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Passive Optical Networks: Lowered Cost and Increased Availability of Fiber-Optic Cabling

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A shortage of fiber-optic lines in metropolitan areas is creating a telecommunications bottleneck for many mid-size and small companies. Telecommunications expert Annabel Dodd explains why passive optical networks may be the answer.

A shortage of fiber-optic lines in metropolitan areas is creating a telecommunications bottleneck for many mid-size and small companies. Telecommunications expert Annabel Dodd explains why passive optical networks may be the answer.

There is a shortage of fiber-optic lines between telephone companies' central offices and customers. This is creating a bottleneck of telecommunications capacity to customer premises. There are many fiber-optic lines in the high-speed backbone portion of the Internet and public networks. However, obtaining high-speed fiber to businesses' premises is often costly and time-consuming, and the capacity is inflexible. The article "OnSite Access Pins Big Hopes on Hooking Up Small Firms, in the October 12, 2000, issue of The Wall Street Journal, stated that 67 percent of office buildings don't have high-speed wiring.

Fiber-optic cabling is located in two portions of carriers' outside cabling: in the backbone, or part of the network that carries traffic between states and between telephone companies' equipment, and between an organization's building and the nearest telephone company equipment. This portion of the network is called the feeder cable. Most of the bottleneck in telecommunications is in this feeder or local part of networks.

What Are Passive Optical Networks?

Passive optical networks use optical access networking techniques to lower the cost of fiber in metropolitan areas. They essentially allow smaller, flexible quantities of bandwidth (capacity) to be delivered to customers. Customers can start with lower-speed services and order more capacity as they grow without changing onsite electronics. Inexpensive passive splitters distribute a single fiber's capacity among multiple fiber strands that go to customer sites. These splitters are located between the customer and the telephone company equipment. Intelligent optical terminals installed at each customer connect to the fiber and to the customer equipment.

One of way of overcoming this bottleneck is the use of passive optical networking technologies. Passive optical networks bring high-speed fiber out to points in neighborhoods. From these points, the capacity on the fiber is shared among up to 32 sites. A passive coupler at the neighborhood node spreads the fiber's capacity to multiple sites by using time-division multiple access. This is analogous to dividing the water from a hose into 32 separate streams. Each stream carries a portion of the capacity to an individual customer. The passive couplers are low in cost, are small (they range in size from a ballpoint pen to a Palm Pilot), and do not require electrical power. The biggest hurdles to proliferation of fiber in metropolitan areas are the cost of digging up the street and obtaining rights-of-way to lay the conduit.

With current fiber optic technology, once the fiber and electronics are in place, adding capacity involves costly hardware upgrades and long delays. With passive optical nodes, capacity to individual organizations is flexible, scalable, and lower in cost. When customers need more capacity, they call their carrier, who, by computer command, allocates more capacity to the individual site. No additional equipment is required, and long waits are eliminated.

Because capacity is available, high-speed traffic consisting of voice, video, and data can easily be carried on these fiber links. Currently PON Intelligent Optical Terminal devices at the customer site can be provisioned for from 64kilobits up to 100 megabits with ports that sense the protocol that the customer uses. The service can be used for local and long-distance calls, Internet access, intranets, or virtual LANs (VLANs), in which separate networks appear as one unified local area network (LAN).

Optical switches at the carrier's central office support local and long-distance services, ATM, Gigabit Ethernet, and IP. The optical switch accepts optical signals and changes them to electrical signals to look at the packet type so that the call can be routed properly. It can change the signal back to optical and hand the signal off to either the Internet or another carrier.

In addition to capacity, widely available, lower-cost fiber increases reliability for commercial customers. Users will be able to have more affordable diverse routes into their buildings. If one strand of fiber is cut or one central office crashes, traffic can be easily rerouted to an alternate carrier. This diverse routing is now affordable only for very large customers.

The development of passive optical networking devices is an example of the telecommunications industry's response to growing demand for bandwidth by business customers. These developments are spurred by both the growth of the Internet and innovations stimulated by competition and opportunities resulting from new local telephone companies started after passage of the Telecommunications Act of 1996.

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