PDF of this tutorialFiber-optic technology, offering virtually unlimited bandwidth potential, is widely considered to be the ultimate solution to deliver broadband access to the last mile. Today's narrowband telecommunications networks are characterized by low speed, service-provisioning delays, and unreliable quality of service. This limits the ability of a consumer to enjoy the experience at home or the ability of workers to be efficient in their jobs. The last mile is the network space between the carrier's central office (CO) and the subscriber location. This is where bottlenecks occur to slow the delivery of services. The subscriber's increasing bandwidth demands are often unpredictable and challenging for telecommunications carriers. Not only must carriers satisfy today's bandwidth demands by leveraging the limits of existing infrastructure, they also must plan for future subscriber needs.
A new network infrastructure that allows more bandwidth, quick provisioning of services, and guaranteed quality of service (QoS) in a cost-effective and efficient manner is now required. Today's access network, the portion of a public switched network that connects CO equipment to individual subscribers, is characterized by predominantly twisted-pair copper wiring.
Fiber-optic technology, through local access network architectures such as fiber-to-the-home/building (FTTH/B), fiber-to-the-cabinet (FTTCab), and fiber-to-the-curb (FTTC) offers a mechanism to enable sufficient network bandwidth for the delivery of new services and applications. ATM–PON technology can be included in all these architectures, as shown in Figure 1.
Figure 1. ATM–PON Architectures
In general, the optical section of a local access network can either be a point-to-point, ring, or passive point-to-multipoint architecture. This tutorial focuses on the passive point-to-multipoint architecture (PON). The main component of the PON is an optical splitter device that, depending on which direction the light is traveling, splits the incoming light and distributes it to multiple fibers or combines it onto one fiber.
The PON, when included in FTTH/B architecture, runs an optical fiber from a CO to an optical splitter and on into the subscriber's home or building. The optical splitter may be located in the CO, outside plant, or in a building.
FTTCab architecture runs an optical fiber from the CO to an optical splitter and then on to the neighborhood cabinet, where the signal is converted to feed the subscriber over a twisted copper pair. Typically, the neighborhood cabinet is about 3 kft from the subscriber's home or business.
FTTC architecture runs an optical fiber from the CO to an optical splitter and then on to a small curb-located cabinet, which is near (typically within 500 ft) to the subscriber. It is then converted to twisted copper pair.
The PON can be common to all of these architectures. However, it is only in the FTTH/B configurations that all active electronics are eliminated from the outside plant. The FTTCab and FTTC architectures require active outside-plant electronics in a neighborhood cabinet or curb. This tutorial will focus on FTTH/B architectures.
When fiber is used in a passive point-to-multipoint (PON) fashion, the ability to eliminate outside plant network electronics is realized, and the need for excessive signal processing and coding is eliminated. The PON, when deployed in an FTTH/B architecture, eliminates outside plant components and relies instead on the system endpoints for active electronics. These endpoints are comprised of the CO–based optical line terminal (OLT) on one end and, on the other, the optical network termination (ONT) at the subscriber premises. Fiber-optic networks are simple, more reliable, and less costly to maintain than copper-based systems. As these components are ordered in volume for potentially millions of fiber-based access lines, the costs of deploying technologies such as FTTH, FTTB/C, and FTT/Cab become economically viable.
One optical-fiber strand appears to have virtually limitless capacity. Transmission speeds in the terabit-per-second range have been demonstrated. The speeds are limited by the endpoint electronics, not by the fiber itself. For the ATM–PON system today, speeds of 155 Mbps symmetrical and 622 Mbps/155 Mbps asymmetrical are currently being developed. As the fiber itself is not the constraining factor, the future possibilities are endless. Furthermore, because fiber-optic technology is not influenced by electrical interferers such as cross-talk between copper pairs or AM band radio, it ensures high-quality telecommunications services in the present and future. In addition, fiber does not exhibit radio frequency (RF) emissions that can interfere with other electronics and is regulated by the Federal Communications Commission (FCC).
While copper-based transport technologies remain ubiquitous, the long-term industry belief holds that it is inevitable that fiber will replace copper throughout the access infrastructure. Because copper infrastructure is embedded in communications systems, this transformation to optical transport is expected to occur over many years. Over time, new builds ("Greenfield") will be all fiber based, and existing builds will be rehabilitated by replacing copper with fiber or by overlaying new fiber on the existing copper infrastructure. Electronic equipment, as well, must be replaced with optical equipment.
