Fiber-to-the-subscriber: current benefits, future growth

Fiber-to-the-subscriber: current benefits, future growth

atm-based passive optical networks promise to provide multimedia services to subscribers economically and efficiently.

JOHN CAMPANELLA and MELANIE GOLDSBOROUGH, nec America Inc.

T he increasing and often unpredictable bandwidth demands from subscribers are one of the most critical challenges facing carriers today. Not only does it require leveraging the current infrastructure, but it also necessitates expanding future bandwidth needs. Ideally, these bandwidth needs should be met without making equipment changes or dispatching technicians on multiple visits. But that is not often possible.

Today`s bandwidth upgrades for subscribers require a site visit and manual reconfiguration at the subscriber`s site, at the serving central office, and often at intermediate locations. The work performed at intermediate sites, such as remote terminals or outside-plant locations, is often the most difficult because of current access platforms. Once the site visits have been completed, the end-to-end path must be reestablished and reprovisioned to actually achieve the bandwidth upgrade requested by the subscriber. Since the upgrade process is dependent on so many different actions that must be coordinated and performed serially, there is a substantial risk of problems that can derail the effort and result in costly delays.

To reengineer this cumbersome process, a new network infrastructure is required. This new network would support bandwidth upgrades without costly and time-consuming efforts in remote locations. It would also achieve a seamless end-to-end, highly reliable, and high-performance solution. The most logical access platform to meet these criteria would be based on fiber optics. It would minimize remote, active system components and would rely on the system endpoints--the central office or point-of-presence on one end, and the subscriber or customer premises on the other--for active electronics.

Why choose a fiber-optic backbone? A fiber-optic infrastructure is desirable because it offers virtually unlimited bandwidth potential. And by relying on the endpoints of a system for locating the active system components, the carrier has greater control over the operation, administration, maintenance, and provisioning of the access system.

fsan initiative

In 1995, telecommunications carriers and manufacturers initiated an international movement to establish a standard for the systems required in the local access network. Such standardized systems would deliver a full range of telecommunications services--both narrowband and broadband. This work, commonly known as the full-services access network (fsan) initiative, is meant to enable the large-scale introduction of broadband access networks.

Since its inception, the carrier and manufacturer membership of fsan has continued to grow, and new contacts are made at each session gathering. Although it is recognized that the needs for each carrier differ due to differing regulatory, business, and structural environments in each country, sufficient similarities exist in the needs for future access networks to suggest that significant benefits can be achieved through adopting common network elements. In so doing, carriers will realize cost benefits for the platform, enabling a reduced price per line for broadband services.

Accordingly, the architecture of choice for the fsan initiative is based on fiber optics. Technological advances and manufacturing economies of scale have reduced the costs for the photonic, electronic, and optoelectronic components that are required to bring fiber to subscribers. An example of the technological advances that reduce costs is the optical interface. Current optical devices that rely on multiple discrete components may be reduced to about 5% of their current costs by the use of integration. Manufacturing economies of scale can reduce costs even in component areas that use mature and stable technology. Manufacturing costs of certain chipsets in the fsan architecture may be reduced to about one-tenth of their costs as the yearly volume approaches the one-million-unit level, which is achievable given the carriers represented in the fsan group. In addition to the cost savings derived from the manufacturing standpoint, the fsan initiative also advances fiber-to-the-subscriber, as well as fiber-to-the-cabinet and fiber-to-the-curb, by using a standard set of network elements.

Fiber-to-the-subscriber system

The Asynchronous Transfer Mode (atm) leased-line service (shown in Figure 1) uses a passive optical network and architecture in the access network and is consistent with the fsan network definitions. A recent deployment of this atm passive optical network technology furnishes up to 32 customer locations with a total bandwidth of 150 Mbits/sec. The shared network is based on a time-division multiplexing (tdm) frame structure in the downstream and a time-division multiple access (tdma) frame structure in the upstream. In addition, wavelength-division multiplexing of both the upstream and the downstream signals offers better use of the fiber-optic access infrastructure.

The system elements that comprise the atm over the passive optical network consist of an optical line terminal, a splitter/combiner, an optical network unit, and the fiber cable; the system uses a passive double-star architecture. The first "star" of the architecture--at the optical line terminal--is where the wide-area network interface to the services is logically split and switched to the optical network unit subscriber interfaces. The second star occurs at the splitter; here, the information is passively split and delivered to each network unit.

The optical line terminal has a switching capacity of 20 Gbits/sec and provides atm leased-line, high-speed digital leased line, switched digital video, and standard analog and Integrated Services Digital Network services. The terminal is located in the carrier`s central office or point-of-presence and is the interface point between the access system and the various service points in the network.

The services enabled by this architecture--voice, video, and data--comprise the full-service menus envisioned by the fsan initiative. In the current network application, fast data services are delivered over an atm backbone and are primarily offered to corporate and business customers. When the data (content) from the network reaches the terminal, content is actively distributed to the passive splitter at 1550 nm using tdm in the downstream. The content is then routed to the network unit on the subscriber location.

The transmission method from optical line terminal to optical network unit also is tdm. The network unit selects the content destined for it and discards the remaining content as shown in Figure 2. In the upstream, the payload from the customer-premises equipment is transmitted to the atm passive optical network unit, which then transmits the information to the terminal as tdma bursts on the 1310-nm wavelength. The terminal performs concentration and statistical multiplexing of user-data using Synchronous Optical Network/Synchronous Digital Hierarchy- (sonet/sdh-) based transmission interfaces and delivers the payload to the services network. The only active components of the system are the optical line terminal and the optical network unit.

The optical network unit may be installed on or near the subscriber`s premises. For business customers, it may be on the premises, in the telecommunications closet, or with the local area network servers. For a residence, it may be located in a network-interface device, or it may be integrated as part of a residential gateway device. In a typical architecture, the network unit contains a 155-Mbit/sec optical interface in a subscriber`s location and bidirectionally communicates with the terminal on one optical cable.

This communication is accomplished through a l:N (N is a maximum of 32 branches) passive splitter installed on the transmission line. The network unit may deliver service via a user network interface and connects the customer-premises equipment to the network. The user interface may be electrical or optical.

The capacity, or bandwidth, offered to each customer of the fiber-to-the-subscriber system can be defined by the carrier. Simply by provisioning at the optical line terminal, the optical network unit is automatically reconfigured and the bandwidth is adjusted to meet the customer`s demand. The bandwidth is shared, both logically and physically, by the subscribers supported by the passive double-star architecture of the terminal and splitter.

Fiber-to-the-subscriber application

A recent deployment by a major carrier member of fsan illustrates the fiber-to-the-subscriber application in providing access to business customers. In this deployment, the atm passive optical network technology using a passive double-star architecture provides 25- and 155-Mbit/sec service drops to medium and large business customers. This system provides the security and high reliability of fiber optics with bandwidth flexibility to achieve upgrades more easily.

The architecture is composed of the active star from the optical line terminal and a passive star through the passive splitter. Customers who use the fiber-to-the-subscriber network rely on an optical network unit at the customer premises to convert the optical signal to an electrical signal and provide the interface to their customer-premises equipment. Typical applications for this equipment would include a router with an atm interface, an atm enterprise switch, or a frame-relay access device. Business customers can choose scalable bandwidth from l.5 to 155 Mbits/sec or several steps in between. The carrier can upgrade simply by provisioning at the central office, with the end-to-end system following the changes.

Application services and economics

The use of the splitter in the passive optical network architecture enables the sharing of bandwidth among the users. The major benefit of bandwidth-sharing is the ability to divide the costs of the fiber-to-the-subscriber system. Bandwidth-sharing systems must also offer methods that guarantee bandwidth to particular subscriber applications at certain times. The ability to prioritize certain traffic flows is a key feature of the atm standard. atm has well-defined service rates for assigning priority and enabling more-equitable sharing of the total bandwidth of the pds architecture. For example, the customer can prioritize the atm cell flows so that a high-quality videoconference with a key client may be assigned a higher priority than a large file transfer of historical trend information; once the conference is concluded, the priorities can be reassigned. Another example may be assigning a higher priority to billing information downloads during critical business cycles.

By sharing bandwidth effectively, high-quality access services can be offered to subscribers at lower costs. The system described here can offer significant cost savings over some existing architectures designed to deliver voice, video, and data services. Through the use of atm concentration and statistical multiplexing, combined with the sharing of active optoelectronic components via the splitter elements, atm passive optical network technology can achieve a 20%-to-40% cost savings over "circuit-based" access systems. An example of a circuit-based system may be a sonet/sdh ring add/drop multiplexing system or a sonet/sdh point-to-point system.

Operational savings will also be realized as a result of the atm passive optical network architecture. Since the active components of this fiber-to-the-subscriber system are not located in remote terminals, but at the central office and customer premises only, labor costs associated with the installation, turn-up, and maintenance of initial systems are reduced. Subsequent upgrades in bandwidth can be provisioned and established remotely, without "truck rolls" and hardware change-outs.

Given the strong international support and the stated benefits of the architecture, it seems that fiber-to-the-subscriber systems are positioned to play a leading role in enabling multimedia applications in the years to come. u

Fig. 2. The only active components in this system are the optical line terminal and the optical network unit. The architecture uses both tdma and sonet/sdh transmission.