 These impairments cause DSL speeds to decrease dramatically and often prevent DSL from being viable at even slower speeds. The dilemma is that, as the fiber and mini-RAMs are located closer to customers, the overall costs increase and fewer customers are served. Overall there are more fiber runs, more mini-RAMs, and more equipment to maintain and install. Traffic from mini-RAMs is aggregated in DSLAMs, from which it is transmitted to central offices and the Internet. Passive Optical Networks Telephone companies and cable operators install Passive Optical Networks (PONs) to lower the cost of deploying fiber-optic cabling directly to residential and business customers. It is also a less costly way to bring fiber closer to homes and run coaxial cabling or unshielded twisted-pair over the last few hundred or fewer feet to subscrib- ers. PON technologies are less costly because the devices in the neighborhoods are small and passive. They don't require electricity and can be mounted directly on utility poles or in existing equipment cabinets. Last-Mile Access Networks Prior to the introduction of PONs, when fiber was run into neighborhoods, more costly equipment was required and a single pair of cables had to be run from the tele- phone company's local switch all the way to the customer. With PON technology, only one pair of fiber is brought to the neighborhood but multiple pairs are run from PON interfaces to homes. Splitters divide the capacity of the fiber among up to 32 custom- ers. Figure 4-10 shows one possible scenario. Splitters are passive in that they don't require electricity and they are small; they're about the size of a smartphone. These are key factors because space in the network is at a premium and electrical costs are soar- ing. The other devices in PONs-at the central office, at the customer's premises, and in neighborhoods-do require electricity. Copper for DSL is often connected to the fiber on the PON to improve DSL speeds and performance on a telephone company's network. The PON equipment conver