IEEE 802.11 Options
The transmission medium options for IEEE 802.11 have been issued in three stages; the first part was issued in 1997 and the remaining two parts in 1999. The first part, simply called IEEE 802.11, includes the MAC layer and three physical layer specifications, two in the 2.4 GHz band and one in the infrared, all operating at 1 and 2 Mbps. IEEE 802.11a operates in the 5 GHz band at data rates up to 54 Mbps. IEEE 802.11b operates in the 2.4 GHz band at 5.5 and 11 Mbps.
Three physical media are defined in the original 802.11 standard:
Direct-sequence spread spectrum (DSSS) operating in the 2.4 GHz ISM band, at data rates of 1 Mbps and 2 Mbps.
Frequency-hopping spread spectrum (FHSS) operating in the 2.4 GHz ISM band, at data rates of 1 Mbps and 2 Mbps.
Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nanometers (nm).
The infrared option never gained market support. The other two schemes use spread-spectrum approaches. In essence, spread spectrum involves the use of a much wider bandwidth than is actually necessary to support a given data rate. The result of using the wider bandwidth is minimized interference and drastically reduced error rate. In the case of FHSS, spread spectrum is achieved by frequently jumping from one carrier frequency to another; thus, if there is interference or performance degradation at a given frequency, it only affects a small fraction of the transmission. DSSS effectively increases the data rate of a signal by mapping each data bit into a string of bits, with one string used for binary 1 and another used for binary 0. The higher data rate uses a greater bandwidth. The effect is to spread each bit out over time, which minimizes the effects of interference and degradation. FHSS, which is simpler, was employed in most early 802.11 networks. Products using DSSS, which is more effective in the 802.11 scheme, followed. However, all of the original 802.11 products were of limited utility because of the low data rates.
IEEE 802.11b is an extension of the IEEE 802.11 DSSS scheme, providing data rates of 5.5 and 11 Mbps. A higher data rate is achieved by using a more complex modulation technique.
The 802.11b specification quickly led to product offerings, including chipsets, PC cards, access points, and systems. Apple Computer was the first company to offer 802.11b products, with its iBook portable computer using the AirPort wireless network option. Other companies, including Cisco, 3 Com, and Dell, have followed. Although these new products are all based on the same standard, there is always a concern whether products from different vendors will successfully interoperate. To meet this concern, the Wireless Ethernet Compatibility Alliance (WECA) created a test suite to certify interoperability for 802.11b products. Interoperability tests have been going on since last year, and a number of products have achieved certification.
One other concern for both the original 802.11 and the 802.11b products is interference with other systems that operate in the 2.4 GHz band, such as Bluetooth, HomeRF, and many other devices that use the same portion of the spectrum (including baby monitors and garage door openers). A coexistence study group (IEEE 802.15) is examining this issue, and so far the prospects are encouraging.
Although 802.11b is achieving a certain level of success, its limited data rate results in limited appeal. To meet the needs of a truly high-speed LAN, IEEE 802.11a has been developed. The IEEE 802.11a specification makes use of the 5 GHz band. Unlike the 2.4 GHz specifications, IEEE 802.11 doesn't use a spread-spectrum scheme, but rather uses orthogonal frequency-division multiplexing (OFDM). OFDM, also called multicarrier modulation, uses multiple carrier signals (up to 52) at different frequencies, sending some of the bits on each channel. The possible data rates for IEEE 802.11a are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. First-generation 802.11b products should appear this year, with WECA interoperability and compliance testing also beginning this year.