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A wide array of outside plant components is used to build FTTH networks. The earliest FTTH networks borrowed from the designs of metro and long-haul networks and became simple extensions of these networks. Soon it became clear to the industry, though, that if FTTH was to become ubiquitous, specialized products and installation methodologies would have to be introduced. Innovation would be required to tackle the high cost of access networks, address deployment velocity, and improve network reliability.
All FTTH networks inherently are designed to deliver an optical fiber to the subscriber. However, their design is highly dependent on the unique nature of the access environment, so product and design flexibility is critical. At their core, FTTH networks contain an optical line terminal (OLT), optical cable, and optical network terminal (ONT). Various other specialized components are added to address the unique nature of the access network.
The OLT is typically at the CO/HE but can also be in a remote terminal in the field. The OLT houses the laser transmitters dedicated to each user in a PTP network or shared across several users in a PON. The OLT is also the aggregation point of voice from the public switch telephone network (PSTN), data from a router, and video via its multiple forms.
The optical fiber carries the signal to the user and is divided into three sections: feeder cable (terminated at the CO/HE), distribution cable (fanning out across the access network and connect to the feeder cable "feeds"), and drop cable (used to physically connect the users to the FTTH network). As a medium, optical fiber's bandwidth is only limited by the transmitters of the OLT and hence future-proofs the access network because of its tremendous bandwidth capacity.
The ONT receives the signal from the OLT and converts it into usable electronic signals that a user's telephone, computer, TV, or any other number of devices can receive. The ONT also serves to communicate IP traffic back to the OLT such that voice conversations can occur, Web pages can be requested, and TV channels can be changed. Typically, the ONT is connected to a battery backup device providing a limited time period (typically eight hours standby) of lifeline services.
As discussed, PTP networks are characterized by their simplicity. A PTP network minimizes the number of components in the field and has all the items described above as well as enclosures used to connect the multiple cables deployed in the field. PON networks more efficiently utilize the optical fiber in the field and the transmitters of the OLT. Therefore, their design is more complex than PTP.
Beyond the OLT, optical cable and ONT, the PON includes many specialized components that serve to address the cost, deployment, and reliability concerns of earlier FTTH deployments (see Figure 7). The most important of these is the optical splitter. Depending on the split architecture chosen, splitters can take the form of 1x32, 1x16, 1x8, 1x4, or 1x2 and can be almost anywhere in the access network. As discussed, many carriers choose the centralized split architecture because of its inherent efficiencies. The aggregation of splitters is typically in a cabinet called a local convergence point (LCP). This is where feeder cable ends and distribution cable begins (from here, each customer has a dedicated fiber). The distribution cable then snakes its way into the neighborhoods and buildings of the access network. When a distribution cable nears a user, a network access point (NAP) is used to access a small number of optical fibers in the cable. From this point, drop cables, usually containing one to four fibers, are used to connect to the subscriber's ONT.
Figure 7
A recent standardized innovation in the drop cable and NAP is the use of environmentally hardened connectors. Legacy networks connected all the optical fibers of all access components with an optical splice, either mechanical or fusion. While typically introducing little optical loss into the network, the splice introduced high cost into the network deployed because of the time involved to achieve one splice and the technician skill level and equipment deployment requirement. Connectors eliminate these costs, greatly improving deployment velocity while introducing little loss into a network because of the short loop lengths inherent in access networks. FTTH network connectors are standardized technology governed by Telcordia GR-3120.
