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Virtually
all telcos have accepted that fiber is the future of broadband
communications and most have Greenfield FTTH (fiber to the home)
deployments in their long-range plans. As a first step, they are building
up the outside plant with fiber where it is economically feasible. This
deployment is typically from the central office to a cross-connect
cabinet. Ultimately, fiber will reach every home in new areas and provide
as much bandwidth as required. However, until that time, and in existing
areas, telcos will provide services through copper and fiber connections.
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The primary factor limiting a telco’s ability to offer full-rate DSL service to its entire subscriber base is DSL's rapid attenuation over a copper pair. This is due to poor transmission line characteristics in the subscriber loop. As such, DSL service is subject to limitations that currently prevent it from being deployed as a last-mile service to many potential end-users.
Currently, an ADSL subscriber must be within approximately 9 kft to receive ADSL at a data rate that is greater than 7 Mbps. Subscribers at distances greater than 9 kft cannot receive ADSL with a data rate high enough to provide the broadband applications they are demanding, yet many copper loops in the outside plant extend from the central office out to 18 kft and beyond.
Remote DSLAMs
In the past, telcos usually relied on remote DSLAMs to provide increased reach. They found, however, that this option was expensive, time-consuming to deploy, and troublesome to maintain. Perhaps worst of all, few remote DSLAMs offer non-blocking architectures and instead, typically act as last-mile bottlenecks; as subscribers are added, the available bandwidth per subscriber usually drops dramatically.
Although remote DSL equipment has enjoyed volume price reductions as of late, most of the expense for this type of solution remains in non-equipment costs such as cabinet construction and placement, extending power to the remote location, municipal permits, civil works, etc.. In this deployment model, 11% of the cost is attributable to fiber, 22% to electronics, and 67% to construction and civil works costs.
The cost of a remote DSLAM solution can be minimized if there is space in an existing cabinet enclosure for an overlay of ‘mini-DSLAMs’ but this does not address the inherent flexibility and scalability problems, nor does it resolve the question of how to power this equipment and shed the excess heat and EMI generated.
DLC upgrades
Upgrading DLCs (digital loop
carriers) or NGDLCs (next generation DLCs) is another option, but in many
cases, multiple 'combo' cards are required and the end result is only
high-speed access for subscribers - no tiered services, video, or capacity
for other future-oriented applications that help justify a massive capital
outlay. The Millennium-Skyline Project report suggests that only 8% of existing U.S. DLCs are broadband-capable. More telling though, is their finding that 49% of U.S. DLCs cannot be upgraded for broadband without a complete overhaul.
Fiber-optic extenders
Increasingly, telcos are extending DSL reach using fiber optics and simple extenders. This option comprises central office and outside plant equipment, and leverages investments in fiber-optic cable placed in the feeder and distribution portion of the network.
Using fiber extenders to enhance the reach of DSL is cost-effective and can provide broadband connectivity to unserved areas 82 kft (15 miles) or more from the central office. This approach removes the need for remote DSLAM equipment and facilitates the delivery of full-rate DSL to all subscribers simultaneously. Moreover, such a solution is massively scalable and supports scores of simultaneous full-rate subscribers on a single fiber pair. With a non-blocking architecture, such a solution is future-friendly and capable of enabling ‘clear-channel’ video delivery and other next-generation, high-margin services.
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