As Brewer points out, the notion of access as a killer app was largely a response to the prevailing trend in 1994 of handheld devices as communication islands. Each device would seek its own killer app as an information organizer.² In 1996, Kleinrock identified the disconnected state as the "usual" one rather than an "exceptional" one.³ Wireless islands are data networks that remain largely separate from telecommunication networks and handheld devices. They communicate with very few (if any) other devices (FIG. 1).

In 1998, Brewer and Stemm proposed a system in which individual mobile devices would no longer be isolated islands in the great wireless sea.^2,4 Instead, they would have the same access to the infrastructure as any other device on the network. In this communication system, handheld devices have gained access to the same network infrastructure as desktops and other devices (FIG. 2). If any two devices wish to communicate directly, however, they must do it via the infrastructure. This requirement is problematic in places where the infrastructure doesn't exist or one or more of the devices lacks the capacity to access the network directly. (Notice that in Figure 2, the network and telecommunication infrastructures are beginning to interact. They are permitting such uses as receiving e-mail via one's cell phone or carrying on a telephone conversation over the Internet.)

The WPAN environment allows devices to connect directly to one another without depending on a network infrastructure for peer-to-peer interactions. Note that this proposed environment doesn't preclude any given device from also making use of a network infrastructure when it has the capacity and need to do so (FIG. 3). In the future evolution of the wireless communication systems, handheld devices can communicate with each other as well as with the network and telecommunication infrastructures.

This systems view of wireless connectivity prompts many questions. For example, what usage models are best suited to WPANs? At the highest level, the usage model is largely unconcerned with technology per se. Instead, it describes the nature of the information that's being exchanged. It also details the manner in which this exchange takes place from a user's perspective. For example, a user may point a PDA at a printer and tap the print function. The data on the PDA is then printed. Perhaps another user turns on a pair of headphones and walks near a laptop on which an MP3 player is ready to launch. When the music starts playing, the sound is heard on the headphones. To understand the usage model in which they participate, the users don't need to understand the technical details associated with the data exchange.⁵

Specific usage models are usually best suited to particular technologies. Infrared, for example, tends to be ideal for dynamic point-to-point object exchange. It fits that role because of its natural spatial separation, limited range, and high bandwidth. In contrast, the longer range of RF makes it better suited to usage models that don't require high bandwidth, but must not be severely limited by either direction or distance.⁶ The headphone example is a good usage model for RF.

WPANs are particularly well suited to a number of usage models. One such model involves device-to-device data exchanges (FIG. 4). In this important class of usage models, data is exchanged in a point-to-point (or point-to-multipoint) fashion between two or more devices. This usage model ranges from simple object exchange to more elaborate synchronization and device browsing.

One of the simplest usage models for device-to-device data exchange is the object push. In this model, one device must be in a receiving state. The user of the other device issues an explicit command to push a particular object. This usage model is typical in printing and the exchange of data objects between PDAs.

A usage model that's related to object push is object pull. Here, one device makes specific requests of the other. In its simplest form, object pull may involve extracting a default object from a remote device. Imagine a meter reader driving up to a house and connecting wirelessly with the water meter. The water meter may have a wireless interface and a default object, which contains the information desired by the meter reader. The meter reader can send a short message that he or she desires the default object from the water meter. In response, the water meter can transmit that object back to the meter reader's handheld data-collection device.

In more elaborate situations, object pull may involve complex objects and structured interactions that transcend the notion of a default object. In order to navigate around a remote device, some kind of remote browsing capability needs to be implemented. In this situation, one device exposes a remote file system to the other device. Depending on the manner in which remote browsing is implemented, it may be made to function as a form of peer-to-peer networking. Here, the full file and folder structure of the remote device is exposed to another device as part of its view of the networked world.

To implement synchronization, these various capabilities can be employed with additional sophistication. In its simplest form, synchronization may involve pushing or pulling objects to establish a baseline backup of data objects. In a more sophisticated form, it could involve elaborate schemes that are designed to leave either of the devices reflecting information that was gleaned from the other.

Another type of WPAN usage model involves the transmission of real-time audio data. Real-time audio capabilities are common in many consumer-electronics devices (televisions, stereos, music players, etc.). Increasingly, these capabilities also are found in mobile computers like laptops or PDAs. Typically, RF is the preferred technology for these usage models. For the user, it allows a full range of motion within the spatial range of the device to which the user connects.

One common use for real-time audio is in wireless microphones. Usually, these microphones connect to a base station that was built specifically for the microphone. One of the objectives of WPANs is that these communication protocols become standardized around usage models. Devices will then be able to interoperate on a large scale with devices of a similar type-irrespective of manufacturer.

Wireless communication between speakers or headphones and sound systems is similar to this model. Wireless speakers would eliminate the need to run wires to remote locations as the consumer positions speakers around a house. The headphones provide mobility for the user as well as flexibility (FIG. 5). After all, the wireless headphones could be used to communicate with a number of devices ranging from the home stereo to the laptop and portable CD player.