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Chapter 22
Choosing Asymmetrical Digital Subscriber Lines

Gilbert Held

Digital subscriber lines (DSLs) enable high-speed transmissions — including interLAN communications, videoconferencing, and mainframe access — over existing copper twisted-pair local loops. By determining the availability, operating rate of the service, and the capability of the carrier’s internal broadband network, users can decide whether DSL service is a cost-effective transport replacement for their current service.

INTRODUCTION

Although essentially all long-distance communications in North America are now transported via fiber-optic cable, the majority of what is referred to as “the last cable mile” — the 18,000-foot local loop from the telephone company central office (CO) to subscribers — continues to be twisted-pair copper cable. Because of the high cost of installing fiber cable from telephone company central offices to individual residential and business subscribers, communications carriers are exploring methods that enable high-speed transmission to occur over their existing copper twisted-pair local loops. The result is the development of a series of digital subscriber lines.

Although several DSL products, including high data-rate digital subscriber lines (HDSL), single-line digital subscriber lines (SDSL), and very high data-rate digital subscriber lines (VDSL) are discussed in this chapter, the focus is on asymmetrical digital subscriber line (ADSL) technology, because it is the most practical transmission facility. In addition, ADSL is being used in a series of field trials conducted by telephone companies around the world.

ASYMMETRICAL DIGITAL SUBSCRIBER LINES (ADSL)

ADSL is a technology for use over telephone twisted-pair loops in which data rates between 1.544 and 6M bps, and in some cases up to 8M bps, can be obtained from the carrier’s central office to the subscriber (referred to as downstream transmission), while a transmission rate of 640K bps to 800K bps is obtained in the opposite direction (referred to as upstream transmission). Because the operating rate differs in each direction, this technique is referred to as asymmetrical transmission.

ADSL modems use either discrete multitone (DMT) or carrierless amplitude/phase (CAP) modulation. DMT was standardized by the American National Standards Institute (ANSI) as standard T1.413. CAP was developed by Paradyne, formerly a unit of AT&T and now an independent company. Although DMT is standardized, CAP is presently deployed in a number of field trials throughout the world for high-speed Internet access and has received high marks for its performance. It is quite possible that communications carriers will select both of the competitive methods to obtain an ADSL capability.

Discrete Multitone (DMT) Operation

DMT permits the transmission of high-speed data over conventional copper twisted-pair wire without adversely affecting voice communications. Through the use of frequency division multiplexing (FDM), a copper twisted-pair subscriber line is subdivided into three parts by frequency. One channel is assigned for downstream data transmission and a second is assigned for upstream data transmission. The first two channels have a variable amount of bandwidth based on the results of the ADSL modems communicating with one another. The third channel, which uses a fixed spectrum of bandwidth from 0 to 4 KHz, is used for normal telephone operations. Thus, an ADSL line simultaneously supports the bidirectional transmission of high-speed data and conventional voice connection.

In Exhibit 1, the ADSL modem is shown with two digital connections, one to a LAN and another to a mainframe. An ADSL modem can support virtually unlimited digital connections; however, most modems used in field trials have been limited to two to four connectors. The ADSL modem supports multiple digital devices through the use of time-division multiplexing (TDM), which can be used to subdivide the upstream and downstream data channels by time into independent digital connections.

Under the DMT standard, the frequency spectrum above 10 KHz is subdivided into a large number of independent subchannels. Because the amount of attenuation at high frequencies depends on the length of the subscriber line and its wire gauge, an ADSL modem based on DMT technology installed at a central office must first determine which subchannels are usable. The modem transmits tones to the remote modem installed at the customer’s premises, where the tones are analyzed. The remote modem responds during this initiation process at a relatively low data rate, which significantly reduces the possibility of the signal analysis performed by the remote modem being misinterpreted.


Exhibit 1.  Using an Asymmetrical Digital Subscriber Line.


Exhibit 2.  ADSL Modem Use of the Copper Twisted-Pair Frequency Spectrum.

On receipt of the returned signal analysis, the ADSL modem located in the telephone company’s central office will use up to 256 4-KHz-wide subchannels for downstream transmissions. Through a reverse learning process, the remote modem will use up to 32 4-KHz-wide subchannels for upstream transmission. Exhibit 2 illustrates the use of the copper twisted-pair frequency spectrum by an ADSL modem. In Exhibit 2, f1, f2, and f3 are variables based on the results of the ADSL modem’s discrete multitone handshaking process; however, the actual limit for f3 is 1.1 MHz, which represents the maximum usable bandwidth on a copper twisted-pair cable.

Exhibit 3. Generalized ADSL Performance
Wire Gauge Subscriber Line Distance Operating Rate

24 AWG 18,000 feet 1.5/2.0M bps
26 AWG 15,000 feet 1.5/2.0M bps
24 AWG 12,000 feet 6.1M bps
26 AWG 9,000 feet 6.1M bps

Note: AWG is the American Wire Gauge standard for measuring connectors.

Benefits of DMT. One of the main benefits of DMT is its ability to take advantage of the characteristics of twisted-pair wire, which can vary from one local loop to another. This makes DMT modulation well suited for obtaining a higher data throughput than is possible through the use of a single carrier transmission technique. Because the transmission rate varies depending on the wire gauge and cable length as well as the physical characteristics of the cable, ADSL performance can vary considerably from one local loop to another. Ignoring bridge taps, which represent sections of unterminated twisted-pair cable connected in parallel across the twisted-pair cable under consideration, various tests of ADSL lines provide a general indication of their operating rate capability, which is summarized in Exhibit 3.

Carrierless Amplitude/Phase (CAP) Modulation

CAP modulation is a derivative of quadrature amplitude modulation (QAM), which was developed by Paradyne when it was a subsidiary of AT&T and which is used in most modems today. Unlike DMT, which subdivides the bandwidth of the twisted-pair wire into 4 KHz segments, CAP uses the entire bandwidth in upstream and downstream channels.

Under CAP, serial data is encoded by mapping a group of bits into a signal constellation point using a two-dimensional, eight-state trellis coding with Reed-Solomon forward error correction. Reed-Solomon automatically protects transmitted data against impairments due to crosstalk, impulse noise, and background noise.


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