Today's Wireless Broadband: 802.11
By Holly Bartlett, Thu Apr 19 00:00:00 GMT 2001

As connectivity shifts from wired to wireless, one short-range broadband industry standard leads the pack: IEEE 802.11. How does it work, and what?s the future hold for the standard?

Originally conceived by 3Com, Lucent Technologies, Apple Computer and Cisco in 1997, the 802.11 standard has a virtual lock on the short-range wireless data industry today. Boasting easy integration with normal Ethernet networks, and security features like encryption, it has quickly become the clear choice for wireless corporate networking.

The consumer markets were until recently somewhat of a different story. A significant challenge to 802.11bís dominance came from HomeRF, a competing high-speed, short-range standard backed by Intel, Proxim, Motorola, and Compaq. HomeRF enables telephony and cost less, but its list of compatible products is comparatively small.

Now, with Intel having jumped ship in favor of 802.11b, HomeRFís fate as a niche player is all but sealed.

Let me explain how 802.11 works its magic.

Through the ether

802.11b data is encoded using DSSS (direct-sequence spread-spectrum) technology. DSSS works by taking a raw data stream of zeros and ones and modulating it with a second pattern, called the chipping sequence. In 802.11b, that sequence is known as the Barker code. The Barker code is an 11-bit sequence (10110111000) that has certain mathematical properties particularly good for modulating radio waves.

The basic data stream is modulated with the Barker code to generate a series of data objects called chips. Each bit is "encoded" by the 11-bit Barker code, and each group of 11 chips encodes one bit of data.

The wireless radio generates a 2.4 to 2.483 GHz carrier wave and modulates that wave using a variety of techniques. For 1Mbps transmission speeds, BPSK (Binary Phase Shift Keying) is used (one phase shift for each bit). To accomplish 2Mbps transmission speeds, QPSK (Quadrature Phase Shift Keying) is used. QPSK uses four rotations (0, 90, 180 and 270 degrees) to encode 2 bits of information in the same space as BPSK encodes 1.

The rule in any radio transmission is that you must increase power or decrease range to improve signal quality. Because the United Statesí FCC restricts the output power of portable radios to 1 watt EIRP (equivalent isotropically radiated power), range is the only remaining factor that can change. Thus, on 802.11b devices, as you move away from the radio, the radio adapts and uses a less complex (and slower) encoding mechanism to send data.

Getting faster

In 1998, Lucent Technologies proposed a standard to the IEEE called Complementary Code Keying. To achieve a much faster 11Mbps, vendors had to change the way they went about encoding the data. Rather than using the Barker code, they used a series of codes called Complementary Sequences. Because there are 64 unique code words that can be used to encode the signal, up to 6 bits can be represented by any one particular code word (instead of the 1 bit represented by a Barker symbol).

The CCK code word is then modulated with the QPSK technology used in 2Mbps wireless DSSS radios. This allows for an additional 2 bits of information to be encoded in each symbol. Eight chips are sent for each 6 bits, but each symbol encodes 8 bits because of the QPSK modulation.

The spectrum math for 1Mbps transmission works out as 11 megachips per second times 2 MHz (the null-to-null bandwidth of a BPSK signal) equals 22 MHz of spectrum. Likewise, at 2Mbps, you are modulating 2 bits per symbol with QPSK, 11 megachips per second, and thus have 22 MHz of spectrum. To send 11Mbps, you'd send 11 million bits per second times 8 chips/8 bits, which equals 11 megachips per second times 2 MHz for QPSK-encoding, yielding 22 MHz of frequency spectrum.

Itís much more difficult to discern which of the 64 code words is coming across the airwaves, because of the complex encoding. Also, the radio receiver design is significantly more difficult. In fact, while a 1Mbps or 2Mbps radio has one correlator (the device responsible for lining up the various signals bouncing around and turning them into a bitstream), the 11Mbps radios must have 64 such devices.

Security issues

Like any technology that becomes widely adopted, 802.11b has been put under the microscope. And as far as security is concerned, it leaves a lot to be desired. Earlier this year, researchers with the Isaac project at UC Berkeley discovered quite a number of problems with 802.11bís WEP encryption.

Not the least of which is a relatively wimpy 40-bit encryption scheme. Upon reviewing this work and the design of 802.11's security, respected Bell Labs security researcher Steven Bellovin was quoted in the Wall Street Journal on February 5th as saying that there were some "real howlers" in the design.

WECA, the Wireless Ethernet Compatibility Alliance that works in conjunction with the IEEE, promptly issued a formal response after the Berkeley researchers announced their findings. Unfortunately, their response did little more than an acknowledge the issue, and downplayed its significance. Their response spent more time focusing on semantic quibbles and how hard it is to perform the attacks than admitting there were fundamental flaws in the protocol in the first place.

On top of all this, a group of researchers at the University of Maryland published a paper of their own outlining even more vulnerabilities in 802.11. Both the quality and quantity of examination of 802.11's security leaves little doubt about its significant shortcomings.

Addressing the issue will be a revision to the 802.11b protocol to be released later this year. The revision adds improved authentication and access control to Ethernet networks, and will significantly reduce the vulnerability of WEP to attackers trying to compromise network data.

But even with the revision in place, WEP remains somewhat vulnerable. To address that issue specifically, yet another revision is being worked on, which, among other things, is slated to add 128-bit AES encryption to fix the 802.11's encryption woes.

The future - still getting faster

The 802.11b standard was designed to operate in the 2.4-GHz ISM (Industrial, Scientific and Medical) band. The 802.11a standard, on the other hand, was designed to operate in the more recently allocated 5-GHz UNII (Unlicensed National Information Infrastructure) band. And unlike 802.11b, the 802.11a standard departs from the traditional spread-spectrum technology, instead using a frequency division multiplexing scheme that's intended to be friendlier to office environments.

The 802.11a standard gains some of its performance from the higher frequencies at which it operates. As mentioned previously, the laws of information theory tie frequency, radiated power and distance together in an inverse relationship. Thus, moving up to the 5-GHz spectrum from 2.4 GHz will lead to shorter distances, given the same radiated power and encoding scheme. In addition, the encoding mechanism used to convert data into analog radio waves can encode one or more bits per radio cycle (hertz). By rotating and manipulating the radio signal, vendors can encode more information in the same time slice.

To ensure that the remote host can decode these more complex radio signals, you must use more power at the source to compensate for signal distortion and fade. The 802.11a technology overcomes some of the distance loss by increasing the EIRP to the maximum 50mW.

Although segmented, the total bandwidth available for IEEE 802.11a applications is almost four times that of the ISM band; the ISM band offers only 83 MHz of spectrum in the 2.4 GHz range, while the newly allocated UNII band offers 300 MHz. The 802.11b spectrum is plagued by saturation from wireless phones, microwave ovens and other emerging wireless technologies, such as Bluetooth.

In contrast, 802.11a has an ace up its sleeve: Its spectrum is relatively free of interference, at least for now. Only time will tell whether the 5-GHz band will become just as crowded as the 2.4-GHz band.

Holly Bartlett is a Seattle-based freelance writer, and covers technology for TheFeature.