% $Id: intro.tex,v 1.4 2005/04/08 21:21:45 gokhale Exp $
\section{Introduction}
\label{sec:intro}
Disseminating data efficiently from source nodes to sink nodes is one
of the main functions in wireless sensor networks (WSNs).  The unique 
characteristics of wireless sensor networks described below makes the
problem of disseminating the data generated by source nodes non-trivial:
\begin{enumerate}
\item To ensure that the mobile phenomenon is accurately tracked, the
      WSNs are usually large and spread out over a wide geographical
      terrain. 
\item The computational power, storage capacity and battery life of
      sensor nodes is limited.  Furthermore, the nodes themselves
      cannot be reconfigured (to add extra resources, for example) once
      deployed. 
\item Frequent topology changes due to node failures or sensor node
      movement is the norm.
\item Sink nodes may be mobile and move randomly, however, the source
      nodes may not have knowledge of the sink movement pattern.
\end{enumerate}


Since a WSN is made up of a large number of low power radio devices
that are distributed throughout the sensing space it follows that the 
data-dissemination protocols must be scalable, efficient, 
fault-tolerant and adaptable~\cite{hull2004mitigating,khanna:04}.  A
number of data-dissemination protocols have been suggested in the
literature, however, they fail to address the following challenges
effectively: 

\begin{itemize}
  \item \textbf{Overhead of network broadcast and resulting power
    consumption} -- Protocols, such as Directed Diffusion
  (DD)~\cite{Estrin_DD:00}, rely on the sink nodes to flood the
  network with an interest message, followed by the source node
  sending a data response.  The Two-tier Data Dissemination
  (TTDD)~\cite{Zhang_TTDD:02} protocol alleviates this overhead to
  some extent by dividing the  sensing space into cells and limiting
  the data query flooding by a sink to its own cell.  However the
  advertisement of availability of data at each source is still done
  by flooding the network.  The sinks can then query the source
  directly for the data of interest.  Both approaches result in
  excessive power consumption. 

  \item \textbf{Overhead of sink mobility} -- In DD the mobile sink
  nodes have to propogate their location information continuously
  throughout the WSN, which is inefficient.  On the other hand, TTDD
  is only suited for 
  intra-cell sink mobility and cannot handle sinks moving faster than
  their local (inside cell) forwarding states getting updated.
  Furthermore, the cell size cannot be increased on-the-fly to
  accomodate fast-moving sinks since the cell sizes are statically
  determined. The Ring-based Index~\cite{RB:03} scheme also addresses
  localized sink mobility patterns, however, it is not suitable for
  fast moving sinks.


% Amogh, are there any problems with GHT or ring-based, which we might
% want to showcase here

\end{itemize}

We propose EASM -- Energy-efficient Arbitrary Sink Mobility --
data-dissemination scheme for WSNs.  It is a rendezvous-based
scheme~\cite{Estrin:03}, 
where the sink and source nodes communicate through well-known 
rendezvous points.
In our scheme we exploit the fact that sensor nodes are aware of their
own location through the use of schemes, such as GPS~\cite{GPS:01}.
Sink nodes indicate the type of data (or topic) they are interested in
to the rendezvous points and thereafter send their requests to
these rendezvous points.  Each sink node uses a hash function to map a
data type of interest with an appropriate 
rendezvous point.  This rendezvous point acts as the {\em source node}
for the sink thereby servicing its requests.  This approach involves
sinks directly making requests to their rendezvous points, and thereby
avoid expensive network-level broadcasts of data request (or {\em
  interest}).

\smallskip\noindent
The rest of the paper is organized as follows: Section~\ref{sec:easm}
describes the EASM protocol architecture and its fault
tolerant capabilities; Section~\ref{sec:eval} describes
experimental results; Section~\ref{sec:relwork} compares our work with
related research; and Section~\ref{sec:conclusions} provides
concluding remarks. 

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