Combining RF over glass with PON technology provides a way for cable operators to cost-effectively deploy fiber where it makes sense – both today and tomorrow.
By Dawn E. Emms and Luis Yu
For cable operators, the last-mile fiber race is on. Telcos are aggressively pushing fiber to the home or curb, while cable operators are relying on coax and hybrid fiber/coax (HFC) network architectures that cover the last mile. Service providers are selling a message that fiber enables greater bandwidth and more advanced services. In addition, builders are looking to boost new home sale prices by including fiber in their greenfield developments. However, the cable industry must balance cost-efficiency with long- and short-term benefits to compete with telcos and satellite providers for subscribers. The ability to offer the best subscriber services to the maximum number of homes while remaining cost-effective is the number one goal for today’s cable industry.
Several alternatives have emerged to keep cable competitive while cost-effectively managing today’s needs and providing the ideal network backdrop to prepare for the future generation of cable networks – which is a push toward all-IP. However, operators currently are undecided about the best FTTP alternatives specific to the industry.
To stay competitive in today’s triple-play environment, cable operators must capitalize on the best architectures that meet the needs of customers – rural, new builds, businesses, and multiple dwelling units (MDUs).
RF over glass (RFoG) recently has emerged as a promising HFC and FTTP bridging architecture for cable operators. It’s an architecture that’s optimized to cost-effectively drive new revenue streams for certain situations, including rural and low-density new build locations.
RFoG architecture provides many of the advantages found in a traditional HFC architecture, but with a more inherent ability to migrate to a future-proof network. Similar to HFC, RFoG operates from the same headend equipment, supports the same services and protocols as the traditional HFC plant, including DOCSIS 3.0, and can interface with the same back office equipment. RFoG architectures run fiber directly to the home to serve single-output RFoG customer premises equipment (CPE). Traditional RF output is maintained, enabling the continued use of current set-top boxes, DOCSIS cable modems, and eMTAs. Having these advantages and this setup allows cable operators to add fiber to the home easily, without the large capital investment required by traditional FTTP deployments.
However, RFoG by itself has several limitations, including limited downstream and upstream reach, fiber management issues, and lack of a route redundancy option (Figure 1). These challenges prevent cable operators from reaching the maximum number of subscribers per node, while increasing an operator’s costs.
Figure 1. RFoG reference architecture, highlighting distance limitations.
Limited downstream and upstream reach: The largest cost in an RFoG system is the RFoG CPE and its associated laser diode for return transmission. There are specific laser transmitters that can drive down the cost of the CPE. However, they severely limit reach to only 10-20 km depending on the actual model and network configuration. Current CPE and laser transmitter configurations cannot cost-effectively provide the appropriate network reach required to make RFoG a mainstream architecture.
Fiber management and route redundancy issues: With the growing importance of high-demand and high-revenue services, route redundancy is now a requirement. Each fiber in a typical RFoG deployment serves only up to 32 subscribers. For example, in a 256-home service area, a cable operator would need to dedicate eight fibers from the headend/hub to that area to ensure service to each subscriber. Given these direct fibers run from the headend, managing fiber bundles becomes a major issue. In addition, there is no existing practical method to provide redundancy in the system.
Merging RFoG and EPON = RFPON
As operators look to deploy RFoG architecture they must plan for cable’s future and ensure that all network upgrades – including RFoG – provide a migration path to the high-bandwidth all-IP requirements anticipated for future generations. Fortunately, the opportunity to combine RFoG and a passive optical network (PON), also known as RFPON, has emerged, promising to finally deliver this evolutionary path and enabling step-by-step, area-by-area upgrades.
The virtual hub and the introduction of EPON are two key elements in a successful RFPON architecture.
Virtual hub: One of the first and most crucial elements for evolving to an RFPON architecture is the introduction of a virtual hub (Figure 2). A virtual hub houses a fully operational hub in standard node housing and moves the functionality of an indoor hub to a weather-proof node enclosure that can be strategically deployed closer to subscribers. Other key features of a virtual hub include:
- Support for multiple plug-in modules such as EDFAs, analog return path receivers, integrated WDM/analog return path receiver functionality, digital transceivers and transponders, optical switches, monitoring transceivers, and optical multiplexers.
- Redundancy and route diversity with very fast switching times (typically less than 5 ms).
- Eight times greater fiber-efficiency than RFoG alone. With RFoG only, one dedicated transport fiber is needed for up to 32 subscribers. With a virtual hub that requirement is reduced to one transport fiber for service to 256 subscribers. Further fiber efficiencies can be gained by using CWDM/DWDM to multiplex the backhaul requirements of multiple nodes on a single fiber.
|Figure 2. Overcoming the limitations of RFoG.|
Introduction of EPON: Gigabit Ethernet PON (GEPON) technology has emerged as the best fit for cable operators and the most efficient method to provide an all-IP network. GEPON provides high bandwidth (up to 1,000 Mbps bi-directional today). Also, GEPON has been deployed worldwide, ensuring operators will have access to cost-effective component and CPE prices.
And GEPON is part of an evolving standard to ensure cable operator investment is protected. The choice of an upstream wavelength for RFoG is not arbitrary; the 1310-nm specification 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 (GEPON) or an ITU G.984 (GPON) system given that these both prescribe the use of 1310 nm for upstream data communications.
The combined frequency channel plan is shown in Figure 3.
|Figure 3. RFPON spectrum.|
To maximize RFPON benefits, operators can take advantage of a temperature-hardened optical line terminal (OLT) module. This enables the availability of GEPON functionality from a standard cable node housing or virtual hub. With this module, cable operators can cost-effectively and selectively migrate an installed RFoG network to a standards-based GEPON FTTP network—if and when the move becomes justified by growth in bandwidth-intensive and revenue-generating services.
An OLT module enables delivery of 1-Gbps Ethernet over fiber, interfacing with standard GEPON optical network units (ONUs). The PON supports a point-to-multipoint fiber architecture with simple optical splitters. On the network side, an OLT module is fully compatible with standard Gigabit Ethernet transport that has been implemented with small-form-factor pluggable (SFP) transceivers.
The GEPON network is supported by existing DOCSIS provisioning systems, configuration servers, network management consoles, and back-office infrastructure. These can be used to provision, manage, and bill for services delivered over this network, enabling new services to be deployed seamlessly.
While the anticipated endgame is the all-IP network delivered to the residential subscriber, a more immediate use for this application is commercial services. Today, in areas where cable operators are deploying RFoG networks, they can cost-effectively tap into the lucrative business market, providing very high speed symmetrical bandwidth to businesses co-located within the network. With the ability to locate the GEPON OLT into the virtual hub using existing fibers for the link back to the headend, new fiber only needs to be run between the virtual hub and the business for service.
RFPON in practice
The example in Figure 4 illustrates how a cable operator could use RFPON to serve a rural area.
|Figure 4. Serving a rural area.|
This RFPON deployment can be viewed as an extension of the installed HFC network. The virtual hub is located at a convenient place within the network and is served from the same headend equipment and ideally the same provisioning system as the existing HFC network. If route diversity is not required, the virtual hub can 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 needs to be installed at the headend/hub, and a dark fiber needs to be installed to the commissioned node (or a wavelength added to an existing fiber).
From the virtual hub there are eight fibers, each connected to the appropriate splitter network, to service widely distributed homes in the area. If the cable operator is looking for a future-proof architecture, 1610 nm rather than 1310 nm should be adopted for the upstream signal. Downstream services such as broadcast TV, downstream data, and video-on-demand traffic 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.)
When additional bandwidth is required, an OLT module could be installed in the virtual hub, introducing dedicated IP services. The corresponding CPE would need an upgrade to service the PON infrastructure. With the OLT module 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 OLT module supporting symmetrical bandwidth of up to 1 Gbps, this approach also is compelling for providing service to businesses co-located in the same rural service area. This strategy enables the cable operator to have a unified network for both residential and business consumers with minimal capital expenditure and no additional operating expenses. At the same time, the operator secures additional revenue streams.
Deploying fiber all the way to the home is very expensive for cable operators. However, there are some scenarios in which it does make business sense – notably rural and low-density areas where network reach is very important. With an RFPON approach, the operator has an optimal way to deploy FTTP today; the virtual hub technology cost-effectively overcomes the limitations associated with other approaches. Importantly, with the introduction of an OLT module in the virtual hub, cable operators now have an evolutionary upgrade path capable of supporting ultra high bandwidth, all-IP services – once it is justified by the revenue opportunity.
Bottom line: RFPON not only gives cable operators a competitive edge today, but also provides a versatile, cost-effective alternative to full FTTP deployment – one that enables them to evolve their networks to meet new market possibilities.
DOCSIS (Data Over Cable Service Interface Specifications) is a trademark of Cable Television Laboratories Inc.
Dawn E. Emmsis director, marketing, andLuis Yuis senior product manager at Aurora Networks.