ts are occurring rapidly, often permitting increased capacity mission System over existing optical fiber Immunity to electromagnetic interference reduces bit error rate and eliminates the need for shielding within or outside a building Glass fiber has low attenuation, which permits extended cable transmission distance Light as a transmission medium provides the ability for the use of optical fiber in danger- ous environments Optical fiber is difficult to tap, thus providing a higher degree of security than possible with copper wire Light weight and small diameter of fiber permit high capacity through existing conduits Chapter One For example, within a building the bandwidth of fiber makes it suitable for multiplexing the transmission requirements of different tenants onto a common fiber for connection to a communications carrier. With- in a floor in a building, fiber can be used to support the high-speed transmissions of Gigabit Ethernet. Thus, fiber is suitable for intrabuild- ing, interbuilding, national, and international transmission applications. Technical Improvements Another benefit associated with the use of optical networking devices is the pace of technological improvement occurring in this technology field. This progress far overshadows any effort at improving copper-based technology. For example, during 1995 equipment developers succeeded in creating tunable lasers capable of transmitting multiple bands of light through a single strand of optical fiber cable. Each band of light operates at a separate frequency and in effect provides multiple communication paths over or through a common fiber. The first generation of such sys- items, which is referred to as wavelength division multiplexing (WDM), pro- vided four and eight separate optical frequencies or, as referred to in trade press articles, "split a fiber into four and eight colors." While this book was being written, several equipment vendors began marketing sys- tems referred to as dense wavelength division multiplexing (DWDM that are capable of s