Jake Bishop

Article 3

xDSL Technology

Article: xDSL Local Loop Access Technology:

Delivering Broadband over Copper Wires

Source: 3Com

                        http://www.3com.com/technology/tech_net/white_papers/500624.html

Article review on DSL technology

Summary:

This article describes the different xDSL technologies in development today and compares them to other current and emerging WAN service technologies. Performance bonuses and limitations are included for each DSL variety, to give a fair comparison of the different types of DSL, and fully describe its capabilities.

It also describes how xDSL technology is being used in various forms world wide, and the current world market for DSL technologies.

The article also describes the investment strategies of 3Com for xDSL as the market for this new technology develops.

Response:

Before reading this article I was not aware of the full capabilities of DSL, or the variety of choices available. I found the information from 3Com very informative, but not too technical.

DSL technology offers relatively inexpensive, high-performance communication, for both private and business use. Its use of existing phone-system copper wiring also makes access to this technology much easier, in comparison to T-1 or ISDN connections.

While it was a little long, I thought reading through the entire document well worth it. I give this 5 out of 5 stars for usefulness as a quick reference for the description of the different variants and their performance specs. I also give 4 out of 5 stars for completeness of content; you would almost require reading a technical manual to get much more detailed information, and at the reading level for someone with novice to advanced technology experience.

xDSL Local Loop Access Technology
Delivering Broadband over Copper Wires

By Robyn Aber

Today’s environment is ripe for the emergence of digital subscriber line (xDSL) technologies. The use of more multimedia information on the Internet and World Wide Web by business and residential users is a major growth factor. Another is the availability of affordable networking equipment that enables larger numbers of users to access corporate information from remote sites.

The opening of the telecommunications industry in the United States and throughout the world is sparking the entry into new service delivery by incumbent local exchange carriers (ILECs), interexchange carriers (IECs), Internet service providers (ISPs), competitive local exchange carriers (CLECs), and satellite and cable companies. Mixed media networking, the need for affordable broadband transmission rates, and a competitive telecom service environment all contribute to making xDSL the right technology at the right time. xDSL services promise to dramatically increase the speed of copper wire–based transmission systems without requiring expensive upgrades to the local loop infrastructure. New xDSL services are being readied to join the bandwidth race.

This paper describes the different xDSL technologies in development today and compares them to other current and emerging WAN service technologies. It also reports on current and future worldwide xDSL deployments and gives some market introduction projections. Finally, the paper describes 3Com’s strategic direction with respect to the emerging xDSL technology market.

What Are Digital Subscriber Line (xDSL) Services?
xDSL services are dedicated, point-to-point, public network access technologies that allow multiple forms of data, voice, and video to be carried over twisted-pair copper wire on the local loop (“last mile”) between a network service provider’s (NSP’s) central office and the customer site, or on local loops created either intra-building or intra-campus. xDSL is expected to have a significant impact in the next three years by supporting high-speed Internet/intranet access, online services, video-on-demand, TV signal delivery, interactive entertainment, and voice transmission to enterprise, small office, home office, and, ultimately, consumer markets. The major advantage of high-speed xDSL services is that they can all be supported on ordinary copper telephone lines already installed in most commercial and residential buildings.

Development History
xDSL was designed initially to provide video-on-demand and interactive TV applications over twisted-pair wires. Interest in copper-based digital subscriber line services was spurred when fiber-based broadband loops proved to be too costly for widespread deployment. Another boost came with the passage of the Telecommunications Reform Act of 1996, which allows local phone companies, long-distance carriers, cable companies, radio/television broadcasters, Internet/online service providers, and telecommunications equipment manufacturers in the United States to compete in one another’s markets. The race to provide broadband bandwidth was on.

In xDSL, telecommunications companies see an opportunity to leverage customer demand for faster data access that has resulted from the explosive growth of the Internet and the advent of IP telephony. xDSL has the potential to deliver high-speed data access and much more. xDSL technology is in the early stages of commercial availability. The key players have agreed on standards and continue to work out interoperability, provisioning, and operations issues.

Different Types of xDSL and How They Work
The “x” in xDSL stands for the various kinds of digital subscriber line technologies, including ADSL, R-ADSL, HDSL, SDSL, and VDSL. To fully grasp the significance of these technologies and the applications for which each is best suited, it is important to understand how they differ. Key points to keep in mind are the trade-offs between signal distance and speed, and the differences in symmetry of upstream and downstream traffic. Figure 1 shows that xDSL is used only in the local loop in an end-to-end remote access architecture.

Figure 1. xDSL in the End-to-End Network

Table 1 compares the different types of xDSL technologies along with competing technologies, including 56 Kbps analog dial-up, cable modems, and Integrated Services Digital Network (ISDN).

Asymmetric Digital Subscriber Line (ADSL)
ADSL technology is asymmetric. It allows more bandwidth downstream—from an NSP’s central office to the customer site—than upstream from the subscriber to the central office. This asymmetry, combined with “always on” access (which eliminates call setup), makes ADSL ideal for Internet/intranet surfing, video-on-demand, and remote local area network (LAN) access. Users of these applications typically download much more information than they send. Downstream, ADSL supports speeds between 1.5 and 8 Mbps; upstream, the rate is between 640 Kbps and 1.54 Mbps. ADSL can provide 1.54 Mbps transmission rates at distances of up to 18,000 feet over one wire pair. Optimal speeds of 6 to 8 Mbps can be achieved at distances of 10,000 to 12,000 feet using standard 24-gauge wire.

Rate-Adaptive Digital Subscriber Line (R-ADSL)
R-ADSL operates within the same transmission rates as ADSL, but adjusts dynamically to varying lengths and qualities of twisted-pair local access lines. With R-ADSL, it is possible to connect over different lines at varying speeds. Connection speed can be selected when the line synchs up, during a connection, or as the result of a signal from the central office.

ADSL Lite
ADSL Lite is a lower-speed version of ADSL that will eliminate the need for the telco to install and maintain a premises-based POTS splitter. Elimination of the POTS splitter is intended to simplify DSL installation and reduce the costs of DSL for NSPs. ADSL Lite is also supposed to work over longer distances than full-rate ADSL, making it more widely available to mass market consumers. It will support both data and voice and provide an evolution path to full-rate ADSL.

The effort to introduce ADSL Lite has been spearheaded by the Universal ADSL Working Group, an industry group that worked to develop a worldwide G.Lite standard within the International Telecommunications Union (ITU) Study Group 15. An ITU standard (G.992.2) was approved in October, 1998. Additional standards work can be expected in ANSI TIE1.4, the ATM Forum, and the ADSL Forum to address issues such as compatibility with home wiring and network interfaces. 3Com is an active participant in these standards bodies working on the development of ADSL Lite.

ISDN Digital Subscriber Line (IDSL)
IDSL provides full duplex throughput at speeds up to 144 Kbps. Unlike ADSL, IDSL is restricted to carrying data only. While IDSL uses the same 2B1Q modulation code as ISDN to deliver service without special line conditioning, it differs from ISDN in a number of ways. Unlike ISDN, IDSL is a non-switched service, so it does not cause switch congestion at the service provider’s CO. ISDN also requires call setup, while IDSL does not (DSL is an “always on” service).

High Bit-Rate Digital Subscriber Line (HDSL)
HDSL technology is symmetric, providing the same amount of bandwidth upstream as downstream. HDSL is the most mature of the xDSL technologies, and has already been implemented in telco feeder plants (lines that extend from central offices to remote nodes) and also in campus environments. Due to its speed—1.544 Mbps over two copper pairs and 2.048 Mbps over three pairs—telcos commonly deploy HDSL as an alternative to repeatered T1/E1. (T1 lines, used in North America, have a data rate of 1.544 Mbps; E1 lines, used in Europe, have a data rate of 2.048 Mbps.) Although HDSL’s 12,000 to 15,000-foot operating distance is shorter than ADSL’s, phone companies can install signal repeaters to cost-effectively extend its useful range. HDSL’s reliance on two and three twisted-pair wires makes it ideal for connecting PBX systems, digital local loops, IEC points of presence (POPs), Internet servers, and campus-based networks. HDSL II is pro-posed as the next-generation HDSL within ANSI and ETSI. It will offer the same performance as HDSL, but over a single pair.

Single-Line Digital Subscriber Line (SDSL)
Like HDSL, SDSL supports symmetrical TI/E1 transmissions, but SDSL differs from HDSL in two important ways: it uses a single copper-pair wire, and it has a maximum operating range of 10,000 feet. Within its distance limitation, SDSL is capable of accommodating applications that require identical down-stream and upstream speeds, such as video conferencing or collaborative computing. SDSL is a precursor to HDSL II.

Very High Bit-Rate Digital Subscriber Line (VDSL)
VDSL technology is the fastest xDSL technology, supporting a downstream rate of 13 to 52 Mbps and an upstream rate of 1.5 to 2.3 Mbps over a single copper-pair wire. VDSL can be viewed as a cost-effective alternative to fiber to the home. However, the maximum operating distance for this asymmetric technology is only 1,000 to 4,500 feet from the central office; this distance can be extended by running fiber optic cable from the CO to an optical network unit and copper from that point to the user location up to 4,500 feet away. In addition to supporting the same applications as ADSL, VDSL’s additional bandwidth could potentially enable NSPs to deliver high-definition television (HDTV), video-on-demand, and switched digital video, as well as legacy LAN extension symmetrical services. VDSL is in the requirements and standards definition stage.

Figure 2. An Amplitude-Shifted Sine Wave

xDSL Delivers Broadband over Copper
The best thing about xDSL technologies is their ability to transport large amounts of information across existing copper telephone lines. This is possible because xDSL modems leverage signal processing techniques that insert and extract more digital data onto analog lines. The key is modulation, a process in which one signal modifies the property of another.

In the case of digital subscriber lines, the modulating message signal from a sending modem alters the high-frequency carrier signal so that a composite wave, called a modulated wave, is formed (Figure 2). Because this high-frequency carrier signal can be modified, a large digital data payload can be carried in the modulated wave over greater distances than on ordinary copper pairs. When the transmission reaches its destination, the modulating message signal is recovered, or demodulated, by the receiving modem.

Technology and Applications Comparison
There has been a lot of speculation in the industry about which remote access technologies will succeed and which will fail. As new local access technologies are rolled out, they do not displace others; actually, the reverse is true. Technologies like analog dial-up, dedicated leased lines, Frame Relay, and ISDN all coexist successfully in the market based on differences in service availability and on their ability to generate incremental revenue by serving different applications.

The fact that so many WAN services continue to coexist often leads to confusion and complexity for enterprise network managers and planners. The range of services will certainly continue into the next century. Factors that will determine the success of one technology versus another include availability, pricing, ease of installation and use, and relevance to users’ applications. Some of the key issues surrounding xDSL and competing technologies are summarized in this section.

56 Kbps Analog Modems
56 Kbps analog modems (ITU V.90 standard) provide a range of midband (28.8 to 56 Kbps) access to the Internet, intranets, and remote LANs.

In order to realize 56 Kbps throughput, there must be a 56 Kbps modem using compatible modulation techniques at each end of the connection. Therefore, NSPs and ISPs must have V.90 modems at their points of presence. A single 56 Kbps modem at the user’s site will deliver the next highest speed with which it can synch up. Even when 56 Kbps modems are installed at both the carrier and user sites, these modems achieve top speeds only if the connection has just a single analog/digital conversion, and actual through-put is determined by line quality.

Another important fact to keep in mind is that this technology is asymmetric. The 56 Kbps rate is only achieved downstream on a digital line from the network to the user. The upstream connection is analog and operates in the 28.8 to 33.3 Kbps range.

ISDN
ISDN is also considered a digital subscriber line service. ISDN and xDSL technologies share some common technical characteristics: use of the existing telephone company copper cabling infrastructure; digital quality-of-service capabilities such as low noise, less interference, and clearer voice transmission; and the security of digital communications, which is inherently more difficult to tap than traditional analog systems.

However, ISDN differs from xDSL technologies in that it is a switched service in which both ends must support ISDN, whereas xDSL is a point-to-point access service. ISDN also requires external power for operation. To ensure continuous operation, customers need either a backup power system or a redundant POTS line. In contrast, xDSL carries its own power on the line. Voice and data transmission is split (multiplexed) on the wire: voice is carried under 4 kHz; data is carried above 4 kHz. If a power failure occurs, xDSL data transmission is lost, but lifeline POTS still operates.

Another key difference is that ISDN is widely available now and has momentum in the marketplace. Telcos, competitive access providers, and ISPs are investing the resources and building out the infrastructure to develop it further. As ISDN modems and terminal adapters become easier for users to configure, customer premises equipment (CPE) prices continue to drop, and tariffs are reduced, ISDN is gaining broader appeal among telecommuters and small office and retail users who require Internet and intranet access, remote LAN access, credit authorization, or database connectivity.

Cable Modems
Designed to provide broadband Internet access, cable modems are primarily targeted at consumers for residential use. Cable modems offer the potential of broadband (up to 30 Mbps) information delivery downstream to users and midband (128 Kbps) to broadband (up to 10 Mbps) connections back upstream to the cable headend. Unlike xDSL and ISDN, cable modems are a shared—not dedicated—access technology. The total available bandwidth is shared among users in a neighborhood as if they were on a LAN. Given that design, not everyone on the network will get the top speeds of 10 to 30 Mbps that are quoted for downstream throughput. Actual rates will vary according to the number of users on the system at any given time and the type of modem that is being used. Security is also an issue on these shared access systems.

The multimedia cable network system (MCNS ) standard for the delivery of data over cable has been defined and is being adopted by major multiple system operators (MSOs) and cable modem manufacturers. Its adoption adds more stability to cable as a data transmission technology. However, the wide-spread introduction of cable modems is still contingent upon the development and implementation of complex, two-way transmission systems and operations systems for management and billing. Today’s systems are primarily telco return, in which phone lines are used to provide upstream transmission.

Another hurdle that cable modems must overcome is negative perceptions about the quality of service delivered by cable systems. Some users are approaching the use of cable modems for data transfer with caution. For cable modem access providers to be successful, they must be able to compete not only on price, but also on reliability of service.

xDSL
For all intents and purposes, xDSL modems can be considered “next-generation” modems, initially targeted for business users. xDSL technologies are being positioned for a wide range of data dialtone, video dialtone, voice, and PBX interconnect applications. For the near term, however, the trend continues to be toward data applications, with voice-over-IP emerging as a new application.

While xDSL technologies hold a lot of promise, there are a number of critical issues to be resolved before they can achieve wide-spread commercial deployment. Standards are now agreed upon. During 1996–1997, standards bodies split along the partisan lines of DMT versus CAP modulation schemes. In January 1998, ANSI re-ratified DMT as the standard of choice, and the ITU adopted it in February 1998.

Other ongoing issues for xDSL technologies include interoperability, spectral compatibility (e.g., interference between different services carried in the same cable binder), near-end crosstalk associated with reverse ADSL provisioning, and loop qualification. A nontechnical but critical factor will be how successfully NSPs move from xDSL technology and market trials to commercial rollout.

Sometime in the next three to five years, xDSL technology could potentially be used to deliver Asynchronous Transfer Mode (ATM) to the home over the existing copper infrastructure or via a hybrid fiber/copper network. Efforts to define the standards for doing this are now under way in ANSI, ETSI, the ADSL Forum, the ATM Forum, and the Full Service Access Network (FSAN) Council. While joint development efforts are proceeding, considerably more cooperative work is needed before these organizations can agree upon a set of standards that will enable the delivery of low-cost, end-to-end ATM to the desktop over xDSL.

ADSL Development and Deployment Progress
Of all the emerging xDSL technologies, ADSL is receiving the most attention because there is a standard (DMT) for it, and its capabilities provide NSPs with a competitive offering to cable modems. But there is increasing interest in symmetrical xDSL offerings such as HDSL and SDSL.

As a local access service, ADSL’s implementation has no critical drawbacks. It can be deployed as an overlay network where there is subscriber demand, eliminating the need for NSPs to risk building out their infrastructure unnecessarily in the hope that the technology will catch on.

ADSL development and deployment is focused primarily in North America, followed by northern Europe and the Pacific Rim. In North America, US West, GTE, Ameritech, SBC, BellSouth, and Edmonton Tel (Canada) are the service providers leading the current wave of ADSL/xDSL deployment. Covad, Northpoint, and a handful of other CLECs are entering high-density metropolitan areas—typically offering a portfolio of xDSL offerings at different classes of service and price points, and competing with incumbent local exchange carriers. Chicago-based InterAccess was the first ISP to offer ADSL. Telia (Sweden), Telenor (Norway), British Telecom (UK), and Telfonica (Spain) are leading xDSL proponents in Europe. In the Pacific Rim, Telstra (Australia), Hong Kong Telecom, and Singtel (Singapore) are deploying xDSL for data and video applications.

ADSL modems have been tested successfully by more than 40 telephone companies, and close to 50,000 lines have been installed in various technology trials and commercial deployments. Increasingly, alternative service providers such as enterprises, multi-tenant building owners, hospitality businesses (hotels and resorts), and office park developers are offering or considering offering ADSL to their users as private network operators.

Getting Started with ADSL
ADSL is not yet generally available. It is an emerging technology that is predominantly in the early commercial deployment stage. NSPs still must put in place the overlay networks to handle commercial service offerings, and network equipment vendors must build production-level DMT systems. Users can expect to see ADSL products and services introduced throughout 1998, followed by more wide-spread deployment in 1999 and 2000.

ADSL Suppliers
xDSL suppliers generally fall into three categories:

• Component manufacturers
• Systems providers
• Service providers

Component manufacturers provide the chips, modems, and POTS splitters used at both ends of a line to receive, send, and process digital data. Systems providers offer end-to-end solutions that include modems, splitters, and multiplexers as well as operations, administration, management, and technical support capabilities. Service providers offer xDSL access services and may or may not bundle products from component manufacturers or systems providers to offer their subscribers turnkey solutions.

Prospective users of ADSL need to determine whether their local service provider offers a turnkey solution, or whether they must work directly with equipment manufacturers, value-added resellers, or systems integrators. It is possible that ADSL modems will be available at retail outlets during 1999 in a number of markets where service is deployed.

Network Design: What’s Needed
Figure 3 illustrates the various components of an ADSL network. It includes both NSP components (central office) and user components (including branch offices, small offices, and home offices).

Figure 3. xDSL Network Elements

Potential users of ADSL will need the following:

• An ADSL modem (compatible with the one at the NSP’s point of presence)
• A POTS splitter to separate voice and data transmissions (unless using ADSL Lite)

Since the ADSL modem essentially front-ends a LAN (or is capable of doing so), branch office or small business users will need a router or hub; home users will need a computer interface.

Providers of ADSL services will need modems and POTS splitters in their digital subscriber line access multiplexer (DSLAM) to terminate and aggregate incoming ADSL lines and redirect voice traffic to the public switched telephone network (PSTN) and data to a high-speed digital line (DS3, OC-3, or OC-12). The DSLAM is the major intelligence component in the ADSL system. It consists of central site modems and a service access multiplexer (SAM) that interfaces to the NSP’s ATM or Frame Relay backbone. The ADSL service provisioning model includes two types of DSLAM: the central office DSLAM is built for high density and concentration, while the remote DSLAM sits in the remote DLC system. Service providers will also need billing systems, testing and diagnostic functionality, and network management capabilities.

Significant development work is still needed by NSPs and equipment manufacturers alike to develop more affordable, scalable, interoperable, and easily provisioned ADSL systems. But this is an exciting emerging technology that will initially provide high-bandwidth local access for enterprise networks and teleworkers.

Conclusion
xDSL technology—with its ability to support voice, content-rich data, and video applications over the installed base of twisted-pair copper wires—is inherently suited to meet user demands for broadband, multimedia communications. The most promising of the xDSL technologies for integrated Internet access, intranet access, remote LAN access, video-on-demand, and lifeline POTS applications in the near term is ADSL or R-ADSL (a rate-adaptive version of ADSL). During the past year, ADSL has concluded trials by more than 40 network service providers throughout the world, primarily in North America and northern Europe.

Service introduction began in 1997, but ADSL service is still being rolled out in many areas. In the meantime, xDSL technologies and standards will continue to evolve, as will user demand for these emerging services relative to other local access service alternatives.

3Com has been shipping its end-to-end, standards-based xDSL solutions since June 1997. With 3Com as a partner, telecom and enterprise NSPs can provision xDSL in a way that leverages their existing infrastructure and service delivery model, adding xDSL equipment as demand grows.

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