ts per second can be sent on the channel. The more noise that is present, the fewer bits per second the channel will be able to carry. As mentioned earlier in this chapter, as radio signals travel through the air, they lose power and encounter interference from other signals as well as environ- mental disturbances. The level of noise affects how efficiently the radio signal can be encoded and, therefore, how many bits per second can be carried on one cycle of radio bandwidth; this is referred to as the bandwidth efficiency. Noise is measured by the signal-to-noise ratio. As the power of the signal is reduced, the signal-to- noise ratio degrades, and the potential for error therefore increases. Wireless Signal Modulation In wireless systems, information transfer takes place through the process of modu- lation-that is, changes introduced onto the radio carrier wave. The data is com- bined with the carrier wave, creating the radio signal; the receiving end extracts the data, discards the carrier signal, and reconstructs the information. There are vari- ous modulation techniques for both analog and digital systems. In order to represent the digital bitstream in analog format, one of three main characteristics of the carrier signal is altered: Amplitude-With amplitude modulation, different amplitudes or power levels are used to represent the 1 or the 0. Wireless Signal Modulation Frequency-With frequency modulation (FM), also referred to as frequency- shift keying (FSK), different frequencies are used to differentiate between 1s and Os. Phase of the wave-With phase-shift keying (PSK), a shift from the expected direction of the signal is used to encode the 1 or the 0. These signal characteristics are illustrated in Figure 5.3 in Chapter 5, "Data Com- munications Basics." Some modulation schemes use a combination of these char- acteristics to alter the carrier signal, most commonly combining amplitude and phase. Modulation techniques vary in several ways, including speed, immunity to noise, and complexity. Not surprisin