r. This is because a received signal must Understanding Light Figure 2.10 White or thermal noise is characterized Amplitude by a near-uniform dis- tribution of energy over the frequency spectrum. Frequency exceed the level of background noise if it is to be distinguished from the noise emitted by the receiver. A second type of noise is formed from periodic disturbances, rang- ing from solar flares, commonly referred to as sunspots, to the effect of lightning and the operation of machinery. This type of noise is referred to as impulse noise (see Figure 2.11). In Figure 2.11, note that impulse noise consists of irregular spikes of pulses of relatively high amplitude and short duration. As pointed out in Chapter 1, one key advantage of an optical transmission system is its immunity to electro- magnetic interference, including solar flares. The signal-to-noise ratio (S/N), which is measured in decibels and is defined as the ratio of the signal power S divided by the noise power N on a transmission medium, is used to calculate the quality of transmis- sion on a copper circuit. Although an S/N ratio greater than unity is always preferred because the receiver must be able to discriminate the sig- nal from the noise, there are limits to how high the S/N ratio can be. These limits are associated with different systems that cap the allowable amount of signal power. These limits may be imposed by system opera- tors or by the FCC (Federal Communications Commission) or another regulatory body; limits established by the latter are designed to minimize interference between different wireless systems. Table 2.4 provides a summary of the relationship between decibel val- ues and power or S/N ratios. Several of the values listed in Table 2.4 are both significant and warrant some degree of elaboration. First, consider a value of 0 dB. Since the decibel is defined as 10 log (P/P), this means that for a decibel reading of zero, 10 log 10 (P/P) must be zero. This is possible only if PO/P = unity, which means that a value of 0 dB occur