Cost-effective fiber solutions for metro-area access overbuilders

One issue that has held up the progress of fiber-to-the-premises (FTTP) optical broadband service in metro areas is how to make an all fiber network scaleable and cost effective in a competitive overbuild environment where customer penetration models initially would be relatively low. The answer is to “overbuild” utilizing a scalable PON topology, which to date has proved to be both onerous and expensive, especially with fiber intensive “point-to-point” deployment models. However, times are changing rapidly and this article will examine two possible solutions—fiber-to-the-curb (FTTC) and FTTP systems.

It is very desirable in a metropolitan overbuild situation to deploy a network that can scale with subscriber penetration rates while minimizing initial capital expenditures. While point-to-point fiber topologies promote scalability of electronics, the thousands of fiber pairs and the associated splicing and termination costs to serve a relatively small geographic area makes that type of deployment scheme less than attractive.

FTTC and FTTP are gaining serious consideration as solutions for high-density areas where a number of subscribers can be reached within the last 100 meters of a city block or Multi-Tenant (MTU) building. This article will demonstrate how the FTTC topology may be utilized as a building block approach to ultimately delivering fiber to the premise or be simultaneously integrated, employing the same product platform to meet disparate end user service requirements. The product platform in this scheme will offer a family of customer premise devices for both residential and business installation to deliver high speed IP Ethernet, voice (POTS or VoIP), E1 or T1, and RF (Analog and QAM) or IP video services to the end user.

The FTTC (also known as “fiber to the building” for MTU situations) model provides the network operator with a passive fiber optic network between the central office, headend or hub and a group of 24 subscribers with each fiber eventually serving 8 subscribers.  The FTTC solution will place a single fiber optic cable near the end user in scalable groups of 24 dwellings for a scalable, “success-based” deployment model. Coaxial cable for RF CATV services and copper pairs for IP Ethernet and telephony services would be used for the final 100 meters to the end users.

FTTP (a.k.a. fiber to the home or business) extends the fiber optic passive network all the way to the customer premise or end user. The FTTP solution will place fiber optic 1x8 passive optical splitters in scalable groups of 24 dwellings, once again, in a success-based deployment model. A single fiber optic cable will extend from the group of splitters directly to the end user and terminate at each user with a terminal device.

FTTC (Fiber to the Curb): Let’s examine a proposed scenario to provide service to 2300 dwellings using an FTTC overbuild architecture as depicted in the figure below. 

Within the central office, the core optical line terminal (OLT) device interfaces with a multi-layer Gigabit Ethernet switch / router via GbE optical interfaces for telephony, T1 / E1 and IP services.  For RF services, the core device interfaces with standard 1550nm CATV transmission equipment.

The FTTC network configuration allows for all of the network components, GbE switch router blades, OLT chassis and PON modules, 1x8 passive splitters, and FTTC terminals to be incrementally deployed with subscriber penetration.  From the headend to the client, the network operator would establish three distinct points within the distribution network where the network components and subscriber connections are scaled as subscriber penetration increases.

1. PON Group:

   The PON Group consists of a group of three 1x8 optical splitters connected back to the PON ports of the OLT device.  Each 1x8 splitter is incrementally placed to match subscriber penetration. Each PON Group location serves up to 192 subscribers.

2. FTTC Terminal Group:

   The FTTC terminal Group consists of a group of three subscriber “gateway” devices, each connected to a single port on the 1x8 optical splitters at the PON Group location. Each gateway is incrementally placed along with the PON splitters to match subscriber penetration.  For example, each gateway would provide eight RF ports, eight 10/100 BaseT ports and eight POTS ports. Each location serves up to 24 dwellings or subscribers.

3. Network termination:

   Provides a connection point for the transition from network cables to the cabling inside the clients home or business. Each location serves eight dwellings or subscribers.

In this example, each location is set up to be scalable in 33% increments. For flexibility, the actual deployment can be optimized to scale in larger or smaller equipment groupings to best suit the dynamics of the service area. In any case, the network can be built to minimize the up front cost of deployment by distributing the points of network scalability as compared to a dedicated fiber solution.

The core OLT is located in the central office or may be remotely placed up to 70 km from the GbE multi-layer switch. Regardless of location, the OLT establishes twelve 1x8 optical PONs for a total of 96 optical ports, which are terminated by the gateway devices. As subscriber penetration increases, OLT and GbE interfaces on the switch / router can be added. Furthermore, PON modules called CLMs (Core Line Modules) are populated with four PON transceivers, each providing 32 optical ports. The CLM modules may be populated as needed as subscriber penetration increases.

A fiber cross connect or patch panel is placed between the OLTs and the outside plant cables feeding the PON groups to facilitate the activation of PON ports. Three fibers are needed to each PON location to ultimately support 100% subscriber penetration. All fibers could be initially placed with one fiber activated and two dark fibers to be activated with each 33% penetration increment.

The space requirement in the “CO” is kept to a minimum as the equipment necessary to serve the 2300 dwellings would be contained in a single 43 RU EIA equipment rack. Only 36 fibers terminate at the headend to serve the 2300 potential subscribers, a dramatic cost difference when compared to a point-to-point solution that would require up to 4600 fibers for IP services and another 2300 fibers for RF based video services. The associated termination costs, network splicing costs, space requirements, civil disruption due to the sheer volume of cables and ongoing fiber management make these types of “fiber to the home” deployments impractical.

Similar to the FTTC network, the FTTP model provides a very scalable entirely passive solution between the central office and the customer premise. Services within the headend are provided in the same manner as the FTTC topology. The equipment utilized is the same and may be utilized for FTTP and FTTC applications simultaneously. However, the network operator deploys fiber deeper establishing two points (instead of three) within the FTTP distribution network to enable the network components and subscriber connections to be scaled as subscriber penetration increases.

1. PON Group: The PON location consists of a group of three 1x8 optical splitters connected back to the PON ports of the core OLT.  Each 1x8 splitter is incrementally placed to match subscriber penetration.  Each PON location serves up to 24 dwellings or more depending on the mix of MTU dwellings served with the gateway devices.

2. Network termination: Provides a final connection point to transition from the network fiber cables to the fiber cabling to each home or business. Each location serves eight dwellings or subscribers.

Once again, the FTTP OLT is located in the central office or may be remotely placed up to 70km from the GbE router/switch switch. Each complete set of core OLT modules establishes twelve 1x8 optical PONs for a total of 96 optical ports, which are terminated by the gateway terminal device located at the subscriber premise. The case for metro FTTP overbuilds has been helped recently by the advent of field-hardened OLTs, which allows network operators to distribute the OLT deep in the network to serve many more customers as compared to “CO alone” OLTs. As subscriber penetration increases, network elements can be added incrementally as subscriber penetration increases just as in the FTTC deployment.

A fiber cross connect or patch panel is placed between the OLTs and the outside plant cables feeding the FTTP PON groups to facilitate the activation of PON ports as needed based on subscriber penetration. Three fibers are needed to each PON location to ultimately support 100% subscriber penetration.

The first 1x8 splitter deployed in the PON group provides service for up to 33% penetration. The PON location can be established within a splice enclosure or cabinet to match the desired location, whether in the street, building or telephone pole. Fibers are placed between the PON group and network termination location allowing individual subscribers within the 24 UI group to be connected to the optical 1x8 splitters.

The network termination provides a final connection point to transition from the network fiber cable to the fiber cables entering each dwelling, or a point to place the gateway device if coax and copper pairs will enter the customer premise. 

Multiple PON groups and termination locations may be positioned along the same fiber route: For example, a network operator can feed a group of 96 end points with a single 24-count fiber cable. In this example one 12 count bundle is utilized for the PON connections to the OLT and the second 12 count tube is re-utilized along the path to connect between the PON group and each group of eight homes.

Furthermore, customer premises devices these days can meet the variety of service requirements often found in metro environments. Depending on the size of the dwelling or business, network operators can offer  2 - 10/100 BaseT, 2 POTS and 1 RF port; 4 - 10/100 BaseT, 4 POTS and 1 RF port and other configurations, such as 8 - 10/100 BaseT, 8 POTS and 8 RF port for MTU applications. For medium business that require T1 or E1 connection, the FTTC or FTTP gateways can provide up to 4 RF ports, 4 POTS, 4 – 10/100BaseT or 1 –1000BaseT ports, and 4 E1 ports.

In summary, both FTTP and FTTC options described above address the biggest sticking point in the deployment of optical broadband services in metro areas for overbuilders--which is deploying a scalable infrastructure cost effectively. Minimizing start up costs while removing deployment barriers associated with an unmanageable fiber infrastructure is a key factor when it comes to deploying an all fiber network. The FTTP solution provides up to a 24 times reduction in the amount of fiber and the FTTC topology reduces the amount of fiber by more than 100 times, as compared to a central point-to-point topology. With a combined network splicing and fiber termination costs which approach $175 per fiber or $500 per subscriber for point-to-point networks, the new FTTP and FTTC options provide a very cost-effective solution enabling operators to engage in the deployment of an all-fiber based topology today.

Paul Whittlesey is the vice president of product marketing at Wave7 Optics and can be reached at paul.whittlesey@w7optics.com.

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