>Aging copper wires? That's a new one. I thought the
>only limitation on DSL
Copper wires getting long in the tooth really doesn't
have anything to do with DSL. In fact, some of the
best performance can be had on some pretty ancient
wires because of the larger guage. (One company
demonstrated their DSL running on barbed-wire fence).
Two main factors are bridge-taps and line-coils.
Bridge taps are where your "line-pair" is split and
directed to some other point in your subdivision. In
other words, the same line-pair is present in two or
more pedestals in your neighborhood. This way the
phone company can run a 200-pair into a neighborhood
of 150 houses, but yet allow any 50 to have a second
line. A coil is a low-pass filter. Coils typically
are placed every 6000 feet or so, but the upper limit
on POTS service without coils is 15000-20000. Coils
and DSL are NOT compatible, since they attenuate the
DSL signal..
Other factors are that some forms of T1 and DSL are
not compatable in the same cable sheath without
serious cross-talk problems. Most of the newer DSL
standards play pretty well with other forms of data
traffic in the same sheath so this is getting to be a
lesser problem unless your provider is running some
legacy equipment (older than 2 years).
When a internet provider sells DSL service into an
area served by an ILEC (such as Verizon), they ask for
a LADS (Local Area Data Service) circuit. LADS is a
point to point, within the same exchange, circuit used
for broadband transmission of digital data signals.
These circuits are either two or four wire cable
facilities, conditioned and absent of loads and bridge
taps. Service is provided between two points that are
no more than two route miles apart. The LADS service
offering is for those customers that have the
requirement for a dry cable pair. (This is quoted
from our tarriff).
There are other issues as to why you can't get DSL.
Although, DSL is an average outward reach of 18,000
feet (with some equipment providing as much as 60,000
feet), your neighborhood might be served by an RLC
(Remote Line Concentrator) or DLC (Digital Line
Concentrator). These are essentially sub-exchanges
served off the host exchange switch. These
sub-exchanges may actually contain the actual
switching equipment (like a Nortel DMS-1), which not
only serves the local with dial-tone, but local call
handling too. In order to serve these "neighborhoods"
with DSL, a DSLAM (Digital Subscriber Line Access
Module) must be co-located with the remote switch.
Unfortunately RLC/DLC sites are traditionally pretty
compact and not environmentally controlled, it can be
a problem to locate additional equipment like a DSLAM
in them without a major construction project. Add to
that, you need either multiple T1's, DS-3 or spare
fiber pairs on the backside of the DSLAM to get to an
ATM switch someplace. Things can get really ugly and
expensive in a hurry.
Depending on the DSLAM platform used (ethernet vs ATM)
the cost per subscriber can be anywhere from $300 to
$2000 not including backbone costs. This is just for
the DSLAM!
Next question comes up is whether or not it costs the
provider more to sell you the higher speeds. (duh,
it's just a setting...). It really does, because we
must provide an adequate backbone to handle the higher
speeds of the subscriber. DSL really does provide the
subscriber high bandwidth at a low cost. However, if
you absolutly must have a set gauranteed bandwidth
then buy your own point-to-point T1's.
Ken N.
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