% $Id: overview.tex,v 1.4 2005/03/01 08:06:33 gokhale Exp $

\section{Overview of Data Dissemination Protocols}
\label{sec:overview}

This section provides a brief overview of the two data dissemination
protocols we study in this paper.

\noindent\paragraph*{\bf Directed Diffusion (DD)} 
Directed diffusion (DD)~\cite{Estrin_DD:00} was the first protocol to
address the problem of sink mobility in the context of sensor networks.
In DD each participating node maintains a dynamic map of the sensing
tasks and the previous hop from which the {\em interest} was 
received. Each sensing task has an associated information, such as the
node(s) to which to forward data reports, the rate of sending this
data and the duration. As soon as the source nodes detect sensing
% Amogh, what does ``detect sensing task'' mean?
%Andy, in DD source nodes try to see if someone is sending 'interest'
% which is defined as a 'sensing task' or a request from the sink
% node to track a specific task. The terminology is used in DD
% paper.
% OK (keeping the comment)
tasks, they start sending exploratory packets to the sink nodes (which
generate these tasks or interests). The sink node chooses one
particular node (in its neighborhood) from which to receive the data
and thus for a given source node, a single path is reinforced, which
is used for all the traffic towards the sink.

\noindent\paragraph*{\bf Two-tier Data Dissemination (TTDD)}
Two-tier Data Dissemination (TTDD)~\cite{Zhang_TTDD:02} uses a
two-tier architecture to disseminate the data to interested nodes. A
grid is formed with the source node at one of the crossing points of
the grid. Thus, the sensing space is divided by the grid into small
cells. Only the nodes at the crossing 
points (called {\em dissemination nodes}) can forward the data. A sink
node, which is interested in receiving a specific data, floods its
local cell in order to discover the closest dissemination node. Each
dissemination node contains forwarding information for each request of
data it receives. On discovery of a dissemination point, the data request is
forwarded to source node by the dissemination nodes, followed by data
report being sent to the sink. Here, the smaller cell is the first
tier, while the higher tier is made of the dissemination nodes at the
grid points, which makes the two tier architecture. 

