RFoG

In some scenarios it may make sense to deploy fiber all the way to the home where FTTP may provide new revenue streams more cost-effectively. Likely venues include:

• New housing developments
• Rural, low-density areas
• MDUs

For 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 (and in the same way) but is actually transported via fiber into the home rather than on traditional coax. 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 out" 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 known as "RF over Glass" (RFoG). Aurora Networks has taken RFoG to the next level with our RFPON solution: next-generation RFoG.

Building Blocks

The reference architecture for an RFoG system is shown in Figure 1.

Figure 1. RFoG reference architecture, highlighting distance limitations

Click on diagram to enlarge

The reference architecture comprises a downstream optical transmitter operating nominally at 1550 nm, optical amplification as required by topology being served and a wave division multiplexer (WDM) for combining downstream and upstream optical signals on a single fiber. It also comprises 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 customers, there is a splitter — with each fiber supporting up to 32 customers.

At the customer site, there needs to be an RFoG CPE, designed for either indoor or outdoor installation, which comprises a WDM to separate the downstream optical signal (at 1550 nm) from the selected upstream wavelength. The downstream receiver then recovers the RF downstream signals from the downstream optical carrier, and the RF signal is fed via coax into the home. In the upstream, 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; the 1310 nm solution today is more cost-effective given the wide availability of components (both active and passive) at this wavelength. However, 1610 nm is more future-proof; it permits an optional overlay with either an IEEE 802.3ah (EPON) or an ITU G.984 (GPON) system given that both these systems use 1310 nm for upstream data communications.

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

Limitations of RFoG

While the 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 reach
• Limited upstream reach
• Fiber-intensive
• No route-redundancy option.

While the downstream reach is important, the system limitation will be driven by the upstream. The major cost element in the system is the RFoG 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 limitions of an RFoG system, using our VHub™. The VHub is a fully operational hub but in a standard node housing. In this application it is designed to serve up to 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. The key VHub features for this application are:

• Support for up to 12 plug-in modules (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 20 milliseconds (typically <5 milliseconds).

The RFoG architecture is shown in Figure 2.

Figure 2. Overcoming the limitations of RFoG

Click on diagram to enlarge

The 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 overcomes the limitations of the RFoG reference design noted above:

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


• Upstream reach limitation: At the VHub, the return signals are received and then digitized for onward transmission. 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 up to 10 km. With up to four analog receivers in the VHub, effectively 64 or 128 subscribers share the upstream bandwidth. Use of the CWDM digital return overcomes the distance limitation (with the reach now becoming >60 km) as well as maintaining a very fiber-efficient solution.


• Fiber-intensive: With the traditional RFoG approach, one dedicated transport fiber from the headend 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 optical switch provides route diversity with switching times less than 20 milliseconds (typically <5 milliseconds).

With the VHub approach, the MSO has an optimal solution to deploy FTTP today, a solution which cost-effectively overcomes the limitations associated with other approaches. With the introduction of our Node PON GEPON module, the MSO now has an evolutionary upgrade path to support all-IP full-duplex services — once justified by the potential revenue opportunity. Aurora Networks — working with MSOs to break access barriers.

Back to top