Asymmetric Digital Subscriber Line

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DSL modem for ADSL and Ethernet
Multifunctional device: wireless access point , switch , router and ADSL modem.

With Asymmetric Digital Subscriber Line ( ADSL , English for asymmetric digital subscriber connection ), the currently most common connection technology of broadband lines designated for consumers. It was developed as a variant of DSL technology with the stipulation that it would function over the existing telephone connection line without impairing telephony via the fixed network connection , and at the same time for the target customer group (private households, small businesses) an asymmetrical (unequal) data transmission rate with a higher suitable thereTo offer receive as send rate .


Opened DSL splitter for connecting telephone and ADSL modem

ADSL can be used on connecting lines of analog ( POTS ) and digital ( ISDN ) telephone connections or as an unbundled data connection ( unbundled DSL ).

With POTS / ISDN connection lines there are frequency ranges which are not used for telephony and are therefore idle. These higher frequency ranges are used for ADSL. Basically, the line attenuation increases with increasing distance from the exchange , which geographically limits the available data rates and coverage with ADSL. Outdoor DSLAMs offer a remedy here, but are not widely used because of the higher costs per port, at least with ADSL.

The functional principles of ADSL technology are frequency division multiplexing , Fourier transformation and Discrete Multitone Transmission (DMT). The essential components of an A DSL modem are a fast DMT modulator / demodulator and a digital signal processor for calculating the Fourier transforms for the individual frequencies.

So that the two types of use of the telephone line do not interfere, the two frequency ranges at the subscriber as well as in the main distributor are separated by a crossover , the so-called DSL splitter . Basically, no voice channel is occupied by using ADSL, so you can use the Internet and make calls using traditional landline telephony at the same time.

In the past century this was not possible because the telephone modem of that time only had the bandwidth for voice transmission. With the frequency band from 300 Hz to 3400 Hz, either voice or data could be transmitted, not both at the same time. The available bandwidth at that time was a maximum of 56 kbit / s.

The data transmission with the ADSL-over-ISDN used in Germany runs in 4312.5 Hz wide bands with a symbol rate of 4 kBd each in the range from 138 to 275 kHz for the upstream and 275–1104 kHz for the downstream . Because of the poor line quality - after all, the telephone lines were not intended for the transmission of signals with a bandwidth of around 1 MHz - the line from the terminal to the exchange is "measured" and individual bands are hidden if the attenuation is too great or reflections occur.

The ADSL2 + standard, which is increasingly being used by ADSL providers, is accompanied by an expansion of the frequency range used up to 2.2 MHz, which enables significantly higher data rates with short connection lines: Generally up to 24 Mbit / s (megabits per second) in receiving direction and up to 3.5 Mbit / s in sending direction; In Germany, however, the low-data rate ADSL-over-ISDN connection only offers up to 16 Mbit / s in the receive direction and up to 1125 Kbit / s (kilobits per second) in the send direction.

ADSL on analog and ISDN connections

Approximate bit rates of various ADSL standards depending on the length of the line to the DSLAM

Usually, ADSL is switched to analog connections and pure data connections according to the ADSL-over-POTS standards ('Annex A'), while ADSL-over-ISDN is used on ISDN connections ('Annex B'). These standards differ in the signaling and the frequency bands used .

The ADSL device technology originally installed in the German fixed network uses ADSL-over-ISDN on all ADSL connections as a worldwide specialty . This has the advantage that only one, instead of two, technical standards must be supported by the ADSL provider, and the subscriber can switch between analogue connection, ISDN connection and pure data connection without changing the installed ADSL technology.

At the same time, the decision to use ADSL-over-ISDN technology for analog telephone connections also results in bandwidth and range disadvantages. As a disadvantageous consequence , the ADSL availability on long connection lines is significantly limited compared to ADSL-over-POTS due to the omission of the ADSL-over-POTS frequency range with the lowest attenuation and thus the longest range, from 26 to 138 kHz, which is the maximum possible data rate on all ADSL connections decreased. Newer ADSL variants such as the Reach Extended ADSL2 (ITU G.992.3 Annex L), which has been used nationwide by France Telecom since spring 2006 and has a particularly high coverage, which again changes the area that can be supplied by an exchange compared to ADSL over POTS / Annex A. Approx. 40% larger or ITU G.992.5 ADSL2 + Annex M with a send data rate drastically increased to 3.5 Mbit / s are also not available for ADSL-over-ISDN.

Telekom plans to switch completely to the so-called IP-based connections ('Annex J') by 2018 and to make the low, low-attenuation frequencies usable for data transmission. That means the end of analog telephone connections as well as digital ISDN telephone connections (and thus for ADSL-over-ISDN, 'Annex B') in the Deutsche Telekom network.


Spectrum of Discrete Multitone Modulation on the transmission path
Opened ADSL router with a description of the individual functional units

When establishing the ADSL connection, the ADSL modem on the subscriber's side and the DSLAM in the central office first agree on the ADSL standard used (see below) and then negotiate the connection parameters of the ADSL connection: the transmission capacity of the individual DMT frequency carriers of the copper wire pair is measured, the downstream and upstream transmission rate is negotiated according to the specifications of the DSLAM profile configured for the connection and distributed to the individual carriers. After the connection negotiation has been completed, the DSL connection remains synchronized (ie connected) until the DSL connection is terminated .

When negotiating the transfer rate is rate adaptive ( English rate adaptive ) negotiation (also rate adaptive mode , RAM) from the fixed or solid distinguished negotiation.

With fixed negotiation, the DSLAM specifies the (upstream / downstream) transmission rate. If the specified transmission rate cannot be achieved due to the current line measurements (for example also due to temporary interference), the DSL connection will fail.

With rate-adaptive negotiation, however, the DSLAM only specifies the maximum transmission rate (or a data rate corridor); If the connection negotiation does not succeed with the maximum transmission rate specified by the DSLAM, the connection is synchronized with the highest possible transmission rate that the current line conditions allow (within the data rate corridor). This means that the transfer rate negotiated when the connection is established adapts to the line conditions. ADSL connections configured in this way are therefore rate-adaptive .

The seamless rate adaptation possible with ADSL2 + also allows the transmission speed to be adapted to the transmission quality of the cable connection when there is an existing connection without losing synchronization (i.e. without disconnecting the DSL connection); this function is currently (2010) largely not yet implemented by the German ADSL2 + providers (with the exception of HanseNet , QSC and M-net DSLAMs).

Because of the high signal-to-noise margin required for the safe operation of ADSL connections with fixed rate switching, only a significantly lower data rate can usually be made available at these connections than would be possible with adaptive negotiation of the data rate. Because of this disadvantage, both nationally and internationally, most ADSL providers have been using rate-adaptive switching for several years.

In Germany, fixed rate switching is still used almost exclusively for DSL connections up to 6 Mbit / s ( T-DSL , T-DSL resale , bitstream access ) that are technically implemented by Deutsche Telekom . The expansion of rate-adaptive switching to the entire ADSL product portfolio from the end of 2008, which she intended in autumn 2007, has meanwhile been repeatedly postponed. The gradual introduction from February 2010 announced in October 2009 was suspended after a few days due to problems in the CRM-T booking system .

While with fixed negotiation the signal-to-noise margin is usually so high that almost all ADSL modems and routers on the market guarantee trouble-free operation, with rate-adaptive switching the quality of the modem used is at the technical limit Longer connection lines have a significant impact on the achievable data rate and the line stability of the ADSL connection.


Additional protocols are used to establish a connection to the Internet, such as the PPP- based: PPPoE (for example in Germany and Switzerland; several PPPoE can be used on the DSL connections based on T-DSL technology from Telekom (also T-DSL resale ) -Connections to different Internet access providers exist at the same time), PPP over ATM in combination with PPTP (for example in Austria, France and Italy) or MPoA (in Germany for routed subnets, in Spain for static IP addresses).


For the purpose of better error correction of the ADSL connection, some providers (such as Telekom's T-DSL) use interleaving . This means that the packets are not simply sent in their original order, but are interleaved. This means that the reception of a packet takes a longer time, and thus increases the latency of the connection. B. after the package 10 is not the same package 11, but the packages 11 to 18 interlocked. If the line conditions permit, the interleaving can sometimes be switched off with these providers using an option ( FastPath ), which is usually subject to a surcharge .

ADSL standards

Frequency ranges

Frequency and channel allocation of the various ADSL standards
  • analog / ISDN
  • Protective tape
  • Upstream
  • Downstream ADSL / ADSL2
  • additional downstream ADSL2 +
  • Receive and send rate

    ADSL standards and maximum user data rates
    standard Surname Receive rate (downstream) Send rate (upstream) factor
    ANSI T1.413 Issue 2 ADSL 8 Mbit / s 0.6 Mbit / s 13.3: 1
    ITU-T G.992.1 ADSL (G.dmt) 8 Mbit / s 1 Mbit / s 8: 1
    ITU-T G.992.1 Annex A ADSL over POTS 10 Mbit / s 1 Mbit / s 10: 1
    ITU-T G.992.1 Annex B ADSL over ISDN 10 Mbit / s 1 Mbit / s 10: 1
    ITU-T G.992.2 ADSL Lite (G.lite) 1.5 Mbit / s 0.5 Mbit / s 3: 1
    ITU-T G.992.3 ADSL2 (G.bis) 12 Mbit / s 1.2 Mbit / s 10: 1
    ITU-T G.992.3 Annex A ADSL2 over POTS 12 Mbit / s 1 Mbit / s 12: 1
    ITU-T G.992.3 Annex B ADSL2 over ISDN 12 Mbit / s 1 Mbit / s 12: 1
    ITU-T G.992.3 Annex I All-digital mode ADSL2 12 Mbit / s 3.2 Mbit / s 3.75: 1
    ITU-T G.992.3 Annex J. All-digital mode ADSL2 12 Mbit / s 3.5 Mbit / s 3.43: 1
    ITU-T G.992.3 Annex L RE-ADSL2 6 Mbit / s 1.2 Mbit / s 5: 1
    ITU-T G.992.3 Annex M ADSL2 extended upstream 12 Mbit / s 3.5 Mbit / s 5: 1
    ITU-T G.992.4 ADSL2 (G.bis.lite) 12 Mbit / s 1 Mbit / s 12: 1
    ITU-T G.992.5 ADSL2 + 24 Mbit / s 1 Mbit / s 24: 1
    ITU-T G.992.5 Annex A ADSL2 + over POTS 24 Mbit / s 1 Mbit / s 24: 1
    ITU-T G.992.5 Annex B ADSL2 + over ISDN 24 Mbit / s 1 Mbit / s 24: 1
    ITU-T G.992.5 Annex I All Digital ADSL2 + 24 Mbit / s 3.2 Mbit / s 7.5: 1
    ITU-T G.992.5 Annex J. All Digital ADSL2 + 24 Mbit / s 3.5 Mbit / s 6.86: 1
    ITU-T G.992.5 Annex M ADSL2 + M 24 Mbit / s 3.5 Mbit / s 6.86: 1

    See also

    Web links

    Commons : ADSL  - collection of pictures, videos and audio files


    1. reduced range of ADSL-over-ISDN
    2. Annex L: 40% more space can be supplied Archive link ( Memento from September 27, 2007 in the Internet Archive )
    3. ^ Statements by Deutsche Telekom on the network of the future
    4. Effects of the different DSL switching rules In: c't 8/2007, p. 86f
    5. The largest national competitor, Arcor , has been switching all ADSL connections rate-adaptively since 2002
    6. BT Wholesale confirms launch of the Max services In:, March 2, 2006
    7. Telekom wants to convert DSL to "Rate Adaptive" In: November 2, 2007
    8. Telekom prepares DSL at maximum speed In: October 17, 2009
    9. Deutsche Telekom starts selling rate-adaptive DSL In: February 2, 2010
    10. Info page on DSL modems and routers as well as line and data rate optimization
    11. ITU-T Recommendation G.992.1: Asymmetric digital subscriber line (ADSL) transceivers
    12. ITU-T Recommendation G.992.2: Splitterless asymmetric digital subscriber line (ADSL) transceivers
    13. ITU-T Recommendation G.992.3: Asymmetric digital subscriber line transceivers 2 (ADSL2)
    14. ITU-T Recommendation G.992.4: Splitterless asymmetric digital subscriber line transceivers 2 (splitterless ADSL2)
    15. ITU-T Recommendation G.992.5: Asymmetric Digital SubscriberLine (ADSL) transceivers - Extended bandwidth ADSL2 (ADSL2 +)