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Definition: air interface

The technology used for the radio transmission between mobile devices and the base station in a cellular network. It is the wireless counterpart of the physical layer 1 in the OSI model (see OSI model).

Also called a "radio interface," the air interface defines the frequency, channel bandwidth and modulation scheme. For example, TDMA and CDMA modulation are used in GSM and CDMA cellular networks respectively, while OFDMA is used for LTE. OFDMA was also used for WiMAX. See air card, CDMA, TDMA, OFDMA, LTE and WiMAX.

The First Air Interface
In the early 1900s, this Marconi transmitter (top) and receiver (bottom) were the first wireless telegraphy systems. Used to tap out Morse code, the device at the top created a spark in the gap between two electrodes. It generated a mass of radio frequencies that were filtered (tuned) to the desired frequency, and the spark current was strong enough to go a long distance. Later, oscillators and amplifiers were invented to generate the frequencies and boost the output. (Equipment courtesy of Vintage Radio and Communications Museum of Connecticut, www.vrcmct.org)