Digital Subscriber Lines (xDSL)

xDSL, a term that encompasses the broad range of digital subscriber line (DSL) service options, has the potential to revolutionize the areas of Internet access and telecommuting by offering a low-cost, high-speed data transport option for both individuals and businesses.

The basic allure of xDSL is that it provides faster data services than today’s top-speed, 56-kbps analog modems—more than 100 times faster—using the local loop, the existing outside-plant telephone cable network that runs right to your home or office.

The appeal is obvious: This technology enables telephone companies to offer broadband service without major network rewiring and can be implemented quickly and profitably, especially because it stands to benefit both the consumer (with faster data rates) and the service providers (with new revenues from old cables). Here’s how it works. Nearly all existing telephone lines can carry frequencies up to 1 MHz. But analog telephone service only requires a maximum frequency of 3.3 KHz, leaving a large amount of the bandwidth unused. xDSL makes use of this otherwise wasted space by piggybacking high-speed data traffic onto the unused bandwidth.

By filtering the frequencies at each end of this wide-open range (4 KHz to 2.2 MHz) and isolating them from the voice-bandwidth channel, the local telco can transport both traditional telephone signals and high-speed xDSL signals over the same old four-wire telephone line that already links your home or business to their central office (CO). At least, that’s the promise of xDSL.

But reality has cast a shadow over the case for xDSL being made by telco providers, competitive local exchange carriers (CLECs), and Internet service providers—all clamoring for a piece of what’s estimated to become a billion-dollar business over the next several years.

Implementing DSL.
Today, with more than 750 million sites around the world wired for telephone service, the basic infrastructure for xDSL is already in place; each twisted-pair circuit is a veritable high-speed, dedicated pathway just waiting to be activated.

Back then, as now, cable modem service challenged xDSL in the beyond-analog broadband market. But the inherent advantages of the widespread copper telco network over CATV coax suggested (and still do) that xDSL is better positioned to deliver more bang for the consumer’s buck—at least in the near future.

Anecdotal reports from early cable modem users, who once enjoyed the lion’s share of the service’s 10-Mbps coax pipeline, suggest data slowdowns are an increasing problem as more people tap into the shared service, leaving smaller portions of the overall pie for anyone to access.

Additionally, with a comparatively modest 65 million sites around the globe wired with the hybrid fiber coax (HFC) cabling system necessary to support cable modem service, xDSL arguably enjoys a strategic market advantage.

Recent standardization efforts have made it easier and more affordable to roll out some varieties of xDSL. In fact, many leading service providers have recently redoubled their efforts to make xDSL available to their customers, desperate for a foot-in-the-door edge over cable companies.
 

DSL alphabet soup.
Understanding the basic premise of DSL service is simple enough. Trying to get a handle on the variations and permutations—the proposed, standardized, and sometimes abandoned incarnations that have been lumped together as xDSL—is a different matter.

By last count, our Technical Support experts identified 16 distinct references to DSL services (including DSL itself), many of which prove to be redundant references to the same technology. Different sources often cite different definitions with different distance limits and data rates.

As with any developing technology, nailing down such essential details is challenging, especially when you consider that some of the xDSL variants are outdated or proprietary versions being put forth by private companies.

With that caveat in mind, consider this xDSL laundry list, drafted in hopes of bringing some semblance of order to this wide-ranging subject.

Digital Subscriber Line (DSL).
DSL is the root of all xDSL services. In fact, DSL is the root physical layer for ISDN BRI service—two 64-kbps bearer (B) channels and one 16-kbps data (D) channel (2B+D) all bundled together to provide a functional 128-kbps pipeline capable of supporting simultaneous transmission of voice and data, fax, or video signals.

DSL service can support high-speed data transmission over a local loop ranging up to 18,000 feet from the CO. As with most xDSL services, data rates will increase near the CO and taper off with distance. One other important thing to consider when discussing xDSL is that top speeds and distances are almost always expressed as theoretical maximums, assuming ideal line conditions.

Various factors such as excessive crosstalk, wire gauge, the physical condition of the line, and hardware issues such as the presence of load coils or bridge taps on the loop all may conspire to limit the effectiveness of, or even preclude the possibility of using, xDSL services.

Asymmetric DSL (ADSL).
More bandwidth is usually required for downstream transmissions, such as receiving data from a host computer or downloading multimedia files. ADSL’s asymmetrical nature provides bandwidth where it’s needed.

ADSL provides transmission speeds ranging from downstream/upstream rates of 9 Mbps/640 kbps over a relatively short distance to 1.544 Mbps/16 kbps as far away as 18,000 feet. The former speeds are more suited to a business, the latter more to the computing needs of a residential customer.

The lopsided nature of ADSL is what makes it most likely to be used for high-speed Internet access. And the various speed/distance options available within this range are one more point in ADSL’s favor. Standardized by ANSI as T1.413, ADSL enables you to lease and pay for only the bandwidth you need.

G.lite.
Also known as ADSL lite, this simplified xDSL offering was standardized as G.992.2 by the ITU in June 1999.

Unlike some xDSL offerings, G.lite is the closest thing to plug-and-play DSL. Other DSL services require a truck roll—an on-site visit from telco service personnel to configure customer premises equipment (CPE), install a DSL modem and a splitter (which keeps the voice and data channels separate), and even install an Ethernet card in the customer’s computer.

In theory, at least, G.lite can eliminate the need for on-site installation by providing the customer with everything to set up the system themselves—without a splitter.

Essentially consumer-grade DSL, G.lite is a direct response to the cable modem challenge. Initially driven by the Universal ADSL Working Group (UAWG), a consortium led by Microsoft®, Intel®, and Compaq® to standardize a mass-market version of DSL that could be implemented quickly and affordably, G.lite offers customers a variety of slower-speed DSL service levels to choose from to suit their particular system requirements.

High-Speed Digital Subscriber Line (HDSL).
Unlike asymmetrical DSL, a high-speed digital subscriber line (HDSL) provides symmetrical pathways for bandwidth—matching upstream and downstream circuits—at speeds ranging from 128 kbps to 1.544 Mbps over distances from 5000 to 18,000 feet.

Based on its speed/distance capabilities, HDSL best serves WAN applications and has the potential to serve as a better physical layer for T1 services. HDSL can also be effectively used for extending a high-speed LAN beyond its normal geographic boundaries. Generally, this kind of a configuration requires the use of bridges or routers and a dedicated DDS or T1 circuit (and all the recurring expenses they entail).

If the LAN sites you plan to interconnect are within roughly three miles (18,000 feet) of each other, you can use lower-cost connectivity devices (and lower cost HDSL) to make the connection over normal telephone wires.

Rate-Adaptive Asymmetric DSL (RADSL).
This variation on the ADSL theme dynamically negotiates maximum top speeds based on varying line conditions. RADSL automatically runs at the highest possible rate and speeds up or slows down as momentary conditions dictate.

RADSL data rates may range as high as 6.1 Mbps downstream and 1.5 Mbps upstream.

Symmetric DSL (SDSL) or Two-Wire DSL.
Examining the convoluted xDSL family tree, it’s difficult to figure out how SDSL (which stands for symmetric DSL) came to represent the two-wire version of HDSL (which actually is symmetric DSL, albeit a four-wire version), but it did.

What’s even more confusing (besides the fact that SDSL is often cited as single-line DSL) is that, according to Newton’s Telecom Dictionary, SDSL has come to be known within the ANSI standards organization as HDSL2—a naming convention that, somehow, in this acronym hodge-podge, almost seems to make sense!

Essentially offering the same capabilities as HDSL, SDSL offers T1 rates (1.544 Mbps) at ranges up to 10,000 feet and is primarily designed for business applications.

Very High-Speed DSL (VDSL).
VDSL is characterized by asymmetrical transmissions that offer very high data rates over very short distances. VDSL’s top speeds are understandably dependent on line length.

Maximum downstream rates are projected to reach 55 Mbps over lines up to 1000 feet long or up to 13 Mbps over segments as long as 4500 feet. Upstream rates will range between 1.6 and 19.2 Mbps.

Like ADSL, VDSL uses forward error-correction to ensure data integrity while limiting the bandwidth-intensive response/acknowledgement interactions that characterize many other error-correction techniques.

Compared to other DSL technologies, VDSL is still in its infancy, mainly because there are so many open issues regarding performance guarantees relating to transmitting high-frequency signals over copper lines.

For instance, otherwise benign factors such as unterminated extension lines in a home have no impact on everyday telephone communications or even data services such as ISDN, but in certain situations they could be crippling to VDSL service.

So, for the time being at least, VDSL remains one of the digital subscriber line services that has potential (and problems) as yet not fully explored.