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\section{Related Work}
\label{sec:relwork}

Sensor networks have been investigated actively over the past few 
years. SPIN~\cite{Heinzelman:99} is a family of protocols used for efficiently
disseminating data over a sensor network. It uses data negotiation
and resource-adaptive algorithms to reduce energy consumption.
Nodes assign a high-level name ({\em meta-data}) to their data
and perform meta-data negotations before data communication to
ensure redundant data is not sent.

GHT~\cite{GHT:02} is a data-centric storage based dissemination scheme
which uses hash function to determine nodes where data are kept.
However, GHT requires data be pushed to these nodes such that
sink nodes can access it. It may thus introduce data transfers
even when sink actually is not querying.

In Directed diffusion (DD)~\cite{Estrin_DD:00}
a sink broadcasts {\em interest} (data request) message throughout
the network. When source receives an interest message it sends
data in a similar fashion. On receiving the data response (
possibly from multiple neighbors) one path is reinforced and is
used for further communication. Multiple paths may be used by 
a sink node for redundancy.

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. 

Similar to TTDD EASM employs a virtual grid, however EASM uses this
grid structure simply to find out the grid center location to be filled
in sink queries while TTDD uses it for localizing sink mobility
impact.

Ring-based index~\cite{RB:03} is a rendezvous based data dissemination
scheme.  A rendezvous point is used as a node to communicate between
sink and source nodes. 
An index ring is used for replication, where nodes are {\em forwarding 
nodes} which forward the data requests to next clockwise neighbor
in the ring and {\em index nodes} which know the location of and 
hence can forward the request to source nodes.
EASM is similar to ring-based index in that it also uses rendezvous
node to query for source node location. However, the way replication
is handled is fundamentally different. Instead of a ring, EASM uses
{\em n} depth replication to elect {\em forwarders} and {\em
interceptors}.  Furthermore, EASM handles local as well as global sink
mobility patterns while ring-based is suited for local sink movements.

Most of the already proposed schemes, which rely on broadcast
(of interest or response messages) may not work well in large
networks, with high number of sink-source pairs, especially when
sinks exhibit high degrees of mobility. Data requests and responses
are always sent to a location in the sensing space, thereby resulting
in significant energy savings. Node replication reduces hot-spot
message complexity and increases fault tolerance. Finally it is able
to handle high sink speeds with marginal increase in energy
consumption. 

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