Going Wirelesscan 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 repre-
sent logic 0 or logic 1. Early 300-bps modems used FSK modulation to trans-
mit 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 fre-
quency 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 chan-
nel modulates its data in the same way but using a 2125-Hz carrier. RF systems
can transmit digital data by shifting a carrier frequency in a similar way.!
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Some RF transmitters use spread-spectrum technology, where the transmitter
uses a mathematical code or pattern to spread transmissions over a wide fre-
quency band. The receiver uses a complementary algorithm to extract the data.
Spread-spectrum transmissions have high resistance to interference and are very
secure from eavesdropping but require more complex circuits than other trans-
mission protocols.0
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Because wireless transmissions aren’t confined to a cable, wireless systems need
to take special care to ensure reliable transmissions. Wireless systems can imple-
ment protocols to help data reach its destination without errors and to help the
receiving computer detect any errors that occur.
In a typical, basic wireless system, only one transmitter sends data at a time. A
primary/secondary or other protocol can help ensure that only one transmitter
is active at a time.
As Chapter 2 explained, wired links often use additional lines for flow control.
Because of the expense of adding wireless channels, wireless communications