xDSL Systems

1 Reference models
2 Overview and common features of all xDSL systems
3 ADSL – Asymmetric Digital Subscriber Line
4 ADSL2 and ADSL2plus
5 VDSL – Very–high–speed Digital Subscriber Line
6 DSL internet access from the perspective of communication protocols
7 Components of DSL Internet access
8 Exercises for the chapter

Reference models

Based on the following general ITU reference model, it can be quickly seen that xDSL is physically a pure access transmission technology that is only used in the local loop network area between the fiber termination point and the network termination at the end customer.

xDSL reference model of the ITU

The basic elements of the xDSL standard are:

the network termination $\rm$,
a subscriber line $\rm$ and
the line termination $\rm $.

There is a lot of freedom for network operators in implementing this reference model in practice. What all previous implementations have in common is that they use existing metallic subscriber lines.

$\text{Examples 1:}$ In an example, the configuration most frequently encountered in Germany is shown according to the graphic. To note:

Reference model according to $\text{1TR112_U-R2-V7.0 DTAG}$

In all xDSL variants deployed today, the data service converted in the modems is combined with the telephone service. This allows transmission over the existing telephone network.
The splitter splits the signal on both sides of the subscriber line.
An important interface is designated $\rm U-R2$ (red marking). This was standardized in Germany in 2001 by Deutsche Telekom AG in order to be able to use any modems on the subscriber side. This means that the customer is no longer dependent on his provider's xDSL modem

.

Overview and common features of all xDSL systems

The technical realization of an xDSL system involves many system components, which can be distributed over several localities. There is a wide range of realization options. To summarize:

The systems for ADSL and VDSL shown below represent the most common implementation at the present time (end of 2009).
Data transport at the protocol level is based on the $\rm ATM$ technology (Asynchronous Transfer Mode).
Despite a large data overhead, ATM still offers advantages over Ethernet in terms of guaranteed quality of service (QoS), i.e. effective bit rate, low delay and jitter.
Ethernet, on the other hand, enables very high data transmission rates, especially through the "10 Gbit/s Ethernet" and "100 Gbit/s Ethernet" (Metro Ethernet) variants. ATM, on the other hand, is more suitable for lower data rates.

There are currently numerous discussions about whether ATM should be replaced by 10 Gbit/s Ethernet in the course of Next Generation Network. However, upgrading the backbone from ATM to Ethernet represents a not inconsiderable investment.

$\text{Summary:}$ As mentioned in the chapter "General Description of DSL" , the most commonly deployed xDSL variants are.

$\rm ADSL$ and $\rm ADSL2$ respectively $\rm ADSL2+$
$\rm VDSL(1)$ and $\rm VDSL(2)$

defined in such a way that simultaneous operation of POTS (Plain Old Telephone Service) or ISDN (Integrated Services Digital Network) on the same line is possible at any time. This is also the basis of the further descriptions

.

ADSL – Asymmetric Digital Subscriber Line

The physical network termination is in the local exchange in the ADSL modem (ADSL Transmission Unit Central Office, $\rm ATU-C)$. Before that, in the splitter the low-frequency telephony spectrum is still separated from the higher-frequency ADSL spectrum by low-pass and high-pass filtering.

Modeling of an ADSL connection from the end customer to the local exchange

The graphic shows an ADSL connection from the end customer to the local exchange, which is described very briefly below. The reverse data connection paths are in each case mirror images.

The splitter forwards the telephone signals to the ISDN/POTS exchange and the ADSL signals to the Digital Subscriber Line Access Multiplexer (DSLAM), in which the ADSL Transmission Unit Central Office (ATU-C) is implemented as a plug-in card.
The DSLAM bundles many ADSL connections and forwards the data after decoding at the ATM level via optical fiber to the ATM Service Access Multiplexer . This sends the data from all DSLAMs over the backbone to the Broadband Remote Access Server (BBRAS).
The BBRAS terminates the point-to-point protocol data link and forwards the IP packets via routers to the destination. The backbone consists of optical components based on the SDH standard (Synchronous Digital Hierarchy).
The splitter connected to the telecommunications connection unit (TAE) separates the signals. The telephone signals are routed to the telephony terminals or to the NTBA, the ADSL signals to the modem (ADSL Transmission Unit Remote, ATU-R). The modem decodes and forwards the binary data to the connected terminals.

During initialization of the ADSL connection, $\rm ATU-C$ and $\rm ATU-R$ perform a so-called "training" in which relevant system parameters such as data rate, interleaved and fast mode, etc. are determined depending on the line conditions. The parameters negotiated in this process are retained until the next check and synchronization. For the transmission of administrative data (overhead), 32 kbit per frame are statically reserved in the ADSL systems.

ADSL2 and ADSL2plus

These two system variants are further developments of ADSL:

The enhanced system variant Asymmetric Digital Subscriber Line Transceivers 2 $\rm (ADSL2)$ was specified in 2002 with the publication of ITU Recommendations G.992.3 and G.992.4 .
2003 was followed by the ITU recommendation G.992.5: Extended-bandwidth ADSL2 $\rm (ADSL2+)$.

Compared to ADSL, the following changes occurred:

In ADSL2, the Seamless Rate Adaption (SRA) was included in the standard. This allows transmission parameters to be changed during operation with time-variant channel quality without loss of synchronization.
For this purpose, ATU-C and ATU-R periodically check the Signal–to–Noise Ratio (SNR) of the transmission channels. If a channel in use deteriorates, the receiver notifies the transmitter of the new data rate and transmission level. After a subsequent sync flag the parameters are adopted.
ADSL2 systems also offer a wide range of diagnostic options even without the modems having been synchronized, a feature that is particularly important for troubleshooting, error analysis and error correction.
In addition, ADSL2 provides the ability to reduce transmit levels when SNR is sufficient, thereby minimizing crosstalk and increasing throughput in the trunk cable. This power cutback can be initiated not only by the DSLAM, but also by the ATU-R.
In ADSL2, the number of overhead bits is no longer fixed, but can vary between 4 and 32 kbit. This increase in the user data bit rate of up to 28 kbit/s per data frame is all the more important the longer the distance between the modem and the DSLAM.

As a result, ADSL2 systems achieve a transmission rate of more than 8 Mbit/s (up to 12 Mbit/s) downstream and more than 800 kbit/s (up to 3.5 Mbit/s) upstream.

With ADSL2+, the transmission rate in the downstream is doubled again; the maximum rate is theoretically 25 Mbit/s.

VDSL – Very–high–speed Digital Subscriber Line

In terms of the basic structure of their components, VDSL systems are identical to ADSL systems, with the only exception that the relocation of the splitter and the DSLAM from the local exchange to a cable branch makes the last section between the network operator and the customer - the so-called last mile - shorter. This measure was necessary because VDSL can only exploit its advantage - the greater transmission speed - over very short distances due to the attenuation of the higher frequencies, which increases sharply with line length.

DSLAM and BBRAS are still connected via STM-1 interfaces. Therefore, the route between the local exchange and the cable branch must now also be laid with optical fiber.

Modeling of a VDSL connection from the end customer to the local exchange

A distinction is made between two alternative VDSL variants:

the $\rm VDSL(1)$ system based on "QAM" (quadrature amplitude modulation), which is predominantly deployed in Asia, and.
the $\rm VDSL(2)$ system based on "DMT" (Discrete Multitone Transmission).

VDSL(1) systems were never deployed in Germany because of their inadequate ability to provide audio/video, telephony and Internet (triple play) with sufficient quality of service. Instead, the VDSL(2) standard was established immediately: Because of higher performance and greater range, the better quality of service as well as the reusability of ADSL(2+) infrastructure.

$\text{Summary:}$ Following are a few characteristics of the $\rm VDSL(2)$ system:

VDSL(2) has achieved a maximum transmission rate of 50 to 100 Mbps since 2006, depending on the standard used.
The specified VDSL(2) transmission bandwidth of 30 MHz was considered the maximum reasonable bandwidth in 2009.
By 2011, with complementary measures such as Dynamic Spectrum Management and Advanced Codes total transmission rates of up to 280 Mbit/s were expected for short line lengths (up to 300 meters).

DSL internet access from the perspective of communication protocols

Some xDSL modems offer an Ethernet interface for connecting the data terminals and a transparent connection to the remote terminal, based on the Internet Protocol. To note:

This option is enabled by the LAN Emulation (RFC2684) and the ATM Adaption Layer Protocol (AAL5). The Ethernet data stream is converted to ATM for this purpose.
This eliminates the need to install ATM equipment and existing Ethernet hardware can be used, greatly simplifying xDSL configuration at the customer site.
The ATM connection extends at least as far as the Broadband Remote Access Server' (BBRAS) and is converted there or continued directly, depending on the backbone data transmission system.

Modeling of an xDSL connection by using an xDSL modem

The following graphics show the communication in an Internet connection according to the OSI model, where $\rm xDSL$ is used only between the $\rm xTU-R$ on the customer side and the $\rm xTU-C$ on the provider side (brown background).

For the first graph, the xTU-R is assumed to be a $\rm xDSL modem$ .

Modeling of an xDSL connection through the use of an xDSL router

In the second graphic, a $\rm xDSL router$ is used as the xTU-R interface.

This enables the connection of multiple terminals in a network with shared xDSL line.
Here, instead of the modem, a router initializes the Point-to-Point-Protocol-over-Ethernet connection.

Components of DSL Internet access

Finally, necessary components for a DSL connection are listed. The graphic shows examples of these, mostly from Deutsche Telekom.

Necessary components to establish an xDSL connection

$\rm NTBA$: The commonly used term is Network Termination for ISDN Basic Rate Access. At the German Telekom, the term also stands for Network Terminator Basic Connection. Tasks of the NTBA are:

With the help of a fork circuit and echo cancellation, the two-wire UK0 interface on the provider side is converted into the four-wire S0 interface on the subscriber side.
In addition, the NTBA manages the ISDN code conversion from the MMS43 code $\rm (U_{K0}$–Bus) to the modified AMI code $\rm (S_{0}$–Bus).

$\rm xTU-R$: The abbreviation stands for xDSL Transceive Unit - Remote and denotes the subscriber-side xDSL unit. At Deutsche Telekom - always known for special naming - the term network termination point broadband access (NTBBAE) is also common.

Due to the widespread use of Ethernet, today's xDSL modems and routers usually have only one Ethernet port for connecting data terminal equipment. Originally, they were used for subscriber-side connection of ATM data terminals.
Thus, this unit must also perform the function of a Layer 2 bridge in order to be able to transmit Ethernet over ATM to the Broadband Remote Access Server (BBRAS) for termination.

$\rm xDSL modem$: With this functional unit, the data connection from/to the data terminal is initialized by a point-to-point protocol (PPP) via a PPP over Ethernet connection (PPPoE) and terminated by the BBRAS. Only data terminal devices that can separately establish a data connection via PPP are eligible.

$\rm xDSL modem router$: This initializes the data connection via PPP and enforces the addresses at IP level. This allows multiple terminals to be connected and allows internal data exchange between them without having to dial into them separately.

$\rm Splitter$: This is basically a combination of high-pass and low-pass with three interfaces, which handles the separation of the high-frequency xDSL data signals (above 138 kHz) from the low-frequency POTS or ISDN telephone signals (below 120 kHz), or their combination.

This broadband access unit (BBAE) - as it is also called in Telekom jargon - is nothing other than a crossover unit.
The sum of the signals is present on the subscriber line side, while the xDSL data and the POTS/ISDN signals are separated from each other by a splitter on both the customer and provider sides.

$\rm xTU-C$: The abbreviation stands for xDSL Transceive Unit - Central office. It is the provider-side xDSL unit and is usually implemented as a printed circuit board insertion (linecard) for the DSLAM. It is sometimes also referred to as network termination point broadband access (NTBBAE). The xTU-C terminates the physical retail xDSL subscriber lines, modulates the ATM bit stream on the subscriber side, and demodulates the xDSL signal on the provider side.

$\rm DSLAM$: The abbreviation stands for Digital Subscriber Line Access Multiplexer. Experts also use the designation MXBBA for the DSLAM, which comes in various forms. In its simplest form, it terminates the physical subscriber lines with its xTU-C line cards. In an extended form, an ATM Service Access Multiplexer is also integrated in the DSLAM.

The task of the DSLAM is to bundle the ATM bit streams of the subscriber lines and to forward them in concentrated form in the multiplexing process via an STM-1 fiber interface into the provider network.
STM is an SDH transmission standard for multiplexing optical channels and stands for Synchronous Transport Module. STM-1 allows a bit rate of up to 155.52 Mbit/s, STM-64 up to almost 10 Gbit/s.

Exercises for the chapter

"Exercise 2.2: xDSL Variants"

"Exercise 2.2Z: DSL Internet Connection"