ADSL and SDSL

The best technologies solve problems with existing equipment. Residences shackled with 56.6Kbit/sec dial-up lines want access to broadband applications, and businesses with expensive T1/E1 connections to the Internet want some budget relief. Where available, enter Digital Subscriber Line (DSL).

DSL connects the customer premises to a service provider's Central Office (CO) using existing copper telephone lines. When complications don't render lines ineligible, DSL modems can boost a poky 56.6Kbit/sec dial-up connection to speeds that meet and surpass 1.54Mbits/sec. However, DSL's major drawback is that availability is contingent upon distance from the service provider's CO.

DSL is not a one- size -fits-all technologythere are many, many sizes, though they won't all be in stock in any given area (see "Key Players in the Extended Family" at the end of this tutorial). Varieties of DSL generally fall along one of two main distributions, though all differ in specifics. The two primary modelsAsymmetric DSL (ADSL) and Symmetric DSL (SDSL)stem from an early fork in the road: Asymmetry favors downstream data flow, while symmetry maintains equal upstream and downstream speeds.

ADSL has found a home in residential circles, and SDSL has made friends among the suit-and-tie crowd . Both systems possess strengths and limitations that directly trace back to their fundamental differences in symmetry.

The Sharing Side Of Asymmetry

ADSL has made considerable inroads to the residential market, where it competes with cable modems for customers starved for high-bandwidth connections. Remarkably tailored to the appetite of the Web-surfing home user , ADSL delivers between 384Kbits/sec and 7.1Mbits/sec of data in the downstream direction. Upstream throughputs hit 128Kbits/sec to 1.54Mbits/sec.

An asymmetrical model complements the residential profile of Internet use: Masses of multi-media and text course downstream, and undemanding levels of traffic make their way upstream. Costs for ADSL generally scale from $40 to $200 per month, depending on expected data rates and service-level guarantees . Cable modem services often cost less, at an average price of $40 per month, but lines are shared among customers, not dedicated like DSL.

Carrier loops accommodate ADSL alongside analog voice service by assigning digital signals to frequencies above the voice audio spectrum (see Figure 1). To do this, a splitter must be installed. A splitter uses a low-pass filter to separate telephone frequencies on the low end of the audio spectrum from the higher frequencies of ADSL signals. Bandwidth available to ADSL remains unaffected whether or not analog voice frequencies are in use.

Figure 1: Asymmetric Digital Subscriber Line (ADSL) sends data at frequencies of 26KHz to 1100KHz, maintaining analog voice service on the same copper wire in the 0KHz to 3.4KHz range. Symmetric DSL (SDSL) uses the entire frequency of the line for data and does not coexist with analog voice service.

Splitters are required at both the customer premises and the CO for full-rate ADSL. They generally do not require power, and therefore do not obstruct "lifeline" voice service in the event of a power outage .

ADSL speeds are not an exact science, although they scale downward at somewhat predictable intervals. Service providers deliver a "best effort" service whose results depend heavily on distance from the CO. "Best effort" generally means that providers only guarantee about 50 percent bandwidth. Attenuation and interference factors such as crosstalk prey on the lines after 10,000 feet, and can render them unsuitable to data transfer at distances greater than 18,000 feet.

ADSL speeds can reach up to 7.1Mbits/sec downstream and 1.5Mbits/sec upstream at distances up to 12,000 feet from the CO. However, DSL Reports (www.dslreports.com) business editor Nick Braak says not to expect the upper limit. "It's virtually impossible to get 7.1Mbits/sec, even in a lab," says Braak. ADSL gets reined back to rates of 1.5Mbits/sec downstream and 384Kbits/sec upstream after 10,000 feet; as the line approaches 18,000 feet, speeds drop yet further384Kbits/sec downstream and 128Kbits/sec upstream.

Service contracts for ADSL may prohibit the use of home networks or any use of Web servers. However, DSL technology does not prohibit operability with household LANs. For example, even if the provider restricts the customer to a single IP address, Network Address Translation (NAT) enables multiple users to share that one IP address.

One DSL connection is sufficient for a home full of computers. Some DSL modems will include an Ethernet hub, and dedicated devices known as "residential gateways" act as bridges between the Internet and home networks.

ADSL is available in two modulation schemes: Discrete Multitone (DMT) and Carrierless Amplitude and Phase (CAP).

DMT slices available frequencies into 256 channels of 4.3125KHz each, within the range of 26KHz to 1100KHz (as noted in Figure 1).

The Copper Bone Connects To The ATU-R Bone

So there's the CO, the twisted-pair copper, and the remote location. What connects to what?

At the customer site is a device known as the ADSL Transmission Unit-Remote (ATU-R). Though originally specified for ADSL, ATU-R now refers to the remote unit for any DSL service. In addition to providing DSL-modem functionality, some ATU-Rs can perform bridging, routing, and Time Division Multiplexing (TDM). An ADSL Transmission Unit-Central Office (ATU-C), located at the CO, terminates the other side of the copper line and coordinates the link on the CO side.

The DSL provider feeds multiple DSL loops into a high-speed backbone network by means of a DSL Access Multiplexer (DSLAM). At the CO, the DSLAM aggregates data traffic from several DSL loops and feeds that traffic to the service provider backbone, which sends the traffic to destinations elsewhere on the network. Typically, the DSLAM connects to an ATM network with PVCs established to ISPs and other networks.

G.Lite: ADSL, Hold The Splitter

A modified form of ADSL, known as G.lite, eliminates the necessity of a splitter at the customer premises.

G.lite throughputs fall short of full-rate ADSL speeds, though they still put 56.6Kbit/sec dial-up modems to shame. Bandwidth capabilities shrink as a result of potentially increased noise interference, and remote management introduces higher levels of interference. (Types of interference that besiege data transmission are discussed in the section "Cut Out That Racket.")

Using DMT, the same modulation technique used in ADSL, G.lite offers maximum downstream full-rate speeds of 1.5Mbits/sec and upstream rates of 384Kbits/sec.

The ITU's G.992.1 recommendation, also known as G.dmt, was published in 1999, along with G.992.2, or G.lite. G.lite became available in 1999 and costs less than ADSL in large part because it doesn't require a "truck roll," the industry term for a service technician visit to a customer site for installation or troubleshooting. Service providers find it difficult to justify hundreds of dollars in labor for a single $49-per-month residential connection, and any modification that reduces costs receives their undivided attention.

Business-Grade DSL

An enterprise has distinct needs from a home user, and SDSL, operating on a symmetric model, lends itself naturally to office applications.

Corporate upstream capacity can fill up quickly due to heavy Web-server traffic and the resource demands of employees who send large volumes of PDFs, PowerPoint presentations, and other documents. Outgoing enterprise traffic can equal, if not exceed, incoming traffic. Furnishing downstream and upstream data rates of 1.5Mbits/sec in North America and 2.048Mbits/sec in Europe, SDSL resembles the T1/E1 connection that pervades enterprise network architectures worldwide.

Where ADSL utilizes unoccupied frequencies and averts conflict with analog voice frequencies, SDSL takes over the whole line. SDSL eliminates analog voice capabilities in favor of full-duplex data transmission. No splitter, no analog voicenothing but data.

A viable alternative to T1/E1 pipes, SDSL has gotten a fair amount of attention from Competitive Local Exchange Carriers (CLECs) seeking to woo a business market ripe for value-added services. SDSL service as such is almost always marketed by CLECs. However, HDSL is commonly employed by ILEC to implement T1 service.

Under optimal conditions, SDSL rivals T1/E1 data rates and triples ISDN speeds (128Kbits/sec) at its furthest reaches . Figure 2 depicts the distance/speed relationship of SDSL. Data rates taper off in direct proportion to distance from the CO; they also vary depending on the vendor.

SDSL distance from the Central Office (CO)	Maximum data rate
Up to 10,000 ft.	1.5Mbits/sec
11,000 ft. to 12,000 ft.	1Mbit/sec
13,000 ft. to 15,000 ft.	784Kbits/sec
16,000 ft. to 18,000 ft.	416Kbits/sec
Source: DSL.net

Figure 2: Symmetric Digital Subscriber Line (SDSL) speeds depend on the distance from the Central Office (CO); they also vary depending on the vendor. SDSL lines are provisioned at a fixed data rate, while ADSL can adapt dynamically to line conditions.

SDSL utilizes the 2 Binary, 1 Quaternary (2B1Q) modulation scheme adapted from ISDN BRI. Each pair of binary digits represents one quaternary symbol. Two bits are sent per hertz (Hz).

SDSL may be better adapted to its business niche than ADSL in the residential domain. Whereas cable modems entice home users with lower prices than ADSL, SDSL keeps pace with T1/E1 data speeds at substantially reduced costs. Typical T1 prices range between $500 and $1,500, depending on distancethe equivalent cost for SDSL ranges from $170 to $450. In the SDSL pricing scheme, the lower end of the spectrum denotes the cost for lower guaranteed data rates.

Cut Out That Racket

The factors that deteriorate signal quality are numerous and varied, though many are not exclusive to DSL. However, some devices that made life easier in the purely public-switched past have come to haunt present-day DSL deployment.

Crosstalk. Electrical energy radiating from bundles of wire converging at a service provider's CO produces an inconvenient disturbance known as Near-End Crosstalk (NEXT). When signals wander between channels of different cables, line capacity takes a dive. "Near end" specifies that the interference derives from an adjacent pair of cables at the same location.

Segregating DSL lines from T1/E1 lines significantly reduces the negative effects of crosstalk, but there is no guarantee that a service provider will apply that design principle.

NEXT has a related counterpart , Far-End Crosstalk (FEXT), an interference phenomenon whose source originates from another pair of cables at the far end of the connection. With regard to DSL, the degree of FEXT interference is significantly lower than NEXT.

Attenuation. Signal intensities fade as they traverse copper cable, especially signals at high bit rates and higher frequencies. This limitation places a cap on the distances DSL is capable of.

Lower-resistance wire can minimize attenuation, but any given service provider may not deem the expense justifiable. Thick wires have less resistance than thin wires, but they also carry a higher price tag. The most popular cable gauges are 24 gauge (approximately 0.5 millimeter) and 26 gauge (approximately 0.4 millimeter). Due to reduced attenuation, 24-gauge copper cable permits conduction over greater distances.

Load coils. When the PSTN was populated only by voice calls, load coils squeezed more distance out of phone linesa wholly commendable pursuit. The problem nowadays is that they interfere with DSL.

Sacrificing frequencies above 3.4KHz for improved transmission of frequencies in the voice range, load coils are mutually exclusive to DSL. Prospective DSL subscribers cannot get DSL service until load coils no longer reside on a copper run.

Bridged taps. When the phone company doesn't feel like totally disconnecting an unused section of cabling, it takes a shortcut by installing a bridged tap. These practices never seemed to bother anyone until the rabid demand for DSL emerged. Bridged taps cripple DSL's ability to make use of the line, however, and they must often be removed to qualify the line for DSL use.

Echo suppressors. An echo suppressor permits transmission in only one direction at a time. The devices inhibit potential echoes but also render full-duplex communication impossible. Modems can send a 2.1KHz answer tone at the beginning of a connection to deactivate an echo suppressor .

Fiber optic cable. Distance limitations and noise interference are not the only potential DSL pitfalls. If fiber, in the form of a digital loop carrier, is employed in the local loop, the route is ineligible for DSL. Fiber supports digital transmission, but DSL was developed only for analog copper line. The local loop of the future will be built on a hybrid fiber/twisted pair foundation, with short copper runs to neighborhood fiber nodes.

Dance On Command

To initiate services, contact one of the following sources: your local telephone company (also known as an Incumbent Local Exchange Carrier, or ILEC), a CLEC, or a local ISP.

Monopolies die hard. An ILEC can supply a splitter and provision ADSL over an existing line, but until recently, customers who brought their business to a CLEC were forced to pay for an additional phone line. Apparently this was the penalty levied for not going to the ILEC. Extra charges sting on a monthly basis, but more frustratingly, the extra hoop can necessitate a pointless wait for installation of an additional circuit.

However, as of June 6, 2000, by FCC ruling , the additional phone line is no longer required, enabling CLECs to reduce prices, if deemed desirable, by approximately $20 per month.

Voiceovers

Everyone wants to extricate themselves from the high costs of local (and indirectly, long distance) voice charges with Voice over DSL (VoDSL). ADSL already supports analog voice frequencies by allocating digital data communications to higher frequencies, but VoDSL takes an alternate tack. VoDSL converts voice from analog to digital format, sending voice as just a fraction of its digital load.

ADSL and SDSL both support VoDSL, but G.lite is generally considered unsuitable for the task.

Life With DSL

DSL does not guarantee that as soon as you reach for the Internet, your bidding will be done instantly. DSL is merely a last mile solution, an attractive option for high-speed Internet access. Once off the DSL circuit at the CO, you're typically on a third-party network, and thus are vulnerable to all the bottlenecks and traffic problems inherent to that service provider.

The wise user will opt for backup with automatic switchover, even when he or she finally receives DSL service, choosing regular dial-up V.90 technology or ISDN, if available. DSL can go down periodically.

A choice based solely on price may eventually provoke regret . The lower the monthly fee, the less responsive you should expect your service to be.

Security is also a concern with DSL, just as with every other data pipe. Unlike cable modems, DSL users enjoy dedicated connections that remain immune to the activities of other users. Neighborhoods do not share lines, as with cable modems, a point which DSL provisioners cite as a security advantage. Both technologies can be at risk from intrusion and Denial of Service (DoS) attacks, however, due to always-on connections and fixed IP addresses.

If data transmission systems ever evolve into living organisms, twisted-pair copper may lead the pack in adaptive survivalism. From within the communications gene pool, ordinary copper stepped up its act and colonized previously underserved local loop niches . The last mile is a large and eager market, especially receptive to affordable technologies that go heavy on the bandwidth.

Free, unlimited broadband access for all may not materialize in this lifetime, but if you can get DSL, you're moving in the right direction.

Key Players In The Extended Family

Other strains of Digital Subscriber Line (DSL) include High Bit Rate DSL (HDSL), ISDN DSL (IDSL), and Very High Bit-Rate DSL (VDSL).

HDSL. The first symmetric form of DSL, HDSL emerged in the early 1990s as an alternative to T1/E1 lines. Its primary benefit is that it requires no signal repeaters. Like SDSL, HDSL achieves maximum rates of 1.5Mbits/sec, but it does not extend beyond distances of 15,000 feet and requires two cable pairs to SDSL's one. As with SDSL, HDSL does not permit line sharing with analog phones.

IDSL. A hybrid of DSL and ISDN technologies, IDSL bypasses the PSTN and travels on the data network instead. IDSL employs the data-encoding technique of ISDN and delivers up to 144Kbits/sec of bandwidth, 16Kbits/sec more than ISDN alone.

VDSL. An asymmetrical technology like ADSL, VDSL operates at rates of 13Mbits/sec to 52Mbits/sec downstream and 1.5Mbits/sec to 2.3Mbits/sec upstream. Currently in the experimental phase, the catch with VDSL is that a recipient must be located within 1,000 feet to 4,500 feet of the Central Office (CO) or other access point.

Resources

An excellent all-around resource for Digital Subscriber Line (DSL) information and peer reviews of DSL service providers can be found at www.dslreports.com. A section devoted entirely to business use of DSL is expected to launch in August 2000.

The xDSL Resource Web site has a comprehensive FAQ listing on each of the varieties of DSL, their modulation techniques, and the interference minefield on the local loop. Go to www.xdslresource.com/xDSLFAQ.shtml.

A special report on DSL appeared in the April 2000 issue of Network Magazine . It covers implementation hurdles and Voice over DSL (VoDSL). It also provides a comparative analysis of DSL costs and deployment from U.S. sites. This special report and other DSL articles from back issues of Network Magazine are available at www.networkmagazine.com.

This tutorial, number 145, by Rob Kirby, was originally published in the August 2000 issue of Network Magazine.