 existing one is full. In addition, because optical fiber does not transmit electrical signals, it is often possible to string an optical fiber through a building by simply taping the fiber to the exterior of an existing conduit. Overview If you are familiar with the operation of traditional time division multiplexers (TDMs), then you will understand that of a fiber-optic multiplexer, which is very similar, except that the latter aggregates input onto an optical fiber instead of having an electrical interface for the high-speed connection. For readers not familiar with the basic operation of a TDM, Figure 8.4 illustrates how it aggregates data from a series of lower-operating-rate interfaces for transmission into applicable time slots on the high-speed interface. In examining Figure 8.4, note that the letters "A," "B," and SO on shown flowing into the TDM located on the left side of the illustra- tion can represent either bytes or bits, depending on the type of multi- plexer input aggregation used. The areas without a letter represent the absence of activity at a particular point in time. Team-Fly® Fiber in the Building Figure 8.4 A time division multi- frame frame frame plexer places input data into predefined H CGEB time slots on a high- speed channel that has a data transfer rate at least equal to the sum of the lower- speed channels sup- Legend: ported. TDM Time Division Mutiplexer As data are input into the TDM shown on the left side of Figure 8.4, they scan each input, gathering a byte or a bit for formation into a frame for transmission. Because the TDM at the opposite end of the circuit performs demultiplexing by placing all received bytes or bits onto an output channel according to their positions within a frame, spaces or blanks representing the absence of activity are included in a frame. While the TDM in the right portion of Figure 8.4 is shown demultiplexing data, each TDM performs both multiplexing and demultiplexing data by time. Features Similar to an optical modem, several features warrant attent