RFPON

Hybrid Fiber/Coax (HFC) networks, and Fiber Deep in particular, can provide all the network capacity needed to provide the services demanded by subscribers and more. In some scenarios, however, it may make sense for MSOs to deploy fiber all the way to the subscriber's home. In particular, FTTP networks may stop cables' competition from securing new footholds and may help displace the competition. Moreover, FTTP may provide new revenue streams more cost-effectively. Specific venues of interest might include:

• New housing developments
• Rural, low-density areas
• Multi-dwelling Units (MDUs)

For new housing developments, the builder may be able to increase the sale price if it can be claimed that the house is "fiber ready." Indeed, in many instances the extra cost of deploying the fiber all the way to the home will be met or heavily subsidized by the builder. FTTP keeps the competition out.

For rural, low-density areas(<30 HP/mile) it is always very difficult to deploy an HFC network cost effectively, with many line extenders or similar devices typically required to reach remote households. In this case, deploying FTTP brings many advantages: limitless bandwidth potential, reduced operating costs (maintenance and powering) and all for approximately the same cost. With today's revenue generating unit (RGU) monthly payment potentially in excess of $100, providing service to these low-density homes now becomes profitable.

MDUs offer another unique market segment opportunity (assuming they are an existing development — while new MDU developments would be categorized as above). Depending on the locale, they may attract a wealthy inhabitant who expects and would be happy to pay for a superior triple play service. For this application, the fiber can be distributed from the basement of the building and fed all the way to the actual dwelling unit, providing true FTTP. However, there are other MDUs where this is not feasible; for example, in a building already cabled with coax, trying to replace that with fiber is prohibitively expensive. In these situations, the fiber can be run into the basement of the building and either distributed via coax from there or further split and the fiber fed via elevator shafts (or similar) to wiring closets on each floor, with coax running from there to the unit. In these latter situations, the MSO is significantly reducing the serving area size while increasing the network capacity.

In these scenarios, the optimal response is an architecture which operates from the same headend equipment as the traditional HFC plant, supports all the same services as HFC, interfaces with all the same back-office equipment (in the same way) but is actually fiber into the home rather than the more traditional coax to the home. The FTTP solution runs fiber all the way to the home to serve a single-output "mini node" customer premises equipment (CPE) so that traditional RF output is maintained, enabling continued use of QAM set-top boxes, DOCSIS^® cable modems and eMTAs. This architecture, first deployed by many in the industry in 2006, is referred to as RF over Glass (RFoG). Aurora Networks has taken this to the next level with our next-generation RFoG solution: RFPON.

RFPON is based on the benefits of Aurora's experience with numerous field deployments, resulting in the development of tools both to solve inherent issues with RFoG implementations and to provide a smooth migration path to an all-IP network of the future.

RFoG Building Blocks

The reference architecture for an RFoG system, as shown in Figure 1, is comprised of a downstream optical transmitter operating nominally at 1550 nm, optical amplification as required by the topology being served, and a wave division multiplexer (WDM) for combining downstream and upstream optical signals onto a single fiber. It also includes an upstream optical receiver which receives the upstream optical signals on either 1610 nm or 1310 nm and converts them to RF. In the field, conveniently located near the end users, would be a splitter — with each fiber supporting up to 32 customers.

Figure 1. RFoG reference architecture, highlighting distance limitations

Click on diagram to enlarge

At the customer site, an RFPON CPE is required, designed for either indoor or outdoor installation, and which comprises a WDM to separate the downstream optical signal (at 1550 nm) from the selected upstream wavelength. The downstream receiver recovers the RF downstream signals from the downstream optical carrier, and the RF signal is then fed via coax into the home. In the upstream path, the RF signal is supplied to an upstream transmitter (with an output at 1310 nm or 1610 nm) for onward transmission to the headend.

The choice of an upstream wavelength is not arbitrary; today a 1310 nm solution is more cost-effective given the wide availability of components (both active and passive) at this wavelength. However, 1610 nm is potentially more future-proof; it permits an optional overlay with either an IEEE 802.3ah (EPON, or GEPON) or an ITU G.984 (GPON) system given that both these systems use 1310 nm for upstream data communications. There is now an emerging IEEE 802.3av (10GEPON) system which could also elect to standardize on 1610 nm, but at this time it is still a work-in-progress with no conclusions as yet.

The associated RFoG reference diagram of a frequency/wavelength spectrum for a typical North America system is shown in Figure 2.

Figure 2. RFoG spectrum

Click on diagram to enlarge

The same suite of products described above can be offered to any subscriber in any area of the cable plant, not just for areas which are fed via fiber. This results in a completely unified headend, significantly simplifying operation for the cable operator.

Limitations of RFoG

While an RFoG system does meet many of the objectives of the MSO to deploy an HFC-compatible FTTP network, technically this solution has limitations, namely:

• Limited downstream and upstream reach
• Fiber-intensive
• No route-redundancy option

While the downstream reach is important, system limitation is dictated by the upstream. The major cost element in the system is the terminating CPE and its associated laser diode for return transmission, hence minimizing the cost of this component is important. The most cost effective laser transmitter for CPE devices is the Fabry-Perot (FP) laser. While FP lasers do drive down cost, they severely limit the reach of the units. Depending upon actual model and network configuration, a reach of just 10–20 km is typical. Unfortunately, this greatly impacts the area which can be served directly from the cable systems' headends/hubs.

In a typical RFoG deployment, each fiber would serve up to 32 subscribers. For example, in a 256 home service area, an MSO would need to dedicate eight fibers from the headend/hub to that area to ensure service to each subscriber. Similarly, with these direct fiber runs from the headend, there is no practical method to provide any redundancy in the system. With the growing importance of high-demand, high-revenue services, this is not an ideal solution.

RFPON: Next-generation RFoG

Aurora has pioneered technology which efficiently overcomes all the limitations of an RFoG system: the RFPON VHub™. Aurora's standard VHub houses a fully operational hub in a standard node housing. For an RFPON application it is designed to serve 256 subscribers. Effectively, it moves the functionality of an indoor hub to a weather-proof node enclosure that can be deployed closer to subscribers in the network. VHubs can be strand or pedestal mounted. The key features of the VHub for this application are:

• Support for up to 12 plug-in modules (which can include EDFAs, analog return path receivers, integrated WDM/analog return path receiver functionality, digital transceivers and transponders, optical switches, monitoring transceivers, and optical multiplexers,)


• Monitoring and control of the VHub via our Opti-Trace™ EMS software


• Redundancy and route diversity with switching times less than 10 milliseconds (typically <5 milliseconds).

Figure 3. RFPON — overcoming the limitations of RFoG

Click on diagram to enlarge

The RFPON VHub system has been successfully deployed world-wide. In addition to its flexibility in placement (it can be located very deep into the network), it addresses the limitations of the RFoG reference design noted above:

• Downstream reach limitation — With EDFAs packaged for installation in the VHub housing, downstream reach is no longer limited.


• Upstream reach limitation — At the RFPON VHub, return signals are processed using an optical-electrical-optical (OEO) conversion while simultaneously being converted from an analog to digital signal format. With the VHub configured with upstream analog return path receivers, the subscriber CPEs only need to transport back to the VHub, a very short distance of typically no more than 5 km. With up to four analog receivers in the VHub, effectively 64 or 128 subscribers share the upstream bandwidth. Once received, the upstream signals are then fed into two "2-fer" digital return transmitters, each transmitting on one of 15 CWDM wavelengths. Use of the CWDM digital return overcomes the distance limitation (now with a reach >60 km) as well as maintaining a very fiber-efficient solution.


• Fiber-intensive — With the traditional RFoG approach, one dedicated transport fiber is needed for 32 subscribers. With the VHub, this is reduced to one transport fiber for 256 subscribers. The traditional approach needs eight times more fiber.


• No route-redundancy option — Aurora's hardened OS42S1S optical switch provides route diversity with switching times less than 10 milliseconds (typically <5 milliseconds). The only way to provide route redundancy with traditional RFoG would be to build two separate systems — totally cost prohibitive.

In addition, Aurora has developed integrated passive modules to simplify the input/output connections to the network, and these modules can also be housed in the VHub. In particular, there is a module to provide a combined optical splitter for the 1550 nm broadcast signals together with 1310/1550 diplex filters. This compact design, with MPO connectors, eliminates most fiber jumpers and minimizes associated losses which are normally created by broadcast splitting and/or 1310/1550 (or 1610/1550 as-needed) mux/demux functions. This integrated module not only saves precious VHub real-estate and improves network reliability but also greatly simplifies the installation and maintenance of the unit. Taking this to the next-level of integration, Aurora has consolidated the return path receivers into the integrated passive module while maintaining the same module form factor. This further simplifies operation and opens VHub slots which can be populated with other modules. There will be eight fibers from the RFPON VHub, with each fiber able to serve up to 32 subscribers. (These subscribers can, of course, be either residential or commercial.)

Evolving the RFPON Deployment

When making the investment to deploy FTTP, it is critical that there exists an established path to take the network from its current cable TV form of today to the "all-IP world" that will be needed for future generations. Aurora's RFPON solution provides that evolutionary path, enabling a step-by-step, area-by-area upgrade with its award-winning Node PON™ technology.

The most efficient way to provide an all-IP network will be with PON technology — of the many standards established today, GEPON is emerging as a great fit for MSOs. It provides high-bandwidth (up to 1,000 Mbps bi-directional today), has been deployed world-wide (ensuring cost-effective component and CPE prices) and is part of an evolving standard to ensure that MSO investment is protected. However, PON standards have agreed on a downlink of 1490 nm with an uplink of 1310 nm. Given the potential overlap with the traditional RFoG deployment, the RF overlay upstream wavelength needs to be 1610 nm, thus making the 1310 nm wavelength available for the PON. The new frequency/channel plan is shown below.

Figure 4. RFPON supports the best of both worlds

Click on diagram to enlarge

Aurora Network's Gigabit Ethernet Node PON Module, the GE4132M, is an OLT module designed to work in all our VHubs and nodes, making PON delivery from an outdoor platform a reality. Using this module, the cable operator can cost-effectively add all-IP services to their network on a service area by service area basis, operating in parallel with traditional cable TV services that are transported over the 1550 nm and 1610 nm wavelengths. Ultimately, but only if and when justified by revenue growth, Node PON equipment can enable full migration of an installed HFC network to a standards-based GEPON FTTP network. With an RFPON VHub which can support one, two or three Node PON modules, the dedicated IP-bandwidth to a group of 256 subscribers can be as high as 3 Gbps full duplex. (This is in addition to all the traditional cable TV services that are received from the traditional RFPON deployment.) Of course, the CPE device at the home will also need to be upgraded to support the new PON services. However, by adhering to the widely-deployed GEPON standard, the expectation is that the CPE device would be cost-effective, with the costs driven down by wide-scale deployment. Going one step further and eliminating the RF overlay would allow five Node PON modules to be supported — up to 5 Mbps dedicated bi-directional bandwidth to each subscriber. With future generation support for the evolving 10GEPON standard, bandwidth potential is almost limitless. This is truly a future-proof solution.

Today Aurora Networks is the only company to provide this seamless evolution from an HFC architecture to a full IP-based network on a service area by service area implementation. In addition, there are notable key features of our Node PON solution:

• Fully-compliant with the GEPON standard — compatible with off-the-shelf GEPON CPE devices
• Bandwidth is 1000 Mbps bi-directional
• Each module can support up to 32 consumers per fiber drop
• Designed to fully interoperate with existing cable modem back-office provisioning systems.

A Practical Example

Figure 5. Serving a rural area with RFPON

Click on diagram to enlarge

Initially this deployment can be viewed as an extension of the installed HFC network. The RFPON VHub would be located at a convenient place, being served from the same headend equipment and provisioning system. If no route diversity is required, the VHub would be served by just one fiber from the nearest fiber node. If the broadcast and narrowcast services are not available on the 1550 nm wavelength then the appropriate transmitter would need to be installed at the headend/hub and a dark fiber to the node commissioned (or a wavelength added to an existing fiber). From the VHub there would be eight fibers, each connected to the appropriate splitter, to service the widely-distributed homes in the area. If the MSO is looking for a future-proof solution, it is recommended that 1610 nm rather than 1310 nm be adopted for the upstream signal. The downstream services (broadcast TV, downstream data and VoD traffic, etc.) are carried on 1550 nm with all the associated upstream traffic on 1610 nm. (The CPE device would also need to mirror these wavelength selections.)

Once network demand exceeds available capacity, a Node PON module can be installed in the RFPON VHub, introducing dedicated IP services. The corresponding CPE would need to be upgraded to service the PON infrastructure. (Typically, GEPON-compatible CPEs will not be deployed until a customer has subscribed to those services; this considerably reduces the upfront capex cost.) The frequency/channel plan for both the traditional and the IP services was shown in Figure 4. With the Node PON seamlessly integrated with DOCSIS provisioning systems, the introduction of this new technology does not cause a disruption to back-office processes and procedures.

Additionally, with each Node PON supporting symmetrical bandwidth of up to 1 Gbps, this solution is also compelling for providing service to businesses which are co-located in the same rural serving area. This could result in the cable operator having a unified network for both residential and business consumers with minimal capital expenditure and no additional operating expenses — and gaining additional revenue streams!

Conclusion

Deploying fiber all the way to the home is typically very expensive. There are, however, some scenarios in which it does make business sense. Aurora has been a pioneer in this space, developing and optimizing solutions specifically for MSOs. With our RFPON approach, the MSO is provided with an optimal solution to deploy FTTP today: a solution which cost-effectively overcomes the limitations associated with other approaches. With the Node PON GEPON module, an evolutionary upgrade path capable of supporting all-IP full-duplex services is ready whenever justified by potential revenue opportunities.

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