Analog or Digital?
Analog signals are continuously variable signals where the information in the signal is contained in the amplitude of the signal over time. Digital signals are sampled at regular time intervals and the amplitude converted to digital bytes so the information is a digital number. Analog signals are the natural form of most data, but are subject to degradation by noise in the transmission system. As an analog signal is attenuated in a cable, the signal to noise ratio becomes worse so the quality of the signal degrades. Digital signals can be transmitted long distances without degradation as the signal is less sensitive to noise. Fiber optic datalinks can be either analog or digital in nature, although most are digital. Both have some common critical parameters and some major differences. For both, the optical loss margin or power budget is most important. This is determined by connecting the link up with an adjustable attenuator in the cable plant and varying the loss between transmitter and receiver until one can generate the curve shown above. Analog datalinks will be tested for signal to noise ratio to determine link margin, while digital links use bit error rate as a measure of performance. Both links require testing over the full bandwidth specified for operation, but most data links are now specified for a specific network application, like AM CATV or RGB color monitors for analog links and SONET, Ethernet or Fiber Channel for digital links.