Optical Network Design
New electronics and standards are enabling cable operators to leverage all-fiber access networks (AFANs). New approaches to the outside plant make all-fiber access more cost effective, as well as a better fit for the cable deployment model.
Segmented splitting takes a new approach to network design that permits smaller groups of homes, thereby creating a reduced footprint and eliminating the need for large splitter cabinets. The distribution network in the neighborhood also has new options. In addition to multiport terminals, new branch- and series-connected terminals make design easier and reduce the overall cost of preconnectorized assemblies, which enables faster deployments. These new components can be placed in traditional outside plant locations such as pedestals, strands and hand holes.
Traditional all-fiber access
Traditional all-fiber access solutions, especially fiber to the home (FTTH) deployed by many telcos, have been primarily built around one architecture: the centralized split. To date, the most commonly deployed version of centralized splitting has been a "concentrated" strategy in which 200 to 800 homes are served from one splitting location or local convergence point (LCP). The implementation of that architecture has meant the use of cabinet-style housings to contain all the splitters. This approach has many benefits, but may not always be conducive to the cable operator space.
Some of the benefits of the concentrated/centralized splitting model include:
• A central testing and management location for many subscribers
• The ability to scale high-value assets, such as optical splitters, to match take rates
• A high degree of future flexibility, including wavelength management and technology overlay/mixing capabilities
Considerations surrounding this approach for cable operators include:
• High-fiber-count distribution cables emanating from the cabinet (many fibers to splice if something is cut)
• A larger physical footprint and need for vaults and pads to mount the cabinets
• Typically, external splicing that requires additional closures and hardware
• Challenges in neighborhood "phasing": scaling infrastructure deployment to match development phases
Many service providers feel the benefits outweigh the negatives in this approach, while others prefer a different aesthetic footprint or one that better fits the space, design and deployment methods to which they are accustomed. Cable operators are one type of provider whose own traditions suggest that a different approach may be required for the passive plant. A brief look at current cable design and environmental aspects will help suggest possibilities for making an AFAN that looks and feels more like a cable installation, while providing the capacity and future-proofing potential of an AFAN.
Comparison to cable
Current cable design trends have been driving fiber deeper and deeper into the network toward the subscriber. This has meant node splitting/segmentation for existing networks and fewer homes passed per node for new builds. Today’s designs have been honing in on the 100 to 125 homes passed-per-node ratio in order to support the capacity and service capabilities that customers want, and which cable operators need to provide to compete with other carriers. This is in contrast to the typical 200 to 400 homes passed per splitter cabinet in traditional concentrated all-fiber design and the 250 to 500 homes/node seen in legacy HFC designs.
At the heart of the 100- to 125-home HFC node grouping is the active node electronics, which are often mounted in a doghouse-style pedestal for new builds or aerially in many developed areas. The pedestal approach requires relatively inexpensive hardware and a minimal amount of time and labor for installation when compared to a cabinet. Amplifiers along the coax path are mounted in similar hardware, and coax taps for drop connection are mounted in pedestals and on the aerial strand. This all makes for a very cost-effective approach to network deployment.
In addition, the typical cable network has power supplies located at each node and, in many cases, at supplemental locations along the coax to power the nodes and amplifiers. These are usually deployed in cabinet form on the ground or may be mounted to a pole where power can be easily connected to the electric utility.
New approaches
Electronics solutions are now available and continually evolving to allow cable operators to leverage AFAN in their new builds and in major upgrades. Both the SCTE’s work on RF over glass (RFoG) and the solutions being offered by major electronics vendors are sensitive to basic requirements for the physical plant.
While the electronics are designed to be compatible with both headend/hub gear and customer premises equipment (CPE), they are also designed to function on the same physical layer as other optical access technologies such as Ethernet passive optical networks (EPONs) and Gigabit PONs (GPONs) and their pending 10 Gbps versions.
Generally, this means a 28 dB maximum loss budget and a 20 km distance reach from the last headend/hub active device to the subscriber (based on a 1×32 split ratio). Optical splitting ratios vary somewhat, but 1×32 and 1×64 seem to be the most common at this time; a 1×32 ratio is assumed for the examples in this article.
Given that these basic elements are satisfied, there are a number of ways to design the passive portion of the all-fiber network.
The concentrated splitting method described earlier is just one approach to network design. The evolution of the HFC network toward smaller subscriber groups per node represents a move to a more segmented design approach. Even existing HFC networks undergo node "splitting" or "segmenting" to increase performance. This segmented approach may be physical (truly separate nodes), but is most often a logical process for existing infrastructures.
To combine the benefits of centralized optical splitting with the cost-effective and minimally aesthetic footprint of the HFC network, it is possible to use a segmented splitting strategy. This approach breaks the neighborhood into smaller design areas-with up to 128 subscribers (four 1×32 splits) – allowing the required hardware to be reduced in size with simplified mounting. This design method has a number of key benefits:
• Reduces fiber counts radiating out and around the splitter housing
• Improves the ability to "pay as you phase" in new neighborhoods
• Retains the asset scaling capabilities of the concentrated split strategy
• Because it supports multiple splitters at an LCP, multiple services or service tiers can be managed on separate splitters
• Offers the future-proofing capabilities of the centralized splitting architecture that may not be readily available in distributed splitting approaches
Figure 1 illustrates the segmented splitting approach applied to a neighborhood map. Traditionally, a single cabinet would be placed near the entry to the neighborhood, with all the splitters for the homes passed. High-fiber-count cables would "radiate out" from this point. In the segmented split model, several smaller LCPs are placed in the neighborhood, in this case, two for phase 1 and two at a later date for phase 2. Fiber is provisioned for the future phase, but no hardware is required until it is built out.
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In addition to segmented splitting as an architectural model, there are new ways to design and deploy the distribution portion of the network between the LCP and the subscriber.
Traditional all-fiber design has relied upon three approaches. First, network access points (NAPs), also called "terminals," which permit drop connections, can be spliced in by creating mid-span access points on the cable. Another spliced approach is to use two to six pre-stubbed NAPs, splicing all of their stubs at one point on the distribution cable, thus reducing the number of splice points. Preconnectorized solutions replace the splicing function with a factory pre-configured access point so that the terminal can just be "plugged in" to the access point when needed.
Of these, combining up to six NAP stubs at one splice or connection point is the most cost effective because it reduces the number of splice/connection points without requiring unduly long terminal stubs. Until now, this had not been practical in preconnectorized solutions. Further, traditional methods have not offered good models for reaching into cul-de-sacs and crossing roads with minimized sheath placement.
Two new approaches, branch-connected and series-connected NAPs, solve these concerns. Because they connect to 12- and 24-fiber access points on the factory pre-configured distribution cable, fewer access points are required. Without the branch and series capability, only two NAPs can be connected at a factory access point, whereas the branch and series approach allows the access point to be leveraged for six terminals. This achieves the same stub-aggregating approach as in the spliced version, but with the speed of deployment of the factory preconnectorized approach at about the same total installed cost. Figure 2 illustrates how the branch and series NAPs are deployed on the map in Figure 1.
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In Figure 2, the series-connected NAP (shown in the blue outlined area) reaches past the end of the distribution cable assembly to finish the run. Likewise, laterals into cul-de-sacs and small home "pockets" can be served from the main distribution cable. The branch-connected terminals (shown in the red outlined area) allow much more "reach" using the NAP stub cables, while using less distribution cable.
Studies indicate that depending on labor and per-foot placement costs, this method is typically more cost effective than preconnectorized solutions in which each NAP has its own connection to the main cable. It is generally on par with costs for spliced-in NAPs, but offers a much reduced deployment time.
Figure 3 illustrates the segmented split model using small LCPs, preconnectorized distribution assemblies and branch- and series-connected NAPs. It is easy to understand and design.
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There are really two key concepts here. First is the segmented split model itself, as a way to use smaller, lower-cost network elements for optical splitting. The second concept is branch- and series-connected terminals, which offer the most benefit when deployed using preconnectorized distribution cables. The two concepts can be deployed independently. For example, segmented splitting can be deployed using spliced-in NAPs, and branch- and series-connected NAPs can be used with concentrated splitting cabinets.
Migration
A careful look at the history of cable operators’ broadband networks shows a continual evolution of technology to meet subscriber needs. Deployment of optical fiber will not likely change this strategy; however, with proper design considerations, the physical layer – cable, hardware, optical splitters – in the AFAN can be the stable foundation for years to come.
Deployment scenarios could begin with RFoG and expand to overlay EPON or GPON technologies, which will be easy to manage when centralized splitting is used. Technologies that seem distant now will be easier to deploy if the one-to-many relationships between headend and subscriber are maintained in one location instead of distributed locations. An example is wavelength division multiplexing (WDM) PONs (WDM-PONs), in which each subscriber enjoys a unique wavelength rather than a share of the optical power from one wavelength. Effort should be made before deploying any architecture to make sure that it is the best one for the application. Very low and rural densities may benefit from alternative approaches.
Conclusion
Two key concepts have been illustrated: segmented splitting plus branch- and series-connected NAPs. Segmented splitting allows all-fiber access design to be carried out using methods and elements that resemble current HFC design. Branch- and series-connected NAPs add a new dimension and support rapid, cost-effective deployment of preconnectorized solutions at a total installed cost similar to more labor intensive spliced-in NAP models.
The all-fiber access network can be designed and deployed in ways that are easily adopted by those accustomed to HFC design. And, with the proper considerations, that network can be the foundation that supports an evolution of delivery technologies, keeping cable competitive in the marketplace and keeping subscribers happy with their access experience.
Mark Conner is market development manager for Corning Cable Systems. He is also chairman of the SCTE Interface Practices Subcommittee (IPS) Working Group (WG) 5.
