The Radio Link

Three physical layers were standardized in the initial revision of 802.11, which was published in 1997:

• Frequency-hopping (FH) spread-spectrum radio PHY
• Direct-sequence (DS) spread-spectrum radio PHY
• Infrared light (IR) PHY

Later, three further physical layers based on radio technology were developed:

• 802.11a: Orthogonal Frequency Division Multiplexing (OFDM) PHY
• 802.11b: High-Rate Direct Sequence (HR/DS or HR/DSSS) PHY
• 802.11g: Extended Rate PHY (ERP)
• The future 802.11n, which is colloquially called the MIMO PHY or the High-Throughput PHY

This book discusses the physical layers based on radio waves in detail; it does not discuss the infrared physical layer, which to my knowledge has never been implemented in a commercial product.

The IR PHY

It is often said that microwave ovens operate at 2.45 GHz because it corresponds to a particular excitation mode of water molecules. This is sometimes even offered as a reason why 802. 11 cannot be used over long distances. If atmospheric water vapor would severely attenuate any microwave signals in rain or in humid climates, then 802.11 is not suitable for use over long distances.

802.11 also includes a specification for a physical layer based on infrared (IR) light. Using infrared light instead of radio waves seems to have several advantages. IR ports are less expensive than radio transceiversin fact, the cost is low enough that IR ports are standard on practically every laptop.

IR is extremely tolerant of radio frequency (RF) interference because radio waves operate at a totally different frequency. This leads to a second advantage: IR is unregulated. Product developers do not need to investigate and comply with directives from several regulatory organizations throughout the world.

Security concerns regarding 802.11 are largely based on the threat of unauthorized users connecting to a network. Light can be confined to a conference room or office by simply closing the door. IR-based LANs can offer some of the advantages of flexibility and mobility but with fewer security concerns. This comes at a price. IR LANs rely on scattering light off the ceiling, so range is much shorter.

This discussion is academic, however. No products have been created based on the IR PHY. The infrared ports on laptops comply with a set of standards developed by the Infrared Data Association (IrDA), not 802.11. Even if products were created around the IR PHY, the big drivers to adopt 802.11 are flexibility and mobility, which are better achieved by radio's longer range and ability to penetrate opaque objects.

 

Licensing and Regulation

The classic approach to radio communications is to confine an information-carrying signal to a narrow frequency band and pump as much power as possible (or legally allowed) into the signal. Noise is simply the naturally present distortion in the frequency band. Transmitting a signal in the face of noise relies on brute forceyou simply ensure that the power of the transmitted signal is much greater than the noise.

In the classic transmission model, avoiding interference is a matter of law, not physics. With high power output in narrow bands, a legal authority must impose rules on how the RF spectrum is used. In the United States, the Federal Communications Commission (FCC) is responsible for regulating the use of the RF spectrum. Many FCC rules are adopted by other countries throughout the Americas. European allocation is performed by the European Radiocommunications Office (ERO) and the European Telecommunications Standards Institute (ETSI). In Japan, the Ministry of Internal Communications (MIC) regulates radio usage. Worldwide "harmonization" work is often done under the auspices of the International Telecommunications Union (ITU). Many national regulators will adopt ITU recommendations.

For the most part, an institution must have a license to transmit at a given frequency. Licenses can restrict the frequencies and transmission power used, as well as the area over which radio signals can be transmitted. For example, radio broadcast stations must have a license from the FCC. Likewise, mobile telephone networks must obtain licenses to use the radio spectrum in a given market. Licensing guarantees the exclusive use of a particular set of frequencies. When licensed signals are interfered with, the license holder can demand that a regulatory authority step in and resolve the problem, usually by shutting down the source of interference. Intentional interference is equivalent to trespassing, and may be subject to criminal or civil penalties.

Frequency allocation and unlicensed frequency bands

Radio spectrum is allocated in bands dedicated to a particular purpose. A band defines the frequencies that a particular application may use. It often includes guard bands, which are unused portions of the overall allocation that prevent extraneous leakage from the licensed transmission from affecting another allocated band.

Several bands have been reserved for unlicensed use. For example, microwave ovens operate at 2.45 GHz, but there is little sense in requiring homeowners to obtain permission from the FCC to operate microwave ovens in the home. To allow consumer markets to develop around devices built for home use, the FCC (and its counterparts in other countries) designated certain bands for the use of "industrial, scientific, and medical" equipment. These frequency bands are commonly referred to as the ISM bands. The 2.4-GHz band is available worldwide for unlicensed use.[*] Unlicensed use, however, is not the same as unlicensed sale. Building, manufacturing, and designing 802.11 equipment does require a license; every 802.11 card legally sold in the U.S. carries an FCC identification number. The licensing process requires the manufacturer to file a fair amount of information with the FCC. Much this information is a matter of public record and can be looked up online by using the FCC identification number.

[*] The 2.4-GHz ISM band is reserved by the FCC rules (Title 47 of the Code of Federal Regulations), part 15.247. ETSI reserved the same spectrum in ETSI Technical Specifications (ETS) 300-328.

The Nonexistent Microwave Absorption Peak of Water

Spread spectrum was patented in the early 1940s by Austrian-born actress Hedy Lamarr. She was certainly better known for other reasons: appearing in the first nude scene on film in the Czech film Ecstasy, her later billing as "the most beautiful woman in the world" by Hollywood magnate Louis Mayer, and as the model for Catwoman in the Batman comics.

It is often said that microwave ovens operate at 2.45 GHz because it corresponds to a particular excitation mode of water molecules. This is sometimes even offered as a reason why 802. 11 cannot be used over long distances. If atmospheric water vapor would severely attenuate any microwave signals in rain or in humid climates, then 802.11 is not suitable for use over long distances.

The existence of a water excitation mode in the microwave range is a myth. If there was an excitation mode, water would absorb a significant amount of the microwave energy. And if that energy was absorbed effectively by water, microwave ovens would be unable to heat anything other than the water near the surface of food, which would absorb all the energy, leaving the center cold and raw. An absorption peak would also mean that atmospheric water vapor would disrupt satellite communications, which is not an observed phenomenon. NASA Reference Publication 1108(02), Propagation Effects on Satellite Systems at Frequencies Below 10 GHz, discusses the expected signal loss due to atmospheric effects, and the loss is much more pronounced at frequencies above 10 GHz. The absorption peak for water, for example, is at 22.2 GHz.

Microwave ovens do not work by moving water molecules into an excited state. Instead, they exploit the unusually strong dipole moment of water. Although electrically neutral, the dipole moment allows a water molecule to behave as if it were composed of small positive and negative charges at either end of a rod. In the cavity of a microwave oven, the changing electric and magnetic fields twist the water molecules back and forth. Twisting excites the water molecules by adding kinetic energy to the entire molecule but does not change the excitation state of the molecule or any of its components.

Use of equipment in the ISM bands is generally license-free, provided that devices operating in them do not emit significant amounts of radiation. Microwave ovens are high-powered devices, but they have extensive shielding to restrict radio emissions. Unlicensed bands have seen a great deal of activity in the past three years as new communications technologies have been developed to exploit the unlicensed band. Users can deploy new devices that operate in the ISM bands without going through any licensing procedure, and manufacturers do not need to be familiar with the licensing procedures and requirements. At the time this book was written, a number of new communications systems were being developed for the 2.4-GHz ISM band:

• The variants of 802.11 that operate in the band (the frequency-hopping layer, both direct sequence layers, and the OFDM layer)
• Bluetooth, a short-range wireless communications protocol developed by an industry consortium led by Ericsson
• Spread-spectrum cordless phones introduced by several cordless phone manufacturers
• X10, a protocol used in home automation equipment that can use the ISM band for video transmission

Unfortunately, "unlicensed" does not necessarily mean "plays well with others." All that unlicensed devices must do is obey limitations on transmitted power. No regulations specify coding or modulation, so it is not difficult for different vendors to use the spectrum in incompatible ways. As a user, the only way to resolve this problem is to stop using one of the devices; because the devices are unlicensed, regulatory authorities will not step in.

Other unlicensed bands

Additional spectrum is available in the 5 GHz range. The United States was the first country to allow unlicensed device use in the 5 GHz range, though both Japan and Europe followed.[*] There is a large swath of spectrum available in various countries around the world:

[*] Europe is obviously not a single country, but there is a European-wide spectrum regulator.

• 4.92-4.98 GHz (Japan)
• 5.04-5.08 GHz (Japan)
• 5.15-5.25 GHz (United States, Japan)
• 5.25-5.35 GHz (United States)
• 5.47-5.725 GHz (United States, Europe)
• 5.725-5.825 GHz (United States)

Devices operating in 5 GHz range must obey limitations on channel width and radiated power, but no further constraints are imposed. Japanese regulations specify narrower channels than either the U.S. or Europe.

Spread Spectrum

Spread-spectrum technology is the foundation used to reclaim the ISM bands for data use. Traditional radio communications focus on cramming as much signal as possible into as narrow a band as possible. Spread spectrum works by using mathematical functions to diffuse signal power over a large range of frequencies. When the receiver performs the inverse operation, the smeared-out signal is reconstituted as a narrow-band signal, and, more importantly, any narrow-band noise is smeared out so the signal shines through clearly.

Use of spread-spectrum technologies is a requirement for unlicensed devices. In some cases, it is a requirement imposed by the regulatory authorities; in other cases, it is the only practical way to meet regulatory requirements. As an example, the FCC requires that devices in the ISM band use spread-spectrum transmission and impose acceptable ranges on several parameters.

Spreading the transmission over a wide band makes transmissions look like noise to a traditional narrowband receiver. Some vendors of spread-spectrum devices claim that the spreading adds security because narrowband receivers cannot be used to pick up the full signal. Any standardized spread-spectrum receiver can easily be used, though, so additional security measures are mandatory in nearly all environments.

This does not mean that spread spectrum is a "magic bullet" that eliminates interference problems. Spread-spectrum devices can interfere with other communications systems, as well as with each other; and traditional narrow-spectrum RF devices can interfere with spread spectrum. Although spread spectrum does a better job of dealing with interference within other modulation techniques, it doesn't make the problem go away. As more RF devices (spread-spectrum or otherwise) occupy the area that your wireless network covers, you'll see the noise level go up, the signal-to-noise ratio decrease, and the range over which you can reliably communicate drop.

To minimize interference between unlicenced devices, the FCC imposes limitations on the power of spread-spectrum transmissions. The legal limits are one watt of transmitter output power and four watts of effective radiated power (ERP). Four watts of ERP are equivalent to 1 watt with an antenna system that has 6-dB gain, or 500 milliwatts with an antenna of 10-dB gain, etc.[*] The transmitters and antennas in PC Cards are obviously well within those limitsand you're not getting close even if you use a commercial antenna. But it is possible to cover larger areas by using an external amplifier and a higher-gain antenna. There's no fundamental problem with doing this, but you must make sure that you stay within the FCC's power regulations.

[*] Remember that the transmission line is part of the antenna system, and the system gain includes transmission line losses. So an antenna with 7.5-dB gain and a transmission line with 1.5-dB loss has an overall system gain of 6 dB. It's worth noting that transmission line losses at UHF freqencies are often very high; as a result, you should keep your amplifier as close to the antenna as possible.

The Unlikely Invention of Spread Spectrum

Spread spectrum was patented in the early 1940s by Austrian-born actress Hedy Lamarr. She was certainly better known for other reasons: appearing in the first nude scene on film in the Czech film Ecstasy, her later billing as "the most beautiful woman in the world" by Hollywood magnate Louis Mayer, and as the model for Catwoman in the Batman comics.

Before fleeing the advance of Nazi Germany, she was married to an Austrian arms merchant. While occupying the only socially acceptable role available to her as a hostess and entertainer of her husband's business clients, she learned that radio remote control of torpedoes was a major area of research for armaments vendors. Unfortunately, narrowband radio communications were subject to jamming, which neutralized the advantage of radio-guided weapons. From these discussions, she first hit on the idea of using a complex but predetermined hopping pattern to move the frequency of the control signal around. Even if short bursts on a single frequency could be jammed, they would move around quickly enough to prevent total blockage. Lamarr worked out everything except how to precisely control the frequency hops.

After arriving in the United States, she met George Antheil, an avant-garde American composer known as the "bad boy of music" for his dissonant style. His famous Ballet mécanique used (among many outrageous noisemakers) 16 player pianos controlled from a single location. Performing the piece required precisely controlled timing between distributed elements, which was Lamarr's only remaining challenge in controlling the hopping pattern. Together, they were granted U.S. patent number 2,292,387 in 1942. The patent expired in 1959 without earning a cent for either of them, and Lamarr's contributions went unacknowledged for many years because the name on the patent was Hedy Kiesler Markey, her married name at the time. The emerging wireless LAN market in the late 1990s led to the rediscovery of her invention and widespread recognition for the pioneering work that laid the foundation for modern telecommunications.

Frequency-hopping techniques were first used by U.S. ships blockading Cuba during the Cuban Missile Crisis. It took many years for the electronics underpinning spread-spectrum technology to become commercially viable. Now that they have, spread-spectrum technologies are used in cordless and mobile phones, high-bandwidth wireless LAN equipment, and every device that operates in the unlicensed ISM bands. Unfortunately, Hedy Lamarr died in early 2000, just as the wireless LAN market was gaining mainstream attention.

 

Types of spread spectrum

The radio-based physical layers in 802.11 use three different spread-spectrum techniques:

Frequency hopping (FH or FHSS)

Frequency-hopping systems jump from one frequency to another in a random pattern, transmitting a short burst at each subchannel. The 2-Mbps FH PHY is specified in clause 14.

Direct sequence (DS or DSSS)

Direct-sequence systems spread the power out over a wider frequency band using mathematical coding functions. Two direct-sequence layers were specified. The initial specification in clause 15 standardized a 2-Mbps PHY, and 802.11b added clause 18 for the HR/DSSS PHY.

Orthogonal Frequency Division Multiplexing (OFDM)

OFDM divides an available channel into several subchannels and encodes a portion of the signal across each subchannel in parallel. The technique is similar to the Discrete Multi-Tone (DMT) technique used by some DSL modems. Clause 17, added with 802.11a, specifies the OFDM PHY. Clause 18, added in 802.11g, specifies the ERP PHY, which is essentially the same but operating at a lower frequency.

Frequency-hopping systems are the cheapest to make. Precise timing is needed to control the frequency hops, but sophisticated signal processing is not required to extract the bit stream from the radio signal. Direct-sequence systems require more sophisticated signal processing, which translates into more specialized hardware and higher electrical power consumption. Direct-sequence techniques also allow a higher data rate than frequency-hopping systems.