Sensor Networks’ Integration - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 93

theoretically be implemented as part of the middle tier; however, such functionality
is currently nonexistent. Although jWebDust claims to provide concurrent access
by multiple applications, mechanisms for resource arbitration are currently lacking.
JWebDust also requires all sensor nodes to implement the same firmware for correct
query processing and service discovery. While most TinyOS-capable platforms are
supported, it requires each sensor node in the network to be reprogrammed and
configured prior participation in the framework. jWebDust also lacks functions for
security, privacy and trust, and accounting and provides no explicit support for
controlling the access to different sensor network resources. The reliance on a
central relational database in the data tier can become a scalability bottleneck,
once the number of participating sensor networks and querying applications
grows.
IrisNet
IrisNet [ 4 ] is one of the first attempts to develop an architecture that is able to pro-
vide integrated access to globally distributed sensor networks over the Internet. The
IrisNet's goal is to reuse the infrastructure of deployed sensor networks by enabling
the sharing of generated sensor feeds among many applications (sensing services).
IrisNet provides sensing services with the view of a distributed database in which
data of different sensor networks can be collected and queried. IrisNet is realized
as a two-tiered architecture with organization agents ( OA ) and sensing agents ( SA )
as fundamental components. A developer of a sensing service provides a database
schema tailored for its application, which is implemented on a possibly distributed
set of OAs. The group of OAs maps to a single sensing service and must collect and
organize sensor data to answer a particular set of service-specific queries. OAs form
a distributed database in which data are hierarchically organized in self-describing
tags based on XML (to naturally reflect the hierarchal organization of existing
geographic and political boundaries). Queries on the database are expressed in
XPATH and select data from a node set in the hierarchy. Each of the distributed
OAs can store a subset (subtree) of the hierarchy in which each node either points
to data sources for sensor streams (represented by SAs) directly providing corre-
sponding service data or other OAs, besides pointing to other nodes, that implement
parts of a missing subtree. The IrisNet infrastructure enables the distribution of the
query to adequate OAs and ultimately to the SAs providing the required data, and
the composition of the final response across multiple OAs on the reverse path. OAs
register a global name and IP address with DNS, so queries across distributed OAs
can be dynamically resolved. Data of queries can be cached at their corresponding
OAs, and repeated requests directly served from cache to improve subject to fresh-
ness requirements of the sensing service. IrisNet also provides replications of OAs
and placement of OAs as additional mechanism to improve system reliability and
query performance. SAs provide a generic data acquisition interface toward sensors
and sensor networks and typically collect raw sensor readings as required. Besides
a database schema for the OAs, developers of sensing services write so-called