Digital Subscriber Line

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Digital Subscriber Line ( DSL , English for digital subscriber line ) describes a series of transmission standards of the bit transmission layer in which data can be sent and received at high transmission rates (up to 1,000  Mbit / s ) via simple copper lines such as the subscriber line . This is a significant improvement over telephone modems (up to 56 kbit / s ) and ISDN connections (with two bundled channels of 64 kbit / s each).

The standard is used for communication between DSL modem and DSLAM in order to usually provide broadband internet access via a broadband access server . DSL negotiates the connection parameters such as frequency and downstream and upstream transmission rate. However, the actual Internet transmission rate depends on the broadband access server.

The actual connection is established using any protocol of the higher layers of the OSI model . The security layer is Ethernet or ATM , and the network layer is IP . The broadband access server of the provider is reached via this connection, which enables Internet access via authenticated connections ( e.g. using PPPoE ). DSL-by-call is possible via this authentication using a prefix and / or suffix in the PPPoE user ID .

DSL is in the industry on its own cables regardless of the phone used in the following, however, spoken mostly by the DSL connection over telephone lines.

So far nothing has to be changed on the existing subscriber line for DSL, because the DSL processes used for the mass market use a frequency band on the already laid copper wire pair of the telephone network that is above the frequency range used for analog voice telephony or ISDN. In Germany, however, Deutsche Telekom plans to switch completely to so-called IP-based connections by 2018. These use Annex J , which adds the frequency range still reserved for ISDN in Annex B to the DSL upstream. With Annex-J connections, the telephony and fax services are therefore also transmitted via DSL.

Second generation
DSL modem (NTBBA), manufacturer Siemens

DSL basic knowledge

DSL differs from an Internet connection via an analog telephone connection ( POTS ) or ISDN in that a much larger frequency range is used for data transmission , which enables a much higher data rate; However, the range of the signal is severely limited by this large frequency band, so that the signal has to be further processed ( (de) modulated ) in the local exchange .

In the case of DSL variants such as ADSL , which are usually intended for marketing to private customers , the frequency range used for landline telephony is left out with a high / low pass ( splitter ), so that DSL can be used in parallel to normal telephones. Fax, analog telephone or ISDN are therefore also available during DSL operation. This results in new applications, because Internet access is now always available like with a leased line .

The digital DSL signal is transmitted over the telephone line to the provider's DSL multiplexer DSLAM between the customer's DSL modem and the exchange, which is only a few kilometers away . Furthermore, the signal is a broadband fiber-optic links from the DSLAM to a broadband remote access server as a concentrator and then in the of backbone of the provider transmitted.

Thanks to the high capacity of the backbone connection, the subscriber line (TAL) can be better utilized than with analog or ISDN data transmission, since the data no longer has to be transmitted over the conventional telephone network. Improved modulation methods and the use of a larger bandwidth are effective in DSL (details below).



While ISDN is primarily used for telephony with several user channels over the same exchange line, but also for simultaneous telephony with an existing narrowband Internet connection , ADSL (Asymmetrical DSL: high data rate towards the user, low data rate towards the Internet) is the first technology that network operators have installed for high-speed Internet access for private customers.

ISDN has thus gained a competitor in the private customer segment through DSL, because with ADSL, in connection with an analog landline connection - as with ISDN - the Internet can be used and telephoned at the same time via the same subscriber connection, with the Internet connection being much faster than with narrowband ISDN Internet access is.

SDSL (symmetrical DSL with the same DSL data rate in send and receive direction; colloquially mostly upstream and downstream ) is mainly used for business customers who also need a fast connection to send data, but was developed by the QSC subsidiary Q-DSL home for a while also marketed for private customers. Due to its long range, SDSL is also suitable for supplying customers with long connection lines who cannot be supplied or only insufficiently supplied by means of the low-range ADSL-over-ISDN used in Germany .

DSL as the basis for the migration to the next generation network

Since the middle / end of 2006, most providers in Germany have been trying to increase customer loyalty with so-called triple-play complete connection packages. The subscriber line is used to transmit three services, typically telephony (often using DSL telephony via unbundled DSL ), Internet access and video / television (see also VDSL , ADSL2 + and bitstream access ). The low price - with full utilization of all services - is offset by a lack of flexibility, especially if individually available offers are pushed off the market.

The traditional landline providers increasingly migrate their circuit-switched services point to a next-generation network platform , the network connection from a fixed line with bundled DSL connection at a lower cost realizable unbundled data connection is converted, bringing the local switching equipment can be reduced and their locations to pure DSLAM locations.


Originally, the term digital subscriber line was understood as the transmission technology for the basic connection of ISDN .

At the end of the 1980s and the beginning of the 1990s, digital signal processors with very high computing power became available, which enabled new processes - known today as DSL. At that time, this technology was still very expensive.

Structure of the existing copper cable network

The first DSL process that was developed with these components was HDSL . Standardization organizations in America ( ANSI ) and Europe ( ETSI ) immediately began to standardize this technology in order to use it on a large scale for dedicated lines . There were major constraints: The already for telephony installed twisted pair copper wires should be used in the United States should have a bit rate of 1.544 Mbit / s ( T1 ), in Europe 2.048 Mbit / s ( E1 ) can be achieved, a range of 3 km to 4 km should be achieved. The standardization was completed in the USA in February 1994 (ANSI TR-28), in Europe in February 1995 (ETSI ETR 152). HDSL has now been largely replaced by SDSL , which only requires one pair of wires (a twin wire ) and consumes less power, but does not come close to the range of HDSL (with signal regenerators).

In the 1990s, other DSL processes were developed, such as ADSL . At the same time, internet usage rose sharply. The expansion of the networks could hardly meet the growing demand for data rates . For this reason, the networks in the background ( backbones ) should be expanded and the end users should be offered higher transmission speeds. ADSL was chosen as the technology for fast internet. ADSL has been approved in the telephone network by many network operators around the world.

In Germany, the name DSL was initially known as a synonym for broadband Internet access via ADSL , so that other broadband Internet access (for example via the cable television network or satellite ) are now marketed as "DSL". In Austria and Switzerland, however, there are clear delimitations; The term ADSL is used in these countries and is not a synonym for other broadband Internet services. However, DSL technologies were also designed for applications other than Internet access. Originally used for leased lines that are not available in large numbers, Internet access was the first mass application. Video applications in particular should open up new markets in the future using advanced DSL technologies with high data transmission rates .

ADSL2 + has been on the market since the end of 2005 . This standard currently offers up to 24 Mbit / s. In Japan, another variant of ADSL2 +, which has not yet been standardized, is used, which expands the reception spectrum to 3.7 MHz and enables data rates of up to 50 Mbit / s.

Since the end of 2006, VDSL / VDSL2 has been offered on various markets (e.g. Switzerland, Germany) with which data rates of up to 100 Mbit / s can be achieved. In order to be able to provide all subscribers with high data rates, is to first in the large metropolitan areas, a hybrid access network built up, wherein the glass fiber - connection from the main distributor - to the cable distributor is preceded -Location close to the customer ( FTTN ).


Germany 2012

In mid-2012, 21.4 million households had a DSL connection, while there were 3.6 million cable Internet connections, which means that the DSL market share in the broadband market was around 86%. According to the quarterly report, Deutsche Telekom switched 12.4 million T-DSL connections in June 2012 . Deutsche Telekom implemented almost half of the DSL and broadband connections in Germany. At the end of 2011, the number of broadband connections was 25.3 million, of which 22 million were DSL connections.

Germany in comparison of the EU and the most important industrialized countries (OECD) 2011

Measured in terms of the absolute number of broadband connections, Germany is far ahead of France at the top of the European countries. In terms of the number of DSL connections per inhabitant, Germany ranks 9th among the 30 OECD countries at the end of 2011 (287 DSL connections per 1,000 inhabitants). Since alternative broadband access such as cable internet is gaining a growing market share in Germany, Germany only ranks 27th in the technology-neutral OECD ranking of broadband access per inhabitant.

DSL is also the most frequently used broadband access in Switzerland (1st place in the OECD broadband rankings) and Austria (18th place). In both countries, however, broadband access via cable Internet also has a strong position on the market, so that around two thirds of all accesses are DSL.

World 2011

In December 2011 there were more than 264 million DSL connections worldwide. The largest shares are distributed as follows: China 96 million, USA 56 million, Japan 22 million, Germany 22 million, France 14 million, UK 13 million. DSL has a market share of almost 61 percent in the global broadband market.


Not every telephone line is DSL-capable. Determine whether DSL is available at a location:

  • DSL-compatible expansion of the local exchange with a sufficient number of ports .
  • Continuous copper subscriber lines between the location and the exchange. The subscriber connection must not be switched via multiplexers ( AslMx , PCMxA , PCM5D ). Pupin coils must be bridged or removed.
  • Length of the line between subscriber and exchange (more precisely: low attenuation, see below)
  • Diameter of the line, which can consist of several line sections with different diameters (larger diameter: less attenuation)
  • the number of DSL subscribers in the same access area, the increase in which the interference between the individual DSL connections intensifies. Due to the so-called NEXT and FEXT crosstalk ( Near End Crosstalk and Far End Crosstalk ) in the distribution cables, the DSL provision is limited to approx. 60% of the lines.
  • Cross-line optimization of signal crosstalk behavior in the form of DSM technology can, however, significantly increase DSL availability.

Since DSL and wired alternatives are not available nationwide, neither in Europe nor in Germany, there is increasing interest in alternative types of access, for example Internet access via satellite, via Wi-Fi or via cellular packet data services ( HSDPA , UMTS , EDGE , LTE ).

For this purpose, hybrid access solutions have become established that combine DSL with an alternative access technology to provide shared access. More than 500,000 households use such connections as a combination of DSL and LTE or DSL and 5G, primarily to improve broadband coverage in rural areas.


According to Deutsche Telekom, around 93 percent of subscriber lines can now be supplied with Telekom DSL. This information from Deutsche Telekom on the degree of accessibility regularly meets with criticism, because the high number is achieved because all connections in the connection areas of the subscriber exchanges equipped with DSLAMs are considered to be supplied. This does not take into account the connections in the extended connection areas, which cannot receive Telekom DSL due to unsuitable connection lines (attenuation, crosstalk, multiplexer, glass fiber).

The broadband atlas of the Federal Ministry of Economics was completely redesigned in 2010 by TÜV Rheinland. In April 2008 , the interest group presented a Germany-wide so-called narrow-band atlas that depicts demand . The needs-based expansion is to be supported by entering the broadband requirements and the bandwidth requirements of those interested in DSL and broadband. Detailed information on the Telekom DSL availability for a large number of especially small and medium-sized local networks based on the access network and the individual cable routing of Telekom is available on the website of a broadband initiative.

Situation in East Germany

The large-scale expansion of the eastern part of Germany with passive fiber optics ( OPAL ) after the fall of the Wall is affecting the installation of DSL. In some metropolitan areas such as Berlin (for example Berlin-Pankow ), Leipzig, Magdeburg and others, outdoor DSLAMs are now being installed for supply, while new copper lines are being laid in parallel elsewhere.

Situation in rural areas

At the turn of the year 2006/2007 around 59% of the rural access areas in Germany with a population density of less than 100 inh. / Km² had DSLAMs , which placed Germany in 15th place in the EU.

Outside the core cities , but especially in rural areas , there is also a high proportion of long connection lines, which is why the low-range ADSL-over-ISDN circuit used exclusively in Germany is better both qualitatively (higher data rates) and quantitatively (provision of more connections) DSL coverage of participants outside of the metropolitan areas is hampered. Technologies for a cost-effective, large-scale and rapid elimination of the DSL supply gaps caused by this would be available with ADSL-over-POTS / RE-ADSL2 and SDSL technologies - also in the course of the switch to NGN connections.

Deutsche Telekom, usually the only broadband provider in these regions , also only uses the outdated fixed rate connection for longer connection lines in these areas , which means that for several million households only connections with data rates of less than 1 Mbit / s are available that are currently available Broadband access requirements are not sufficient.

To supply the range victims , Deutsche Telekom has in recent years relied on a gradual expansion of the range of its narrowband ADSL-over-ISDN variant with fixed data rates of 384 kbit / s in the downstream and 64 kbit / s in the upstream (so-called DSL Light or Village DSL ); on the other hand, outdoor DSLAMs are also used here, which enable higher speeds, but due to the high investment costs are only installed if several hundred subscribers can be connected and the backhaul (the central connection) can be implemented cost-effectively. So-called ADSL extenders were tested by Deutsche Telekom in 2007 . These are micro-DSLAMs connected to the exchanges via G.SHDSL , which can supply up to eight households with ADSL. For the time being, however, their use will be limited to the Hungarian TAL network of Deutsche Telekom.

Since 2006 there has been state funding in Germany for the expansion of broadband infrastructures. Schleswig-Holstein was the first federal state to adopt a broadband directive and is providing 3 million euros between 2006 and 2009 as part of the Schleswig-Holstein Fund. Joint funding from the federal government and the federal states is imminent. In July 2007, the Federal Ministry of Economics provided a handout on the use of subsidies in accordance with European law, in order to make it easier for municipalities to access funds to support infrastructure measures. Funding, which can be used specifically for the development of a broadband infrastructure from 2008 onwards, is also to be made available by the Federal Ministry of Food, Agriculture and Consumer Protection and the federal states . The project is currently being discussed with regard to state co-financing; The funding amounts mentioned so far have met with criticism as being too low. In view of the controversial demand from Deutsche Telekom to have municipalities subsidize the expansion, these funds can support an expansion. Telecom competitors are also more likely to be active in such regions if support services are available for expansion (for example in Alveslohe).


In April 2010, according to company information, 97% of households in Austria could be supplied with DSL by Telekom Austria .


A broadband connection with 600 kbit / s downstream and 100 kbit / s upstream has been defined as a public service for all citizens from 2008, which is unique in the world in Switzerland . Since 1 March 2012, the minimum bandwidth of 1 Mbit / was increased s, since 1 January 2020 which is 10 Mbit / s (down) or 1 Mbit / s (up) The contract for the basic service , the Swiss universal service Obtained a license in the telecommunications sector Swisscom (Switzerland) AG , which was already able to supply 98 percent of the Swiss population with DSL at the end of 2003. For the remaining two percent, Swisscom intends to use Internet access via satellite to implement broadband access in addition to further DSL expansion and mobile communications .


The term DSL tariff has become increasingly common for the costs of DSL offers from Internet Service Providers (ISPs), since providers have increasingly started to offer their products as complete offers (including DSL packages). If you look closely, however, the tariff describes the possible forms DSL time tariff , DSL volume tariff and DSL flat rate . The DSL flat rate is establishing itself more and more as the standard because of its unrestricted usage options and the fixed price that has now fallen sharply.

Strictly speaking, the costs for the DSL connection are not yet taken into account in a DSL tariff. The costs for the subscriber line are priced into the basic telephone connection fee for DSL offers that require a conventional line-switched landline connection, whereas in the case of pure data connections it is included in the price for the DSL connection.

Complete offers consisting of a telephone connection, DSL connection and flat rates for both landline telephony and DSL access are increasingly being offered.

Provider purchasing costs

Which tariff model a provider offers depends largely on the conditions under which he can purchase pre-products or offer them himself.

As an established operator of German subscriber lines, Deutsche Telekom is obliged to make these lines accessible to other providers via unbundling . This is currently done either by means of collocation and rental of the complete or partial ( line sharing ) access line or with the bundle of offers consisting of Telekom DSL or T-DSL resale connection and optionally T-DSL-ZISP , ISP-Gate , T -OC-DSL for connection to the provider's network, which is gradually being replaced by bit stream access.

In all price models of DSL providers, the following components are priced in one way or another:

DSL connection / DSL line

In the same way as a telephone connection, a monthly flat fee has to be paid by the customer for the line via the DSLAM in the exchange to the broadband PoP . Depending on the offer, this line rental is priced into the DSL offer or has to be ordered and paid for separately. The price that customers ( basic Telekom DSL connection fee ) or providers (either connection line rental , line sharing rental or T-DSL resale or bitstream access connection rental) have to pay for this is largely subject to regulation in Germany by the Federal Network Agency .

For certain bandwidths, some providers offer the so-called Fast Path option. This option significantly reduces latency at the expense of error correction.

DSL access

DSL access (or DSL tariff in the narrower sense) usually refers to the provision of infrastructure on the provider side ( backbone from broadband PoP etc.) as well as the required resources (IP addresses, data volume, support, etc.). The provider of DSL access does not have to be the provider of the DSL connection at the same time.

End devices on the customer side

DSL modem and possibly a router are provided by some providers at no extra charge (especially with SDSL lines), with other providers the customer has to provide these devices himself.

DSL variants

There are different types of DSL technologies, which are summarized under the designation "DSL" or "xDSL" ( x as a placeholder for the specific procedure):

  • ADSL - Asymmetric Digital Subscriber Line , an asymmetric data transmission technology, for example with data transmission rates of 8 Mbit / s to the subscriber ( downstream ) and 1 Mbit / s in the opposite direction ( upstream ) ;
  • ADSL2 + - An extended form of ADSL with data transmission rates of up to 25 Mbit / s to the subscriber (downstream) and up to 3.5 Mbit / s in the opposite direction (upstream) , the speed is negotiated dynamically;
  • HDSL - High Data Rate Digital Subscriber Line , a symmetrical data transmission technology with data transmission rates between 1.54 and 2.04 Mbit / s;
  • SDSL ( G.SHDSL ) - Symmetrical Digital Subscriber Line , a symmetrical data transmission technology with data transmission rates of up to 3 Mbit / s symmetrically, i.e. in both the receiving and transmitting directions; with a four-wire connection (two copper twin wires) a maximum of 4 Mbit / s can be transmitted. Alternatively, the range can be increased at the expense of the data rate.
  • VDSL or VDSL2 - Very High Data Rate Digital Subscriber Line , a data transmission technology that theoretically offers data transmission rates of up to 210 Mbit / s in symmetrical operation.
  • UADSL - Universal Asymmetric Digital Subscriber Line

Other methods known as "DSL"

  • ISDN Digital Subscriber Line uses existing ISDN technology and enables data rates of up to 160 kbit / s
  • cableDSL - brand name of TELES AG for special internet access via cable connection
  • skyDSL - brand name of TELES AG for a Europe-wide internet access via satellite with up to 24 Mbit / s in the downstream
  • T-DSL via satellite - Deutsche Telekom brand name for Internet access via satellite. Originally, access via satellite only enabled the products mentioned to receive data ; a conventional modem or ISDN connection was used for transmission. Thanks to the use of iLNBs , an additional connection is no longer required.
  • Wireless Digital Subscriber Line (WDSL) uses radio technology and enables data rates of up to 10 Mbit / s. It is used under this name by FPS InformationsSysteme GmbH .
  • mvoxDSL - brand name for an Internet via radio - offered by mvox AG
  • FlyingDSL - brand name for an Internet via radio - offered by Televersa online
  • PortableDSL - brand name for Internet via radio - offered by isomedia and Airdata
  • AvioDSL - brand name for Internet via radio - offered by overturn technologies GmbH
  • smart-DSL - brand name for Internet via radio - offered by smartup solutions GmbH
  • - Brand name for Internet via radio in the BFWA band (5.8 GHz) - offered by
  • intersaar WDSL - brand name for Internet via radio link - offered by intersaar GmbH

Limited range

There are some factors that affect the range or data transmission rate of the copper line. Above all, the cable length and diameter of the copper wires are decisive. The copper wires laid in Germany have a diameter between 0.25 mm and 0.8 mm, depending on the length of the cable. For long cables, i.e. cables 6 km in length and more, the thicker copper wires are usually used.

Among the disturbing factors belongs especially the crosstalk (crosstalk). In order to prevent neighboring pairs in a cable harness from being impaired by a DSL transmission due to crosstalk , not all pairs of a cable harness are usually connected to DSL connections. With a new interference cancellation technology (IFC) will in future in real time analyze crosstalk interference and targeted compensation signals are balanced, with DSM - servers should be minimized crosstalk in adjacent veins through optimized, coordinated DSL signaling.

In general, the following applies: the further away a subscriber is from the exchange , the lower the maximum achievable data transmission rate. The condition for the availability of DSL is a low attenuation of the subscriber line (measured in dB ) - the lower this is, the higher the maximum data transmission rate.

The different xDSL methods have different ranges, depending on whether and to what extent the lower-range and lowest-attenuation frequency ranges of the copper wire pair are used:

  • The SDSL / G.SHDSL technology, which as a pure data connection can use all frequencies, has proven to be the longest- range (up to 8 km cable length) . This technology is used nationwide by QSC for private customers in Germany . With the establishment of NGN telephony, this technology can also be used for combined voice and data connections .
  • This is followed by Reach-Extended-ADSL2 , which uses the lower, high- range frequency spectrum above POTS usage to a greater extent through increased transmission levels. This standard has been used by France Telecom , for example, for long connection lines since spring 2006.
  • The conventional ADSL / ADSL2 / ADSL2 + variants in accordance with the ADSL-over-POTS standard, which also use the entire frequency range above POTS, then follow.
  • The ADSL / ADSL2 / ADSL2 + variants according to the ADSL over ISDN standard , which are used exclusively in Germany (and Bosnia-Herzegovina ) exclusively (i.e. also on analog connections and pure data connections ), have the lowest range, because here the entire low-attenuation range is below 138 kHz is not used by DSL. Depending on the wire diameter, from a cable length of approx. 4 km, only a significantly limited bandwidth can be used and the data rate is available.

Bandwidth, data transfer rate and attenuation

Transmission method Bandwidth Data transfer rate
POTS (analog) 300 Hz - 3.4 kHz up to approx. 56 kbit / s, typically 4.5  kByte / s - 5 kByte / s
ISDN 0 Hz - 120 kHz 2 × 64 kBit / s user channel + 16 kBit / s control channel
ADSL (ADSL-over-ISDN) 138 kHz - 1.1 MHz Down: up to 8 Mbit / s, Up: 1 Mbit / s
ADSL2 + (ADSL-over-ISDN) 138 kHz - 2.2 MHz Down: up to 24 Mbit / s, Up: 1 Mbit / s
ADSL2 + (ADSL-over-POTS; not in use in Germany) 26 kHz - 2.2 MHz Down: up to 25 Mbit / s, Up: 3.5 Mbit / s
ADSL2 + (Annex-J) 0 Hz - 2.2 MHz Down: up to 25 Mbit / s, Up: 3.5 Mbit / s
VDSL 138 kHz - 12 MHz Down: up to 50 Mbit / s, Up: 10 Mbit / s
VDSL2 - 30 MHz Down: up to 200 Mbit / s, Up: 200 Mbit / s with VDSL profile 30a.

Factors that affect the data transfer rate are:

  • Line attenuation (depending, among other things, on the length and diameter of the copper lines and the frequency spectrum of the signal)
  • Modulation method
  • Line code


The line attenuation represents the reduction of the transmitted energy of a signal in the course of a transmission link and is therefore a decisive value for DSL. The longer the line, the lower the data rates that can be achieved with DSL processes.

The line attenuation that is decisive for the pre-qualification of connection lines is calculated by the network operators using the line routing of the connection line entered in the Kontes-Orka line database . For ADSL circuits, the attenuation is calculated based on a frequency of 300 kHz, for T-Home's Entertain connections based on 1 MHz and for the SDSL attenuation calculation based on 150 kHz.

Copper wires attenuate the signal depending on the wire diameter by a certain value per km of cable length at a certain frequency. Deutsche Telekom assumes the following values ​​at 300 kHz for its ADSL wiring:

Wire diameter Ø 0.35 mm Ø 0.4 mm Ø 0.5 mm Ø 0.6 mm Ø 0.8 mm
Line attenuation per kilometer 14.0 dB / km 12.0 dB / km 8.5 dB / km 7.5 dB / km 5.7 dB / km

The sum of the attenuation of the individual line sections of the connection line determined on this basis results in the attenuation value that is decisive for the Telekom DSL circuit. The attenuation values measured by the ADSL modems and displayed in the user dialog represent the averaged attenuation over all carrier frequencies used during line negotiation in the transmit or receive direction and therefore deviate significantly. As a rough guide, the mean value of the displayed transmission and reception attenuation of the ADSL modems is roughly in the range of the line attenuation at 300 kHz.

Achievable data rates with national DSL network operators with given line attenuation

If a DSL line with a real rate-adaptive circuit (RAM) is operated at the technical limit, the line capacity available at the connection can be used as far as possible. This switching method in connection with ADSL2 + (increasingly also VDSL2, ADSL mostly only in stock) has been predominant with DSL network operators for a number of years, whereby the data rates in the upper range are limited by the respective marketed tariff; Most providers market on an ADSL2 + basis max. a data rate of 16,000 kbit / s in the receiving direction.

  • Depending on the line conditions and the signal-to-noise margin when negotiating the connection, the rate-adaptive circuit, especially when using DSL modems of poor quality, can impair the transmission quality (for example, high response times due to bit errors , unstable connections and broken connections). In this case, the provider can use a so-called security profile to set the max. Reduce the DSL data rate or increase the specified signal-to-noise margin when negotiating the connection; some DSL modems can also be configured accordingly by the user. The decision for security profiles depends on the provider, e.g. T. already in advance of the circuit due to the line data and the calculated line attenuation as well as the possibly existing increased interference potential in adjacent wires due to crosstalk or afterwards if line interference occurs.
  • For ADSL connections technically implemented by Deutsche Telekom (Telekom DSL incl. Resale and Telekom Bitstream ) with a calculated line attenuation at 300 kHz of more than 18 dB (usually corresponding to a connection line more than 1.5-2 km in length) , which applies to around half of households with DSL availability, Deutsche Telekom has so far only switched connections based on the older ADSL standard according to ITU G.992.1 instead of its rate-adaptive ADSL2 + and VDSL2 connections, even with new contracts Usually up to 4 km of additional usable ADSL2 + frequency carriers on such lines lie fallow.
    • Telekom switches these ADSL connections according to ITU G.992.1 at a fixed data rate, whereby the max. bookable data rate results from the calculated line attenuation at 300 kHz:
Data rate 384 kbit / s 768 kbit / s 1,024 kbit / s 1,536 kbit / s 2,048 kbit / s 3,072 kbit / s 6,016 kbit / s 16,000 kbit / s ADSL2 +
damping up to 55 dB up to 46 dB up to 43 dB up to 39.5 dB up to 36.5 dB up to 32 dB up to 18 dB below 18 dB
Due to the exclusive orientation to values ​​calculated from the line database without measurements of the signal-to-noise ratio, these switching limits are set conservatively and the availability of higher data rates is significantly limited with increasing line length, since the signal quality can be estimated much more poorly isolated with the calculated line attenuation as the line length increases the fixed rate switching in itself already necessitates protection against temporary interference. Taken together, this usually leads to high signal-to-noise margins of 15-25 dB on the majority of the connections and, as a result, limited data rates with discounts in the range of one to several Mbit / s compared to the line capacity in addition to the discounts caused by not using the ADSL2 + frequency carriers are to be accepted.
A switching of higher data rates that deviates from these specified attenuation limits, whereby the provider accepts no responsibility for any restrictions in connection quality that may arise (so-called risk switching), can not be booked for DSL connections via Deutsche Telekom's connection technology, in contrast to DSL connections switched via collocation providers , because the ADSL-SV server system from Telekom does not allow this due to the possible increased risk of service cases with their form of fixed rate switching.

DSL devices

The following hardware components are required for DSL access:

Customer side

  • DSL modem , generally known as Customer Premises Equipment (CPE) or in the special case ADSL ADSL Transceiver Unit - Remote (ATU-R); integrated in the so-called DSL routers.
  • On ADSL connections that are not purely data connections and the connection line is also used by a conventional voice telephony connection ( POTS or ISDN ):
    • Broadband connection unit (BBAE), colloquially called "splitter", one of the following, depending on the line type:
      • POTS splitters are passive crossovers to separate the data and voice frequency bands. Its limit frequency is formed from the bandwidth required to transmit the voice band and the charge pulse and is 16 kHz.
      • ISDN splitters have the same function as POTS splitters, but their cutoff frequency is 138 kHz.
      • In Germany, ISDN splitters are generally installed, even if the underlying telephone connection is not an ISDN connection, since only ADSL-over-ISDN is used on all ADSL connections . Pure POTS splitters are not common, but some older devices have an internal switch called Analog / ISDN.

Provider side

  • If necessary, if a PSTN connection is available, a splitter that is permanently integrated into the Deutsche Telekom network in the case of Telekom DSL connections and line sharing connections as an ISDN splitter in the main distributor of the Telekom switching center (so-called "MDF-integrated splitter “) And for reasons of cost has no option to switch between ADSL-over-ISDN and ADSL-over-POTS.
    • An offer of ADSL-over-POTS to telecom DSL and line sharing connections would therefore only be possible with some effort in terms of exchanging these MDF-integrated splitters.
  • DSLAM ( D igital S ubscriber L ine A ccess M ultiplexer) or ATU-C ( A DSL T ransceiver U nit - C entral Office ), also in general terms COE ( C entral O ffice E called quipment). The modems are integrated in the DSLAM.
  • DSL-AC ( D igital S ubscriber L ine A ccess C oncentrator) or broadband PoP (BB-PoP).

Depending on the technical implementation, additional components such as RADIUS servers for user registration and user administration and billing (consumption data storage for the purpose of invoicing) can be added.

Interfaces and specifications

Examples of interfaces and specifications for DSL technologies are:

  • U-R2 (1TR112) - interface defined by Telekom at the end of 2001 for the interoperability of ADSL terminals
  • ITU-T G.991.2 ( SHDSL )
  • ETSI TS 1010338 and ETSI TS102 080 Annex A (ADSL over POTS for everywhere except Germany) and Annex B (ADSL over ISDN); the trap: Annex A does not apply to ADSL over POTS in Germany, but Annex B G.992.1 # Annex B does
  • ITU-T G.992.1 ( Annex A and Annex B , G.dmt)
  • ITU-T G.992.2 (G.lite)
  • ITU-T G.992.3 ( ADSL2 )
  • ITU-T G.992.4 (splitterless ADSL2)
  • ITU-T G.992.5 ( ADSL2 + )
  • ITU-T G.993.2 ( VDSL2 )


Protocols for ADSL technologies are for example:

  • PPP over Ethernet protocol (PPPoE) , which regulates the encapsulation of PPP packets in Ethernet frames. PPPoE is used, for example, by Deutsche Telekom for Telekom DSL connections (also for Telekom Bitstream and T-DSL resale connections and for T-DSL Business Symmetrical on SDSL basis); Several (up to 10) PPPoE sessions to different Internet access providers can exist at the same time at these Telekom DSL connections (exception: VDSL-based access including ADSL2plus routes implemented via these DSLAMs) if these can be terminated at DTAG-BBRASs ( via OC , Gate or Z-ISP )
  • PPP over ATM protocol (PPPoA) , which regulates the encapsulation of PPP packets in ATM cells.
  • Point-to-Point Tunneling Protocol (PPTP) , which creates a tunnel via a PPP connection. PPTP is widely used in Austria, Italy and Belgium, but rarely in Germany.

Broadband providers and market shares


Market shares in fixed-line broadband connections March 2017
providers Customers proportion of
Deutsche Telekom 12,596,000 40.4%
United Internet ( 1 & 1 including Freenet ) 4,270,000 13.7%
Unitymedia Kabel BW 3,044,600 10.4%
Vodafone DSL + Kabel Germany 5,586,000 19.5%
Telefónica Germany (incl. O2, Alice) 2,103,000 6.5%
Tele Columbus (including Primacom , Pepcon) 3,600,000 1.7%
other ( EWE TEL , M-net , NetCologne etc.) 2,700,000 7.8%
total 30,300,000 100.0%


  • A1 Telekom Austria (aon and eTel Austria )
  • UPC Telekabel ("chello" and "Inode")
  • Silver Server (short: sil )
  • (an ESSgroup company)
  • Tele2
  • Brennercom Tirol GmbH
  • GmbH
  • Anexia
  • net4you
  • next layer (only for business customers)
  • ASCUS Telecom (formerly HAPPYnet and Technix) (an ESSgroup company)
  • (an ESSgroup company)
  • iPlace
  • Salzburg AG / cablelink
  • VOL - Vorarlberg Online
  • NA-NET Communications GmbH
  • LinzNet GmbH
  • Stadtwerke Hall in Tirol GmbH (citynet @ hall)
  • (formerly Lenz-Moser) (an ESSgroup company)
  • Telematica


See also


  • Oliver Komor, Mathias Hein: xDSL & T-DSL. The practice book. Franzis, Poing 2002, ISBN 3-7723-7134-5 .
  • Andreas Bluschke, Michael Matthews: xDSL primer. VDE-Verlag, Offenbach 2001, ISBN 3-8007-2557-6 .
  • Remco van der Velden: Competition and Cooperation on the German DSL Market - Economics, Technology and Regulation. Mohr Siebeck Verlag, Tübingen 2007, ISBN 3-16-149117-3 .

Web links

Individual evidence

  1. The first Giga-DSL prototype comes from Huawei .
  2. ^ Statements by Deutsche Telekom on the network of the future
  7. a b
  8. DSL distribution worldwide 2011. In: Archived from the original ; accessed on February 10, 2018 .
  9. , April 16, 2007
  10. Use of DSM minimizes crosstalk interference .
  11. a b Report on the Broadband Atlas 2007, p. 17 ( Memento from September 28, 2007 in the Internet Archive ) (PDF)
  12. Federal Ministry of Economics and Technology Broadband Atlas
  14. The nationwide broadband demand atlas of the community of interests
  15. Telekom DSL availability statistics for small and medium-sized local networks
  16. DSL to Pankow: ( Memento from June 24, 2007 in the Internet Archive ) T-Com built over Pankow fiber optics by the end of 2006
  17. EU: Annual iSociety Report 2008 - staff working paper: (PDF; 449 kB) EU figures on DSL coverage in rural areas
  18. EU conference "Briding the Broadband Gap" 2007, IDATE study (PDF; 368 kB)
  19. a b Heise: c't DSL FAQ 2/2010
  20. DSL switching rules in comparison Heise
  21. Telekom wants to switch DSL to "Rate Adaptive" . , November 2, 2007 - Note: the change has been postponed
  22. Telekom is preparing DSL at maximum speed . , October 17, 2009
  23. Deutsche Telekom starts selling rate-adaptive DSL ., February 2, 2010
  24. Bundestag wants to close gaps in broadband coverage ., March 7, 2008
  25. State Secretary Pfaffenbach at Cebit 2008: Millions of connections with insufficient data rates ., March 6, 2008
  26. WIK study warns against opening broadband scissors ., March 27, 2008
  27. DSL connections from Deutsche Telekom with a transmission rate of 384 kbit / s in the downstream are not broadband internet access due to the upstream data rate being reduced to 64 kbit / s without technical necessity (corresponding to 1-channel ISDN ) according to the broadband definition of the Federal Ministry of Economics
  28. Hegensdorf - In the village of DSL graves
  29. Broadband extenders bring broadband to the country
  30. ^ No DSL: Broadband funding in Schleswig-Holstein
  31. Federal Ministry of Economics and Technology: ( Memento from September 15, 2011 in the Internet Archive ) (PDF) The use of public funds to close broadband gaps in Germany
  32. BMELV: ( Memento of September 30, 2007 in the Internet Archive ) The federal and state governments are committed to improving broadband coverage in rural regions
  33. German Association of Towns and Municipalities (DStGB) ( Memento of January 8, 2009 in the Internet Archive ): Broadband funding in rural areas essential - funding approaches too low
  34. : Segeberg district between a snail's pace and the speed of light
  35. 97% DSL coverage according to information from Telekom Austria
  36. OFCOM: The Federal Council is adapting the benefits of the universal service
  37. OFCOM: Faster Internet in the basic service from 2020
  38. OFCOM: Universal service license granted to Swisscom
  40. a b c Diagram with maximum data rates in the receiving direction of various ADSL methods with given ADSL line attenuation in the receiving direction under optimal line conditions ( Memento from February 29, 2008 in the Internet Archive ) Interference on the line path such as crosstalk from neighboring connection lines can significantly limit the actually achievable data rate
  41. Attenuation calculation and attenuation limits at Telekom
  42. different attenuation calculation from providers
  43. More broadband for Germany . (PDF; 2.4 MB) Deutsche Telekom, p. 3 .: DSL ranges of Deutsche Telekom
  44. Telecom attenuation limits. (PDF)
  45. As part of pilot tests, since spring / summer 2007, Telekom DSL has been switched with 384 kbit / s up to a line attenuation of 60 dB in selected access areas.
  46. Fixed-rate DSL connections from Deutsche Telekom with a transmission rate of less than 1 Mbit / s in the receiving direction do not meet the minimum requirements for broadband access
  47. a b c For increased transmission data rate 4 dB less
  48. Vierling Infomagazin , 02/2003 (PDF) p. 2: Telekom's line database-oriented DSL line prequalification requires conservative planning specifications for its fixed switching limits
  49. 1TR112 Technical Specification of the U-Interfaces of xDSL Systems in the network of Deutsche Telekom . (ZIP archive) Version 12.2, as of February 2014 (English)