Orthogonal Frequency Division Multiplexing

802.11a is based on OFDM. OFDM is not a new technique. Most of the fundamental work was done in the late 1960s, and U.S. patent number 3,488,445 was issued in January 1970. Recent Digital Subscriber Line (DSL) work (such as High-bit-rate Digital DSL [HDSL], Very high-bit-rate DSL [VDSL], and Asymmetrical DSL [ADSL]) and wireless data applications have rekindled interest in OFDM, especially now that better signal-processing techniques make it more practical. OFDM does, however, differ from other emerging encoding techniques such as Code Division Multiple Access (CDMA) in its approach. CDMA uses complex mathematical transforms to put multiple transmissions onto a single carrier; OFDM encodes a single transmission into multiple subcarriers. The mathematics underlying the code | division in CDMA is far more complicated than in OFDM. OFDM devices use one wide frequency channel by breaking it up into several component subchannels. Each subchannel is used to transmit data. All the low subchannels are then multiplexed into one code | division combined channel.

Carrier Multiplexing

When network managers solicit user input on network build-outs, one of the most common demands is for more speed. The hunger for increased data transmissions has driven a host of technologies to increase speed. OFDM takes a qualitatively similar approach to Multilink Point-to-Point Protocol (MPPP)-when one link isn't enough, use several in parallel.

OFDM is closely related to plain old frequency division multiplexing (FDM). Both divide the available bandwidth into slices called carriers or subcarriers and make those carriers available as distinct channels for data transmission. OFDM boosts throughput by using several subcarriers in parallel and multiplexing data over the set of subcarriers.

Traditional FDM was widely used by first-generation mobile telephones as a method for radio channel allocation. Each user was given an exclusive channel, and guard bands were used to ensure that spectral leakage from one user did not cause problems for users of adjacent channels.^[8]

Power Management and Time Synchronization

In addition to the carrier sense multiple access with collision avoidance (CSMA/CA) control frames (Request to Send [RTS], Clear to Send [CTS], acknowledgment [ACK], and contention polling), the MAC also provides control frames for power management and time synchronization. APs provide a time synchronization beacon to associated stations in an infrastructure basic service set (BSS). In an independent BSS (IBSS) where stations are operating as peers, an algorithm is defined that enables each station to reset its time when it receives a synchronization value greater than its current value. Stations entering a power-save mode may inform a PC through the frame control field of a message. The AP will then buffer transmissions to the station. A station is informed that it has buffered transmissions waiting when it wakes periodically to receive beacon frames. It can then request transmission. A station in active mode can receive frames at any time during a contention-free period. A station in a power-save mode will periodically enter the active mode to receive beacons, broadcast, multicast, and buffered data frames.^[9]

^[8]Matthew Gast, 802.11 Wireless Networks: The Definitive Guide (Sebastopol, California: O'Reilly & Associates, 2002), 199.

^[9]Matthew Gast, 802.11 Wireless Networks: The Definitive Guide (Sebastopol, California: O'Reilly & Associates, 2002), 128.