Maximizing the efficiency of optical line terminal (OLT) cards in passive optical networks (PON) in low-density and rural FTTx installations can be a major challenge. In most PON designs, it is considered ideal to connect 32 subscribers to a single OLT for maximum cost efficiency.
In rural installations, however, the density can range from a home every mile or two, to 20-30 homes scattered in random patterns. In these instances, connecting 32 users to a single OLT may be not be economical using standard optical splitters. Fortunately, tap splitters provide an easy method of maximizing OLT usage without being penalized by the number of subscribers connected.
Most PON networks use one of three physical topologies when using optical splitters.
- The home run topology places the splitter within a service provider’s facility and is recommended for high density, short distance designs.
- The centralized topology places a single 1:32 splitter in the OSP inside cabinet, pedestal, or even a splice closure.
- The distributed topology has two or more splitters that are cascaded off one another, i.e., 1:2 x 1:4 x 1:4 = 32 subscribers. It is this topology that provides the best solution.
A single 1:32 splitter adds an average of 15.8 dB of loss to the span, on top of the attenuation added by the fiber, splices, and connectors. In a standard distributed design, each 1:2 optical splitter would have a loss of 3.4 dB. After the sixth 1:2 splitter, the total attenuation would be 20.4 dB. After adding the fiber, splice, and connector attenuation values, the total loss would limit the number of subscribers to six or seven. Therefore, the OLT’s utilization rate would be only 20% rather than the desired optimum 100%.
For this reason, tap splitters offer a unique solution for low-density installations. These products can taper the split percentages in increments ranging from as low as 1/99 percent up to the standard 1:2 (50/50) types. With a tap splitter in place, the cable near the first subscriber would have a drop cable spliced to the 1% leg of the splitter, and the other 99% of the optical power would be transmitted down the span.
As each subscriber would have different attenuation levels due to the splitter and span variations, a loss budget would need to be calculated for each subscriber between the OLT and their ONT. The maximum loss allowed per subscriber would vary as the distance from the OLT increases. This — along with splitter attenuation differences based on the split percentage — requires attention to detail when planning the system.
This first subscribers on each fiber would use the 1/99 tap splitters until the loss budget required a larger split percentage such as a 2/98 splitter. The percentages would continue to increase to maintain the optical power level at the ONT until the last splitter is installed, which would normally be a 1:2 (50/50) split.
This technique helps to provide greater OLT utilization while still making use of standard OSP cables and closure products. Since tap splitters are the same size as most heat shrink protectors used to protect fusion splices, they can fit into standard splice trays, offering a cost effective technique for fiber access in low-density rural applications.