% $Id$

% Amogh, what is EASM?
\section{Overview of EASM}
\label{sec:overview}

This section provides a brief overview of EASM. First we 
define the terms used during the discussions:
\noindent\paragraph*{\bf Source Node} 
A sensor node which contains data of a specific type.
\noindent\paragraph*{\bf Publisher}
A sensor node that is aware of the current location of the
source node.
\noindent\paragraph*{\bf Subscriber}
A sensor node that acts as a routing-point for all the data
coming from various publishers, to the sinks in that area.
\noindent\paragraph*{\bf Sinks/Sink Node}
A (mobile) sensor node that requests data after every fixed 
interval of time.

Next we list down the assumptions made during the design of EASM:
\begin{enumerate}
\item We assume that sensor nodes which take part in routing 
      the administrative/data packets are stationary. Thus
      source nodes, publishers, subscribers are stationary 
      while sinks may be mobile and make data requests on the 
      move.
\item Each node is aware of its own location, which can be 
      achieved through ilocalization techniques such as 
      GPS~\cite{GPS:01}.
\item The links are symmetric, thus a node B sending data to 
      node B is within the radio range of and may receive data
      from A.
\item For a given data type, the corresponding data is stored
      at a sensor node, which is the source node for the data
      requests for that data type.
\end{enumerate}

The scheme uses the GPSR routing system~\cite{GPSR:02}.
The data is sent to the sinks interested in data, only when such
sink has previously requested for the data. Data are only sent to
the sink nodes interested in it. Furthermore, the sink nodes use the
subscriber as a 'routing-point' for sending all its data requests.
Thus, each sink sends its request to a well-defined location (
subscriber which can handle that request) avoiding flooding the 
network.

We use hash functions to determine the subscriber and publisher
locations in this dissemination scheme. This is used to find the
end-point for routing a sink data request (to appropriate subscriber)
and find the publisher, which can respond to data requests of a specific
type. Obviously, a large amount of energy is saved since request-
response does not use broadcast.

The scheme provides for dynamically increasing the subscribers in a
region. This is essential for handling subscriber overloading which 
may be caused by the following: 

\begin{enumerate}
\item Sinks are essentially mobile nodes and hence may concentrate
      in a small region in the sensing space increasing sink per
      unit area. 
\item The subscriber might be used as an ordinary sensor node 
      participating in routing traffic for some other sink-source
      pair(s).

If the existing subscribers can not effectively route data requests
generated by sinks in that specific region, new subscribers are 
appointed.

