The HFC architecture is remarkably nimble; a cable operator can create additional bandwidth in many ways. Regardless of the selection path for finding the RF spectrum, leveraging DOCSIS 3.0 over HFC may make strong economic sense to the cable operator for high-bit-rate data services, minimizing the need for costly fiber builds. The operator may begin creating and allocating DOCSIS 3.0 channels across the its footprint for residential data and voice service, and since business services usage patterns are dissimilar from residential service usage peaks, these same DOCSIS 3.0 channels, cable modem termination system (CMTS), and network infrastructure may be leveraged for high-bit-rate commercial and residential data services. The use of DOCSIS 3.0 over HFC provides business services as a cost-effective and efficient solution. However, if DOCSIS 3.0 over HFC is not desirable or possible, there is another outside plant option to explore that leverages DOCSIS 3.0 over an all fiber to the premises approach, called RF over glass (RFoG). DOCSIS 3.0 over RFoG If fiber solutions are preferred for business customers – to resolve operational concerns regarding active outside plant networks or to support future customer bandwidth and data throughput growth – an emerging technology called RFoG may offer a second alternative to traditional HFC solutions. Essentially, RFoG is similar to an HFC architecture with the coaxial portion of the network residing only in the inside wiring at the customer premises and thus extending fiber throughout the outside plant network to the customer premises. RFoG supports all existing cable services, including all video and DOCSIS services. Overview The SCTE Advanced Plant Architecture Study Group is using the term RFoG. The RFoG solution uses existing business support system/operational support system (BSS/OSS) and headend elements (network elements and provisioning systems for video, data, and voice) in the cable operator’s network. The traditional cable installation practices are leveraged as well as customer premises equipment (CPE) and services such as analog, digital, and DOCSIS services. An RFoG service group size will likely be in the range of 32 homes passed; however, combining at the headend will remain an option as with traditional HFC. This allows CMTS equipment to be shared over a larger pool of service groups for even greater economies of scale. The reasoning behind RFoG may be found in service providers deploying an all-passive optical network, like RFoG or passive optical network (PON) systems, which find lower operational costs than fiber-to-the-node approaches with actives or copper wiring in the outside plant. RFoG architecture If we consider an HFC architecture with a 500 homes-passed node, the architecture connects the headend and the node with fiber, and from the node transmits signals to its 500-homes-passed serving area using coaxial cable for the so-called last mile. The RFoG architecture is essentially 32 mini-nodes terminating fiber connections not in the outside plant like a traditional HFC node, but rather at the customer premises locations, with a sort of mini-node, and these mini-nodes will receive the fiber connections from the headend and convert them to electrical signals to be transmitted over coax in the home or business. Fundamentally, this is similar to HFC but with fiber extending to the home or business and with the coax portion of network at the customer premise (inside wiring). All of the existing services and technologies delivered over HFC can be delivered over RFoG, and the benefits include smaller service groups for greater effective bandwidth per sub, an all-passive optical network to the home, and a migration strategy for even higher bit rate technology over Ethernet PON (E-PON) or Gigabit PON (G-PON) while still using RFoG.

Figures 1 and 2 illustrate a traditional HFC system and the RFoG architecture. Essentially, the node in the outside plant of the HFC network is replaced with an optical splitter. The RFoG systems have a wavelength division multiplexing (WDM) mux at the headend allowing the optical wavelengths to be combined on a single fiber down to the customer. The RFoG CPE is essentially a media converter, receiving and transmitting the optical signals and interfacing with the coax network at the customer premises. It is unlikely that RFoG would be a replacement technology for existing HFC deployments, but it may be an alternative for new residential builds, and business service units may consider RFoG builds in existing HFC deployments and new build areas.

If fiber solutions are preferred for business customers to resolve any operational concerns about active outside plant networks or to support future bandwidth growth, new technologies like RFoG may offer a great alternative to traditional optical solutions. The DOCSIS and RFoG solution for business services supports video, data, and voice on one network. With this architecture, symmetrical data rates could exceed 100 Mbps. This fiber to the business solution is meant to leverage the DOCSIS network elements at the customer premises and headend—the embedded multimedia terminal adapter (EMTA)/cable modem and CMTS respectively. The RFoG solution leverages all back office systems for triple play and the existing voice over Internet protocol (VoIP) solutions at the regional data center (RDC) or headend, as well as the customer premises.

The RFoG optical architecture in the outside plant will likely use 1,550 nm on the forward path and 1,310 nm on the return path. Both optical interfaces have competitive price points. However, the traditional PON systems often use 1,310 nm. So, if the RFoG system is deployed using 1,310 nm lasers as well, then a traditional PON system could not share the same fiber. Alloptic has an RFoG option to use 1,590 nm on the reverse path instead of 1,310 nm, which will allow for the traditional PON overlay (1,490 nm and 1,310 nm) to share the same fiber with an RFoG system. Figure 3 is an example of an RFoG and DOCSIS system and shows the leverage of fiber to the business (FTTB) using RFoG technology and traditional DOCSIS systems at the headend and CPE.

The emergence of RFoG enables an FTTB or fiber to the home (FTTH) passive optical network solution to use all-passive fiber networking that carries all services traditionally delivered over HFC. These services include the carriage of analog and digital services as well as the transport of data and voice services over DOCSIS. If an operator prefers to deploy fiber solutions to resolve operational concerns regarding active outside plant or to support future capacity growth of the business customer, RFoG may offer an alternative to traditional fiber solutions. The use of RFoG and DOCSIS may be an excellent combination because this includes an all-passive optical network direct to the business, which incorporates the redundancy schemes found on high-end CMTS equipment. These redundancy schemes may not always be available on Ethernet switching or PON network elements. The business service unit will also consider back office implications of each network technology alternative and the costs and time to market to support the expected explosive growth in coming years. Business class Business customers expect high availability and performance – this is part of the service level agreement (SLA) between the operator and the business customer. The network architecture, whether over traditional HFC or RFoG, should be designed to meet the performance thresholds of the SLA. This section will cite the DOCSIS-based system performance and explain how some systems have been engineered from the ground up, maintaining carrier class performance.

As mentioned previously, integrated redundancy schemes may not be found on some Ethernet switching or PON systems. The CMTS, however, is capable of supporting lifeline telephone services, and these performance expectations are shared in large part with business services. High-end CMTS equipment has hitless access line cards sparing for the cable access side of the shelf. This hitless cable access module sparing is implemented in the case of a failure or for card replacement. This CMTS feature allows the access line card, interfacing to the customer, to be mapped to another access line card upon a failure, with no dropped calls and no modem re-registration.

In addition, high-end CMTS products have the capability of supporting business class services with no single point of failure and hitless software upgrades (HSUs). The HSU feature allows software upgrades without impacting customer services; if the goal is to achieve 99.999 percent availability or 5.26 minutes of unavailability per year, a single upgrade without HSU will miss the mark.

Carrier class features should extend to the CPE – especially if that equipment supports lifeline voice services and data. Today’s EMTAs support local loop diagnostic capabilities to test impairments such as AC cross-wiring, incumbent local exchange carrier (ILEC) cross-wiring, wiring shorts, receiver-off-hook, and excessive number of phones. In an effort to assist troubleshooting, voice quality metrics can be collected and retrieved or displayed upon request. These metrics include listening quality and conversational quality mean opinion score (MOS), RF signal and noise levels, residual echo return loss, and packet loss.

The DOCSIS network infrastructure needs to be carrier class to support the demands of voice and business services. Conclusions Cable operators have built an extensive HFC network infrastructure. This, combined with the arrival of DOCSIS 3.0, may be sufficient to support the majority of small to medium business (SMB) service needs, minimizing costly fiber builds. However, if fiber solutions are preferred for business customers to resolve any operational concerns about active outside plant networks or to support future bandwidth growth, new technologies like RFoG may offer a great alternative to traditional optical solutions. High-end DOCSIS CMTS equipment has already integrated redundancy schemes that may not be found on Ethernet switching or PON elements, and this provides assurance to the operators that the access layer is engineered at 99.999 percent availability. The customer SLA may place demanding requirements on the entire network end-to-end solution, so DOCSIS CPE should also be required to have integrated test and service assurance features.

Cable operators have made considerable investments in back office systems for DOCSIS voice and data services that would be leveraged for DOCSIS 3.0 technologies. As a result, DOCSIS 3.0, coupled with traditional HFC technologies and/or emerging RFoG technologies will provide an excellent end-to-end solution architecture for business services. The resulting system will be capable of supporting data services with data rates of 100 Mbps or more while leveraging existing data and voice networks and systems.

Michael Emmendorfer is senior director, Solution Architecture and Strategy, for ARRIS. Reach him at

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