There’s no question that many service providers are exploring upgrades to their network infrastructure. One of the biggest drivers of these new deployments is the customers’ desire for next-generation services and the associated bandwidth demands. The growth of social networking sites, online gaming, and applications such as Internet television are pushing the current access network infrastructure to its limits. Service providers have a number of options when designing the network architecture to address these needs. The network architecture that is seeing the largest growth of course is FTTx.

Whether it’s the attempt to realize operational savings by reducing the ongoing cost of maintaining an outside plant (OSP) based on twisted copper pair, taking part in the $7.2 billion Broadband Stimulus portion of the American Recovery and Reinvestment Act, or future-proofing the network by using fiber, many service providers are developing plans to upgrade their OSP to a fiber-based network.

The Fiber-to-the-Home (FTTH) access network is primarily made up of two types of equipment: the Optical Line Terminal (OLT), located in a remote terminal or Central Office (CO), and the Optical Network Terminal (ONT), located at the customer premises. The ONT is the most expensive component of a FTTH build out. Obviously, economics heavily drives the ONT decision-making process, but the technologies available on the ONT also play into the decision. While OLTs on the market today offer some level of service differentiation, such as extended reach capabilities and various port densities, the greatest level of service differentiation is located on the ONT. A number of different technologies can be implemented on the ONT side of the circuit to allow the service provider to customize the type of service delivered as well as utilize some of the existing capital-intensive equipment in the network.

Getting Schooled on ONTs: Easy As ABC
When a service provider is trying to choose which type of ONT to deploy, five questions should be posed and considered:

Question #1: One of the most important questions to be answered is what type of service should be deployed? If the service provider desires to deliver Triple Play services, an ONT with standard interfaces may be all that is necessary. Using an ONT with POTS and Ethernet interfaces, a service provider can deploy voice, high-speed Internet, and television using IPTV. For those providers who are looking to deploy services beyond POTS, these ONTs provide the flexibility of adding modern services such as Video-On-Demand and Pay-Per-View. Obviously, the additional costs of IPTV headends, middleware, and set-top boxes (STBs) need to be factored into the equation, but standard ONTs allow the service provider to deliver services and compete with MSOs in a given area. (See Figure 1.)

Question #2: Another question that the service provider should address is: where will the ONTs be installed? While qualitative data is not widely available, Infonetics Analyst Jeff Heynen notes that approximately 70 percent of ONT deployments in the Tier 3 service provider space are going into brownfield applications.

Unfortunately, the homes in these existing neighborhoods may not have been constructed with 21st Century data needs in mind. These homes are not likely to have CAT5 wiring much less drops near TV sets for an IPTV STB Ethernet connection. While CAT5 wiring is still somewhat scarce in older homes, the majority, if not all homes built over the past few decades are wired with twisted copper pair to deliver traditional POTS services or coaxial cable for television services. Technologies such as those championed by Multimedia over Coax Alliance (MoCA) and HomePNA allow service providers to utilize existing in-home wiring, such as twisted pair or coax, to deliver both high-speed Internet and IPTV. While MoCA has taken the lead in deployments in North America, driven almost entirely by Verizon, HPNA is an equally viable option. ONTs that provide both interfaces are widely available today.

Question #3: Another consideration for the service provider is whether or not it will be possible to utilize existing network equipment and still provide modern services to meet customers demand. For those service providers who may already have an RF headend in their network and have typically deployed cable television services over a Hybrid Fiber Coax (HFC) infrastructure, migrating to FTTH is a way to consolidate other services across the same physical plant without the limitations of HFC.

ONTs that implement RF overlay allow the service provider to offer voice and data services using a PON infrastructure while also providing a delivery method for standard cable television services. With the addition of a third wavelength in the downstream direction, the RF signal from the headend can be inserted onto the PON by a Wave Division Multiplexer (WDM). The ONT optics strip out the third wavelength and output the RF signal on a standard f-connector that is attached to the customer’s current coaxial wiring. This solution is ideal for providers who are looking to offer basic cable television service, where no return path is necessary. If the provider wants to offer services that require a return path back into the network such as Video-On-Demand or Pay-Per-View, this can be achieved without changing ONTs. By transitioning television services to an IPTV architecture, the service provider can utilize the existing Ethernet interfaces on RF Overlay ONTs and upsell their customers to a premium service package. (See Figure 2.)

Additionally, if the service provider has a Scientific Atlanta (SA) headend, a return path from the STB can be achieved with the addition of a Single Wire Return Device (SWRD) located at the customer premises. The SWRD receives the signal from the STB, converts it to data, and places it on the Ethernet network going back to the ONT. Utilizing the same upstream wavelength, the information traverses the same network as the remainder of the customer data traffic but is switched toward the customer's headend after leaving the OLT.

Another option is ONTs that implement RF over Glass (RFoG). RFoG can coexist on the same physical fiber plant as other technologies such as GPON. Utilizing two additional wavelengths on the PON, RFoG allows the service provider to use the existing headend to deliver cable television outside the traditional HFC network. RFoG allows a service provider to upgrade the network to the home, but keep the headend, which is very expensive, in place. As time goes by, the provider can later make the transition to GPON to deliver other differentiated services. RFoG is largely seen by cable operators and Tier 3 ILECs as a transitional step from traditional HFC networks to a network designed around either EPON, GPON, or Active Ethernet. (See Figure 3.)

Question #4: In addition to the type of service that is to be delivered, there are other considerations that a service provider must take into account when choosing an ONT. One factor is the ease of installation. Most vendors allow the ONT electronics to be installed separately from the outdoor housing. This allows the provider to install housings, and then come back later and install the ONT electronics when the service is ready to be turned up. Additionally, the provider must consider what type of fiber termination will be used in the network. Housing options for field splice and pre-connectorized fiber are available.

Question #5: Another factor that should be considered is the troubleshooting ability on the ONT. Today, some ONT vendors are adding the ability to view important statistical information such as Ethernet statistics, metallic loop tests for POTS statistics, and in the cases of VoIP deployments, SIP and MGCP debug information. This level of insight into the ONT performance allows the service provider to quickly identify problems and improve the customer's overall experience with the service.

Masters Degree in ONTs?
As you can see, the wide breadth of ONTs on the market today allows the service provider to deliver a variety of services. Technologies such as MoCA, HPNA, and RFoG each allow the service provider to tailor the deployment to meet the customer's need or provide services that fit in better with the service provider's current network. Ultimately the choice to deploy a FTTH architecture affords the service provider the greatest flexibility in delivering next-generation services. The ONT may be the last piece of the OSP deployment, but it is the equipment which the customer is most familiar with and the choice of which ONT to use should be made so that the customer's expectations are fully met.

About the Author
James Britnell has more than 13 years experience in telecommunications. James is the ONT product manager for ADTRAN's Enterprise Networks Division, responsible for the design, development, and product direction for the ONT product line. Prior to joining the Product Management team, he served as a design engineer for ADTRAN's Total Access line of Integrated Access Devices. For more information email james.britnell@adtran.com or visit www.adtran.com

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