Digital Subscriber Loop (DSL) Technologies

Though many service providers have great expectations for the DSL technologies, penetration as of March 1999, was far less than one percent of the 177 million phone lines in the U.S.

Deployment Issues
There are many issues that are slowing this technology, including several that contribute to an increased cost of deployment:

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 for ADSL.

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Digital Loop Carrier (DLC)
In order to reach locations that couldn't otherwise be reached, a technique called Digital Loop Carrier (DLC) is being used by some providers. DLC, much like the hybrid fiber/coax of cable technologies, moves the termination point for local-loop subscribers out into the neighborhood, typically to a pedestal painted a bland color. A fiber optic cable carries the aggregated traffic back to the local central office (CO). This effectively reduces the length of the local loop, but not without cost:

• Deployment Costs: Because this requires special ADSL termination equipment, this increases deployment costs even more. And, the CO DSLAMs may not be designed properly for the pedestal operation.
  

• Link Aggregation: Once the DSL line hits the first DSLAM device, the DSL access lines are aggregated onto an oversubscribed backhaul trunk into the carrier's switched/routed network. That trunk represents a potential bottleneck.

  All networks are oversubscribed — it's how much that is the issue. Oversubscription by providers varies from about 2 to 1 to as much as 100 to 1. Traffic might be aggregated yet again onto another carrier's backbone, itself an oversubscribed link.

ADSL modulation methods leave the frequency band below 25 KHz untouched. This means that an analog telephone call can share the line without interference. This is discussed further under "ADSL" and "G.Lite, below.

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Variants of DSL
The following "flavors" of DSL have been defined:

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 7 Mbps.

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 feet.

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 4,500 feet.

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.