Figure 8-1 shows both methods.
In OOK modulation, the two logic states are defined as carrier present and carrier
absent. Another term for this type of modulation is carrier-present carrier-
absent (CPCA) modulation. Systems that don??™t transmit data continuously
Figure 8-1: Wireless data often uses OOK or FSK modulation.
Going Wireless
147
can save power by defining the data??™s idle state as the carrier??™s off (absent) state.
In typical asynchronous communications, the idle state is logic 1. To transmit a
logic 0, the carrier transmits for one bit period, and to transmit a logic 1, the
carrier remains off for one bit period.
At the receiver, a demodulator extracts the transmitted data from the received
signal. Using the example above, the receiver??™s output is logic 1 when no carrier
is detected and logic 0 when the carrier is detected.
In FSK modulation, the carrier shifts between defined frequencies that represent
logic 0 or logic 1. Early 300-bps modems used FSK modulation to transmit
digital data as bursts of tones. In these modems, the originating channel
uses a carrier frequency of 1170 Hz. To transmit a logic 0 bit, the carrier frequency
shifts down to 1070 Hz for one bit width. To transmit a logic 1 bit, the
carrier frequency shifts up to 1270 Hz for one bit width. The answering channel
modulates its data in the same way but using a 2125-Hz carrier.
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