In How Do Optical Taps for FTTH Work? Part I, we introduced why network design professionals may choose to deploy optical taps in rural applications where customers can be spaced far apart. These taps pull a targeted amount of signal from a fiber optic strand and then allow a specific amount of signal to drop off to the subscriber and the remainder of the signal to transmit along the fiber optic strand for other users. In part II, we will review how optical taps distribute that signal, differ from Passive Optical Splitters (PON), and why they are a popular choice for rural network designs.
Taps allow the service provider to deploy fewer optical fibers in rural applications since multiple users can tap into a single optical fiber.
Higher-density urban and suburban FTTH networks often use a PON design. Standard PON networks are typically made with passive optical splitters. These splitters have a single incoming fiber (input) and split its signal up to typically 32 subscribers with a single fiber to each user (output). The passive optical splitter is used to take the signal and split it downstream to multiple users. The splitters are in ratios of 1:2, 1:4, 1:8, 1:16 or 1:32 (1:64 is available but less common). They take the single input and distribute it into symmetrical outputs (e.g., a 1×8 splitter takes one signal in and 8 signals out.) These splitters can be centralized or cascaded and distributed in the network. A distributed splitter-based PON network is a cost-effective design for urban and higher-density FTTH networks as it utilizes fewer switch ports, less fiber cabling, and uses unpowered splitters. This reduces capital and operational expenses for these point-to-multi-point networks compared to point-to-point Active Optical Networks.
However, in rural markets, the subscriber density is lower, and the distances to connect them are longer. In these applications, a TAP network may be a more suitable design. In a TAP network, the fiber is deployed through the service area, and fiber optic taps divert optical signals to the subscriber. These taps can be thought of as distinct from the splitter. The output is not symmetrical like the splitters we noted above. The optic tap is spliced onto the fiber which splits off a portion of the signal, say, for example, 10%, and it passes the remaining 90% along to the next drops. The tap allows a pre-determined ratio of the signal to continue down the line to the next home or business. Multiple taps can be spliced onto the line and “daisy chained” along the route until the signal or loss budget is fully used up. Taps allow the service provider to deploy fewer optical fibers in rural applications since multiple users can tap into a single optical fiber.
UCL Swift PSPL taps use asymmetrical 1×2 FBT couplers (Pass-Through) and a 1xN PLC splitter (Drop ports) in which the input signal is divided in two directions. The signal is directed to the Pass-Through port and also directed to the PLC splitter, where it is distributed to subscriber drop ports (N= 4,8,16 split ratios). To aid with network installations, UCL Swift PSPL hardened tap terminals are designed with color-coded caps to designate the tap port function. Blue Cap=Input Port. Orange Cap=Through Port. Black Cap=Output/drop port.
Asymmetric optical tap solutions like UCL Swift’s require less fiber than a distributed splitter configuration.
As mentioned in Part I, when you add more taps to the circuit, you reduce the amount of signal passing from one to the next. Eventually, as the circuit is populated with more and more taps and the signal is siphoned off for the associated users, the signal will become too weak to meet the system’s minimum performance requirements. At that time, the fiber optic strand is fully utilized and another strand must be used for additional users. So as with all fiber networks, it is important for the network designer to calculate the link loss budget to make sure the design does not exceed the limits.
Rural areas provide different challenges than urban or suburban topologies in FTTH design, and the network engineer needs to consider all the options best suited for the location being served. The long distances to connect rural areas can contribute to a high amount of fiber required to serve that area. A key benefit of tap architecture is the reduction in the amount of fiber required, which allows the region to be served at a lower cost. Asymmetric optical tap solutions like UCL Swift’s require less fiber than a distributed splitter configuration. By using signal splitting, taps with established split ratios allow the use of fewer fibers than conventional centralized and distributed splitter architectures. This provides a network that reduces expenses on equipment and labor and is easy to maintain and expand. As such, optical tap networks are a cost-effective, operationally efficient design option for these applications.