Wireless solutions will not only need to provide this kind of seamless connectivity but since they will operate in the unlicensed bands, they will have to be robust to prevent potential interference. Interference resistance is going to be a "must have" feature for all radios that operate in unlicensed bands. But the problem is not the band-it's building the right solution.

All three types of networks are differentiated by range, data rates, power consumption and cost. WPANs, such as Bluetooth piconets, provide short-range connectivity for peripherals such as keyboards, speakers and headsets, and support low data rates and limited ranges to achieve low cost and minimal power drain. WLANs, such as 802.11b wireless Ethernet, offer higher speeds and longer ranges in office buildings and homes. WWANs, such as cellular or PCS networks, work over a large area, but offer much lower data rates than WPANs and WLANs. Many common and emerging usage models require simultaneous connections to two or more of these networks, with many companies working on architectures that will enable computing and communication devices to automatically detect all connectivity opportunities, select the ones needed and seamlessly connect to them.

Given the high cost of licensed spectrum, most of these data networks will be implemented in unlicensed bands, hence creating potential for interference. In addition, other applications such as cordless phones will share these unlicensed bands, creating further potential for interference. However, this interference should come as no surprise. In fact, the FCC requires every device operating in these bands to have a label stating that the device can cause interference.

False alarm

So is there cause for alarm? Certainly not. The FCC has also put together a set of rules that allow multiple devices to share the spectrum. These rules allow room for considerable innovation in building radios that can resist interference.

Bluetooth and 802.11b serve significantly different connectivity needs, and consequently have very different operating characteristics. Because of this, they are more complementary technologies than competing technologies, and are likely to be co-located in many computing and communication devices. This presents a problem. When both technologies are operating at the same time, but are separated by more than 3 meters, they don't typically interfere with one another to a great degree. However, within 3 m, and especially within one-half a meter, they can degrade each other's performance significantly. This is particularly relevant to devices such as laptops or Internet appliances enabled with both technologies, as they will have far less than one-half a meter separating the two.

The requirement of simultaneous operation raises the question: "What usage scenarios will require both technologies to operate at precisely the same time?" While there is not a multitude of simultaneous operation scenarios in today's typical environment, the mere threat of debilitating interference in the future requires savvy corporate IT managers to put safeguards in place. However, several important technological developments will entrench simultaneous operation in our everyday lives. For example, it will become increasingly common for users to be connected to the network from their 802.11b-enabled PC in the presence of Bluetooth devices. This will very likely present serious interference problems. The interference is dramatic-similar to the experience of downloading e-mail over a 28.8-kbit/s phone-line connection, instead of a high-speed Ethernet LAN.

But unsatisfactory end-user experiences are not the only potential drawback to WLAN and Bluetooth interference. It is very likely that once a user encounters a problem, he will return the product, or call a vendor's customer support representative. With increasing margin pressure on wireless solutions, either of these scenarios results in an unprofitable sale for the vendor, and an unhappy customer. The above issues have driven the industry to take comprehensive steps to resolve interference problems before they affect end users, retailers and the overall market.

Various technical approaches can solve coexistence issues. One, simultaneous operation, is at the pinnacle of performance and technical achievement and provides the best user experience. Other techniques, explored below in more detail, represent steps to improving coexistence and provide important evidence of the many efforts to provide combination 802.11b/Bluetooth products to the market. However, each technique has strengths and limitations that must be considered.

For example, co-location without a coexistence mechanism may achieve rapid time-to-market, but provides a single-card reference design only. Consequently, it can result in significant degradation to both radios' performance. The extreme proximity of two radios with no coexistence mechanism will likely produce the worst-case scenarios. This will lead to a poor user experience and elevated vendor support costs.

Because dual-mode radio switching does not require changes to the silicon, it could be relatively quick to market. Its coexistence mechanism requires that while one radio is operational, activity in the other is completely suspended.

It is implemented primarily through two methods. In the first, the system simply shuts off the non-operating radio and does not signal its absence to other nodes in the network. That generates difficulties for the network and can drop performance levels below that of simple co-location without a coexistence mechanism.

The second approach does signal other network nodes that it is suspending one of its radios. Performance of this approach will still be 60 percent lower than that of unhindered radios, because of its modal nature (one on/one off), but is better than simply shutting the radios off. Neither of the methods supports switching while Bluetooth voice (SCO) links are in operation.

Driver-level transmit switching generally describes an approach in which application transmit requests are mediated at the driver level, thereby avoiding simultaneous transmission. Intuitively, the approach degrades throughput by some measure simply because of its modal transmit structure. More important, however, are its limitations in avoiding collisions with incoming packets. This is caused by the technique's dependence on the host operating system, which is generally non-deterministic in its response time (non-real-time). The resulting transmission of one protocol during reception of the other causes loss of received packets, interference, and potential user difficulties. The approach does not switch quickly enough to support Bluetooth SCO links, and it will have great difficulties mitigating the substantial interference from Bluetooth piconet master/slave polling activities.

While Bluetooth adaptive hopping certainly improves simultaneous performance under limited penetration scenarios, its widespread adoption will likely require intervention from regulatory organizations. Even under a fast-track program, that can be a time-consuming process. The problem is that 802.11b systems and unmodified Bluetooth devices will continue to roll rapidly to market, creating a situation where Bluetooth devices with adaptive hopping can't communicate with unmodified devices. Many Bluetooth devices, for one reason or another, might not include the functionality. Further, in the presence of more than one Bluetooth piconet or 802.11b network, it is uncertain whether adaptive hopping will produce positive or negative results.

MAC-level switching is the most effective of the modal/switching style approaches. It is a collaborative technique accomplished by exchanging information between the two protocols at the medium access control (MAC) level and managing transmit/receive operations accordingly.

Because MAC-level switching is performed in the baseband, it can switch between protocols at a much faster rate than driver-level approaches. Consequently, it is able to mitigate many of the problems that driver-level switching cannot.

Simultaneous operation is a system-level solution that employs technical enhancements to every aspect of the wireless subsystem, from antenna to application. It incorporates aspects of many techniques described above, as well as other advanced technology not addressed here. The solution maximizes each technique's effectiveness by leveraging system-level design practices.

Mobilian's engineers are developing this system-level solution, called TrueRadio.