Fiber Construction, Part 3: Certifying PON with Unbalanced Splitter Architecture
As we know, the demand for broadband services is on the rise and demand is not restricted to towns and cities. Folks living in rural locations need good-quality, high-speed services just as much as their urban cousins. For the majority of cases, especially greenfield sites with no existing infrastructure, Passive Optical Networks (PON) and Fiber-to-the-Home (FTTH) services are seen as the future-proofed approach.
This is an important issue in a lot of countries and has led many national and regional governments to launch projects to encourage providers to build networks and deliver FTTH services for both urban and rural areas.
The problem for service providers is that the business case for doing so doesn’t always add up, so they look at alternative PON architectures to overcome the challenges and make the business cases work for both urban and rural applications.
Until recently, the majority of PON deployments have been based on a traditional architecture with a feeder fiber connecting a local or central office to an optical splitter mounted in a street cabinet. Then, feeder fibers connect splitter ports to the end customer via a hub or drop terminal, again mounted in a street cabinet or some similar, ruggedized enclosure. The splitters in these single-split architectures are usually 1×16 or 1×32, equating to one common port and either 16 or 32 splitter ports feeding service to customers. Quite often these splitters are spliced to the feeder and distribution fibers. This helps to reduce optical losses over, say, using connectors (optical loss budget in PON is a critical issue), but does take a lot more time and effort during the construction phase.
This is fine for the majority of applications or environments but not always the most attractive option for service providers, especially in more dense urban environments where there are a larger number of potential customers in a smaller geographical area. In these situations, service providers can opt for higher-split ratios and adopt a cascaded splitter architecture. For example, with a 1×8 splitter feeding multiple 1×8 splitters or a 1×16 splitter feeding 1×4 splitters, you can achieve an overall 1×64 split ratio – meaning that a single Optical Line Terminal (OLT) port can serve up to 64 customers instead of 32, providing more revenue per OLT port and better ROI.
There are drawbacks to this approach. One concerns the maximum distance (or reach) of the PON network, typically no more than 20km, which means that these cascaded splitter and higher-split ratio architectures are not well-suited for rural environments where you have longer distances to cover and smaller groups of homes. Another drawback is the upfront cost to build the network. With splitters being spliced to fibers, it means you have to build your entire PON in one go. There is no option to build out stage-by-stage and according to demand for service. In addition to upfront investment for fiber and components, the cost also includes labour.
The answer to these issues lies in a more recently-adopted approach, one based on an unbalanced (or tapered) splitter architecture. The splitters mentioned above could also be described as balanced splitters as they divide optical power arriving on a common port evenly between the 4, 8, 16 or 32 splitter ports. An unbalanced splitter, which is typically a 1×2 device, will divide optical power unevenly between the two splitter ports. For example, this divide will start with something like an initial 10/90 split with 10% of the optical power fed through to one port and 90% to the other. The 10% that is “tapped off” will deliver service to a customer or small group of customers while the 90% that passes through will feed the next splitter in the cascade. As you move down the cascade of splitters, the ratio of power split will change in order to tap off the correct amount of power to deliver service; for example, 10/90 to 15/85 to 20/80 and so on:
The benefit of this unbalanced splitter approach is the ability to cover longer distances between splitters and drop (or tap) off service to smaller groups of customers. This makes it ideal for rural or longer-reach applications. Another benefit is that the unbalanced splitters are connectorized, meaning that building a PON can be quicker, something analogous to a “plug-and-play” approach (no extra time splicing). This also means that you don’t have to build the whole PON in one go; you can phase the build as and when demand is there. Unbalanced splitters are also smaller, requiring smaller enclosures meaning they can be pole mounted. Both factors contribute to lower initial deployment costs.
Challenges definitely exist in this environment, too. More connectors mean more potential points of failure, so connector inspection becomes even more critical than before. The big issue comes when characterizing and certifying the final end-to-end build with an OTDR. Any decent, modern OTDR will be able to recognize a balanced splitter in a trace and report it as such. However, unbalanced splitters produce a slightly different event signature and unless your OTDR is optimized to automatically recognize it, the likelihood is that the unbalanced splitter will be misinterpreted as a bad connector or fiber bend. This plays out as a certification failure and unnecessary investigation or re-work (or worse, still-manual manipulation of OTDR results to change misinterpreted event types), all of which is time and money.
Luckily, VIAVI has already added unbalanced splitter support to its PON OTDR products and FTTH-SLM application, forming the industry’s most advanced solution for certifying traditional and new unbalanced (or tapered) splitter PON architectures end-to-end. This ensures network deployment, activation and maintenance tasks happen right – the first time.
For more information, check out the FTTH-SLM solutions brochure or for a guide to what test is required through all phases of the networks lifecycle (build->network activation->service activation->maintenance), download/order a copy of our Understanding PON Testing poster.
Douglas Clague is currently solutions marketing manager for fiber optic field solutions at VIAVI. Doug has over 20 years of experience in test and measurement with a primary focus on fiber optics and cable technologies, supporting the telecommunications industry. Prior to VIAVI, Doug held positions as manufacturing engineer, solutions engineer and business development manager. Doug has participated on numerous industry panels around fiber and cable technology trends. He attended Brunel University in London and graduated with an honours degree in electrical and electronic engineering.