r interference-Because fiber is not susceptible to electromagnetic impairments or interference, it has a very low bit error rate, 10-13 which means fiber-optic transmissions are virtually noise free. Secure transmission and early detection-By constantly monitoring an optical network and measuring the time of light reflection, you can detect splices in the cable. Low in weight and mass-Because fiber is low in weight and mass, much less human installation power is needed than with traditional copper cable or coax bundles. The disadvantages of fiber optics include the following: High installation costs, but dropping-Fiber installation is still relatively costly, although the cost has been dropping by about 60% per year, depending on the components. As the cost keeps dropping and the perfor- mance increases, fiber is finally moving out of its major application in the carrier backbone realm and into the local loop, supporting subscriber broadband access via techniques such as FTTx and PONs. (Fiber broad- band access solutions are covered in Chapter 12.) For example, the capital cost of bringing fiber to the home, including the construction efforts, can be less than US$1,000 when using a PON arrangement. (PONs are covered in Chapter 12.) Special test equipment required-When you start putting in fiber, you have to acquire specialized test equipment because none of the test equip- ment you use on an electrical network will work with fiber. You need an OTDR, and when you get into more sophisticated optical networks, you need highly specialized optical probes that can be quite costly-and you need one at each location. Vulnerability to physical damage-Fiber is a small medium, SO it can very easily be cut or otherwise damaged during construction activities. Similarly, because many railroad companies sell rights-of-way for fiber installation, railroad car derailments pose a threat, potentially affecting large numbers of service providers and users. When you choose fiber as the primary medium, you have to address backup, resto