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The deployment of optical fiber in an access network can be achieved in multiple ways. In fact, many access technologies are commonly referred to as FTTx when in fact they are simply combinations of optical fiber and twisted pair or coaxial cable networks. These technologies do not provide for the inherent capability of an FTTH network. Nonetheless, it will be useful for us to discuss them later in this tutorial.
Figure 2
FTTH is simply the 100 percent deployment of optical fiber in the access network. It is commonly deployed in two specific configurations. In the first one, fiber is dedicated to each user in the access network. This is called a point-to-point (PTP) network. In the second, one fiber is shared (via a power splitter) among a set number of users, typically between sixteen and thirty-two. This is called a passive optical network (PON). There are advantages and disadvantages to the deployment of PTP and PON networks based on financial, bandwidth, and component considerations.
PTP networks are characterized by the use of one fiber and laser per user. They are the simplest FTTH networks to design. PTP networks are sometimes referred to as all-optical Ethernet networks (AOEN). Figure 2 illustrates several examples of how PTP architectures might be deployed. Again, a dedicated fiber is terminated at the subscriber and active devices at the central office (CO) for a telecommunications provider or head end in the case of a cable TV (CATV) operator or a remote device in the field. The remote device or switch in the field is always an active device and must be powered. Single-mode or multimode fiber media can be used throughout the network. PTP networks have active electronics in the field, are inherently simple, are fiber-rich and require no sharing of fiber or bandwidth for the subscriber.
Figure 3
PONs are characterized by the "splitting" of the optical fiber one or more times in the field, resulting in the sharing of the optical fiber among multiple users. The fiber in a PON is typically shared by sixteen to thirty-two users. Hence the bandwidth of the fiber originating at the CO/HE is shared among a group of users. The splitting of the network is accomplished by an optical splitter. These splitters can split the fiber one to thirty-two times and, by their nature, introduce inherently high losses in the network. Therefore, their use is limited because of the power budget considerations of the network. A PON will have less optical reach than a PTP network, which does not use splitters. Typically a PON is capable of reaching subscribers 20 kilometers (km) from the original transmitter, which will cover 98 percent of the population. A PON uses no electronics in the field and is supported by a set of mature standards and is the most widely deployed FTTH architecture in the United States. Figure 3 illustrates the multiple configurations of a PON. The individual components of a PON will be discussed in more detail in the FTTH Outside Plant Components section of this tutorial.
Carriers deploying PONs have additional architectural choices to sort through-most notably, deciding between a centralized splitter and a distributed/cascading splitter arrangement. Both are deployed for different reasons depending on the tradeoffs of their specific characteristics.
Figure 4 - Centralized Split
A centralized split provides for a "central" location for all the PON splitters-typically in a passive, field-rated cabinet (see Figure 4 for example). Carriers looking to maximize port efficiency in the CO/HE and use of 1x32 splitters to maximize the shared capacity of the fiber plant will be drawn to a central split configuration. This results in minimizing the number of transmitters used in the CO/HE and optical splitters and fiber in the field. Centralized split architecture also provides for a better overall loss measurement for the PON, thereby increasing network reliability. A single 1x32 splitter has less loss than 1x2 and 1x16 or 1x4 and 1x8 cascaded splitters or any combination of 1x16, 1x8, 1x4 and 1x2 splitters in the network. This improves optical reach and the reduction of optical components is directly proportional to increased reliability of the network via the reduction in points of failure. In addition, centralized split has been shown to minimize capital expenditures (CAPEX) of splitters initially in the network, facilitating a "pay-as-you-grow" approach because of the higher splitter output port efficiency at low to medium take rates. Centralized split also provides for simplification of network troubleshooting and fault location that directly translate into labor savings.
Figure 5 - Distributed Split
A distributed/cascaded split configuration results in pushing splitters deeper into the network (see Figure 5 for an example). As the splitters are not centralized, the requirement for field cabinets is reduced or removed as splitters are commonly incorporated into modified enclosures or even back in the CO/HE. The sharing of a CO/HE transmitter among 32 users is still achieved through the distribution of multiple splitters along the optical path-for example, a 1x4 followed by a 1x8, at different locations in the network, results in bandwidth sharing among 32 users. The deep positioning of splitters can result in the "stranding" of splitter assets as the carrier awaits new subscribers on the network or take rates are low. Network testing and fault location can be more difficult with a distributed/cascade split configuration as it is difficult for test equipment to see through an array of splitters along the optical loop. Network reliability can be affected by increased optical components.
