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To: Tim McCormick who wrote (2671)2/8/1998 12:45:00 AM
From: SteveG  Read Replies (1) | Respond to of 9236
 
[From Cooper and Lybrand's report on xDSL]

The Role of xDSL Technology

Posted January 27, 1998 12:00 PM PST

A group of technologies generically referred to as xDSL (Digital
Subscriber Line) allows the economical provision of highspeed digital
access services over existing copper telephone distribution cable.
These technologies will play a significant role in making digital
services available to widely dispersed residential subscribers over
the next several years. However, they also have significant technical
and economic disadvantages that restrict their role in the long run-at
least as it applies to extending the useful lifetime of copper cable.

In spite of copper's current usefulness, there are theoretical
capacity limits in the telephone plant that cannot be overcome
according to the laws of information theory. The limits are in terms
of the amount of information (bit rate) for a given distance (measured
in kilofeet):

ÿÿÿÿÿÿ
Data Rate MaximumÿDistance Comments

1.5 Mb/s 18 Kft Can theoretically reach most, but not
all, homes. Practical limits mean fiber
feeder is often required.

6.0 Mb/s 10 Kft Shorter than the average loop length.
Generally requires fiber feeder.

24 Mb/s 3 Kft Requires fiber well into the
distribution plant.

55 Mb/s 1 Kft. Basically, implies fiber-to-the-curb.


ÿ
Service at 1.5 Mb/s may be sufficient for the average Internet user
today, and 6.0 Mb/s would be a dream. However, these rates will be
sufficient in the future only if the technological progress we have
seen over the past 15 years in computers, multimedia applications, and
the Internet diminishes significantly. This seems unlikely. Historical
and current trends in modem rates, computer performance, and memory
usage suggest that performance requirements will double-on
average-every two years. Assuming that 1.5 Mb/s is acceptable today
and using this rate of improvement going forward, users will be
demanding 24 Mb/s and above within a decade. ÿ The interaction of the
technical limits and the increasing demand for higher data rate
digital services is treated in our recent report, Transforming the
Local Exchange Network, 2nd Edition. A range of scenarios were
developed which capture alternative strategies for application of
xDSL. The middle scenario, for example, assumes a very substantial
role for xDSL, with a rapid early rollout in the next five years and
subscribership to xDSL-based services reaching 10% of all homes by
about 2007. However, the combination of higher data rates and the
constantly improving economic advantages of fiber begin to force LECs
to fiber-intensive solutions well before 2007.

One should also be aware that xDSL does not come free of charge.
Because of a combination of theoretical and engineering limitations,
even the 1.5 Mb/s and 6 Mb/s rates will require extensive deployment
of fiber in the feeder, as well as the replacement of existing circuit
equipment. Further, in some cases, distribution and drop facilities
will need to be rehabilitated. In addition, the cost of the xDSL
electronics-in both the network and the customer premises-is
significant.

Given the limited willingness-to-pay of residential customers, there
will be only so much revenue to cover these costs. While costs are
likely to come down, so will the costs of competing technologies such
as cable modems. Thus, xDSL is a generally attractive interim solution
to a particular problem, but not a panacea that overcomes all of the
competitive challenges facing telephone companies today-especially in
the long run.