Chapter 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.