As the deployment of FTTH (fiber-to-the-home) solutions increases, real world network topologies are challenging the abilities of traditional passive optical network (PON) architectures. PON technology was conceived and standards were developed within the Institute of Electrical and Electronics Engineers (IEEE) and International Telecommunication Union (ITU) around a centrally located optical line terminal (OLT) delivering services over a single-fiber architecture to 32 optical network units (ONU) at a 20 kilometer range. Standards-based optics have been developed to extend that range to 30 and 40 kilometers, providing good solutions for simple range extensions while keeping the optical distribution network (ODN) truly passive.
Now, it should be noted that 64 and 128 splits are supported in the traditional PON architectures as well. Those traditional PON architectures are quite suitable for delivering the benefits of PON to the majority of network operators’ subscriber base; however, there are situations for which those solutions are not a perfect fit. For example, areas with limited fiber availability and deployments needing to provide service beyond the range of traditional PON are driving the need for alternative answers. This challenge demands a solution and here are three ways CommScope is actively engaged in solving these problems.
Put the OLT in a Node
For brevity, let’s call this a Node-based OLT or N-OLT. By doing this, the OLT can be placed practically anywhere in the outside plant with traditional PON distribution from that point on. By using available Ethernet-over-fiber technologies, the N-OLT uses its fiber feeds efficiently, requiring as few as one fiber to serve the OLT. Those same technologies also allow the N-OLT to be located long distances from the headend and close to subscribers where short drops and high split counts can optimize the network. The trade offs of this architecture include the challenges of active elements in the outside plant, such as powering/back-up power and increased maintenance logistics.
Temperature-harden a Smaller Remote OLT
Let’s call this one the Remote OLT (R-OLT). This is so it can be deployed outside of an environmentally-controlled headend environment, but in a rack mounting form factor. Hardening enables the R-OLT to be installed in equipment rooms, OSP cabinets and similar locations where temperatures are not controlled as they are in a headend, but are protected from rain and snow. This accomplishes several things. Similar to the N-OLT, the unit is closer to the end user, allowing for shorter drops and less fiber from the splitter to the ONU. And like the N-OLT, feeds from the network are fiber-efficient and have a great deal of range flexibility. The tradeoffs for the R-OLT are that deployment locations are more limited than with the N-OLT and require an external enclosure, but power is typically available where they can be deployed, accessibility can be limited (e.g., when R-OLTs are in customers’ premises), and the expense/complexity of a weather-sealed housing is avoided.
The PON Extender
This design concept is to leave the OLT in the headend; however, instead of standard PON optics with fixed wavelengths, you use multiple wavelengths to multiplex a number of PON streams onto a single fiber. That single fiber connects to the PON extender where the PON streams are recovered and retransmitted from the extender to ONUs using standard PON optics. The following diagram illustrates this idea. This could easily be termed a “PON concentrator” because of its ability to carry multiple PONs on a single fiber between the OLT and the PON extender. The benefits of this approach are:
- Fiber utilization between the OLT and the PON extender is good—only a single fiber is needed for every eight PONs
- It is less complex than an OLT, reducing maintenance and increasing reliability in the OSP
- It can be placed virtually anywhere in the OSP (like the N-OLT) and is in an OSP friendly node enclosure, allowing it to be placed close to power and/or subscribers
There are tradeoffs with this architecture as well, such as distance limitations because of timing requirements for the OLT-to-ONU PON circuit. The PON extender doubles the number of optical transceivers in the PON ODN. And like the N-OLT, it is an active device in the ODN, with powering and maintenance logistics challenges.
All of these solutions have a place in FTTH/fiber-to-the-building networks and expand the ability of PON to be a “universal” service platform. Do they have a place in your network?