With FTTH, ops can prepare for the future and for an all-IP world.

Since its inception and subsequent adoption by the Society of Cable Telecommunications Engineers Standards Program, the RF over Glass architecture has emerged as a compelling technology in the cable industry's fight against its telco and satellite competitors.

To date, one of the most immediate opportunities for RFoG has been the new-build housing market, where planners have been calling for fiber-to-the-home systems in many new developments. RFoG has become a viable option for cable operators in these situations because it meets the needs of building planners while allowing operators to add the benefits of fiber to targeted service areas within their existing systems without the costs associated with a full fiber deployment.

In 2006, Bresnan Communications, a broadband telecommunications provider, adopted RFoG, or HFC FTTH as it was then called, for these select applications. However, in these times of economic uncertainty, mortgage crisis and scaled-back home construction, cable operators are now looking to an equally viable, yet often overlooked, market for RFoG – the previously underserved rural, low-density areas within their franchises.

RFoG implementations -  virtual hubs (VHub)
Figure 1: With few or no active elements to power, RFoG implementations are becoming economical for reaching less-densely-populated rural markets. Aurora extends the reach of RFoG with its virtual hubs (VHub).

Rural, low-density areas are defined as those with less than 30 households passed (HHP) per mile, where it has been typically very difficult to deploy HFC networks cost-effectively.

RFoG networks, however, require significantly less power than traditional HFC networks and bring the transport advantages of fiber, which is basically loss-less, making it easier and less costly to reach these rural areas. With no RF amplifiers to sweep and balance every year, the operating costs are also significantly reduced. What's more, an RFoG architecture can operate from the same headend equipment as a traditional HFC plant, support all the same services as HFC and interface with the same back office equipment in the same way. The only difference is the delivery medium – fiber-to-the-home rather than coax-to-the-home.

Network reach is another important consideration for rural subscribers, and here again RFoG boasts an advantage. The RFoG technology enables cable operators to extend fiber deeper into the network. In a typical RFoG deployment, each fiber will serve up to 32 subscribers, meaning that in a 256-home service area, up to eight fibers would need to be dedicated from the headend/hub to that area to ensure service for each subscriber. With new technologies increasing the downstream and upstream reach of RFoG architectures, the same number of rural subscribers can now be serviced with only one fiber – increasing the fiber benefits eight-fold.

An RFoG distributed, or virtual, hub moves the functionality of an indoor hub to a weatherproof node enclosure that can be deployed closer to each subscriber in the network. The location of the RFoG virtual hub can be strategically chosen based upon deployment cost, availability of powering, required access reach and equipment access – among other considerations.

The RFoG virtual hub can also be configured with upstream analog return path receivers, meaning that the subscriber's CPE device only needs to transport the signal back to the virtual hub, a relatively short distance, where analog receivers allow subscribers to effectively share upstream bandwidth.

Once received, upstream signals are then fed into digital return transmitters, each transmitting on an xWDM wavelength. With the use of CWDM digital return technology, the reach is now extended to greater than 60 km. With DWDM, that reach can extend beyond 100 km. In addition, the xWDM implementation ensures a very fiber-efficient solution.

Initially, rural RFoG deployment should be viewed as an extension of the installed HFC network, with the virtual hub located in a convenient place and served from the same headend equipment and provisioning system currently in use.

If no route diversity is required, the network can typically be served by just one fiber extended from the installed HFC network, although if broadcast and narrowcast services are not available on the 1550 nm wavelength, then the appropriate transmission equipment may be installed at the headend/hub and a dark fiber to the virtual hub commissioned (or a wavelength added to an existing fiber). From the virtual hub, eight fibers, each connected to the appropriate splitter, would service the widely distributed homes in the area.

For a future-proof solution, it is recommended that 1610 nm rather than 1310 nm be adopted for the upstream signal, freeing the 1310 nm wavelength for the seamless implementation of PON services at a future date over the same fiber drops. The downstream services (broadcast TV, downstream data and VOD traffic, etc.) are carried on 1550 nm with all of the associated upstream traffic on 1610 nm. The CPE device also needs to mirror these wavelength selections.

The ability of the RFoG network to support PON services is a major consideration. Today, PON services, typically GEPON, are finding cost-effective application serving businesses that are co-located with the cable plant. However, going forward, a properly planned RFoG system can also be smoothly transitioned to a full PON deployment for all subscribers – residential and business. With the utilization of the SCTE’s recommended wavelengths, it is possible to construct a network today that can evolve to a PON deployment in the future. The key point is that construction only needs to happen once – paving the way to seamlessly integrate PON technology without the cost of rebuilding the network. Truly future-proof!

When building a cable network to accommodate new, rural housing areas, cable operators require a network architecture that can cost-effectively support these projects. The key criteria to be determined when deciding on a fiber deep or RFoG architecture for a rural housing environment include:

  • Distance of each home from the headend/hub
  • Number of dark fibers available in the area
  • Total number of homes in the development
  • Density of homes per mile
  • Size of each lot
  • Future expansion needs

For fiber deep under normal considerations, the average node size would typically be approximately 3,000 feet, serving 51 homes with an average home density of 91 HHP per mile and a maximum distance from the node to the home of approximately 1,000 feet. With RFoG, the average node size varies depending on the application, but optimally has an average home density of 30 HHP per mile or less and can accommodate a distance of approximately 10,000 feet from the virtual hub to the home.

As a result of this comparison, and based on RFoG’s cost benefits compared with fiber deep, specifically in a rural environment, RFoG is a great architectural choice for rural builds. That said, fiber deep is still the cost-effective architecture of choice for both medium- and high-density areas.

The RFoG architecture is exactly the tool operators need in today’s environment, while being ready to accommodate future network expansion in the most cost-effective way. With the RFoG design, one transport fiber can be used to feed up to 256 homes, as opposed to a traditional RFoG-based headend design where one fiber only feeds up to 32 homes in an ideal environment. In rural areas, it is not always possible to reach even 32 homes where the distance to each home consumes loss budget. The RFoG implementation using a virtual hub provides cable operators with a tool to overcome the reach limitations.

Today, the RFoG virtual hub architecture delivers a number of key benefits:

  • No need to lay the last drop of fiber until a customer signs up for service
  • Longer reach of fiber-to-the-home
  • Services up to 256 homes on one fiber
  • Low take rate for pay-as-you-grow model due to no sub-scriber CPE or drop fiber being required until the customer has signed up
  • Taps only deployed as needed; not a necessity for initial set-up
  • HFC network evolution capabilities to preserve in-home CPE devices and headend systems to offer similar service provisioning and operations support
  • Enhances network capabilities with 1 GHz bandwidth, improves return path performance to facilitate DOCSIS 3.0 and reduces the serving group size to accommodate as low as 32 subscribers per group
  • Meets builders’ FTTH demands
  • Provides a migration to PON without having to “rip and replace” the network, saving huge upfront costs

Rural locations can be viewed as underserved or never served areas for cable services, and with revenue per subscriber at ~$120 per month, RFoG has emerged as a preferred tool to reach these rural subscribers. Through the adoption of this fiber-to-the-home architecture, cable operators can prepare for the future and become ready for a seamless transition to the all-IP world of tomorrow.