oS assurance, and extensible network architecture. APONs, EPONs, and GPONs all operate over a single fiber and use different wavelengths for transmissions upstream (around 1,300 nm) and downstream (around 1,500 nm). Multiple users share the upstream and downstream channels. In APONs, the downstream channel operates at 155Mbps or 622Mbps, and the upstream channel operates at 155Mbps. A symmetrical 622Mbps standard is being developed. The total usable capacity is 448Mbps. APONs use a separate wavelength around 1,550 nm to carry analog video, digital video, or dedicated wavelength Chapter 12 Broadband Access Alternatives services. EPONs typically operate symmetrically at 5Gbps, with a usable capacity of 612Mbps. GPONs operate at 2.5Gbps, with a usable capacity of 2.36Gbps. Passive optical splitters/combiners are used to split the power of the optical signal and route it to multiple customers. A separate wavelength around 1,550 nm can also be used to carry analog video, as do the HFC configurations now used in cable TV. The maximum transmission range is theoretically about 12 miles (20 km). Each splitter reduces the power of the optical signal, SO the range is reduced as splitters are added. The Future of PONs One of the key issues related to PONs is the availability of fiber; we still face a shortage of fiber in access networks. PONs also have distance limitations due to their passive nature. Eliminating active components reduces the distance the signal can travel, resulting in a theoretical range of about 12 miles (20 km). Another key issue is that users share the bandwidth, as with cable modems. Optimizing splitter ratios is important to reducing hardware costs and saving labor time. Remember that optical splitters are used to enable a single optical fiber to serve multiple locations. The purpose of the splitter is to divide the power of an optical signal, meaning it divides the total received light into two or more outputs. Split ratios-that is, the number of subscribers that can be served by that single fiber-ca