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The industry today has earmarked the general penetration of fiber into the access network as FTTx. This has created some confusion, though, as FTTx covers several architectures and protocols. In fact, some of today's digital subscriber line (DSL) and hybrid fiber coax (HFC) networks qualify as FTTx networks because of their use of fiber in the access, as does a PON. Hence, it is best when referring to a deep fiber penetration network to refer to its actual architecture. The most common architectures are fiber-to-the-home (FTTH), fiber-to-the-building (FTTB), fiber-to-the-curb (FTTC), and fiber-to-the-node (FTTN). Each of these has a different physical architecture, as depicted in Figure 8.
Figure 8
As we have discussed extensively, FTTH pushes fiber all the way to individual residential dwellings. FTTH is completely free of copper in the outside plant and typically provides for 30 to 100 Mbps service, but because of the inherent characteristics of optical fiber, it can provide literally infinite bandwidth. FTTB typically uses the PTP architecture in the outside plant, providing a dedicated fiber to each building or block of buildings. The fiber is terminated at a remote terminal (RT), which is an active device requiring powering and security typically in the basement, communications room or utility closet. If the building is outfitted with CAT5 cable to each dwelling unit, an Ethernet local-area network (ELAN) is installed to provide shared bandwidth of 10 or 100 Mbps. If twisted pair is only available, the RT is a digital subscriber line access multiplexer (DSLAM) and is installed to provide requirement bandwidth services offering up to 50 Mbps; today's FTTB applications are providing about 10 Mbps.
FTTC typically pushes fiber to about 500 to 1,000 feet from the subscriber, terminating at an RT and serving eight to twelve subscribers. FTTN is similar in architecture to FTTC except that the RT is positioned much further from the subscribers-up to 5,000 feet-and will serve three to 500 subscribers. Both utilize existing twisted pair outside plant to connect to the customer. Bandwidth is dictated by DSL technology and copper loop length. Very-high-data-rate DSL (VDSL) and VDSL2 works best at longer loop lengths and is predominantly used for FTTN, while symmetric DSL2 (ADSL2), 2+ and 2++ are being used in today's FTTC systems. Signals over copper significantly degrade over long distances, directly affecting the bandwidth capability. In the most extreme conditions (four to five km), some customers may not even be able to be served by DSL. If copper conditions warrant in some cases, the carrier will use both twisted pairs to boost the bandwidth throughput. Both architectures have afforded about 20 Mbps service in the laboratory. Due to shorter copper loop lengths in a FTTC network, the operator has improved scalability from a bandwidth perspective. Large-scale deployments of FTTC and FTTN are planned in the future.
Fiber penetration directly correlates to the bandwidth throughput of each defined architecture and, therefore, the service capability for the operator. As discussed earlier, the bandwidth requirements of each carrier differ, but all are growing. The carrier must take this into account as it deliberates over the desired architecture to deploy. Fiber penetration is also an indicator on the capital expenditures (CAPEX) and operating expenditures (OPEX) expected. Deep fiber will result in a higher CAPEX for existing neighborhoods, but is actually near cost parity with all architectures for new builds. Deep fiber will deliver the maximum amount of OPEX savings comparably. FTTH enables the delivery of savings because of reductions in cost for network, central office, and outside plant operations as well as customer service. Network reliability dramatically increases as well, with FTTH ensuring a steady stream of revenue and enhanced customer satisfaction.
