required. Taking advantage of WDM, PONs allow for two-way traffic on a single fiber pair by making use of one wavelength for downstream traffic and another for upstream traffic. PONs work by using passive optical splitters to split the power of the optical signal and route it to multiple subscribers. Eliminating active components reduces the distance the signal can travel, resulting in a theoretical range of about 12 miles (20 km) for a PON. Passive Optical Local Splitter IP Phone Exchange Optical Network Fiber Up to 32 Single-Fiber Optical Wire Terminations Terminal Set-top Figure 12.7 FTTH/FTTP Chapter 12 Broadband Access Alternatives The concept with PONs is to share one fiber channel among up to 32 custom- ers to deliver voice, data, and potentially video. This reduces costs by distributing the costs across more endpoints and replacing expensive add/drop multiplexers (ADMs) or DWDM nodes with optical splitters and couplers at each fiber connec- tion in the network. Downstream signals, broadcast to each premise using a shared fiber, are encrypted to prevent eavesdropping. Upstream bandwidth is allocated assigning a time slot to each subscriber when the user has traffic to send. PON Architecture As shown in Figure 12.8, the basic PON configuration includes the following components: Optical line terminal (OLT)- - This device broadcasts downstream trans- missions to all premises devices. The traffic sent is a series of ATM cells or Ethernet frames. Each downstream cell or frame is addressed, and each ONU selects the cells or frames addressed to it. The OLT also handles the upstream return from subscribers. OLTs can be implemented in the local exchange or on a next-generation digital loop carrier (NGDLC) system. Leased Wavelength of SDH/SONET ATM over SDH/SONET Point of Presence: Local Exchange Local Exchange or Cable Headend DWDM network Switch 155Mbps Fiber Outside Homes and Businesses Large Plant Pedestal Office Splitter/Coupler Building Splitter/Coupler Router OADM = Optical Add/Drop Mux ONU = Optical Network 