure}

Cyber-based solutions can codify this mathematical relationship to
make runtime decisions for adapting the radio power levels. This may
become necessary based on the spacing between the deployed RSUs so
that the scheduled data communication tasks can reliably complete the
communication and meet the QoS.
\fi

Figure~\ref{fig:power_vs_snir} shows how the time window for
communication with RSU can be adjusted by varying the power level of
the radio, which is an example of how cyber artifacts can control
physical factors.
\begin{figure}[htb]
  \centering
  \includegraphics[width=0.6\columnwidth]{Power_1_Veh_NoActScan_SNIR}
  \bfcaption{Variation in Signal to Noise Ratio with Changing Radio Power}
  \label{fig:power_vs_snir}
\end{figure}
The implementation techniques are similar to the ones used to overcome
the impediments imposed by speed.  This paper, however, does not
integrate the control of speed and power simulataneously, which is the
focus of our future work.

\noindent\textbf{(4) Overcoming impediments imposed by higher layers.}
Section~\ref{sec:handoff_impact_app_comm} revealed how routing tables
must be updated when handoffs occur.  Note that routes must be changed
only at those switches and routers that are closest to the RSUs.
Since the next RSU along the route is stored in the map at the
vehicle, the cyber layer can send a message to the closest router or
switch with the address of the next RSU so that the routing table
entry can be updated.

It is conceivable that the system may get overloaded with such
messages if every vehicle attempts to send route update messages.
Although we do not address this problem in this paper, a potential
solution is to leverage the platoon effect and elect one vehicle as a
leader who alone uses V2I communications while other vehicles use V2V
communication.

Our simulation-based implementation leverages the NotificationBoard
module to sense the signal-to-noise ratio values.  When the value is
close to falling below the sensitivity threshold, an event is
scheduled for the router to trigger a table update.

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