Another interesting application that relates to wireless speakers involves communication between a cellular phone and an in-dash automobile speakerphone. Most cell-phone manufacturers sell hands-free phone kits that include a headset with both microphone and ear piece. This headset could certainly be made wireless, thereby introducing some improvement in the current usage model. In addition, the same technology that connects the phone to the headset could connect the phone to an in-dash microphone and speaker in an automobile. Thus, a user could utilize a cell phone in the car but dial through a steering-wheel-based touchpad. He or she could then communicate via an in-dash speakerphone.

Finally, real-time audio usage models may involve various forms of cordless telephony. Examples include a home intercom system, a cordless phone that connects to a wired base station, or a point-to-point capability for cell phones. For instance, Bluetooth includes a usage model that's called the three-in-one phone. It includes three usage models for a single phone. The first usage model is traditional cellular capability. For local calls, the cell phone can function as a cordless handset and connect to the wired base station (thus saving cellular connection minutes). Lastly, two phones can make a direct, point-to-point connection. They would then function as a sort of mobile ad-hoc intercom system.

A number of usage models also involve the control of peripheral devices. The most common version of this model is the television remote control. Typically, the remote control utilizes unidirectional diffuse infrared as its transmission medium. Because these devices don't require communication in both directions, diffuse infrared may provide an excellent solution for them. It can flood an immediate area with a signal while not interfering with devices outside the room. For peripheral devices that require communication at distances beyond 1 m and in both directions, however, RF solutions may be better suited. For a peripheral device, the longer distance and omni-directional nature of RF allows greater freedom of orientation.

Any peripheral device that's traditionally connected via a wire is a candidate for a wireless solution (whether RF or IR). Keyboards, mice, and other peripherals offer great value when they're more mobile. Laptop docking stations can be similarly liberated from the need to fit the external bus connection from the laptop into the docking station. Any arbitrary consumer-electronics device can be controlled wirelessly. Although this control could extend to toasters, alarm clocks, microwave ovens, and home-security systems, it may be difficult to see the exact value in controlling these things wirelessly.

Although access to wired infrastructures is not a prerequisite for WPANs, it isn't forbidden or prohibited either. For many devices, a connection to the Internet is a very valuable usage model. The access-point usage model will then provide a mechanism for connecting wirelessly to a device that functions as a bridge to the rest of the world. Cellular towers for cellular communications play this precise role. The towers communicate wirelessly with cellular phones while providing wired access to the telecommunications infrastructure. Any wired infrastructure can be reached via such an access point. For example, a network access point can provide connectivity to a local-area network. Presumably, it would provide Internet access as well.

A WPAN technology also may act as a bridge between a mobile device and another infrastructure. For example, a cell phone that's equipped with either RF or IR capability could establish a connection with a similarly equipped laptop. It would permit the phone to function as a modem for the laptop. In this case, the connection between the laptop and the phone is wireless-but not the sort of wireless that is used for the cellular connection.

In every situation in which data can be transmitted via a wire, that wire can be removed and replaced with a wireless solution. Some of these wires, such as phone wires, can be replaced with cellular technology. Other wires-like network cables-can be replaced with wireless-Ethernet technology. Many other wires go to speakers, headsets, PDA cradles, mice, keyboards, and a thousand other devices. Those wires also need to be eliminated.

When it comes to wireless solutions, one size need not fit all. Each wired technology fills different needs. Hence, the wireless replacements will differ in many respects. Cellular and wireless-LAN technologies do fill a great need. For many usage models, however, those two wireless technologies are not necessary-nor sometimes viable-solutions. These short-range, wire-replacement technologies are part of the broad set of WPANs.

Now that the various WPAN usage models are clear, it's time to examine the factors that favor independence connectivity over traditional network infrastructures. Many discussions of mobility make a tacit assumption about a largely homogeneous transport. In some cases, discussions involve heterogeneous transports. But they still assume a consistent usage model (namely mobile Internet access). They also assume a consistent underlying transport protocol (IP).

In "Vertical Handoffs in Wireless Overlay Networks," the topologies of the overlay networks are presumed to be different.⁴ But the devices that are participating in these vertical handoffs are assumed to be mobile IP nodes in a networked environment. For that environment, access to the infrastructure is a key function. The type of wireless transport may vary, but each device must be capable of supporting the various protocol layers that are required to implement TCP/IP. In addition, each device must be capable of communicating at varying distances via different physical media.

In reality, consumer devices tend to be built with capabilities that are sufficient to implement the usage models that are required by users in a particular target market. Makers of handheld devices tend to be concerned with three factors: size, cost, and power consumption. Additional computing power generally requires more expensive processors, a larger product footprint, and greater power consumption. Hence, it's not common for manufacturers to overbuild handheld devices. In other words, the notion of every handheld device being an IP node on the network is simply not tenable. But devices that are part of WPANs are only constrained to provide sufficient capability to implement their specific usage models.

When it comes to mobile devices, some issues are usually constrained. One such issue is power consumption. Two basic aspects tend to influence the power consumption of mobile wireless devices. The first one involves the capability of the processor. In general, the power consumption is greater if the processor is faster and more powerful. Because users tend to be dissatisfied with devices that run out of power quickly, manufacturers are extremely concerned with battery life.