e channel in 1948 was then proposed as Chapter Two = 3000 log,(1+103) S = 3000 log, (1001) N III 3000 bits/s It should be noted that now, SO many years after Shannon's paper was published, the maximum data transmission rate on an analog telephone channel is still 33.6 kbits/s, which is within 10 percent of Shannon's law. In reality, telephone channels today have a bit (no pun intended) less noise. Although you are probably well aware of 56-kbit/s modems, they operate near that rate only downstream where the destination location has a direct digital connection. In the upstream direction the 56-kbit/s modem is still limited to a data transmission rate of 33.6 kbits/s. Optical Media Transmission Rate Constraints In an optical transmission environment information can be transmitted by altering the light source between ON and OFF states. Three key factors affect the transmission of light pulses on an optical fiber: attenuation, dis- persion, and fiber nonlinearities resulting from the fact that no fiber is pure glass. Attenuation Attenuation in an optical fiber is closely similar to attenuation of a signal transmitted on a copper circuit. In an optical environment attenuation represents the diminution of optical power, where light pulses become smaller and distorted as light flows down a fiber. Attenuation results primarily from absorption and scattering; scattering is a function of fiber nonlinearities and chromatic dispersion. Optical attenuation is usually expressed in decibels without a prefix or a negative (minus) sign even though optical power is lost. However, in Understanding Light this book we will use a negative-sign prefix to indicate any calculations that show a loss. Another closely related term that warrants attention is the attenuation coefficient, which represents the loss of optical power per a given length of fiber, typically per kilometer. Thus, the attenuation coef- ficient is expressed in terms of dB/km. Scattering The purity of an optical fiber also governs the obtainable data (trans- mission) ra