What's the state of xDSL?

xDSL was supposedly destined to crush ISDN. Where does it stand now? What's the holdup?

By Robert E. Lee

December  1997
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Where does xDSL fit into high-speed networks today? We explain how it works, how much it costs, and who's conducting the trials. (1,900 words)

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After several years of hype and continuing market misconception about the intended end users of Digital Subscriber Line (xDSL) technologies, the lowest member of this technology family, Asymmetric Digital Subscriber Line (ADSL), is entering the market with a solid set of solutions and a potentially narrower application market than originally envisioned.

It has been nearly three years now since xDSL technologies first made a splash in the technology arena. At the time, it was certain that xDSL would bring down ISDN and other high-speed technologies. The evolution of the technology is proving otherwise; xDSL has entered a tenuous phase. There is a conflict between market promise and the harsh realities of implementation, and telephone companies and network providers, both Internet and corporate, will have to implement a whole new set of technologies to bring the two together. The good news is that everyone seems to be coming to the table, and xDSL is progressing much faster than ISDN did when it first entered the market.


The technology revealed
At a recent briefing by Pacific Bell, ADSL was referred to as turbo ISDN, a definite misnomer. At the heart of xDSL is the division of the copper wire pair into a single analog phone service and a digital transmission channel that is based on asynchronous transfer mode (ATM) cell-based services. This isn't the land of ISDN, with its sophisticated offering of voice and data services. Starting at a speed of 192 kilobits per second (kbps) and going up to 70 megabits per second (mbps) in yet-to-be offered products, this technology is a simple data circuit overlaid on a voice circuit, which in itself isn't very special.

Table 1 reflects the various service types for xDSL. Distance is the nemesis of every transmission technology, even DSL, nonetheless, it offers a much better solution than we have seen outside of fiber-based facilities. The nominal distance is 18,000 feet, but it can be extended out to 36,000 feet, providing nearly seven miles of distance, and thereby opening a much wider range of customers to high-speed data connections.

xDSL Service Types
ADSL - Asymmetric Digital Subscriber Line
32 kbps to 8.192 mbps downstream, 32 kbps to 1.088 mbps upstream. As you can see, this technology fits a demand for surfing the Internet or delivering content with less of a need to move information off of the remote site.
RADSL - Rate Adaptive Asymmetric Digital Subscriber Line
This is a variation of the SDSL and ADSL technologies, which allow for pre-defining or self-adjusting the bandwidth of the circuit. This technology opens the door for additional bandwidth levels to be tariffed, starting with a low-speed home or small office offering, then moving on to remote office speeds and then high-speed Internet connections.
HDSL - High-bit-rate Digital Subscriber Line
Either 1.544 mbps or 2.048 mbps, to implement T1 or E1 line speeds without the overhead of these older transport technologies.
SDSL - Symmetric Digital Subscriber Line
160 kbps to 2.084 mbps in a symmetrical implementation, with downstream and upstream data rates being equal.
VDSL - Very high-bit-rate Digital Subscriber Line
Up to 70 mbps based on the distance from the central office. At 3,000 feet the rate drops to 26 mbps while at 5,000 feet the data rate is down to 13 mbps.

In order to implement a xDSL circuit, a modem must be placed on the customer's premises, which provides the initial transport. It then multiplexes the analog circuit with the digital transmission. At the phone company's central office, the circuit is split into the analog voice service and an ATM circuit. The ATM cells are then routed to the ATM network for transport to the final destination, via a permanent virtual circuit that has been configured for the specific application. In this sense, the xDSL implementations are a point-to-point solution across a technology, ATM, which could support connectionless applications. At the remote end, it is necessary today for an ATM connection to be present. In the future, it is expected that frame relay circuits will provide the central site connections.

Because this technology is much simpler than ISDN, it does away with many of the implementation headaches associated with configuring ISDN. In particular, the need to configure circuit identifiers is gone. This is a hard-wired technology that requires more effort to configure into your LAN than onto your phone company's network. Because of the high speeds this technology brings, most modems will have an Ethernet port; some will even act as a router for the network.

Application possibilities
With a point-to-point technology design, xDSL opens the door to a somewhat guaranteed bandwidth performance between the remote client and the host network. Outside of the host network some problems may arise, which we'll discuss later. Let's begin with remote LAN access. Pacific Bell is in trial with ADSL in Northern California on a symmetrical 384-kbps circuit at a price of $150 for businesses and $80 for consumers. This is strictly client-end pricing; the host network must implement an ATM connection to the phone company's ATM network, thus raising the per-client price. Compared to the present costs of frame relay or T1 circuits, which start at $500 and work their way up, this technology is a bargain.

In a bold move to provide this technology, DSLNET is targeting the southeastern Wisconsin area with an asymmetrical service at 1.5-mbps downstream and 1-mbps upstream, plus a host of e-mail and Web services, priced from $649 to $889. US West has opened the technology to the Phoenix area, under the MegaBit Services brand. In a combined access and Internet service package, installation is only $199.95, and the monthly recurring charges under contract will be $59.95 for a 192-kbps circuit. Contrast this to the ISDN circuit in California, which runs $49.95 for Internet access and $38.50 for the telephone line, all for only 128-kbps bandwidth.

It is important to note that xDSL is only going to provide a simple voice-grade line for SDSL (Symmetric Digital Subscriber Line) and ADSL, and this line will only supply the same services available on other analog lines today. That places xDSL into a market niche of data-only applications. But we're not talking about ordinary data.

How about video conferencing over the network? Video conferencing is considered a data-only application. With typical access speeds of just 384 kbps, full-motion videos using the video conferencing compression technologies available today can be easily and effectively transported through xDSL. With a higher-rate ADSL circuit, complete data and videoconferences can be broadcast over this network simultaneously, with little impact to existing applications. Distance learning, traditionally a videoconferencing-based solution, is a natural for xDSL.

Even small office/home office (SOHO) and advanced home Internet users will benefit from this technology. Many areas are creating home-based tariffs to reduce the entry-level pricing for this technology. Like the US West offering, Pacific Bell has reduced the home ADSL rate to $80 for 384 kbps symmetrical, plus the cost of Internet access. Bell South's trial is bringing this technology to the market at just $49.95 for the circuit and unlimited Internet access at 1-mbps download speeds.

State of the market
Only US West has really entered the market beyond the trial level -- but this has been limited to 14 areas in the Phoenix metropolitan area. Everyone else is in the midst of expanding the scale of their trials. The trial periods have been extended for several reasons. First, the impact of this amount of bandwidth intruding on the existing networks has carriers concerned. The math makes this point very clear. Just a thousand customers for an ISP, all signed up for a 1.5-mbps service would require 1.5 gigabits per second (gbps) of bandwidth into the ISP to have complete bandwidth guaranteed inbound. The ISP would need to duplicate that circuit capacity out to the Internet for complete coverage. On a linear progression, 10,000 customers would scale the needs to 15 gbps -- well beyond anyone's current capacities in anything less than the largest of ISPs. US West is advising ISPs to plan for seven to 15 to one ratios for access, reducing the inbound requirements for the 1,000 customers to as little as a single 155-mbps ATM circuit. This is more affordable, but clearly places this technology into the realm of large, well-funded ISPs who can afford to implement large circuits like this.

Because xDSL rides the ATM networks, it is necessary to confirm with the local phone carrier that an appropriate interlata agreement is in place if the circuit needs to span latas. As a local area transport, xDSL should be easy to implement. A second issue is the interoperability of xDSL modems. These will need to be matched to the requirements of the phone companies. US West has gone with PairGain while Pacific Bell is going with Alcatel. The interoperability is between your site and the central office that you connect to, not the remote host network -- ATM solves that problem for you. However, if you are implementing across phone company boundaries, this issue could increase your support costs.

The cost of the modems is encouraging. Prices start as low as $495 for the HotWire 5216 from Paradyne, which provides 1-mbps symmetrical service, and $1,195 for its HotWire 5446, which supplies a 7-mbps data rate. PairGain, Westell, and Alcatel are some of the early market leaders in this technology.

Your next steps
Unless you are in a market trial area, don't expect to run out and get this technology today. If you need to leverage the circuit at your site into more than a data line, either plan on voice over IP or go with ISDN to mix and match these application needs.

If on the other hand you need to boost your bandwidth, and you can wait for the service to arrive (we're really talking early to mid 1998), then definitely begin your investigation into xDSL technology and the impact it will have on your network plans. Start with the links below and plan a visit to a major technology trade show near you (Comdex, NetWorld+Interop, etc. You know the list!). Call your phone company and get the latest information on its plans. Then talk with your ISP if it will be one of the segments in your network. This will take a fair amount of coordination at first, but it won't be any more complex than most networking projects today.


About the author
Robert E. Lee is a technology consultant, speaker, columnist, and author who has been in the computer industry for 20 years. He specializes in networking, Internet strategies, systems analysis and design activities, and has participated in the Windows NT and Internet Information Server betas since the start of those products. In addition to several other recent feature stories, Rob wrote the June 1997 SunWorld news story, "Cisco throws its support behind Microsoft's directory service vaporware."

[Amazon.com Books]You can learn more about Robert E. Lee's The ISDN Consultant: A Stress-Free Guide to High-Speed Communications and Serving the Net: Using the Power of Microsoft Internet Information Server at Amazon.com Books. Reach Robert at rob.lee@sunworld.com.

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