DSLAMs: The RBOCs need to retrofit their central
offices to add DSL Access Multiplexers (DSLAMs) and cabling.
Length Limitations: Once that has been done,
the length limitations still must be dealt with. Twisted-pair cabling
rapidly attenuates frequencies much higher than one MHz. The following
length limits exist:
Carrier Serving Area (CSA): Within the CSA,
ADSL can deliver 6 Mbps or greater downstream and as much as 800
Kbps upstream. The CSA range is 12,000 feet for 24-gauge wire and
9,000 feet for 26-gauge wire. About 50 percent of U.S. telephone
lines are in this range.
Revised Resistance Design (RRD): Within
the RRD range, ADSL can deliver 1.5 Mbps or greater downstream and
384 Kbps upstream. The RRD range for 24-gauge wire is 18,000 feet
and for 26-gauge wire, it is 15,000 feet. This includes approximately
80 percent of U.S. telephone lines.
DSL Extenders: There are a couple of products
on the market that extend the distance that DSL can reach from the
CO. At least one of the products is a repeater that is installed
midway through a copper loop.
Loading Coils: About 15 to 20 percent of the
local loops in the US. contain "loading coils", inductors that cancel
noise during voice calls. The problem is that they also cut off any
frequencies above 4 kHz and DSL technologies work at 400 kHz and higher.
These loading coils must be removed in order to support DSL. (These
coils also limit the bps rates available with V.34 modems, often to
about 26,600 bps or less.)
Bridged Taps: Sometimes when a given telephone
line is disconnected, the lines that run to the demarcation point
are left connected to the main distribution line. This is called a
"bridged tap". Multiple bridged taps can degrade a line beyond use
ADSL (Asymmetric DSL): The "asymmetric" refers
to the fact that it allows more bandwidth downstream than upstream.
Downstream, ADSL supports speeds of 1.5 to 8 Mbps, depending on line
quality, distance, and wire gauge. Upstream rates range between 16
and 640 Kbps.
To use a line for both analog telephone and ADSL,
local exchange carriers install a splitter, generally at the demarcation
point. One inside line goes to the telphone, while the other goes
to the ADSL modem. This requires a technician go to the site at installation
time, a significant cost.
A modulation scheme for ADSL has not yet been agreed
to. This is causing its own problems: Some competitive carriers are
still, as of April 1999, installing ADSL lines encoded with carrierless
amplitude phase (CAP) modulation, which is easier to deploy but interferes
with transmissions on other cables in the binder, causing bit errors
on T1 and ISDN circuits. Most incumbent local exchange carriers use
the standards-based discrete-multitone (DMT) technology, which doesn't
cause much interference.
G.Lite: In February of 1998, a group of vendors
announced a commitment to a variant called Universal ADSL. The idea
was to trade the potential for bandwidth greater than T1 speeds in
order to enable "splitterless" installation. If the functionality
of splitting off the analog voice can be built into the ADSL modem,
then it wouldn't be necessary to dispatch a technician for installation.
This technology has been, at various times, also called
"consumer" and "splitterless" DSL. Although now standardized by ITU
as G.Lite, is unclear if it will ever be widely deployed, since service
providers are currently holding trials with another splitterless variation
on ADSL that is faster.
Splitterless DSL: But, watch out! Recently,
the term splitterless DSL began to be used to refer to what
is sometimes called full-rate DSL and has a maximum speed of
RADSL (Rate Adaptive DSL): RADSL has the same
transmission limits as ADSL, but it automatically adjusts transmission
speed according to the length and quality of the local line. Connection
speed is established when the line syncs up and varies between 600
Kbps and 7 Mbps downstream, and between 128 Kbps and 1 Mbps upstream.
Maximum distance varies from 12,000 to 18,000 feet.
HDSL (High bit rate DSL): HDSL is symmetric
it furnishes the same amount of bandwidth both upstream and
downstream. This is the most mature of the xDSL approaches and has
already been implemented by the RBOCs as a T1 replacement. HDSL typically
supports 768 Kbps full-duplex over a single twisted pair, T-1 rate
over two pairs, and E-1 over three pairs. Maximum distance is 12,000
SDSL (Symmetric DSL): SDSL, also called HDSL2,
is a single-line version of HDSL. In fact, it is sometimes called
Single-line DSL. SDSL transmits T-1 or E-1 over one twisted pair and
supports standard telephone transmission at the same time. Maximum
distance is again 12,000 feet.
G.shDSL: This is based on a new ITU standard.
When it becomes available, it will offer speeds and symmetric services
similar to SDSL, but with a 30% greater service range!
VDSL (Very-high bit rate DSL): VDSL is the
fastest DSL technology, with downstream rates of 13-52 Mbps and upstream
rates of 1.5-2.3 Mbps over a single wire pair. VDSL can also operate
in symmetric mode at 26 Mbps. The maximum distance, however, is only
IDSL ( ISDN DSL): IDSL is essentially an always-on,
unswitched version of ISDN. The rate is only 144 Kbps (2 64 Kbps channels
and 1 - 16 Kbps channel). The maximum distance is 12,000 feet.