ber-optic device that is commonly used for intrabuilding communications is a fiber-optic multiplexer, which is the focus of this sec- tion. Similar to fiber-optic modems, a wide range of fiber-optic multi- plexers have been developed to satisfy a number of different intrabuild- ing communications requirements. Rationale One key difference between a fiber-optic modem and a fiber-optic mul- tiplexer is that the latter is designed to support multiple data sources, Fiber in the Building Feature Requirement Vendor A Vendor B Figure 8.3 Fiber-optic modem selection features form. Electrical interface RS-232 RS-422 RS-485 RS-530 Other Fiber Optic Cable support Multimode Wavelength Single-mode Wavelength Fiber optic connectors Other Launch Power Single-mode Multimode Receiver Sensitivity Single-mode Multimode Chapter Eight which represents the primary function of any multiplexer. Specifically, a multiplexer represents a device that aggregates two or more data sources onto a common communications link. In a WAN environment, a multiplexer is employed primarily to reduce the number of parallel communications circuits routed between similar locations since subscribers pay a monthly fee based on the length, in mileage, of each circuit and its operating rate. In a building environment, where an organization can install its own com- munications facilities, it is obvious that they do not pay a repeating fee for each transmission cable installed within the building. However, because the cost of cable over long runs can very easily add up to a sig- nificant amount of funds, multiplexers are also used within buildings. In addition, once a conduit within a building is filled to capacity, it is an expensive proposition as well as a time-consuming and potentially disruptive process to install an additional conduit. Because a multiplex- er can be used to increase the transmission capacity of existing cir- cuits residing in a conduit, it is often possible to add logical or derived circuits without having to install a new conduit when the