PON architecture 'futureproofs' FTTH

A passive optical network supports broadband services today but is ready for the baseband services of tomorrow with minor equipment upgrades.

The explosive growth in the demand for access bandwidth requires scalable networks. Long viewed as the end game for access networks because of its many advantages, fiber-to-the-home (FTTH) offers virtually unlimited bandwidth to the subscriber. Moreover, fiber-optic components ensure a long service life relative to metallic-access technologies.

While many experiments have been tried with FTTH architectures over the past 10 years, up until now, the next step to wide commercial deployment has not been taken. Previous FTTH products cost too much and only supported digital services. These digital services did not prove to be a good match with consumer demand for analog telephone service and cable TV.

Recently, there has been increased activity in FTTH, spearheaded by vendors supporting the full-services access network (FSAN) initiative and the International Telecommunications Union (ITU) G.983 standards (see "FSAN initiative propels broadband access worldwide.") Their collective mission is to define standards for optical-access networks. In recent months, a variety of FSAN-compliant product deployments have been announced. These deployments still provide only a subset of the services demanded by consumers, however. The products do not address critical needs such as outdoor deployment and serviceability.

A new approach for FTTH that is economical and upgradable for the future is required. Such an approach would initially use a passive optical network (PON) architecture for broadband service delivery (broadband PON or BPON). Then, as demand warrants, the network would provide baseband service delivery Asynchronous Transfer Mode (ATM) PON-APON. APONs are emerging as the future standard for high-speed, optical-access networks. This FTTH architecture would allow simultaneous use of the PON for BPON and APON, a key point for network and service migration.

The B/APON approach will support all current services on the same optical-access network required for the all-digital future. The architecture is consistent with FSAN and ITU G.983 directions, which enables standards-based solutions. It also incorporates extensible end-point equipment that, with the underlying architecture, provides a transition path to the future.

The BPON is a cost-effective, end-to-end solution that delivers all of today's high-demand services. "Plain old telephone service" (POTS)-with customer local-area signal service (CLASS) and custom-calling capabilities-high-speed scalable data, and cable TV are transported optically from the central office (CO) or headend (HE) to an optical-network termination (ONT) at the consumer's site. The ONT in this architecture is called a broadband network-interface device (BNID) because, in addition to ONT functions, it implements NID features which are important in mass-market deployment (see Fig. 1).

Fig. 1. In the BPON, telephone, data, and video services are transmitted downstream from the central office or headend to the BNID at the customer premise, using 1:8 and 1:4 optical splitters. The network shown here is an illustration of the FiberPath BPON, the first such architecture available to the market.

The CO/HE equipment for telephony, data, and cable TV is modular, yielding low initial cost, incremental upgrades, and investment protection. Today, the BPON approach translates all services into 6-MHz radio frequency (RF) channels. Downstream (CO/HE-to-BNID) services are carried in the 50- to 860-MHz spectrum. Upstream services are carried in two separate regions, 5 to 42 MHz for traditional cable TV and hybrid fiber/coaxial (HFC) return-path services, and 150 to 300 MHz for voice and data.

The use of RF transmission is very important because it supports today's voice and data digital services and facilitates delivery of cable TV in a format more easily distributed and used by the consumer. It also allows rapid introduction of high-speed data and other consumer services using commercial off-the-shelf components and systems designed for coaxial-based networks.

The PON between the CO/HE and the BNID has no active electronics and is the key strategic feature of the B/APON. It consists of a point-to-multipoint downstream fiber network and a multipoint-to-point upstream fiber network. The current implementation of the BPON uses 1310-nm optics which is an implementation decision rather than a choice dictated by architecture. All receivers and passive optical components are specified to operate at both 1310 nm and 1550 nm. The downstream PON delivers signals to all BNIDs. Frequency-division multiplexing (FDM) and time-division multiplexing (TDM) are simultaneously employed to control the specific services delivered to the subscriber. FDM and TDM are also used in the upstream PON to support transport of a variety of service types across different frequencies. It also delivers effective bandwidth utilization within any single frequency.

Security of the voice and data services is not an issue. It is ensured in the downstream transmission because BPON management controls the BNID's access to this traffic. Upstream security isn't an issue because the optical signals reflected back to a BNID are too insufficient to allow it to extract information.

The architecture used in a typical BPON deployment of 32 BNIDs placed 5 mi (or more) from the CO/HE is shown in Figure 1. The use of 1:8 and 1:4 optical splitters represents the most typical case, but others are possible. For closer distances of 1 to 2 mi, homerun fibers from the CO/HE deployed using 1:32 optical splitters to the BNID are cost-effective and yield important advantages in optical performance and flexibility. In this scenario, it is not difficult to support multiple cable-TV providers, each with access to the entire range of channels, on a subscriber-by-subscriber basis.

The PON also has inherent economic advantages. "Passing" a home doesn't involve field deployment of electronics. Costs can be further reduced when the optical splitters are deployed at the CO/HE because passing a home doesn't require CO/HE electronics. The PON enables incremental electronic investments as service penetration increases.

A BPON for combined RF transmission has many advantages:

A PON provides better service quality, higher reliability, and lower maintenance than HFC networks or digital-loop carrier (DLC) systems. Service outages and truck-rolls due to an active plant are eliminated.
HFC ingress noise and amplifier degradations are eliminated allowing full use of 5- to 42- MHz and 50- to 860-MHz bands.
The optical network will provide more than 30 years of service compared to the 15 to 20 years expected with coaxial or copper plants.
The BPON is "futureproof" and allows evolution to service levels beyond what's possible on HFC networks or DLC systems, with upgrades to the end-point electronics.
The BPON supports node sizes ranging from 32 to 128 subscribers; this range offers a good balance between economical service delivery and subscriber bandwidth needs.

The dual-downstream and dual-upstream PONs of the B/APON will provide a smooth migration path from the broadband approach to the future baseband system. Dual PONs allow separate design optimization of both downstream and upstream architectures. The downstream PON design-the number of splits, the optical loss elements, the number of RF channels, and the types of signals-has a critical impact on the characteristics and cost of the downstream transmitter. The upstream PON operates under a different set of engineering rules due to the number of channels, the types of services, and the optical combining that takes place. However, the BPON can be just as easily implemented on a single-fiber PON using 1550-nm downstream and 1310-nm upstream if circumstances warrant.

Fig. 2. The BNID contains the optical transmitter and receiver, RF tuners, and service interfaces that translate the provisioned services.

The BNID placed at the customer premises consists of the optical transmitter and receiver, RF tuners, and service interfaces that translate the provisioned services from the BPON to existing customer-premises wiring via integrated NID functions (see Fig. 2).

This modular BNID is designed for outdoor operation and allows subscriber access to wiring interfaces while controlling access to network-provider equipment. The optics/RF module maps the downstream and upstream optical signals to electrical signals. Cable TV and other external RF channels are passed transparently to the subscriber-wiring interface. BPON digital services are terminated by the service modules, which, in turn, connect to subscriber wiring interfaces.

The service modules decouple PON transport technology from inside wiring and subscriber service protocols. For example, data delivered to the BNID in a common format can either be mapped to a 4-wire interface for Ethernet or 2-wire interface for 10 Mbits/sec over conventional inside phone wiring. This key capability is exploited in the transition from BPON to APON and future services.

The modular BNID provides video, high-speed data, and up to six POTS lines. It can be configured and upgraded as required for new revenue-generating services and technologies. A minimal configuration of the optical receiver and power supply is sufficient for a cable-TV-only delivery system. Incorporating the optical transmitter adds upstream communication. Addition of the control card and service modules provides POTS, Ethernet data, and other services.

Power is a critical issue in FTTH, as it is in other architectures that require a network provider's electronics at the customer premises. This issue evokes strong positions among service providers. Network power, local power without backup, and local power with backup are all common approaches; which one used depends on the provider. The BNID can be configured with either a network or local power supply. Network power is delivered via a composite optical-fiber/metallic cable. Local power assumes commercial AC power. Conventional battery technology provides battery backup of lifeline service; it offers more backup than the published requirements.

Fig. 3. FnTH amortizes the cost of the BNID across four homes. Twisted-pair and coaxial drops are used to deliver all services to the homes served by the FnTH BNID. A subscriber can migrate to FTTH through a simple four-step process.

The BPON also supports fiber-near-the-home (FnTH) deployment. In this scenario, the BNID serves a small cluster of four homes (see Fig. 3). FnTH amortizes the cost of the BNID across four homes while providing delivery of voice, video, and data. The key to FnTH's migration to FTTH is the use of a fiber, copper, and coaxial hybrid drop. Twisted-pair and coaxial drops are used to deliver all services to the homes served by the BNID.

A FnTH subscriber migrates to FTTH through a simple four-step process:

Placing an optical splitter inside the existing enclosure housing the FnTH BNID.
Activate the fiber drop.
Place an FTTH BNID at the subscriber's premise.
Move the feeder fiber to the appropriate splitter.

The capacity of the FnTH BNID is available to the remaining subscribers. At the end of the migration, the FnTH BNID can become the FTTH BNID. The ability to migrate to the target PON without any stranded optoelectronics is a critical difference from conventional fiber-to-the-curb (FTTC) approaches.

The strategic value of the B/APON solution is that the BPON architecture deployed today enables migration to a very-high-speed APON infrastructure. This capability is a critical point: An access network with a 30-year physical life is valuable, but an access network with a 30-year competitive service and revenue life is invaluable.

The trigger for the migration from BPON to APON will be the market, not a technical event. Today, the economical delivery of all services helps validate the BPON infrastructure. The revenue leader is entertainment video. The vast majority of the consumer entertainment video infrastructure is oriented toward RF delivery of primarily analog and, increasingly, digital programming. Eventually, this orientation will change although there is considerable variance in opinion as to when. The ideal migration path will reflect, not dictate, the change.

Fig. 4. The mechanics of migration from today's BPON to the future APON.

The combination of CO/HE modularity, dual PONs, and BNID modularity enables a migration from an all broadband architecture to a hybrid broadband/baseband network to an all-baseband PON (see Fig. 4). Most service providers delivering entertainment video expect to continue to do so via RF channels for at least the next 10 years. The broadband-to-baseband transition depends on the penetration and the cost of alternative digital-transport options and video appliances. Therefore, it is important that the broadband-to-baseband evolution provides simultaneous digital voice and data services and RF video.

The APON migration is enabled by the modular CO/HE equipment, which allows the addition of the APON optics interface. The upstream PON is then converted to a bidirectional APON. This configuration continues to support distribution of RF-channel entertainment video via the BPON. The APON is used for telephony and data services. Return-path cable TV is provided by telephony or by the use of a digital channel on the APON.

Currently, the price of upstream broadband transmission for digital services is on par with the expected price of APON transceivers. Pricing is expected to shift in favor of the APON as FTTH deployment drives higher volumes for these components. That means the price of the hybrid system will remain constant, or drop, compared to the current broadband system.

The modular nature of the BNID also facilitates the evolution to the APON. Deployed BNIDs can be upgraded with APON transceivers; the continued use of broadband downstream is optional. The simplest evolutionary path is to upgrade entire 32-node PONs, which is a reasonable course given the service provider's motivation to upgrade entire market areas. Nevertheless, it is possible to upgrade individual BNIDs using wavelength-division multiplexing and optical-isolator components to operate both an upstream BPON and APON on the same physical PON.

Ultimately, entertainment video will be delivered over the APON. Given the ease with which the B/APON supports both baseband and broadband services, the service provider will have a high degree of control over this transition. When it happens, the downstream broadband PON can also be converted to an APON to provide more consumer capacity.

The B/APON approach delivers POTS, high-speed data, and analog and digital cable-TV services at competitive prices, which enables service providers to justify FTTH deployment now for new builds. The access plant is based on a PON that, once deployed, doesn't change even as the electronics at the end points evolve. The BPON uses the same architecture as the future APON. The CO/HE, PON, and BNID allow a migration from today's services to future services as market demand dictates. Delivery of future services is only limited by the bandwidth capacity of fiber optics, a very critical advantage.

Bob Lund is the chief technology officer at Optical Solutions (Minneapolis, MN).