A great number of usage models fall under the "wireless" umbrella. For some users, access to a network infrastructure is the killer application. For others, the "killer app" is the capability to communicate independently between devices within a wireless personal-area network (WPAN). Usage models for WPANs include device-to-device data exchange, real-time audio, device control, and network access.

The mobile devices that implement these WPAN usage models are constrained by a number of factors including power consumption, bandwidth, and security. Because of these factors, the devices favor independence over global network connectivity. Some technologies manage to meet these constraints while accommodating the aforementioned usage models. Among those technologies are Bluetooth and IrDA.

At its most fundamental level, wireless data communication consists of computing devices (large or small) that communicate in some fashion without the aid of wires. Little else can be stated that will apply consistently across all types of wireless data communications. The specific nature of the communication between devices depends upon the usage models for which these devices were engineered.

Over time, a number of usage models for wireless data have come into being. For the individuals who depend upon wireless-data technologies, those usage models have come to define "wireless." For example, some people think that "wireless" means cellular-telephone communications. To others, "wireless" means Internet connections for mobile devices-whether those devices are associated with a cellular phone or not.

To quote Imrich Chlamtac, "This myriad of new devices spans the technological and budgetary spectrums, making it harder to attach the definition of mobile computing to any single device or application. However, there is one underlying similarity among all these devices and applications that allows us to simply define a mobile computer as a computing device, which can communicate through a wireless channel."¹ Hence, no particular usage model can claim a monopoly on what is meant by "wireless."

Wireless data communications can be analyzed along several spectrums:

1. The size and capacity of the device
2. The nature of the transmission medium
3. The nature of the information that's being communicated

It's common for the nature of the transmission medium to be somewhat dependent on the device's capacity. It also will be affected by the nature of the data communication in which it engages.

For wireless data communication, one of the most compelling usage models is to extend the Internet's reach. Television and radio advertisements speak of sending messages from device to device and making Internet purchases with a handheld phone or PDA. These days, efforts to enable mobile computing abound. This usage model has captured a great deal of attention in the mobile-computing community. In fact, it led to the now-common mantra (originating from the BARWAN project at Berkeley): "Access is the killer app."² I agree strongly with this statement as long as it is qualified with the addendum, "…for certain usage models."

If we presume that access to an infrastructure is the killer app, such an assumption can slant our perspective. It also can lead us to ignore other critical usage models, which are of great value-even if they have little or nothing to do with extending the reach of the Internet or the mobile office. Indeed, after providing the definition of mobile computing that was quoted above, the authors allow the "access" usage model to dominate their view of the wireless landscape: "A fundamental requirement of today's computer is network connectivity. This implies, mostly, radio-frequency wireless communications for mobile devices." Thus, they unnecessarily limit "wireless" to both a usage model and a transmission medium. In doing so, they limit the discussion of other significant and powerful usage models. They also omit relevant wireless transmission media that should be valued in the dialogue concerning mobile computing.

The notion of wireless personal-area networks implies a number of possible usage models. Most of those models don't depend upon access to a communication infrastructure. For example, imagine a pair of headphones that wirelessly connects to a home stereo, portable CD player, laptop, or television with very little user intervention. Because of the limited range of these headphones, a great deal of simplicity is introduced in the device hardware and communication protocols. This simplicity permits the headphones to more accurately and effectively meet a user's needs without a great deal of configuration.

Now imagine that those headphones require an IP address. They also demand some means of authenticating and connecting to an audio device via a wireless network within the home. As a result, the complexity, cost, and power consumption of the headphones will rise. To allow the headphones to function, greater configuration and infrastructure will be required. In addition, the ability for headphones to connect to consumer devices will always depend upon a network infrastructure that can't be guaranteed to exist everywhere that the headphones and CD player might wander.

In this situation, access to the broader network infrastructure is not the killer application. Rather, the killer app is a point-to-point connection between the headphones and CD player. This connection doesn't depend upon any other network infrastructure. Similarly, handheld personal digital assistants (PDAs) like the Palm, Handspring Visor, and Pocket PC can exchange information via IrDA infrared connections in a dynamic, ad-hoc, point-to-point fashion. They don't need to depend upon any other enabling wireless infrastructure.

Whether or not the "beam-me-your-business-card" or "mobile-headphone" usage models are the end-all, be-all of wireless communications, they require solutions that are suited to the following: the capabilities of the devices, the type of data that they need to communicate, and the nature of that data exchange. One size need not fit all. The fact that one user finds value in beaming a business card in a dynamic, ad-hoc, point-to-point fashion over infrared should in no way be seen as a threat to other usage models. These other scenarios might find users communicating from a cell phone over several miles via radio to a cellular tower. Or they could be communicating from the tower to the wired telecommunications infrastructure. Both scenarios have a place in wireless communications. WPANs provide an umbrella for wireless usage models that don't require access to a network infrastructure.