Definition

Mobile computing devices have changed the way we look at computing. Laptops and personal digital assistants (PDAs) have unchained us from our desktop computers. A group of researchers at AT&T Laboratories Cambridge are preparing to put a new spin on mobile computing. In addition to taking the hardware with you, they are designing a ubiquitous networking system that allows your program applications to follow you wherever you go.

By using a small radio transmitter and a building full of special sensors, your desktop can be anywhere you are, not just at your workstation. At the press of a button, the computer closest to you in any room becomes your computer for as long as you need it. In addition to computers, the Cambridge researchers have designed the system to work for other devices, including phones and digital cameras. As we move closer to intelligent computers, they may begin to follow our every move.

The essence of mobile computing is that a user's applications are available, in a suitably adapted form, wherever that user goes. Within a richly equipped networked environment such as a modern office the user need not carry any equipment around; the user-interfaces of the applications themselves can follow the user as they move, using the equipment and networking resources available. We call these applications Follow-me applications.

Typically, a context-aware application needs to know the location of users and equipment, and the capabilities of the equipment and networking infrastructure. In this paper we describe a sensor-driven, or sentient, computing platform that collects environmental data, and presents that data in a form suitable for context-aware applications.

Context-Aware Application

A context-aware application is one which adapts its behaviour to a changing environment. Other examples of context-aware applications are 'construction-kit computers' which automatically build themselves by organizing a set of proximate components to act as a more complex device, and 'walk-through videophones' which automatically select streams from a range of cameras to maintain an image of a nomadic user. Typically, a context-aware application needs to know the location of users and equipment, and the capabilities of the equipment and networking infrastructure. In this paper we describe a sensor-driven, or sentient, computing platform that collects environmental data, and presents that data in a form suitable for context-aware applications.

The platform we describe has five main components:
1. A fine-grained location system, which is used to locate and identify objects.
2. A detailed data model, which describes the essential real world entities that are involved in mobile applications.
3. A persistent distributed object system, which presents the data model in a form accessible to applications.
4. Resource monitors, which run on networked equipment and communicate status information to a centralized repository.
5. A spatial monitoring service, which enables event-based location-aware applications.
Finally, we describe an example application to show how this platform may be used.

The ultrasonic location system is based on
the principle of trilateration position finding by measurement of
distances (the better-known principle of triangulation refers to
position finding by measurement of angles). A short pulse of ultrasound
is emitted from a transmitter (a Bat) attached to the object to be
located, and we measure the times-of-flight of the pulse to receivers
mounted at known points on the ceiling. The speed of sound in air is
known, so we can calculate the distances from the Bat to each receiver
- given three or more such distances, we have enough information to
determine the 3D position of the Bat (and hence that of the object on
which it is mounted).


By finding the relative positions of two or more Bats attached to
an object, we can calculate its orientation. Furthermore, we can deduce
some information about the direction in which a person is facing, even
if they carry only a single Bat, by analysis of the pattern of
receivers that detected ultrasonic signals from that transmitter, and
the strength of signal they detected.

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