General Description of DSL

# OVERVIEW OF THE SECOND MAIN CHAPTER #

$\rm D$igital $\rm S$ubscriber $\rm L$ine – $\rm DSL$ for short – was synonymous with high-speed Internet access in the local loop to the end customer, although "high-speed" must be put into perspective today (2018). In what follows, we will predominantly use the industry term $\rm xDSL$ to refer to high-speed access, with "$\rm x$" as a placeholder for the specific process, for example $\rm H$DSL, $\rm A$DSL, and $\rm V$DSL.

xDSL has been significantly standardized by the standards bodies "ANSI" (USA) and "ETSI" (Europe) as well as the "ITU" (worldwide). Due to different pre-existing technical conditions and preferences of developers and operators, a large variety of nationally different versions of nominally identical xDSL standards resulted. In the following, we will restrict ourselves primarily to the German xDSL versions.

This chapter contains in detail:

an overview of the historical development and standardization of xDSL,
the differences between ADSL and VDSL as well as statistics on their penetration,
a brief description of xDSL from a communications protocol perspective,
the bandwidth allocations for the two xDSL variants ADSL and VDSL,
a detailed description of the DSL transmission methods QAM, CAP and DMT,
the problems of digital signal transmission over copper twisted pairs in general,
the relationship between SNR, range and transmission rate,
the error correction measures used to reduce the bit error rate.

Motivation for xDSL

The different variants of $\rm xDSL$ - Digital Subscriber Line arose from the need to provide end customers with low-cost, high-rate digital data access. The design had to take into account:

The so-called Last Mile - the last line section leading to the subscriber household and referred to as the Subscriber Line - represents the largest cost factor in a communications network, since the network branches out to the maximum in the local loop area.
Considerations to replace the estimated 130 million kilometers of copper twisted pairs in the local loop network with fiber optic lines (fiber-to-the-home, FttH) have failed to date due to the enormous costs of the mostly underground laying work.
A viable solution was to offer a broadband connection with somewhat lower data rates than in a fiber optic network by using the existing telephone line network and by cleverly combining different messaging techniques and coding methods.
The telephone service - either analog or digital (ISDN) - should be able to operate simultaneously on the same network.

Subscriber line area of a telecommunications network

$\text{Example 1:}$ The graphic shows the last mile

between local exchange
and the end customer.

xDSL types and terms

Before we turn to the historical development of DSL up to the current state, the various types of xDSL must first be defined and some terms explained. The technical features will be covered in depth in the next chapters.

$\text{ADSL}$ – Asymmetric Digital Subscriber Line:
Asymmetric data transmission technology with data rates of 8 Mbit/s to the subscriber (downstream) and 1 Mbit/s in the opposite direction (upstream).
$\text{ADSL2}$ and $\text{ADSL2+}$:
Extensions of ADSL with data rates of up to 25 Mbit/s to the subscriber and up to 1 Mbit/s upstream. The data rate is dynamically negotiated depending on the channel state.
$\text{Re-ADSL2}$:
Another extension of ADSL with about 30% range gain at a data rate of 768 kbit/s downstream.
$\text{HDSL}$ – High Data Rate Digital Subscriber Line:
Symmetrical data transmission technology - i.e. equal rates in downstream and upstream - with data rates between 1.54 Mbit/s and 2.04 Mbit/s.
Note: The name "HDSL" suggests higher data rates than ADSL; however, this is not the case.
$\text{SDSL}$ – Symmetric Digital Subscriber Line:
Symmetrical data transmission at rates of up to 3 Mbit/s. With four-wire wiring (two copper twisted pairs), a maximum of 4 Mbit/s can be transmitted. Alternatively, the range can be increased at the expense of bandwidth.
$\text{VDSL}$ – Very High Data Rate Digital Subscriber Line:
A newer transmission technology based on QAM that operates in the asymmetrical variant with bit rates of 25 to 50 Mbit/s downstream and 5 to 10 Mbit/s upstream. The symmetrical variant has the same data transmission rates in the upstream and downstream.
$\text{VDSL2}$ – Very High Data Rate Digital Subscriber Line 2:
Transmission technology with the currently (2009) highest total data rate of up to 200 Mbit/s. The process is based on DMT (Discrete Multitone Transmission).
$\text{UDSL}$ or $\text{UADSL}$ - Universal (Asymmetric) Digital Subscriber Line.

There are also many products circulating under "DSL" that are not part of the xDSL standard. They are often only intended to make it clear that fast data access is involved. These include:

$\text{cableDSL}$: Brand name of the German company TELES AG, which offers high-speed Internet access via cable. The name was chosen for marketing reasons only.
$\text{skyDSL}$: Brand name for Internet access available throughout Europe via satellite with up to 24 Mbit/s downstream. The upstream here is via POTS (Plain old telephone service) or ISDN (Integrated Services Digital Network).
$\text{T-DSL via satellite}$: Brand name for a downstream Internet access of Telekom via satellite; uses a conventional modem or an ISDN connection for transmission.
$\text{WDSL}$ - Wireless Digital Subscriber Line: Brand name of a German company that uses wireless technology to enable data rates of up to 108 Mbit/s in DSL-free areas.
$\text{mvoxSatellit}$: Brand name of an Internet access with "WiMAX-like radio technology", which, like WDSL and PortableDSL, is only an auxiliary construct for DSL-free areas.

Historical development of xDSL - standardizations

The need for digital subscriber lines to improve line utilization and increase customer convenience was recognized as early as the 1970s. After the ISDN specification in the early 1980s, the actual development of DSL began.

This development was influenced by the findings of many groups located around the world. Accordingly, standardization proceeded in an unstructured manner. From the list below ("Milestones"), it is clear that different committees around the world were in charge of the various standards.
In the industry, the technical realizations of the individual xDSL standards often deviated noticeably from the specification. For example, some standards were started as projects even before the specification, since the industry parties were also represented in the standardization committees.

Industrial xDSL development

The graphic illustrates relationships between.

milestones in the theoretical and practical design of transmission systems,
parallel advances in semiconductor development, and the
realization of the individual xDSL standards with the corresponding data rates.

Here are thenbsp; Milestonesnbsp; of DSL development in short form:

1986 A first concept for HDSL (High-bit-rate Digital Subscriber Line) is defined by AT&T, Bell Laboratories and Bellcore.
1989 First HDSL prototypes appear. # Bellcore meanwhile works on the conceptual definition of ADSL.
1992 In February, first publication of ANSI Technical Report E1T1/92-002R1: "High Bit-rate Digital Subscriber Line - HDSL". # The first prototypes for ADSL (Asymmetric Digital Subscriber Line) appear.
1994 The VDSL concept (Very-high-speed Digital Subscriber Line) is discussed for the first time.
1995 Publication of ETSI Technical Report ETR 152: High-bit-rate Digital Subscriber Line (HDSL) and Transmission Systems on Metallic Local Lines.
# First field trials with ADSL in the USA. # Publication of ADSL Standard ANSI T1.413: "Asymmetric Digital Subscriber Line (ADSL) Metallic Interface".
1996 First publication of ETSI Technical Report ETR 328: Asymmetric Digital Subscriber Line (ADSL) and Transmission and Multiplexing (TM).
1998 In April, first publication of the ETSI Technical Specification TS 101 270-1 V1.1.1: "Very-high-speed Digital Subscriber Line (VDSL)".
# Almost simultaneously, first publication of ANSI Draft Technical Document T1E1.4/98-043R1: "Very-high-speed Digital Subscriber Lines". # In October, first publication of ITU Recommendation G.991.1: "High-bit-rate Digital Subscriber Line (HDSL) Transceivers". # Almost simultaneously publication of ETSI Technical Specification TS 101 135: "High-bit-rate Digital Subscriber Line (HDSL) - Transmission Systems on Metallic Local Lines". # In November, publication of ETSI Technical Specification TS 101 388 V1.1.1: "Asymmetric Digital Subscriber Line (ADSL) - European Specific Requirements".
1999 In June publication of ITU Recommendations G.992.1: Asymmetric Digital Subscriber Line (ADSL) Transceivers and G.992.2: "Splitterless Asymmetric Digital Subscriber Line (ADSL) Transceivers". # On July 22, Deutsche Telekom AG offers ADSL in Germany for the first time (T-DSL 768).
2001 In February, publication of ITU Recommendation G.991.2: "Single-pair High-speed Digital Subscriber Line (SHDSL) Transceivers".
# In November, publication of ITU Recommendation G.993.1: Very-high-speed Digital Subscriber Line transceivers (VDSL).
2002 First publications of ITU Recommendations G.992.3: Asymmetric Digital Subscriber Line Transceivers 2 (ADSL2) and G.992.4: "Splitterless Asymmetric Digital Subscriber Line Transceivers 2 (splitterless ADSL2)" .
2003 First publication of ITU Recommendation G.992.5: Asymmetric Digital Subscriber Line (ADSL) Transceivers - Extended-bandwidth ADSL2 (ADSL2+) .
2006 In February, publication of ITU Recommendation G.993.2: "Very-high-speed Digital Subscriber Line Transceivers 2 (VDSL2)". # In October, Deutsche Telekom AG offers VDSL2 to end customers in selected cities for the first time.

Development of ADSL and VDSL in Europe

From the above compilation it can be seen that the ADSL standardization was predominantly driven by "ANSI" (American National Standards Institute) and that the "ETSI" (European Telecommunications Standards Institute) followed shortly thereafter in each case:

The first ADSL standard (ANSI T1.413) from 1995 was predominantly optimized for video-on-demand services, which is also made clear by the ratio of the downstream and upstream data rates defined herein: 1.5 Mbit/s and 16 kbit/s, 3 Mbit/s and 16 kbit/s, and finally 6 Mbit/s and 64 kbit/s.
The frequency range was originally defined in such a way that ADSL could only be used to operate an analog telephone on the access line. ETSI published a technical report in 1996 (ETR 328) with only a few detailed changes and the possibility to transmit 2048 kbit/s.
Since the second version of the ANSI standard also allowed only one additional analog telephone, ETSI then defined an ADSL system that differed both in bit rates and in the possibility of using an ISDN basic access on the same twisted pair.
The ANSI and ETSI standardization efforts of the previous years resulted in the "ITU" recommendation G.992.1 in 1999, which includes both standards and thus allows many options for implementation.

However, the many options led to major conceptual differences at the end of the 1990s - worldwide, within Europe and also nationally, depending on the semiconductor manufacturer, among other things. Only a few systems, modems and measuring devices interoperated with other manufacturers.

To counteract this proliferation, Deutsche Telekom AG passed the Technical Guideline 1TR112 at the end of 2001, defining all the necessary interface parameters to ensure the interoperability of different manufacturer modems on the provider and customer side. Due to Telekom's market power, this became the quasi-standard for Germany. Furthermore, only those ADSL variants were used in Germany that allowed simultaneous operation of ISDN at any time. Thus, when switching from POTS to ISDN, it was not necessary to change the ADSL version as well.

The VDSL standardization relevant for Europe was decisively shaped by ETSI and often happened in parallel to the American activities. Overall, VDSL standardization proceeded in a more orderly fashion than ADSL. The 3-step plan adopted by ETSI provided for:

Stage 1: Functional and electrical requirements for VDSL systems,
Stage 2: Transmission coding and access method requirements,
Stage 3: Interoperability requirements.

These efforts culminated in April 1998 in the publication of the ETSI Technical Specification TS 101 270-1, which defines as modulation methods both "DMT" (Discrete Multitone Transmission) and "QAM" (quadrature amplitude modulation). The semiconductor manufacturers could not agree on a worldwide line code standard for a long time and there was even talk of the VDSL Line Code War. In 2003, at the so-called VDSL Olympics, the decision was made in favor of DMT and against QAM or the slightly modified variant "CAP" (Carrierless Amplitude Phase Modulation), namely.

because of the robustness of DMT against narrowband interference sources,
although QAM (CAP) would allow faster call setup.

Exercises for the chapter

Aufgabe 2.1: Grundsätzliches zu xDSL