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)".
# Nahezu zeitgleich erstmalige Veröffentlichung des ANSI Draft Technical Document T1E1.4/98–043R1: „Very–high–speed Digital Subscriber Lines”. # Im Oktober erste Veröffentlichung der ITU–Empfehlung G.991.1: „High–bit–rate Digital Subscriber Line (HDSL) Transceivers”. # Nahezu zeitgleich Veröffentlichung der ETSI Technical Specification TS 101 135: „High–bit–rate Digital Subscriber Line (HDSL) – Transmission Systems on Metallic Local Lines”. # Im November Veröffentlichung der ETSI Technical Specification TS 101 388 V1.1.1: „Asymmetric Digital Subscriber Line (ADSL) – European Specific Requirements”.
1999 Im Juni Veröffentlichung der ITU–Empfehlungen G.992.1: „Asymmetric Digital Subscriber Line (ADSL) Transceivers” und G.992.2: „Splitterless Asymmetric Digital Subscriber Line (ADSL) Transceivers”. # Am 22.07. bietet die Deutsche Telekom AG erstmals ADSL in Deutschland an (T–DSL 768).
2001 Im Februar Veröffentlichung der ITU–Empfehlung G.991.2: „Single–pair High–speed Digital Subscriber Line (SHDSL) Transceivers”.
# Im November Veröffentlichung der ITU–Empfehlung G.993.1: Very–high–speed Digital Subscriber Line transceivers (VDSL).
2002 Erstmalige Veröffentlichungen der ITU–Empfehlungen G.992.3: „Asymmetric Digital Subscriber Line Transceivers 2 (ADSL2)” sowie G.992.4: „Splitterless Asymmetric Digital Subscriber Line Transceivers 2 (splitterless ADSL2)” .
2003 Erste Veröffentlichung der ITU–Empfehlung G.992.5: „Asymmetric Digital Subscriber Line (ADSL) Transceivers – Extended-bandwidth ADSL2 (ADSL2+)” .
2006 Im Februar Veröffentlichung der ITU–Empfehlung G.993.2: „Very–high–speed Digital Subscriber Line Transceivers 2 (VDSL2)”. # Im Oktober bietet die Deutsche Telekom AG erstmals für Endkunden in ausgewählten Städten VDSL2 an.

Europäische ADSL– und VDSL–Entwicklung

Aus der obigen Zusammenstellung erkennt man, dass die ADSL–Standardisierung vorwiegend von ANSI (American National Standards Institute) vorangetrieben wurde und dass jeweils kurz danach die ETSI (European Telecommunications Standards Institute) nachlegte:

Der erste ADSL–Standard (ANSI T1.413) aus dem Jahr 1995 war vorwiegend für Video–Abrufdienste optimiert, was auch durch das Verhältnis der hierin definierten Down– und Upstream–Datenraten deutlich wird: 1.5 Mbit/s und 16 kbit/s, 3 Mbit/s und 16 kbit/s, und schließlich 6 Mbit/s und 64 kbit/s.
Der Frequenzbereich war ursprünglich so festgelegt, dass man mit ADSL nur ein analoges Telefon auf der Anschlussleitung betreiben konnte. ETSI veröffentlichte 1996 einen technischen Report (ETR 328) mit nur wenigen Detailänderungen und der Möglichkeit, 2048 kbit/s zu übertragen.
Da die zweite Version des ANSI–Standards ebenfalls nur ein zusätzliches Analogtelefon zuließ, definierte die ETSI daraufhin ein ADSL–System, das sich sowohl in den Bitraten als auch in der Möglichkeit der Nutzung eines ISDN–Basisanschlusses auf der gleichen Doppelader unterschied.
Die ANSI– und ETSI–Standardisierungsbestrebungen der Vorjahre mündeten 1999 in die ITU–Empfehlung G.992.1, die beide Standards beinhaltet und somit viele Möglichkeiten der Realisierung zulässt.

Die vielen Optionen führten allerdings Ende der 1990er Jahre zu großen konzeptionellen Unterschieden – weltweit, innereuropäisch und auch national, unter anderem abhängig vom Halbleiterhersteller. Nur wenige Systeme, Modems und Messgeräte interoperierten mit anderen Herstellern.

Um diesem Wildwuchs entgegenzuwirken, verabschiedete die Deutsche Telekom AG Ende 2001 die Technische Richtlinie 1TR112, in der alle nötigen Schnittstellenparameter festgelegt werden, um die Interoperabilität verschiedener Herstellermodems auf Anbieter– und Kundenseite zu gewährleisten. Durch die Marktmacht der Telekom wurde diese zum Quasi–Standard für Deutschland. Des Weiteren wurden in Deutschland auch nur solche ADSL–Varianten eingesetzt, die jederzeit einen gleichzeitigen Betrieb von ISDN ermöglichten. Somit musste beim Wechsel von POTS auf ISDN nicht auch noch die ADSL–Version gewechselt werden.

Die für Europa relevante VDSL–Standardisierung wurde maßgeblich von der ETSI geprägt und geschah oft parallel zu den amerikanischen Aktivitäten. Insgesamt lief die Standardisierung von VDSL geordneter ab als bei ADSL. Der von ETSI beschlossene 3–Stufen–Plan sah vor:

Stufe 1: Funktionale und elektrische Anforderungen an VDSL–Systeme,
Stufe 2: Anforderungen an die Übertragungscodierung und die Zugriffsmethoden,
Stufe 3: Interoperabilitätsanforderungen.

Diese Anstrengungen mündeten im April 1998 in der Veröffentlichung der ETSI Technical Specification TS 101 270-1, die als Modulationsverfahren sowohl DMT (Discrete Multitone Transmission) als auch QAM (Quadratur–Amplitudenmodulation) zuließ. Die Halbleiter–Hersteller konnten sich lange nicht auf einen weltweiten Leitungscode–Standard einigen und man sprach sogar vom VDSL Line-Code War. 2003 wurde bei den so genannten VDSL Olympics zugunsten von DMT und gegen QAM bzw. der leicht modifizierten Variante CAP (Carrierless Amplitude Phase Modulation) entschieden, und zwar

wegen der Robustheit von DMT gegenüber schmalbandigen Störquellen,
obwohl QAM (CAP) einen schnelleren Verbindungsaufbau ermöglichen würde.

Aufgabe zum Kapitel

Aufgabe 2.1: Grundsätzliches zu xDSL