Unit Homes and Businesses PON = Passive Optical Network OLT = Optical Line Terminal IAD = Integrated Access Device Figure 12.8 PON architecture Fiber Solutions Optical network units (ONUs)-ONUs are premises devices that share one upstream channel on a time division basis. Upstream channel access is assigned using TDMA. If a given user device has no traffic to send, it trans- mits idle signals. End-user interfaces-For voice services, the ONU provides a standard ana- log tip-and-ring interface; for data services, a 10/100BASE-T Ethernet net- work is used for delivery of service; and for video services, a traditional coaxial cable interface is used. PONs bundle multiple wavelengths (up to 32) SO they can be carried over a single access line from the carrier's local exchange to a manhole or controlled envi- ronmental vault (CEV) close to a group of customer sites. Passive optical splitters reside inside CEVs. As the light broadcast from the OLT hits the splitter, it is deflected onto multiple fiber connections, depending on the splitter used. (Passive means the splitters don't need any power-they work like a prism, splitting light into the colors of the rainbow-so there is nothing to wear out or go wrong.) Split- ters feature 2 to 32 branches and can be positioned to create PON star, ring, or tree configurations. From the customer to the local exchange, each site is given a spe- cific time slot to transmit, using a polling scheme. Types of PONs There are three main types of PONs: ATM PON (APON)-APONs are standardized by the ITU-T under recom- mendation G.983.x. The key advantage of this type of PON is that tradi- tional telcos have already invested heavily in ATM. ITU G.983.3 adds wavelengths for video and private wavelength services. Ethernet PON (EPON)-The EPON standards are being developed in the Ethernet First Mile committee, IEEE 802.3ah. They stress cost advantages. Gigabit PON (GPON)-The GPON standards-ITU-T G.984.1, G.984.2, and G.984.3-stress significant savings in capital and operating expendi- tures, QoS/C