A second critical issue for power consumption in wireless devices involves the chosen wireless topology. As a general rule, RF consumes more power than infrared. If the distance is longer for either technology, the power consumption will be greater. As a consequence, mobile devices tend to be fitted with a wireless technology that best suits the desired usage model. But they're seldom equipped with more capability than they absolutely need. Table 1 shows the power consumption for common WPAN technologies at their standard operating distances.^7,8,9 Note that while IEEE 802.11b is a wireless-LAN technology, it is included in this table for comparison with the WPAN technologies.

Another key constraint for mobile-device designers is bandwidth. Different usage models have differing bandwidth requirements. For example, real-time audio can be reasonably achieved with relatively low bandwidth. So for effective real-time audio, raw throughput may actually be counterproductive. Smooth responsiveness is the feature that's most needed for real-time audio. This responsiveness may be achieved with reasonably low transmission rates. In contrast, moving a photograph wirelessly from a megapixel digital camera requires a fairly generous bandwidth.

Generally speaking, handheld devices tend to provide relatively low bandwidth when compared to larger computing platforms. As an example, Bluetooth provides a capability of 1-Mbps raw data. This data rate can be maintained simultaneously by multiple piconets, depending on the number of collisions that occur during frequency hopping. But in an environment that's saturated with RF traffic in the 2.4-GHz band, each Bluetooth device may have available bandwidth that is considerably less than this amount.¹⁰

In contrast, IrDA infrared devices can achieve fairly high bandwidth (up to 16 Mbps) in a very small footprint with relatively low power consumption. Even slower serial-infrared (SIR) transceivers accommodate dedicated links between devices. They achieve speeds of up to 115.2 kbps using off-the-shelf serial UARTs.¹¹

In addition to power and bandwidth, cost remains one of the most critical constraints for mobile wireless devices. WPAN devices tend to be more cost constrained than other technologies. For instance, an IrDA infrared transceiver may cost around $1 per unit. Bluetooth transceivers are more expensive (currently around $20 with hopes of falling to $5 in the next few years).¹² Wireless-Ethernet transceivers are even more expensive. For example, the cost of an IEEE 802.11b-compliant unit ranges from $50 to $100.¹³

In today's market, secure systems are becoming mandatory. As a general rule, the less connected a device is, the more secure it will be. Obviously, security must be balanced against access. If data is in a sealed box and buried in the ground, it's pretty secure. But it's also of little value. For WPANs, however, limited range and specific usage models certainly provide greater security. If two devices are interacting via a 1-m infrared link, a security breach would have to occur within arm's reach of the individuals who are sharing information. Of course, a breach also could occur if the reflected light is detected and the surrounding ambient noise is filtered out.⁶

Due to the physical characteristics of the transmission medium, short-range RF solutions aren't as secure as infrared. But they still provide a relatively limited range and hence a limited opportunity to breach security. Due to the insecurity that's inherent in the Bluetooth physical layer, Bluetooth implements link-level security like authentication and encryption. Some critics feel that Bluetooth's security measures aren't sufficient. In particular, they feel that link and encryption keys can be stolen by exhaustively searching through PINs or mounting a middle-person attack.¹⁴

In summary, WPANs tend to be cheaper, smaller, simpler, more secure, and less power hungry than the technologies that provide infrastructure access. They also tend to have lower bandwidth and shorter range. The key driver in WPAN design is the suitability of a protocol to a particular purpose (or usage model). In addition, users benefit from the ease with which these local wireless devices can dynamically configure to meet their needs. With WPANs, it's essential to remember that the focus isn't a single technology, but rather a set of technologies. Each one is adapted to the particular usage models that are required for a variety of devices.

While the future will undoubtedly bring advances in WPAN technology, two solutions are currently available for consumer products: Bluetooth and IrDA. These two technologies provide a study in contrast. Bluetooth is an RF technology that operates in the 2.4-GHz ISM band. It provides the ability for up to eight devices to form a dynamic piconet. To avoid interference from other radio sources, Bluetooth employs rapid frequency hopping between 79 channels. Such interferers include microwave ovens, 802.11, HomeRF, and other Bluetooth devices. Currently, Bluetooth provides a range of 10 m at raw speeds of up to 1 Mbps. In the future, it promises to increase both that distance and the bandwidth. Presently, Bluetooth's greatest strength may be its grassroots support (with approximately 2500 SIG members). It also has a highly effective marketing machine.

As a WPAN technology, Bluetooth shines when users need mobility within a somewhat limited space (10-m sphere). It also flourishes when relatively low data bandwidth is acceptable for the usage model. Bluetooth is largely targeting consumer electronics. It includes usage models for mobile headsets, remote printing, cell-phone-to-laptop connectivity, and cell-phone-to-cell-phone connectivity.

In contrast, IrDA is a standard for data communications from the Infrared Data Association. Using pulses of infrared light, it communicates at speeds as high as 16 Mbps over distances of up to 1 m. IrDA defines a cone of light with a minimum 15° and maximum 30° half angle. Hence, it is a "point-and-shoot" technology.¹¹

IrDA is ideal for situations in which dynamic ad-hoc connections need to be made quickly. It also shines when high data rates are needed or privacy is a necessity. In all cases, the usage models that involve IrDA must be able to work well under the constraint of a very short distance. Effective usage models for IrDA include wireless point-of-sale (POS) terminals, point-and-shoot object push (including the exchange of objects between PDAs and laptops), and the movement of large data (such as the transmission of pictures from megapixel digital cameras).