Chapter 7. VDSL | DSL Advances

7.1 Basic VDSL Concepts

7.1.1 ADSL Extension

7.1.2 VDSL Architecture

7.1.3 VDSL Spectrum Issues

7.1.4 The Grand Debate

7.2 Applications and Their Interfaces

7.2.1 VDSL Applications and Speed Types

7.2.2 Common VDSL Reference Configurations

7.2.3 Operations

7.3. DMT Physical Layer Standard

7.3.1 Basic Multicarrier Concept

7.3.2 Digital Duplexing

7.3.3 DMT VDSL Framing

7.3.4 Initialization

7.4. Multiple- QAM Approaches and Standards

7.4.1 Profiling in SCM VDSL

7.4.2 Operation of the DFE

7.4.3 FMT

7.5 Ethernet in the First Mile ( EFM )

7.5.1 Multiline FEXT Modeling

Very-high bit-rate digital subscriber lines (VDSL) is currently the highest-speed DSL, with standardized symmetrical data rates as high as 26 Mbps and asymmetrical rates up to 52 Mbps downstream with 6.4 Mbps upstream. VDSL is an evolutionary successor to earlier DSLs in that VDSL increases DSL data rates to support an ever-increasing customer demand for faster access to all types of information. Before exploring the technology, a brief history of VDSL is helpful:

The VDSL concept was first published in 1991 [1] and was a result of a joint Bellcore-Stanford research study into the feasibility of 10+ Mbps symmetric and asymmetric data rates on short phone lines. The study specifically searched for the potential successors to the then more prevalent 1.5 Mbps HDSL and the then relatively new (then also only 1.5 Mbps) ADSL. ^[1] The first serious suggestions that VDSL be standardized came almost simultaneously in the American T1E1.4 group from Amati Communications Corp. in [2] and in the ETSI group from British Telecom [3] as a function of the first ADSL trials in Britain at 2 Mbps and 6 Mbps (supplied by Amati to British Telecom) where discussions on the potential of higher speeds on shorter fiber-fed optical network unit (ONU)-based copper loops were active between the two companies. VDSL history also has connection to early high-speed ATM network studies in the ATM Forum [4] and DAVIC [5], which attempted to transmit 26 and 52 Mbps symmetrically on one or more twisted pairs over very short distances (less than 100 meters ) for local area networks. Although the latter ATM and DAVIC efforts are somewhat forgotten, in that 100BASE-T and now Gigabit Ethernet became the methods of ubiquitous use for internal computer networks on twisted pair, these early ATM and DAVIC efforts did also provide useful information to the development of present-day VDSL standards. The DAVIC group went so far as to consider modification of the early ATM systems for very short-phone-line subscriber access.

^[1] The author (Cioffi) would like to acknowledge gratefully encouragement and support from then- retiring Dr. Joseph Lechleider of Bellcore, who encouraged and financially supported this early VDSL study.

Today, an architecture and transmission technology has emerged from years of VDSL study, and draft documents are just completing formal standardization. A special issue of the IEEE Communications Magazine has several articles on VDSL [35],[36]. Section 7.1 describes the basic VDSL concept and architecture as well as some important spectral issues. Section 7.2 then explores some applications for VDSL and the consequent implications on interfaces and basic architecture, of VDSL. Sections 7.3 and 7.4 then provide greater detail on the transmission details of VDSL implementation. The reader is also referred to Chapter 11, which looks at advances projected in DSL. Section 7.5 provides some early information on the area of VDSL known as Ethernet in the first mile (EFM). Recent interest in 100 Mbps symmetric transmission is discussed as VDSL enhancement in Chapter 11.

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