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xDSL Access Networks


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xDSL Access Networks



Current Telco Services



Digital Loop Carrier Overview



xDSL Variations



Asymmetric DSL



G.Lite (ITU G.992.2)



Very High Data Rate DSL



High Data Rate DSL



ISDN Digital Subscriber Line



Single-Line Digital Subscriber Line



Early Provisioning for xDSLs



Factors in Choosing Which DSL to Offer



Technical Challenges to xDSL



Industry Challenges to xDSL



Success Factors







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Residential Broadband, Second Edition

From: Residential Broadband, Second Edition
Author: George Abe
Publisher: Cisco Press (53)
More Information

4. xDSL Access Networks

This chapter covers the following topics:

  • Current Telco Services

  • Digital Loop Carrier Overview

  • xDSL Variations

  • Asymmetric DSL

  • G.Lite

  • Very High Data Rate DSL

  • High Data Rate DSL

  • ISDN Digital Subscriber Line

  • Single-Line Digital Subscriber Line

  • Early Provisioning for xDSLs

  • Factors in Choosing Which DSL to Offer

  • Technical Challenges to xDSL

  • Industry Challenges to xDSL

  • Success Factors

  • Summary

Cable operators offer their HFC networks for RBB services. How are the telephone companies to counter? Some telcos have toyed with the idea of building HFC networks of their own or buying cable operators. Among these have been PacBell, Telecom Australia, and Ameritech. However, most telcos prefer to capitalize on their existing multibillion-dollar embedded base of subscriber lines by pushing the evolution of a new technology called digital subscriber line (DSL). The subscriber line, often referred to as the local loop, is a pair of wires that connects each subscriber to a local telephone building called the central office (CO). DSL uses existing subscriber lines to transmit data at speeds of over 100 or even 1000 times current modem service. Hence, it is the vehicle by which telephone companies can compete against cable for video and high-speed data services.

Before proceeding, we note a semantic distinction between loop and line. DSL refers to digital subscriber line. For the purposes of this chapter and book, loop refers to a physical wire pair between the home and the carrier premises, either a central office or remote terminal. A line is a service concept that denotes a capability purchased by a customer. Hence, xDSL refers to line because it is a service concept.

DSL is deployed by U.S. telephone operators to modernize their wiring to households. DSL service extends fiber deeper into the neighborhood and closer to the consumer. U.S. phone companies have enormous maintenance requirements for their 171 million wired connections to customers. Regulatory relief and business practice make more than $20 billion per year available for telco maintenance budgets. In the maintenance process, certain upgrades are permissible that have the effect of increasing the bit rate and lowering operating costs.

To make matters complicated, multiple flavors of DSL technologies are emerging. The various DSL technologies discussed in this chapter are high data rate DSL (HDSL), asymmetric DSL (ADSL), single-line DSL (SDSL), ISDN digital subscriber line (IDSL), and very high data rate DSL (VDSL). Collectively, these are referred to as xDSL. Before considering xDSL technologies, a quick look at the history and architecture of the current telco services will be useful.

To date, xDSL services trail cable modem deployment. At the March 1999 meeting of the ADSL Forum, it was reported that, as of December 1998, there were 45,000 domestic U.S. ADSL customers, compared with more than 500,000 cable modem subscribers (330,000 with @Home alone) and 75,000 customers worldwide.

Current Telco Services

Conventional wisdom is that phone wire is too restrictive for high-speed service because customers are accustomed to phone conversations and 56 Kbps modem service. However, the basic characteristics of phone wire have the capacity to carry millions of bits per second. The reason they don't is that present voice service is provided in analog mode and filters are installed on phone interfaces and loops that suppress signals above 3400 Hz. Given these filters, the data-carrying capacity of phone wire is limited to 56 Kbps, using the latest generation of V.90 modems. Other services, operating in a digital mode, occupy bandwidth such as those shown in Table 4-1.

Table 4-1. Bandwidth Comparisons for Telco Services


Upper Limit Bandwidth

Voice service

3400 Hz


80,000 Hz

ISDN (Germany)

120,000 Hz

T1 using 2B1Q

400,000 Hz


About 1,000,000 Hz

Bandwidth-limiting techniques in the voice network enable economical frequency multiplexing over long-distance lines. If users could have 20,000 Hz per phone line, which is the frequency response of a modern stereo system, bandwidth on the backbone would be reduced by a factor of 6. In addition, as noted in Chapter 2, “Technical Foundations of Residential Broadband,” pushing very high frequency through wires creates problems of attenuation and distortion that negatively impact the integrity of the signal.

Today, new modulation, equalization, and error-control techniques make subscriber lines capable of transmission rates greater than 6 Mbps for distances up to 3 miles. How much greater depends on the length of the phone wire between the subscriber and the telephone office, the physical condition of the wires (corrosion, insulation, bridged taps, and crosstalk), and the thickness of the wires (the thicker the wire, the less the resistance and the greater the distance served). Table 4-2 shows the trade-off of distance and speed over 24-gauge phone wire (0.5 mm in diameter).

Table 4-2. Distance Versus Speed over 24-Gauge Phone Wire

Phone Wire


Estimated Distance

DS1 (T1)

1.544 Mbps

18,000 feet


2.048 Mbps

16,000 feet


6.312 Mbps

12,000 feet


8.448 Mbps

9000 feet

1/4 STS-1

12.960 Mbps

4500 feet

1/2 STS-1

25.920 Mbps

3000 feet


51.840 Mbps

1000 feet

Source: Alcatel at

Before examining how technology is pushing this envelope, it will be useful to overview two current telephone services—plain old telephone services (POTS) and Integrated Services Digital Networks (ISDN)—as well as the rationale for DSL systems.

Plain Old Telephone Services

Telephone switching equipment, which establishes phone connections, is located in the CO. Customers are connected to the CO over thin-wire pairs, also referred to as local loops. These thin-wire pairs are segmented in lengths of 500 feet. Lengths are spliced together as needed to reach from the CO to the customer's home.

The first 500-foot segment from the CO, 26-gauge wire, is normally 0.41 mm in diameter. This is the thinnest type of phone wire. After the first segment, the phone wire is often a thicker diameter, such as 24-gauge (0.50 mm) or the thickest, which is a 19-;;gauges (0.82 mm) wire. Because resistance is inversely related to thickness, the thicker-gauge wire is reserved for customers who are farther from the CO. Resistance in the wire is a function of temperature as well as wire thickness. At 70 Fahrenheit, 26-gauge wire has about 40 ohms of resistance per thousand feet; 19-gauge wire induces 10 ohms per thousand feet.

Local loops are bundled in large cables called binder groups. The job of the field technician is to cross-connect a drop wire to your home and then to a phone wire in a binder group. Fifty phone wires to a binder group is a typical configuration near the subscriber. Some binder groups contain as few as 20 local loops, and others contain as many as a few hundred. Feeder cables—roughly the first 9000 feet coming out of a central office—can have hundreds or even thousands of pairs bundled together. These are referred to as binder groups as well. Figure 4-1 shows a simplified connection between a CO and customers.

The copper in phone lines, although very thin, adds up when considered network-wide. U.S. telcos are the largest single consumer of copper in the world. The local loop consumes nearly 50 percent of telco capital cost; that is capital cost of a local loop is $1500 to $2000 with about 50 percent being used for materials, including copper. Therefore, a premium is put on having the thinnest possible copper, consistent with transmission fidelity. Thinner wires mean less real estate and fewer material costs. Thinner wires are lighter, also reducing the cost of aerial runs. Generally, minimizing the consumption of copper is a goal in the design of phone systems and is one reason for telcos' interest in fiber technologies.

Figure 4-1. Wiring Schematic pre-DSL

Eventually, a single-wire pair is extracted from feeder cables and distribution cables of fewer wire pairs until the single-wire pair is connected to your house over the drop wire. The drop wire connects to your home in a junction box called the network interface device (NID). The NID is typically a passive device that serves to couple external phone wire to internal phone wire in the home.

Basic Rate ISDN

Basic Rate ISDN (BRI) is a digital service that provides 160 Kbps over phone wire and up to 18,000 feet of 24-gauge wire. Its standard implementation (ANSI T1.601 or ITU I.431) employs echo cancellation to separate the transmit signal from the received signal on a single pair of wires. It uses bandwidth from 0 to about 80 kHz. European systems use 120 kHz of bandwidth. Therefore, provisioning of ISDN and analog POTS on the same local loop is not possible because both services utilize frequencies less than 3400 Hz.

This is not a big deal in the United States because there are less than 1 million ISDN lines provisioned. However, in Europe, ISDN is more widely deployed and therefore there needs to be a coexistence and migration strategy to move ISDN users to xDSL. In Europe, there are more than 6 million ISDN lines, most using a form of ISDN that uses 0 to 120 kHz. The intent is to migrate ISDN users without changing their local loops. The current idea within the European Telecommunications Standards Institute (ETSI) is to have ADSL start at 140 kHz and proceed upward and to use an ISDN splitter instead of a POTS splitter. This puts ADSL on a different frequency plan than in the United States.

Limitations of Current Telco Networks

Certainly there is new interest in high-speed services over existing subscriber lines. However, even low-speed data service using telephone modems creates new problems for the phone system. These limitations associated with traditional telco networks are also driving interest in xDSL services.

Space and Distance Constraints

Currently, there are more than 22,000 COs in the United States, serving 171 million lines. Hundreds more offices are served by long-distance carriers (AT&T, MCI, Sprint, and so on), also referred to as interexchange carriers (IXC). The average CO serves 7600 lines; a few serve as many as 100,000 lines. The problem of limited space in conduits in and out of COs is becoming more severe as consumers add second lines for home use. About 20 percent of U.S. homes have added second lines for use in home offices and for talkative teenagers. In Beverly Hills 90210, there is an average of nearly 4 subscriber lines per household (that is, main number, fax, teenager, maid).

Central offices are big, expensive buildings. Reducing real estate needs by using more compact electronics can account for noticeable savings. In places where real estate is at a premium, such as Rome and Tokyo, the sale of telco property can generate enough money to fund digital buildouts of telephone services. In other words, telcos can trade buildings and real estate for new digital infrastructures. Therefore, an incentive exists to move to more compact facilities. One way to facilitate such a move is by installing fiber for new services and distributed switching systems, hence the move to DSLs.

Users obtain services by connecting to the CO. Their distance from the CO dictates the cost of providing the service and, in some cases, the type of service received.

Two standardized range limits exist. Revised resistance design rules (RRD) limit distance to 18,000 feet for 24-gauge and 15,000 feet for 26-gauge wire. RRD rules are used for ISDN. Another range limit, carrier serving area (CSA)—used for HDSL service—limits service to 12,000 feet for 24-gauge and 9000 feet for 26-gauge wire. Half of U.S. lines are within 9000 feet of a CO or remote terminal; 80 percent are within 15,000 feet. (The RRD and CSA distances are estimates because the actual distance is a function of line quality. If a particular local loop has severe impairments, the distances will be shorter than these estimates.)

Internet Problems for POTS and ISDN

Internet service over POTS and ISDN lines is readily offered because of the ubiquity of phone service and the widespread availability of modems and voice switches provisioned for ISDN. The problem for telcos is that both POTS and ISDN use the CO voice switch, and data has a different usage profile from voice.

The major difference is that data sessions are longer than voice calls. A 1996 study conducted by U.S. West showed that voice calls are an average 5.64 minutes in length. Calls to Internet service providers (ISPs) are an average 32.47 minutes—six times the length of a voice call. Longer sessions might mean that the number of ports on the switches is insufficient, causing busy signals. When busy signals become excessive, the telco is obliged to buy more switching equipment. At $500 to $1000 per line, this is an expensive proposition. The economic and practical impacts of these estimates are significant, particularly when extrapolated to reflect the continuing growth of Internet usage.

The difficulties associated with this discrepancy in call length are particularly noticeable in areas of the country that have a lot of Internet dial-up activity, such as California. Because of the inordinate costs to their infrastructure caused by data sessions, Pacific Telesis (now SBC) has asked that ISPs be subject to access fees.

Telcos are searching for ways in which data traffic can be offloaded from voice switches to specialized data communications equipment. xDSL services are a method to do this. This is the basic difference between ISDN and xDSL: ISDN goes through a voice switch; xDSL bypasses it. This makes rollout costs incremental for xDSL, whereas ISDN requires telcos to upgrade their voice switches (at a cost of up to $500,000 a pop) before ISDN can be offered. Therefore, xDSL can be considered a lower-cost platform for data service, even though it offers faster bit rates than ISDN.


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Residential Broadband, Second Edition
by George Abe

Introduces the topics surrounding high-speed networks to the home. It is written for anyone seeking a broad-based familiarity with the issues of residential broadband (RBB) including product developers, engineers, network designers, business people, professionals in legal and regulatory positions, and industry analysts.


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