FTTx : What, when, and where?

Sifting through the current optical access options requires careful consideration, particularly when attempting to serve existing neighborhoods.

By MARK LABBÉ
Critical Telecom

Everywhere you go these days service providers are talking about FTTx. But what is the best strategy to deploy FTTx? Is it fiber-to-the-node or fiber-to-the-curb or fiber-to-the-premises? And what are the relative business merits of pushing fiber to the different outside plant locations? Is there an evolutionary plan where service providers can push fiber deeper into the network yet leverage their huge investment in copper infrastructure in existing neighborhoods?

The end game is for service providers to provide new broadband service sets to their customers that will be better than the service sets offered by their competitors. There are two key attributes that the access network needs to have for this to be accomplished. One is a specific amount of physical bandwidth capacity supplied directly to the user's home, and the other is an architecture that is flexible enough to allow new services to be introduced and managed both quickly and easily.

This article will focus on practical approaches to enhance the physical bandwidth of the network in the large base of existing neighborhoods.

Today's neighborhood network

For greenfield applications, fiber-optic cable to the home is considered a viable solution to provide the physical bandwidth capacity for new consumer service sets. However, in existing neighborhoods the choices for fiber cable deployment are more complex. The luxury of an open trench to the home is not available and the copper cable has already been placed in the ground.

As a result, a fiber-to-the-home application in these neighborhoods can be extremely expensive when you begin to dig open trenches, bore a direction hole, or even use a compact trencher for placing additional fiber. As you move closer to the consumer's home the difficulties increase with right-of-way concerns and the unknown obstacles on the consumer's private property. It becomes a challenge to determine the most cost-effective and practical solution for pushing fiber deeper into these established neighborhoods.

The outside-plant structure

In the existing underground outside plant today there are three cable sections: feeder, distribution, and drop wire (Figure 1). At the junction of each cable section is a cabinet that contains wire jumpers to connect the different cable sections. The feeder cable carries large-pair-count cable from the central office, typically in an underground duct structure. The cabinet where the feeder cable meets the distribution cable is called the "cross-connect cabinet." The cross-connect cabinet serves between 300 and 600 subscribers and is also known as an SAC (Serving Area Concept) box, JWI (Jumper Wire Interface), and/or SAI (Serving Area Interface).

There are four to six distribution paths from each cross-connect cabinet. Along each path the distribution cable meets the drop wire at a small cabinet called a "pedestal." The pedestal serves between 4 and 12 subscribers. The distribution wire is directly buried.

Fiber can be pushed from the central office to three distinct locations: to the cross- connect cabinet/node (fiber-to-the-node, FTTN) or to the curb/pedestal (fiber-to-the-curb, FTTC), or all the way to the home/premises (fiber-to-the-premises, FTTP). The deeper the fiber is pushed into the network, the higher the bandwidths that can be delivered-but the higher the cost. The service sets that consumers demand dictate the bandwidths required, which in turn affects the location where fiber meets copper in the access network.

The following rates and service sets can be achieved at the respective fiber-copper transition locations:

• FTTP: Full new-generation network services, 100 Mbits/sec, n channels.
  
• FTTC: Multi-HD services, VDSL2 from the curbside, 35 Mbits/sec, three high-definition or eight standard-definition channels with one video-on-demand (VOD) channel, voice over IP (three calls), and high-speed Internet access (5 Mbits/sec).
  
• FTTN: Broadcast TV services, ADSL2+ from the cross-connect, 15 Mbits/sec, one high-definition or three standard-definition channels with voice over IP (three calls) and high-speed Internet access (1.2 Mbits/sec).

Driven by the demand of their consumers for various service sets, carriers now need to take a closer look at what fiber deployment method will best meet this demand while providing them with a revenue stream to cover their costs.

The cost of doing business

Table 1 provides a cost analysis for FTTx deployment scenarios that includes bringing fiber to the cross-connect cabinet, to the curb-side enclosure, and to the home. Of course, every distribution area is different, but this model uses an average North American topology and assumes 384 customers in the area with four curb-side links off the cross-connect cabinet. The FTTN case assumes a DSLAM port per every customer. The FTTC and FTTP cases assume a port for every home in the neighborhood regardless of whether they are currently a customer or not.

Comparisons are also made with the type of DSLAMs deployed -- a rack-mount DSLAM design that requires an environmental subsystem versus a hardened outside-plant DSLAM that requires no environmental design. The results indicate that deployment costs increase significantly as you move from an FTTN to FTTC to FTTP scenario. It then becomes critical that the ability to recognize increased revenue for the various deployment types is a feasible objective.

FTTN deployment: FTTN represents the least fiber-intensive FTTx deployment option; fiber provides high-capacity transport trunk capacity, while the existing copper infrastructure is fully leveraged for last-mile connectivity.

FTTN provides a viable, near-term means of enabling high-margin IPTV service bundles, which will both increase revenues and reduce churn. Furthermore, risks of stranded investment are minimized with FTTN, as DSLAM systems can be efficiently sized and subsequently adjusted easily to meet take rates or service requirements. FTTN equipment can be relocated and fiber cable can even be reused for FTTC and FTTP in later years.

FTTC deployment: FTTC deployments are an ideal way of both enabling high-value services in the near term and establishing a future-proof position from which to evolve. It is a logical expansion of the fiber network and enables full-rate DSL service to every subscriber.

In a curbside/pedestal deployment installation not every enclosure will be broadband-enabled. The network planner can determine which sites make the most advantageous locations for a broadband curbside enclosure. Of course, the tradeoff is always the cost of broadband-enabling several curbside enclosures versus the benefit of the shorter loop lengths attained.

FTTP deployment: In the FTTP deployment option, a passive optical network (PON) is contrasted with an active optical network (AON). In the AON, a 100-Mbit/sec optical-to-Gigabit Ethernet optical aggregation device is installed in the curbside cabinet. The AON has electronics in the curbside cabinet whereas the PON architecture has optical splitters and combiners at the cross-connect cabinet location. The AON can achieve mean-time-between-failure (MTBF) rates in the range of the PON devices but the benefit of the AON is both higher and less complicated fan out routes, as well as 100-Mbit/sec or greater service.

Viable growth

The FTTx architectures analyzed are progressive extensions of each other and not mutually exclusive. FTTN and FTTC strategies do not carry the risk of stranded investment, because carriers will use the feeder fiber in the future if they are aiming for FTTP. In the meantime, they leverage carriers' existing copper access infrastructure, while providing sufficient backhaul capacity to enable the same current services as FTTP much more quickly.

Providing carriers with immediate revenue sources and the time to consider how to budget for their eventual FTTP deployment, FTTN and FTTC deployment options are viable growth strategies to counter competitive threats from cable providers.