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All-fiber access network design using branch-connected terminals

Tue, 06/30/2009 - 8:05pm
Teresa Bazzle, Systems Engineer & Mark Conner, Market Development Manager, of Corning Cable Systems

“I want my MTV.” That line from the 1985 Dire Straits song “Money For Nothing” defined a decade’s worth of consumer demand for more content (which is to say, more bandwidth). MSOs responded by rolling out hybrid fiber/coaxial (HFC) networks. A quarter-century later, the new battle cry is, “I want my HDTV” – several feeds of it, in fact, along with 20 Mbps Internet, streaming video and a single provider for everything. The growing appetite for bandwidth is changing the business model again.

Traditional systems leveraging coax cables are being challenged for capacity, so for new builds, MSOs are considering all-fiber access networks (AFAN) as an alternative. Because optical fiber offers such high-bandwidth capability, it is well positioned as a universal foundation for both business and residential applications.

Technologies to access the bandwidth in optical fiber are numerous, and finally new technologies compatible with existing MSO equipment are now emerging. Because of this, the need for a cost-effective design of the optical distribution network (ODN) becomes even more important. Almost a decade of development and deployments has improved this crucial element, making fiber deployment faster, easier and less expensive. These innovations are driving the costs of fiber toward parity with traditional hybrid fiber/coax. Millions of homes have now been connected by fiber, and the data is gradually rolling in – showing lower operating costs through fewer service calls and higher overall reliability. MSOs can expect to reduce or eliminate network powering, amp balancing and proof testing. These new strategies and solutions offer simpler design methods and potential cost savings to MSOs.

Network access point strategies
A key lesson learned from past deployments is the impact that the network access point (NAP) placement strategy has on deployment costs. Placement strategy impacts deployment velocity, installation costs, and the ability to defer component installation until service is needed and revenue generated. Deferring costs in itself is advantageous, and it’s even more important in situations where the anticipated initial take rate will be low, or where the plant construction may occur in phases.

At first glance, it may seem that using 12-port NAPs would be most advantageous (Figure 1). After all, this would reduce the number of times the cable must be accessed. However, this strategy increases drop cable lengths, as well as the number of drop cables in front of the homes, ultimately increasing cable material and installation costs. This strategy also multiplies the number of homes at risk in buried deployments should a homeowner inadvertently cut the cables when landscaping. Reducing the NAP size (Figure 2) significantly reduces these costs and the risks to any buried plant.

Figure 1: 12-fiber NAP

Figure 2: Three four-fiber NAPs

Table 1 shows a comparison of the amount of drop cable required when using a 12-fiber NAP versus three four-fiber NAPs. This model assumes each lot front is 75 feet wide. Note that the single 12-fiber NAP requires 1,800 feet more of drop cable than three four-fiber NAPs – an increase of 133 percent.

  Lots Total
Length
Drop Length
1
2
3
4
5
6
7
8
9
10
11
12
One
12-fiber NAP
450'
375'
300'
225'
150'
75'
75'
150'
225'
300'
375'
450'
3150
Three
4-fiber NAPs
150'
75'
75'
150'
150'
75'
75'
150'
150'
75'
75'
150'
1350
Table 1:  Comparison of drop cable requirements

This issue is amplified when longer lot fronts occur. A comparison of the approximate material cost per subscriber with varying lot lengths and NAP sizes is shown in Figure 3.

Figure 3:  Cost impact of lot frontage and port count

Note that typically the lowest costs per subscriber occur with four- and six-fiber NAPs. And, as expected, the minimum cost for each port count occurs with the narrowest lot fronts. In the 50 feet lot front scenario, the eight-fiber NAP has the lowest cost; however, eight-fiber NAPs tend to complicate fiber splice plans. Notably, the 12-fiber NAP was also very good when looking at 50 feet lot fronts; but again, the number of drops consolidated in one location is hard to manage and vulnerable to damage.

Preconnectorized cables and multiports
The most significant innovations in the AFAN have been preconnectorized cables and multiport terminals, which are often referred to as terminal distribution systems (TDS). This system employs the use of hardened multi- and single-fiber connectors in the OSP. The optical connectors in these components utilize seals to protect the interface between fibers and offer the advantages of connectorization – quick access, simple testing and ease of deployment by non-skilled labor. The system consists of a custom cable with factory-installed NAPs, which use a multi-fiber connector to provide access. The connector joins a terminated multiport, which creates the actual NAP and provides the access ports for the single-fiber drop cables (Figure 4).

Figure 4:  Preconnectorized cable and multiports

Prior to the use of TDS systems, distribution cables installed in the neighborhood (similar to the coax cable in HFC networks) would need to be ring-cut or mid-spanned and fusion-spliced at each NAP location. The drop cables that emanated from these locations would either be spliced in on day one (and their total length stored), or the NAP closure would be re-entered, and a drop cable would be spliced each time service was required. The latter requires skilled labor and specialized equipment in the field, adding to the high cost of fiber network implementation, as well as the inherent risk to subscribed fibers during re-entry.

When TDS systems are used instead of splicing, the drop cables are connected to the TDS cable via a multiport terminal and a single-fiber connector. There is no longer a requirement to ring-cut the cable and fusion-splice only a few fibers at each NAP.

To demonstrate, Figure 5 shows a neighborhood layout. Note that a distribution cable has been installed on one side of the street. Based on the previous analysis, NAPs have been grouped to serve four homes each and are placed on both sides of the street.

Figure 5:  Neighborhood with four-fiber NAPs

In Figure 6, the distribution cable is ring-cut, and terminal distribution products, including multiports with raw-ended cable stubs, are spliced to the distribution cable at central points. This strategy reduces the number of cable access points. However, it requires that the entire distribution plant be installed initially. This is advantageous for high take rate applications, but does not provide the ability to defer product installation until needed.

Figure 6:  Cable mid-spanned with spliced-in multiport terminals

With TDS systems, cable ring-cuts are replaced with factory configured access points. The desired distribution fibers are accessed through 5 to 15 feet tethers terminated with a multi-fiber connector. Each access point can support one or two tethers of up to 12 fibers each.
           
Once the TDS is installed, factory-prepared NAPs (multiport terminals with multi-fiber connectors on their stub cables) are connected to the tethers (Figure 7). This solution guarantees fast fiber access and provides the ability to defer the installation of the multiport and drop cables until the first home at that NAP needs service. If the multiport is not needed at cable install, the tether can be stored in a vault, pedestal or on the strand until service is requested.

Figure 7:  Preconnectorized cable with traditional multiports

In many situations, having one or two NAPs originating from each cable access point provides a cost-effective solution. However, this scenario has some limitations if the streets are already in place prior to cable placement. If the distribution cable is brought down on only one side of the street, having two four-fiber taps requires more street crossings, which adds significantly to the installation costs since boring under the street at each point is required. The only other option in this scenario would be to run cables down both sides of the street, eliminating the street crossing, but significantly increasing the amount of cabling required and losing the ability to take advantage of locating multiple tethers or NAPs off each cable access point. These factory access points are most cost effective when the tethers are at their individual maximum 12-fiber capacity, and there are two tethers attached at each access point combining for a total or 24-fiber extracted at any given location.

Branch-connected terminals
To combine the design flexibility of multiport splicing as shown in Figure 8 with the convenience of preconnectorized cable, branch-connected terminals were developed. This innovative solution contains four single-fiber drop ports as seen with traditional multiports, but also includes two four-fiber adapters. These adapters are designed to accept two connectorized “branch” multiports, also serving four subscribers each.

Figure 8: Schematic of branch-connected terminals
installed with sister connectorized multiports

The branch-connected terminal is stubbed with a 12-fiber connector, which plugs into the TDS system tether. Using this component, three four-fiber NAPs can be accessed from a single 12-fiber cable tap. In this way, the seemingly opposite objectives of maximum fibers per tether and smaller four-port NAPs are brought together in an optimal solution.

Figure 9:  Preconnectorized cable with branch-connected terminals

Because two 12-fiber taps can be pre-installed at a single access point in the cable, 24 fibers can be accessed from that point using two branch-connected terminals. This provides the opportunity for fast and inexpensive deployment. And because these solutions use connectors, they do not have to be installed initially and can be deferred until needed. A typical product configuration using branch-connected terminals is shown in Figure 10.

Figure 10:  Installed branch-connected terminals

Branch-connected terminal cost analysis
Component and installation cost analysis has been performed comparing these three solutions – mid-spanned cable with spliced-in multiports, TDS assemblies with a single multiport per tether and TDS assemblies with branch-connected terminals. Using the spliced-in multiports as a baseline, TDS assemblies that connect a single multiport per tether were found to be 15 to 20 percent greater in cost than the base. However, because deferral is possible, the initial installation cost is reduced, and is often less than for the spliced baseline. For TDS assemblies using branch-connected terminals, fewer access points are required on the TDS assembly itself, further reducing initial installation costs. There are more fibers per access point, but a lower cost per fiber being accessed. The overall cost for the TDS using branch-connected terminals was found to offer as much as a 5 percent savings compared with the spliced baseline, while providing much faster deployment times and increased crew productivity. Both TDS approaches allow terminals to be deferred, contributing to their lower initial cost and allowing the balance of costs to be scaled with take rate and revenue growth.

Conclusion
The branch-connected terminal is a new and innovative addition to the continually evolving solution set for the optical fiber access network. This solution, in conjunction with TDS cables, provides the opportunity for flexible designs, fast cable installation and deployment and the ability to defer costs if needed. This brings MSOs one step closer to the competitive infrastructure needed to provide customers “their MTV, HDTV, screaming Internet”… and more.

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