Wireless Systems Design

What You Need To Know About WLAN

High-Speed Wireless Internet And Seamless Interoperability Drive 802.11 Deployment Beyond The Fad Stage.

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 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.

Another challenge for the 802.11 community is the number and variety of 802.11 task group extensions. To name a few, they are: 802.11a, 802.11b, 802.11g, 802.11d, 802.11h, 802.11e, 802.11i, and 802.1x. Such a variety often presents a confusing and contradictory picture to the public, potential investors, and even the hard-core wireless engineer. Here's a quick look at what you need to know about each extension:

802.11c - Access Point Bridging
The 802.11c working group defines the protocols/procedures necessary to bridge 802.11 access points across networks within relatively short distances.

802.11d - Internationalization
This group addresses issues and/or procedures to make 802.11 compliant with spectrum-use regulations around the world.

802.11e - QoS Extensions
The 802.11e group is defining a series of QoS extensions to allow 802.11 networking to handle up-video streaming. It still falls far short of broadcast-quality video, but it's much further along than pre-802.11e QoS support. 802.11e ratification is anticipated in the second half of 2003.

802.11f - Inter-Vendor Access Point Handoffs
This group is cultivating a set of standards that will enable handoffs to be done in such a way as to work across access points from a number of vendors.

802.11h - Power Control for 5-GHz Region
The 802.11h working group is looking into the tradeoffs involved in creating reduced-power transmission modes for networking in the 5-GHz space. This would potentially allow 802.11a to be used by handheld computers and other devices with the limited battery power available to them.

802.11i - Enhanced Security
This group is tasked with improving PHY-level security.

The increasingly high level of "siliconization" is promising to forever change the economic and technical landscape for 802.11. Discrete parts and boards are giving way to higher and higher levels of on-chip integration. Think back to when expensive and cumbersome Ethernet cards gave way to single-chip solutions on the motherboard. 802.11a will bring about a similar revolution, once it has time and the attention of the firms focusing on this area. Just as 10/100 Ethernet quickly dominated the Ethernet scene, so will combination 802.11a/802.11b solutions.

Three companies—Atheros, Broad-com, and LinCom Wireless—represent the silicon players in this space. While each company has market and technology advantages, challenges have so far prevented one from becoming the volume leader.

Atheros was first to announce an 802.11a+b combo chip set. Its solution uses two separate RFICs. One RFIC is a converter that upbands 2.4-GHz operation to 5 GHz. Broadcom has a three-chip 802.11a+b offering on the way. To date, neither Atheros nor Broadcom have developed an integrated 802.11a/b RFIC, though it's an important step in reducing the cost and power consumption of a combined 802.11a/b solution.

LinCom Wireless comes close to offering an integrated combination 802.11a/b device. The company's ComboLink 1 is a two-chip offering that includes an integrated dual-mode baseband/MAC and a dual-band, dual-mode RFIC. Its architecture auto-senses and auto-switches between 802.11a/
802.11b operation, achieving optimal performance based on user-preference settings. It can therefore bridge the gap between 802.11a and 802.11b, while avoiding the power and integration tradeoffs facing users of competing technologies.

As 802.11 products continue to evolve, they will apply to an increasing number of applications. Take, for example, the wireless distribution of broadcast-quality video. WLAN is clearly a young industry with a very bright future.

TAGS: Intel
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