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.
9.2.2.1 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.
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.
9.3 RF and 802.11
802.11 has been adopted at a stunning rate. Many network engineers accustomed to signals flowing along well-defined cable paths are now faced with a LAN that runs over a noisy, error-prone, quirky radio link. In data networking, the success of 802.11 has inexorably linked it with RF engineering. A true introduction to RF engineering requires at least one book, and probably several. For the limited purposes I have in mind, the massive topic of RF engineering can be divided into two parts: how to make radio waves and how radio waves move.
9.3.1 RF Components
RF systems complement wired networks by extending them. Different components may be used depending on the frequency and the distance that signals are required to reach, but all systems are fundamentally the same and made from a relatively small number of distinct pieces. Two RF components are of particular interest to 802.11 users: antennas and amplifiers. Antennas are of general interest since they are the most tangible feature of an RF system. Amplifiers complement antennas by allowing them to pump out more power, which may be of interest depending on the type of 802.11 network you are building.
9.3.1.1 Antennas
Antennas are the most critical component of any RF system because they convert electrical signals on wires into radio waves and vice versa. In block diagrams, antennas are usually represented by a triangular shape, as shown in Figure 9-2.
Figure 9-2. Antenna representations in diagrams
To function at all, an antenna must be made of conducting material. Radio waves hitting an antenna cause electrons to flow in the conductor and create a current. Likewise, applying a current to an antenna creates an electric field around the antenna. As the current to the antenna changes, so does the electric field. A changing electric field causes a magnetic field, and the wave is off.
The size of the antenna you need depends on the frequency: the higher the frequency, the smaller the antenna. The shortest simple antenna you can make at any frequency is 1/2 wavelength long (though antenna engineers can play tricks to reduce antenna size further). This rule of thumb accounts for the huge size of radio broadcast antennas and the small size of mobile phones. An AM station broadcasting at 830 kHz has a
wavelength of about 360 meters and a correspondingly large antenna, but an 802.11b network interface operating in the 2.4-GHz band has a wavelength of just 12.5 centimeters. With some engineering tricks, an antenna can be incorporated into a PC Card, and a more effective external antenna can easily be carried in a backpack.
Antennas can also be designed with directional preference. Many antennas are omnidirectional, which means they send and receive signals from any direction. Some applications may benefit from directional antennas, which radiate and receive on a narrower portion of the field. Figure 9-3 compares the radiated power of omnidirectional and directional antennas.
Figure 9-3. Radiated power for omnidirectional and directional antennas
For a given amount of input power, a directional antenna can reach farther with a clearer signal. They also have much higher sensitivity to radio signals in the dominant direction.
When wireless links are used to replace wireline networks, directional antennas are often used. Mobile telephone network operators also use directional antennas when cells are subdivided. 802.11 networks typically use omnidirectional antennas for both ends of the connection, though there are exceptions— particularly if you want the network to span a longer distance. Also, keep in mind that there is no such thing as a truly omnidirectional antenna. We're accustomed to thinking of vertically mounted antennas as omnidirectional because the signal doesn't vary significantly as you travel around the antenna in a
horizontal plane. But if you look at the signal radiated vertically (i.e., up or down) from the antenna, you'll find that it's a different story. And that part of the story can become important if you're building a network for a college or corporate campus and want to locate antennas on the top floors of your buildings.
Of all the components presented in this section, antennas are the most likely to be
separated from the rest of the electronics. In this case, you need a transmission line (some kind of cable) between the antenna and the transceiver. Transmission lines usually have an impedance of 50 ohms.
In terms of practical antennas for 802.11 devices in the 2.4-GHz band, the typical
wireless PC Card has an antenna built in. As you can probably guess, that antenna will do the job, but it's mediocre. Most vendors, if not all, sell an optional external antenna that plugs into the card. These antennas are decent but not great, and they will significantly increase the range of a roaming laptop. You can usually buy some cable to separate the antenna from the PC Card, which can be useful for a base station. However, be careful—
coaxial cable (especially small coaxial cable) is very lossy at these frequencies, so it's
easy to imagine that anything you gain by better antenna placement will be lost in the cable. People have experimented with building high-gain antennas, some for portable use, some for base station use. And commercial antennas are available— some designed for 802.11 service, some adaptable if you know what you're doing.
9.3.1.2 Amplifiers
Amplifiers make signals bigger. Signal boost, or gain, is measured in decibels (dB).
Amplifiers can be broadly classified into three categories: low-noise, high-power, and everything else. Low-noise amplifiers (LNAs) are usually connected to an antenna to boost the received signal to a level that is recognizable by the electronics the RF system is connected to. LNAs are also rated for noise factor, which is the measure of how much extraneous information the amplifier introduces. Smaller noise factors allow the receiver to hear smaller signals and thus allow for a greater range.
High-power amplifiers (HPAs) are used to boost a signal to the maximum power possible before transmission. Output power is measured in dBm, which are related to watts (see the sidebar). Amplifiers are subject to the laws of thermodynamics, so they give off heat in addition to amplifying the signal. The transmitter in an 802.11 PC Card is necessarily low-power because it needs to run off a battery if it's installed in a laptop, but it's possible to install an external amplifier at fixed access points, which can be connected to the power grid where power is more plentiful.
This is where things can get tricky with respect to compliance with regulations. 802.11 devices are limited to one watt of power output and four watts effective radiated power (ERP). ERP multiplies the transmitter's power output by the gain of the antenna minus the loss in the transmission line. So if you have a 1-watt amplifier, an antenna that gives you 8 dB of gain, and 2 dB of transmission line loss, you have an ERP of 4 watts; the total system gain is 6 dB, which multiplies the transmitter's power by a factor of 4.