The Directive G.992.1 Asymmetric digital subscriber line (ADSL ) transceivers ITU-T describes the physical layer of the interfaces of transmitting and receiving devices for ADSL over copper wires of telephone networks. As DMT modulation method is used - Directive G.992.1 is therefore also referred to G.dmt.

In G.992.1 different transmission channels in conjunction with three realizations are specified:

The ADSL has described as a function of the technical design and environmental data transfer rates of approximately 8 Mbit / s downstream and 1 Mbit / s upstream.

Annex A

The specifications of Annex A ( " ADSL over POTS " ) may be used only with analog phone lines or pure data connections. In this case, a larger frequency range for DSL is used, the area below (under 25 kHz) is still sufficient for analog telephony, but not for ISDN.

Even with the ADSL developments ADSL2 / 2 according to ITU G.992.3 and G.992.5 Annex A describes the specification for operation on analog lines and pure data connections. The service used for the upstream frequency range is at Annex A of 25 kHz to 138 kHz, for the downstream frequencies from 138 kHz to 1.1 MHz ( for ADSL2 up to 2.2 MHz ) was used.

Annex B

Annex B is for digital phone lines of the ISDN designed ( " ADSL over ISDN" ). ADSL is on a high frequency band, the area below ISDN or analog signals ( both works because this frequency range is wider than in Annex A: the required bandwidth for ISDN is 120 kHz, the bandwidth for analog telephony only about 4 kHz ). The upstream area is located at Annex B between 138 kHz and 276 kHz, the downstream range between 276 kHz and 1.1 MHz ( 2.2 MHz for ADSL2 ).

Even with the ADSL developments ADSL2 / 2 according to ITU G.992.3 and G.992.5 Annex B describes the specification for operation on ISDN lines with limited broadband frequency band.

Annex J

See Asymmetric Digital Subscriber Line

This variant is used for example by Deutsche Telekom for splitter-less DSL.

Since October 2012, all the so-called IP-based connections Telekom can be operated without a splitter.

Annex L

See Asymmetric Digital Subscriber Line 2

Annex M

See Asymmetric Digital Subscriber Line 2


Annex A and Annex B are physically configured differently.

The cutout of the most far-reaching, damping poorest frequency spectrum at Annex B for its use for the ISDN signals the reach of Annex B with respect to Annex A by an average of ½ km lower or the attainable downstream data rate drops with identical subscriber line to about 1.5 Mbit / s and at longer connecting cables, with about 50 dB line attenuation at 300 kHz up to 1 Mbit / s lower.

The newer, more far-reaching ADSL protocol Reach Extended ADSL2, which is used in long connecting cables, for example, since the spring of 2006 in France, the country of France Telecom, is also defined exclusively for ADSL-over- POTS. The ADSL2 / 2 variants according to ITU G.992.3 / 5 Annex M with higher Upstream up to 3.5 Mbit / s are not available for ADSL-over -ISDN.

Some DSL modems (such as the Fritz! Boxes), it is possible to change the variant used by Annex load another firmware. Since the hardware of the DSL modem is agreed upon in principle but either Annex A or Annex B, the performance of such a modem is limited by such a firmware change.


Germany is the world's only country that is not used in the Annex A, ie Annex B is also connected to analog connections as well as in ADSL circuit used without traditional voice telephony ( unbundled DSL, NGN bitstream access ). The German Telekom had allowed only ADSL-over- ISDN circuits in their affiliate network for a long time, so that collocation competitors were able to implement only Annex B; addition, since 2006, there is an approval for ITU G.992.5 Annex M, which by the summer of 2009 but remained unused.

In the rest of Europe, Russia and the United States is predominantly Annex A or in countries with high ISDN market share ( for example in Switzerland, Scandinavia, Belgium, the Netherlands and Austria ) mixed operation of Annex A ( on analog connections and pure ADSL / Bitstream ) and high-reach and bandbreitenschwachem Annex B ( only used on ISDN connections ), which is not a problem, according to Swisscom.