Chapter 3. ADSL
Asymmetric digital subscriber lines (ADSL) service is by far the most popular and deployed DSL service with nearly 30 million subscribers active worldwide by the end of 2002 and the number keeps growing. ADSL is asymmetric, with a downstream-to-upstream data-rate ratio of about 4:1 in early deployments (500 kbps downstream and 128 kbps upstream) at the longest ranges. As the ADSL connection speed increases to a maximum of 8 Mbps downstream, the data-rate asymmetry ratio may increase to as much as 10:1. Higher speed ADSL connections (> 1.5 Mbps), while feasible and implemented in available systems and standards, are used less frequently than lower speed connections for two basic reasons: (1) lower speed can be achieved on a greater percentage of loops, and phone companies thus transmit intentionally at lower speeds to try to ensure successful connection even on the longest of loops , and (2) Internet connections/gateways to DSL often cannot presently accommodate higher speeds per user . However, some cities in Asia (e.g., Tokyo) have mainly short lines; 8 Mb/s ASDL service is the norm in these cities. As with any relatively new service, a number of improvements and issues have become known. ADSL was first standardized in 1995 by the ANSI T1E1.4 group . The early ADSL standard had its own internal protocol (ATM not being in use at the time and Ethernet not having the required functionality at the time) that allowed multi-line voice, data, and video traffic. This early standard was used largely in trials that evaluated all these data types, and many issues and improvements were learned. The ADSL Forum (now DSL Forum) was formed in 1995 to focus on the data/fast-Internet application of ADSL and successfully ignited worldwide interest in ADSL for this application. A second issue of the American standard appeared in 1998 [2] and the ITU's international versions G.992.1 and G.992.2 appeared a year later [3] in 1999. Following G.992.1 and G.991.2 in 1999, the ITU developed a second generation set of ADSL Recommendations in 2002 that are known as G.992.3 (previously G.dmt.bis) and G.992.4 (previously G.lite.bis). The new aspects found in the second generation ADSL Recommendations are described in this chapter. ADSL's internal protocol was reduced in later standards, and external channelization of the services began when widespread deployment commenced. The basic DMT engine, first tested in 1993, remains essentially unchanged in all the standards and is today implemented very cost effectively in silicon that is interoperable and widely available from multiple suppliers. The DSL Forum now has also successfully promoted the multi-line voice application through both IP and circuit-switched ATM protocols. Video has yet to become widely deployed with ADSL in 2002. This chapter is not meant to be a comprehensive review of ADSL. The authors have previously published considerable material in a predecessor book [1] on DSL. This chapter is intended to update that earlier material and to focus on some of the enhancements and issues. Section 3.1 begins with some basic trends in the performance improvement of ADSL, and Section 3.2 discusses the potential use of more advanced coding and framing methods in conjunction with the DMT physical layer. Channel time variation and the robustness of ADSL are addressed in Section 3.3, whereas the important area of mutual compatibility with AM radio appears in Section 3.4. Sections 3.5 to 3.9 complete this chapter with a number of topics/issues that enhance deployment and applications. |
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EAN: 2147483647
Pages: 154