Portable devices and wireless networks are becoming commonplace in our
daily lives. This has created a demand for the development of
location-based services and applications, i.e.\ Location-based Services.  
In parallel, several positioning technologies enabling such services have been
developed, where GPS is the most popular one.  The main drawback of this
and other technologies is that they require the deployment of a
specific infra-structure for the positioning, such as sensors and
satellites, which in turn raises their cost or restricts their
applicability.  Therefore, other positioning technologies, which are
based merely on the wireless infra-structure, have been explored.

In this work we present a service which infers the location of mobile
devices using the radio-frequency signal strength of Access Points of
IEEE 802.11 wireless networks.  We have also verified the
viability of its use by applications that require location
information in terms of symbolic regions, instead of coordinates, and
which tolerate inference errors of a few meters.  The service also
supports the aggregation of regions into larger symbolic regions using
the notion of a hierarchy of symbolic regions.