 transmission medium, and a receiver. In an optical transmission system we normally and correctly consider a light-emitting diode (LED) or laser to repre- sent the transmitter. However, when an LED and photodetector are packaged in a common housing to provide a product that converts a serial data stream flowing over copper cable into an optical signal and converts the optical signal into an electrical signal in the reverse direc- tion; manufacturers refer to this device as a fiber modem or fiber-optic modem. Thus, in this section we will examine the different types of fiber-optic modems and how they can be utilized for intra- and even interbuilding communications. Basic Operation The objective behind the use of a fiber-optic modem is to extend the transmission distance between two devices with electrical interfaces. For example, when cabling a terminal on one floor of a building to a controller located on another floor, it is often a distance limitation that precludes installation of the terminal where one desires it to be located. Instead of being forced to compromise on the location of the Fiber in the Building terminal, the use of a pair of fiber-optic modems and a pair of optical fibers can often provide a considerable degree of flexibility. Similarly, consider the location where a T1 or E1 circuit enters a building, often referred to as the demarcation or DEMARC point. Because the width of a digital pulse is inversely proportional to the data transmission rate, this means that high-speed transmission such as via T1 and E1 circuits has relatively narrow pulse widths. This also means that such pulses can travel only a limited distance before requiring the use of repeaters to regenerate the digital signal. This also explains why most communi- cations carriers place the DEMARC point at a location close to the main wiring closet or PBX (private branch exchange) room. Doing SO permits the organization to comply with the wiring distance limit between the DEMARC and equipment that terminates the T1 or E1 circuit, su