The Last Mile

ISDN has its detractors, most of whom rally behind other methods for going digital directly to the subscriber. The alternatives include existing copper-wire digital services, such as T1 at 1.544 megabits per second (Mbps), Frame Relay (FR) at 56Kbps to 1.544Mbps, Asynchronous Transfer Mode (ATM) at 25Mbps to 100Mbps, and Switched Multimegabit Data Service (SMDS) at 35Mbps. Most of these services cost several hundred to several thousand dollars per month and are out of reach for small-scale users. Another alternative is to replace copper wire with fiber-optic cabling; that would reach out and touch every subscriber with essentially unlimited bandwidth. However, there is a single overriding problem with all these alternatives: They suffer from an inability to conquer a physical barrier that telephone companies euphemistically call The Last Mile.

The Last Mile, also called the local loop, is the twisted-wire pair between the central office and the subscriber. This part of the telephone network has remained virtually unchanged. Each telephone user requires a dedicated pair of copper wires. The length is usually more than a mile, but less than 20 miles, and averages about five miles in metropolitan areas. Faster digital services (such as T1, fractional T1, ATM, and SMDS) require digital repeaters at least once per mile. But normal copper pairs--buried perhaps 50 years ago--don't have such repeaters. What's worse, they often have analog conditioning equipment that actually impedes digital signals! If you want high-speed digital service, your telephone company will cheerfully run conditioned lines to your office--for a hefty fee.

How about replacing all the copper with fiber optics? The 130 million phone lines in the US use 650 million miles of copper pairs. This is a large amount of wire in anybody's book: According to a 1987 Bellcore study, the cost of replacing all the existing copper with fiber would be $250 billion (and several decades of labor). This is about 10 times what it would cost to replace every telephone switch in the US with digital equipment and lines.

How about replacing just the business lines, or metropolitan-area lines, with fiber? This will probably happen. But even such limited deployment won't be cheap or fast: It could cost about $10,000 per subscriber and still require decades to complete. All the while, information technology will continue to decentralize the workplace, increasing the demand for faster communications.

ISDN can serve users until fiber (or some other technology) is ready for prime time. It isn't as fast as you might like, but it's a lot faster and cheaper than what we've got. And it's ready for delivery now!

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