Clearly, the remarkable penetration of a simple, always-on wireless-Internet connection enabled by the IEEE 802.11 wireless local-area-network (WLAN) standards is no longer just a trend. In-Stat currently estimates more than 15 million regular 802.11 WLAN users. Some big players like Microsoft, IBM, HP, Intel, and Toshiba are unveiling ambitious plans to mainstream the technology. A number of countries even have plans to deploy nationwide WLANs. In addition, thousands of "hot spots" now located in city centers, airports, hotels, and coffee shops are both operational and extremely popular among the computing public.

To keep the momentum going and prevent competing standards from gaining traction, the 802.11 WLAN standard must continue to deliver on its two primary benefits (see table). Specifically, it has to deliver high-speed wireless-Internet connectivity, while at the same time assuring seamless interoperability between equipment from all vendors.

The principal WLAN standards in use today are 802.11, HomeRF, and—some would argue—Bluetooth. In the future, Ultra-Wideband (UWB) could potentially be a contender as well. But first, it has to overcome regulatory restrictions and interoperability deficiencies. This will probably be a difficult task, though, due to the non-aligned interests of UWB's primary intellectual-property (IP) holders.

Presently, HomeRF appears to have limited applications. As such, it will probably continue its quiet descent into obscurity. As a technology that was created as a cable-replacement solution, Bluetooth will likely fulfill its original mandate without providing a real alternative in the WLAN space. Its limited data rate and range make it a highly unlikely WLAN candidate.

Sure, Bluetooth has achieved widespread publicity resulting in significant name recognition. Yet it still hasn't made noteworthy inroads in actual deployments or delivered on the grand expectations bestowed upon it. Bluetooth is based on frequency-hopping spread-spectrum (FHSS) technology. In one of its most common implementation examples, it serves as a wireless link between Bluetooth-enabled earpieces and cellular phones. On the downside, Bluetooth is limited by low data rates with a maximum rate of 720 kbps. A range that reaches roughly 10 m also hampers the technology.

At any point in time, the Bluetooth signal occupies only 1 MHz. It changes center frequency (or hops) deterministically at a rate of 1600 Hz. Bluetooth hops over 79 center frequencies. Over time, then, the Bluetooth signal actually occupies 79 MHz.

802.11a radios transmit at 5 GHz. They offer data rates as high as 54 Mbps using Orthogonal Frequency Division Multiplexing (OFDM). 802.11a defines one of several different 802.11 physical layers (PHYs). The actual name of 802.11a is the "High Speed Physical Layer in the 5-GHz Band," commonly referred to as the "OFDM PHY."

The most popular WLAN PHY is 802.11b, which has been widely implemented since its ratification in 1999. 802.11b operates in the 2.4-GHz frequency band at data rates up to 11 Mbps. It uses direct-sequence spread-spectrum (DSSS) modulation. The gap between the data rates and modulation schemes of 802.11a and 802.11b could be bridged in the digital baseband by integrating a second modem. The two technologies would still be incompatible, however. They operate in different frequency bands: 5 GHz for 802.11a and 2.4 GHz for 802.11b (see figure).

No matter which 802.11 PHY is deployed, the medium-access-control (MAC) Layer coordinates access to a shared radio channel. Unlike the PHY, the MAC Layer is actually a program that runs on a processor. In contrast, the PHY involves digital communications circuitry and an RF modulator to prepare data for transmission.

OFDM'S INNER WORKINGS
OFDM divides the data signal across 48 separate subcarriers to provide transmissions of 6, 9, 12, 18, 24, 36, 48, or 54 Mbps. Of these transmissions, 6, 12, and 24 Mbps are mandatory for all products. For each of the subcarriers, OFDM uses Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM) to modulate the digital signal. The choice depends on the selected data rate of transmission. In addition, four pilot subcarriers provide a reference to minimize frequency and phase shifts of the signal during transmission. This form of transmission enables OFDM to operate extremely efficiently, which leads to higher data rates and minimizes the effects of multipath propagation.

The Wireless Ethernet Compatibility Alliance (WECA) acts as a certification organization for 802.11 products. It makes sure that products from different vendors interoperate with one another. To date, WECA has a certification program for 802.11b only. It is uncertain whether WECA will provide a certification for 802.11a, as it's inherently incompatible with 802.11b.

The wireless LANs based on the IEEE 802.11 standard and the short-range radio system based on Bluetooth share the same 2.4-GHz ISM band. Bluetooth is designed for device-to-device connectivity on an ad-hoc basis, whereas the 802.11-compliant system targets a wireless extension to the wired-LAN infrastructure. Because it is likely that the two will operate simultaneously within the same areas, the potential exists for serious interference issues. Bluetooth devices hop over 79 MHz of the ISM band, whereas IEEE 802.11b devices require approximately 16 MHz of bandwidth to operate. As a result, it's not advisable to have both Wi-Fi and Bluetooth operate within the same area simultaneously.