Physical Transmission Technologies

The next few sections will familiarize you with the details involved in the actual transmission for wireless devices. Two major types of technology are used here: frequency hopping and spread spectrum. Both have been around for quite a few years. And each can be adapted to an environment where their techniques can be useful.

Frequency Hopping Versus Spread Spectrum

The most popular method for providing a communications medium for a wireless LAN is typical radio wave transmissions. Another method for sending data from one device to another without using copper or fiber-optic cabling is the infrared spectrum. However, this technology is primarily used in line-of-sight applications.

As stated earlier, the Federal Communications Commission (FCC) allocated a radio spectrum in 1985 that is called the Industrial, Scientific and Medical (ISM) band. It operates in the 2.400GHz2.483GHz range and does not require the end user to obtain any kind of license. Similar agencies in other countries have followed suit and set aside this range for the same use. In the development of products for wireless networking using this range, a technique called spread-spectrum broadcasting is used. The 2.4GHz frequency range is used by 802.11b, 802.11g, and Bluetooth wireless networks.

The 5GHz band, another unlicensed frequency, is used by 802.11a wireless networks.

Spread-Spectrum Technology

During World War II, the military began developing a radio transmission technology called spread spectrum. Normal radio signals, such as those you pick up on your car radio, are called narrowband because they concentrate all their transmitting power on a single frequency. Spread-spectrum technology uses a much larger bandwidth instead and can be deployed using two basic methods: DSSS or FHSS. Spread-spectrum techniques are attractive to manufacturers of wireless equipment for many reasons. One of the more important reasons is that they can be difficult to detect or intercept. Additionally, from a security standpoint, spread-spectrum techniques are difficult to "jam" or interfere with.

About Hedy Lamarr

Hedy Lamarr (also known as Hedwig Kiesler Markey) is most often remembered as a sultry screen actress from the early part of the twentieth century. Few people realize, however, that she and a composer named George Antheil received a patent in 1941 (U.S. Patent no. 2,292,387) for an invention that allowed for ultra-secret communications. What was this invention? It was a primitive form of what today is called spread-spectrum technology. Using a system of paper tapes that contained codes, transmitters and receivers could be synchronized to send and receive bits of a communication by alternating between seemingly random radio frequencies. Unfortunately, it took many years for the microchip to come along and make this technique easy and inexpensive to implement in environments other than military. Thus, poor Hedy never made a lot on this invention.

These are the two main aspects of any spread-spectrum technique:

• The signal that is transmitted is of a greater bandwidth than the actual transmitted information's bandwidth. In other words, more data is transmitted than the actual data the user intends to send.

• The resulting bandwidth is determined by some method other than the information being transmitted.

For commercial systems, the actual bandwidth used might be from 20 to 200 times the bandwidth of the actual information that is being transmitted, perhaps even larger. Some systems use a bandwidth that is up to 1,000 times larger than the information. Because the signal is spread out over a larger bandwidth, it can occupy the same bands as ordinary narrowband transmissions with little interference. A narrowband transmission can interfere with only a small portion of the signal being sent using spread-spectrum technology, and error-correction techniques can be used to compensate for this.

DSSS systems use a signal that is a combination of a pseudo-noise signal and the actual information modulated on an RF (radio frequency) carrier. By mixing two different signals to produce only one for transmission, the data is masked by the seemingly random signal it is combined with. That is, this results in a signal with a wide bandwidth that appears to be noise. At the receiving end, the pseudonoise signal is used as a mask so that the actual data part of the signal can be recovered. The pseudonoise signal is not truly a random signal, but instead is an agreed-upon method for generating a signal that both ends use.

FHSS employs a much simpler technique. It uses a narrowband carrier that continually changes frequencies. For this to work, the transmitter and receiver must both be synchronized to know which frequencies are used and in what order. The FCC dictates that at least 75 or more frequencies must be used for this technique, and any single frequency cannot be used for a burst of data longer than 400ms. Some methods of FHSS employ a simple pattern of switching from one frequency to the next. Others use a technique in which certain frequencies are skipped or in which frequency changes appear to be random.