Migration to GPON: Practical considerations from the central office to the outside plant

Nov. 27, 2007
By Patrick J. Sims and Pat Thompson, ADC -- Practical considerations—based on informed decision-making—provide the foundation for a cost-effective transition between legacy and future access technologies.

Practical considerations—based on informed decision-making—provide the foundation for a cost-effective transition between legacy and future access technologies.

By Patrick J. Sims and Pat Thompson, ADC

Service providers need not look too far in the past to find examples of networks built without consideration for future technologies. While our telecommunications forefathers may not have predicted today's broadband revolution as they designed the copper telephone network, this legacy infrastructure still enabled a rough deployment of xDSL technologies.

However, the unpredictable performance of xDSL over load coils, splices, varying gauges and conditions of the legacy copper network produced costly lessons in the importance of network flexibility. Fiber to the premises (FTTP), by contrast, enables service providers to deploy new subscribers with a "clean slate."

As many sources predicted, GPON now promises to dominate the access market by offering a bandwidth boost and enabling higher split ratios. GPON's entry as the "latest and greatest" PON flavor also coincides with challenges service providers face in delivering high-speed, high-bandwidth packaged services to business and residential customers. The pressure is on for providers to make their networks GPON-ready from the central office (CO) to the outside plant (OSP).

Migration-ready PONs

There are several critical parameters to consider when designing a PON for ease of migration to GPON, including fiber-optic cable characteristics, optics classes, and split ratio implications. When increasing split ratios from 1×32 to 1×64 or even higher, for example, spectral attenuation will become an important factor. Optical link budgets are determined by the individual vendor's active components—PON chips within the electronics, lasers, and receivers. The loss range for each class is as follows:

  • Class A: Min. 5 dB to max. 20 dB
  • Class B: Min. 10 dB to max. 25 dB
  • Class B+: Min. 10 dB to max. 28 dB
  • Class C: Min. 15 dB to max. 30 dB

Traditional BPON equipment has typically used Class B optics, but it was determined that some 20-km PON networks were actually stretching the limits of the existing optical budget, forcing active equipment manufacturers to increase budgets to 26.5 dB. These increased budgets, coupled with the possibility of further increasing split ratios with GPON, have resulted in an increase in the Class B receiver photodetectors to allow for a 28-dB loss budget, thus establishing the Class B+ optics category.

Connectorization also plays a huge role in a migration-ready FTTP network. With the addition of next-generation video requirements, GPON systems will likely require higher power, necessitating the superior performance of angled physical contact (APC) connectors—particularly in the PON portion. The APC connector is the best choice for high-bandwidth applications and long-haul links since it offers the lowest return loss characteristics of any connector currently on the market.

In an APC connector, the end-face of a termination is polished precisely at an eight-degree angle to the fiber cladding to reflect most of the return loss into the cladding where it cannot interfere with the transmitted signal or damage the laser source. As a result, APC connectors offer a superior return loss performance of –65 dB. For nearly every application, APC connectors offer the optical return loss performance that broadcasters require to maintain optimum signal integrity.

Systems should also ensure built-in laser safety features for use in networks with Class B and C lasers. Laser safety must be considered with high-power lasers typically used in the analog video optical line terminal (OLT). Since infrared lasers are not visible to the human eye, it's important to take precautions when exposure is possible. Fiber distribution frames need to have built-in laser eye safety features—features that ensure connectors don't point directly at technicians. Designs that have connector ports contained within a tray or other enclosure and pointing side to side—rather than straight out of the panel—help protect technicians, regardless of their level of training or awareness.

Advantage of centralized splitters

The ease of migration from earlier PON architectures to GPON will be dependent on the design of the fiber distribution portion of the network or the link between customers and the central office. This refers mainly to the splitter configuration and how efficiently each OLT card is used.

The two common splitter approaches are centralized and distributed or cascaded configurations. The centralized splitter approach uses 1×32 splitters in OSP enclosures, such as fiber distribution terminals. Each splitter is connected to an OLT in the central office with 32 split fibers routed from the optical splitter through distribution panels, splice points, and/or access point connectors to the optical network terminals (ONTs) at 32 homes.

The distributed or cascaded splitter approach is typically configured with a 1×4 splitter residing in the OSP enclosure and connected directly to an OLT in the central office. Each of the four fibers leaving the 1×4 splitter is routed to an access terminal housing another splitter, either a 1×4 or 1×8. Optimally, there would eventually be 32 fibers reaching the ONTs of 32 homes.

A centralized approach offers several advantages in terms of flexibility. First, it maximizes the efficiency of expensive OLT cards. A cascaded architecture will strand unused ports in areas of low take rates or where customer premises are not grouped tightly together. Other advantages to a centralized splitter architecture include easier access for testing and troubleshooting (it's very difficult for an OTDR to "see" down individual fiber lengths through a series of splitters) and a reduction in splitter signal loss by eliminating extra splices and/or connectors in the distribution network.

More importantly, however, a centralized splitter configuration provides the best means to futureproof the network by offering the flexibility to migrate to next-generation PON technologies, such as GPON, particularly with the likelihood of increasing split ratios from 32 to 64 or higher.

Implications of split ratios

Since much of the GPON standard already revolves around centralized 1×32 splitter architectures in the OSP, GPON's ability to enable 1×64 splits is a huge benefit—servicing twice the homes from a single splitter. However, upgrading a cascaded architecture to a 1×64 centralized architecture will involve significant investment and deployment of additional fiber to take advantage of the full capabilities of GPON.

A network built with the minimum number of connections, including splitter ports, will minimize optical loss while maintaining the flexibility necessary to ensure that equipment and customer churn can be quickly and cost-effectively accomplished. Splitter loss depends mainly on the number of output ports. Each splitter configuration is assigned a particular maximum split ratio loss, including connectors, defined by the ITU G.671 standard and Telcordia GR-1209.

Since the GPON standards have not yet defined the current split ratio maximum for 1×64 splitters, network designers must use a single 1×2 splitter interfacing two 1×32 splitters to make up the 1×64 configuration. Although this is allowable with today's packaging, using Class B optics leaves only 5.35 dB of "head room." Therefore, even with the best fiber manufactured, where the spectral attenuation is 0.31 dB per km, only a 17.25-km PON network is achievable without including any of the connectors within the CO or the splices in the OSP.

Still, the design engineer does have some options. In designing the network, premium splitters and low-loss connectors can be deployed, and fusion splices must be kept well below 0.05 dB of loss per splice. These and other techniques will be used until the standards line up with the technology for 1×64 and higher split ratios. In any case, it is easy to see that moving to a 1×64 split ratio from an existing centralized configuration will offer the best flexibility, easier test access, and the greatest overall cost efficiencies in most FTTP applications.

GPON-ready: From CO to OSP

Within the CO, flexibility is the key. A network should never be built for a single application. Rather, it should be built as a flexible long-term network that can adapt to changes in equipment and technology. A crossconnect network offers excellent flexibility for configuration points. The output connector side is an important consideration and should include high-quality connectors that can accommodate higher power. Again, as optical output levels increase, APC will offer both flexibility and adaptability in a migration to GPON technology.

Cable management is critical in the CO, particularly bend radius protection. Serving more and more subscribers requires careful consideration of loss budgets and physical fiber management methods that protect the optical signal from any degradation. The CO considerations for GPON can be summarized in three words: flexibility, quality, and protection.

The same architectural principles for the CO can be applied to the OSP portion of the network to ensure a smooth migration to GPON; the emphasis should be on centralized splitting. As described earlier, it's much easier to upgrade to a higher split ratio from a centralized approach than a cascaded approach. There is some serious doubt as to whether cascaded systems can even be converted to GPON without significant expense and overhaul.

The selection of connectors in the OSP is another important element to GPON upgrades. Some vendors may tell customers that APCs are too expensive and not necessary for GPON networks. That may have been true at the onset, but the economies of scale in recent years have resulted in SC/APC becoming more cost effective.

The trend toward pushing fiber all the way to the customer premise has established the need for high-performance hardened APCs that can withstand the rigors associated with OSP implementation. Connectors must perform in severe environments and varying temperature extremes. Today, cost-effective APCs are available and specifically designed to meet the highest OSP performance standards—minimizing loss budgets and mitigating reliability issues such as endface geometry and temperature variation.

While service providers strive to meet the challenges of upgrading their FTTP networks to GPON, equipment vendors should seek to make any migration as seamless as possible. Flexibility is always the key to achieving upgrades as easily, quickly, and painlessly as possible.

Patrick J. Sims, RCDD, is a senior principal systems engineer for ADC (www.adc.com) and serves as a board member of the FTTH Council. He is responsible for project management and operations of network design and systems integration for ADC's OmniReach fiber-to-the-premises (FTTP) solutions.Pat Thompsonis program manager for central office fiber products in ADC's Global Connectivity Solutions Group. He has worked with service providers around the world to design, engineer, and manage fiber-optic networks.