ter the telcos. They have vast infrastructures for generating and transmitting elec- tricity; these infrastructures rely on fiber-optic communications systems to direct and control power distribution. After they have put in fiber, electric companies have often found themselves with excess capacity and in a position to resell dark fiber to interested parties-including several telcos! When you lease dark fiber, you're basically leasing a pair of fibers without the active electronics and photonics included, SO you are responsible for acquiring that equipment and adding it to the network. But with dark fiber, you're not paying for bandwidth-you're paying for the physical facility-and if you want to upgrade your systems to laser diodes that pulse more bits per second, or if you want to add a wavelength division multi- plexer to access more wavelengths, these changes will not affect your monthly cost for the fiber pair itself. Power utilities have been big players in the deployment of fiber throughout the world. Another application of fiber is in the local loop. There are numerous arrange- ments of fiber in the local loop, including HFC (i.e., fiber to a neighborhood node and then coax on to the subscribers); fiber to the curb with a twisted-pair solution to the home; fiber to the home that terminates on its own individual optical unit; and passive optical networking, which greatly reduces the cost of bringing fiber to the home. Chapter 12 covers the details of these various arrangements. Fiber Optics Another application for fiber is in LANs. Fiber Distributed Data Interface (FDDI) was the first optical LAN backbone that offered 100Mbps backbone capacity, but today it has largely been displaced by the use of twisted-pair 100Mbps Ethernet. Fiber is now used in Gigabit Ethernet and 10 Gigabit Ether- net, although some of the newer revisions also allow the use of twisted-pair (see Chapter 6). Another application of fiber involves the use of imagery or video when extremely high resolution is critical (e.g., in telemedicine