A DSL modem, in the lingo " NTBBA " (Network Termination Broadband Access, German: network termination for broadband access), is a device for transmitting data via a subscriber line using DSL technology. It is the network termination for the DSL line at the subscriber and thus represents the counterpart to the DSLAM dar. ADSL modems are in technical jargon as ADSL Transceiver Unit - Remote or abbreviated ATU -R.

Connection to the public telecommunications network

About the normal subscriber line of the ISDN connection or analog connection of the subscriber receives a mixed signal (telephony and DSL), which is using a splitter ( broadband connection unit ( BBAE ) ) divided into broadband and telephony and output on two separate lines in the rule. On the line for DSL, the DSL modem is connected. To connect from the splitter to the DSL modem reaches a two-wire cable, either two wires of a normal telephone installation cable or an RJ -45 cable in the middle pins 4 and 5 are occupied. ( See also Broadband Unit: Installation notes )

In the case of SDSL and unbundled DSL, outside Germany, even when there usual Annex A ADSL modem with integrated DC block, eliminating the splitter and the DSL modem is connected directly to the cable.

Modulation method

The ADSL standards to use for communication via the running as a twisted pair copper wire cable between the DSLAM at the local exchange and the DSL modem, a modulation method with different carrier frequencies (Discrete Multitone ); with conventional ADSL eg 256 carrier frequencies spaced every 4.3125 kHz. In SDSL TC -PAM is used instead.

Types and designs

The classical external DSL modem (for example via USB ) or network connected either directly to a PC (for example using a router ). According to the OSI model, the modem an Ethernet bridge and on Layer 2 is located. The Ethernet traffic is transparent but only on an ATM layer placed for the local port and converted back into Ethernet DSL -AC. For the PPPoE client it is like follows, as if the DSL -AC is the actual network. He can be reached both under its own MAC address as well as under the broadcast address. The latter is necessary to find him at all. For the fitting end on the Ethernet Internet protocol (IP) traffic is transparent. Limited to pure bridge functionality, working on the principle of black box modem are increasingly rare. In addition to the design as an external device offers high speed modems as a PCI card.

DSL modems often have a more or less well-documented access to the configuration (for example, via telnet or web interface), through which the user can query the measured out line values ​​( line attenuation, noise margin ) and make adjustments to the intricacies of broadband signaling can.

Are usual in the meantime combinations of (A) DSL ​​modem and ( wireless ) router - so-called (A) DSL ​​router. Some as pure DSL modem devices sold are in fact also such a DSL router, but are from marketers - usually identical to the DSL line provider - limited to the modem function to keep support costs within limits. These devices sometimes have value an amazing hack. With the advent of VoIP telephony and the migration of fixed networks towards NGN analog telephone adapter modules are being increasingly integrated and reported as the device Integrated Access Devices.

1TR112 - the U-R2 interface of Deutsche Telekom

In Germany, the German Telekom has disclosed the interface between their network equipment ( DSLAM / splitter) and to be purchased separately by the customer since January 2002 DSL Modem; the appropriate policy means 1TR112 (Technical Directive). As a " civilian " name of this standard U-R2 has become the norm. Strictly speaking, this is not entirely correct, since U -R2 is actually just the technical name of the splitter output ( the "R" stands for " remote ", ie the customer side, "U" for " user". Splitter output on DSLAM is, according to U -C2 for " Central Office "). More specifically, ie the 1TR112 specifies the signaling at this interface, to which a U-R2 -compliant DSL modem has to hold. Essentially, one has thereby oriented to the ITU-T standards G.99x.x, but included some telecom - specific changes. 1TR112 now contains provisions for ADSL, ADSL2, ADSL2 , VDSL2 and SDSL. Some of the essential contents of the 1TR112 standard are as follows:

  • Since DSL itself must share the line with POTS or ISDN, the lowest 32 DMT frequencies are not used (up to 138 kHz). If the modem is to be used on an outdoor DSLAM must (also known and standardized in ITU standard G.997.1 Downstream Power Back- Off) are supported DPBO. DPBO reduces the transmit power in the downstream direction in order to protect the highly damped signals supplied from the central office DSL connections before crosstalk.
  • The separation of the high frequency ADSL signals from the telephone frequencies takes the so-called splitter, which consists essentially of a low-pass filter, the signals can pass up to 80 kHz. The range of 80 kHz to 138 kHz serves as a "safety belt ". Therefore Analog telephones and ISDN equipment does not ADSL signals. However, since no high-pass filter is installed in the splitter, are on the DSL modem quite phone signals, which has to ignore the device by suitable measures. The modem may be only when powered off disruptive to current ISDN transmissions ( the norm is speaking here of 320 allowable bit errors without the ISDN connection is canceled or if the ISDN UK0 interface reset ).
  • The modem must be able to transmit certain data rates over specified distances ( for " normal" T-DSL 768 are the 864/160 kbit over 2,800 m standard cable) and with attenuation losses at a certain level (<40 dB at frequencies of up to 1,104 MHz) deal 30 kHz.
  • The modem has the DSLAM on demand vendor ID, firmware version and serial number notify (which may be used to adjust better to the DSLAM, the modem on the other side ). In U-R2 is no longer possible to strike up from the DSLAM from a new firmware on the modem, as was the custom in pre- U-R2 era.
  • Another major section of the standard is devoted to the ATM implementation that must provide the modem, and have to look like the ATM cells.

All sold in Germany since October 2001 DSL modems are U-R2 -capable. Caution is advised when the modem manufacturer Siemens and ECI Inovia, which date from the DSL field trial Telekom. Externally, the last produced at this time U-R2 - capable modems often differ little from the older devices that do not comply with the U-R2 standard.

DSL modems, different DSL standards and rate-adaptive DSL circuit

While narrow-band transmissions are problem- free even over long cables, the high frequency DSL signals are strongly attenuated by the line. For this reason, the range of ADSL is limited around the central office of the telephone company to a few kilometers, depending on the cable cross-section of the connecting cable, used high-speed process (Annex B - ADSL as used in Germany about 5 km line length, Annex A - ADSL on analog lines outside Germany in addition about ½ to 1 km, RE- ADSL2 and G.SHDSL up to 8 km ) and the quality of the DSL modem technique used. The increasingly established in Germany rate adaptive ADSL connection negotiation, the quality of the DSL modem technology especially at medium and longer connecting cables, again more to the fore, whereas in the formerly widespread fixed negotiation via the existing local high SNR safety margins of the competition of DSL devices takes place mainly through the additional integrated functions (see above) and the quality of the actual DSL modem technology is neglected.

Transnational compatibility

Basically, there is no guarantee that the same ADSL modem standard are applicable across countries, as manufacturers make country-specific adjustments to the ADSL line code in the firmware in some cases.

Energy demand

Usual DSL modems have a power requirement of approximately 5 watts. Usually they run continuously, so that each year 5 W × 24 h / d · 365 d = 43.8 kWh of electrical energy are required. At a price of 0.25 € / kWh (2011 /2012) to operating costs of about 11 € per year result.