About DSL

What is DSL?

Types of DSL

DSL History




DSL Deployment

What's the Problem With DSL Deployment?

Copper lines have it rough. Twisted pairs get spliced and respliced. They're subject to a wide variety of vibrating, radiating, noise-making elements in the environment, not to mention bad weather if they're aerial, digging cuts and contamination if they're buried. Loop design parameters such as working length, cable gauge, and the number and location of add-on physical components can be expected to affect DSL performance in a variety of ways. These are some of the most common factors that disqualify loops for DSL:

Cable length - DSL was designed deliberately to use low power, to avoid interfering with other transmissions. But that also means its signals can get lost in the background noise if they have to travel over too long a cable. Right now, 12 to 15 thousand feet is the usual limit, as much as 18 thousand feet under otherwise ideal conditions.
Load Coil - is a piece of electrical hardware that was widely installed in an earlier time to improve voice transmission by effectively boosting low frequencies, especially over lines longer than 15 kilofeet. In so doing, however, it tends to suppress high-frequency signals, such as DSL. A load coil essentially disqualifies a loop for DSL service - if you can see that it's there.
Bridged taps - have been included on many loop lines to increase efficiency in use of the outside plant. They create multiple signal paths that can pose special transmission challenges for high-speed transmissions. A signal can hit the bridged tap, split in half, and go in both directions as a weaker signal. A signal can also "reflect" off the end of a bridges tap, creating electronic echo and inversion effects. It's even possible for an inverted signal to collide with its own original such that the two cancel each other at least partially.
Unshielded cable - is typically found only in the drop from a telephone pole to the customer's premises, but that's enough to make the line vulnerable to high-frequency "ingress noise" that can disrupt DSL. A typical source of ingress noise in originally AM radio.
Binder groups - running on one of many twisted pairs in one cable, high-frequency transmissions such as DSL are susceptible to various kinds of "coupling," when signals jump across a twisted pair or from one pair to another. ISDN, HDSL, or T1 signals in a binder group are all capable of coupling into a pair that's carrying DSL and interfering with the DSL signal.

The presence of any of these factors in a loop's design is a good clue to disqualification, but loop design does not guarantee loop performance. Anyway, loop records often don't match actual loop configurations. Records that start out accurate can suffer through incomplete updates or simple data-entry errors. An out-of-date diagram, a typo in the written records, or an unrecorded backhoe incident of 20 years ago might turn a loop that functions fine for voice into an DSL booby trap. And even if you can establish the accuracy of loop records, that accuracy may have a very short shelf life: It disappears the moment anything in the record or the loop itself changes. When loop records are known to be inaccurate or unavailable, loop qualification may have to rely on heuristic extrapolation, using postal mileage records to estimate loop length and rules of thumb to postulate the loop's likely makeup.

The big problem with loop qualification by estimation is the likelihood of false positives - loops that the records say should work for DSL, but that in fact will not without sometimes significant remediation investments. The falseness of a false-positive qualification is usually not known until the supposedly loop-qualified customer has been sold the service and is therefore in a position to be frustrated by and unsympathetic to the service provider's need to fix what's wrong. The design-based approaches are expected to yield false findings (positive or negative, in which you say no to a loop that actually could support DSL) as often as 15 to 30 percent of the time; the industry's top current DSL providers have experienced false-finding rates as high as 50 percent. Extrapolating from what it costs to bring up to part for ISDN, $1500 in 1998, even the lower rate means you need to allocate another $300 for each new DSL customer you can sign up based on prequalification.

It's been much smarter, so far, to not even try to sign up any customer whose loop is the least bit questionable, even though you've already invested the cost of qualifying that loop. And if such a customer asks for DSL, it's smarter to just say "You can't have it" and let the customer's frustration end there. You're in the very awkward and unwelcome position of having to try to discourage customers from buying a service they want and you want to sell them. You're lopping off chunks of your customer base at both ends, maybe losing a quarter to half of your potential DSL market before you've started.

Until now, the only real alternative to this risky business was to apply the loop-testing technique that many local exchange carriers use to identify the sources of imported troubles and analyze other existing conditions in the loop. But these methods are not intended or very well suited to predicting future performance of a loop for DSL. For instance, when measuring loop length, they can't distinguish a straight loop from one that has bridged taps on it.

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