Digital Subscriber Line Access Multiplexer

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Cable distributor (left), multifunctional housing with outdoor DSLAM (right)

A digital subscriber line access multiplexer ( English for "DSL access multiplexer ", or DSLAM for short , pronounced [di: slæm]) is part of the infrastructure required to operate DSL . DSLAMs are located at a location where subscriber lines come together. Usually it is an exchange , but sometimes also decentralized connection points, e.g. B. in large office or residential complexes.

If the DSLAM is located within the exchange, it is called an "indoor DSLAM", in the other case of an "outdoor DSLAM". The latter are also called Access Remote Units ( ARU ) in Austria . In technical jargon , the exchange is referred to as the Central Office (CO), the DSLAM is sometimes referred to as Central Office Equipment (COE).


The DSLAM terminates with its line card , the subscriber lines, collects (or distributed) at the local level to high-traffic retail and hands it over the so-called concentrator to a regional broadband remote access server on which the IP - Routing and PPPoE terminator configuration responsible is.

The DSLAM is the counterpart to the DSL modem at the subscriber. In the training phase (synchronization phase) with the DSL modem, the DSLAM determines which frequencies can be used for DSL transmission. Since different connections can influence a cable, it may not be possible to use all frequencies. In the training phase, all frequencies are tested (for ADSL according to ITU-T G.992.1 Annex B the frequency spectrum is from 138 kHz to 1100 kHz) and the frequencies at which incorrect packets arrive or which do not arrive at all are "marked" as unusable . This training phase is important to ensure a high quality DSL connection that is free from synchronization losses and interruptions. A profile is also stored in the DSLAM which stores the maximum speeds at which the DSL connection is synchronized in the upstream and downstream. In this profile, that minimum is further deposited, SNR -Margin is required, and whether the data is interleaved or not to transmit ( fastpath ). If the values ​​can not be adhered to due to excessive attenuation on the line, depending on the profile (e.g. fixed-rate), constant interruptions may occur, or lower transmission speeds are negotiated (flexible-rate profile or RAM, i.e. Rate Adaptive Mode).

With ADSL2 + connections, it is technically possible to switch off or re-activate individual frequencies while a connection is running (synchronization). However, this technology is currently not used everywhere because this mode is poorly implemented in many modems and leads to errors.

DSLAM function.svg


The ADSL data traffic is transmitted in both directions in ATM cells. The IP data packets are transported in the payload of the ATM cells. The task of the ADSL DSLAM is to switch the ATM cells coming from the network to the correct subscriber ports and vice versa. In the OSI model , this ATM switching function corresponds to a layer 3 function. The DSLAM is transparent to the Ethernet and thus also to the Internet Protocol (IP) that is based on it with PPPoE. With some ADSL modems you can also access the ATM layer directly. This is even required for PPPoA . Normal ADSL modems, however, have fixed ATM parameters and pass through the Ethernet layer on top. Although this has the disadvantage of a slightly higher overhead, it has the advantage of being easy to use with normal Ethernet network cards.


IP-DSLAM stands for Internet Protocol Digital Subscriber Line Access Multiplexer . IP DSLAMs terminate the IP data traffic directly and route it into an IP network. They can also take on a routing function.


The DSLAM has three essential components: line cards , a network interface and a clock feed .

Line cards

Old VDSL -DSLAM from Siemens ; Technology: VDSL DSLAM SURPASS hiX 5625; 24 connections (max. 48 connections possible with two line cards.) Yellow, the multiple copper wires to the subscriber connections, the power supply unit below. The fiber optic cables are in the back of the housing.
Current VDSL -DSLAM from Deutsche Telekom AG with Huawei VDSL vectoring remote station and KVZ

The DSLAM is equipped with slots for so-called line cards . The ports for the lines that go to the participants are grouped on these line cards. Depending on the construction, a line card has 2, 8, 12, 16, 24, 32, 48, 64, 72 or 96 ports. A port essentially consists of a transceiver unit (called ADSL Transmission Unit Central Office for ADSL ATU-C) and a splitter if the line is used for DSL data traffic and a conventional telephone connection (analog, ISDN) at the same time . The ATU-C is used to receive the upstream signal and send the downstream signal. Depending on the type, the splitter can be accommodated on the line card or mounted externally .

Network interface

The second essential component of the DSLAM is its network interface , with which it is connected to the rear of a DSL-AC . In the past, ATM over fiber optic cables were mostly used for this , for example an STM-1 connection with 155 Mbit / s bandwidth, rarely also directional radio . More modern DSLAMs ("IP-DSLAM") use a Gigabit Ethernet interface (electrical or optical) to connect to the backbone network . A DSLAM master can share the bandwidth of the uplink with optional slaves, which can be connected to the master (cascading of the slaves). With some products, this means that the DSLAM master cannot be fully equipped with line cards, since the corresponding modules are required for the data traffic of the cascading. Another possibility for cascading is daisy chaining .


In addition, some models have a LAN interface (LCT, Local Craft Terminal) with which the DSLAM is connected to an administration network, from which it gets its configuration and can be managed . Alternatively, many DSLAMs can be managed in-band . The management data for ATM uplinks are transmitted within a specific PVC (see virtual connection ) or for Ethernet uplinks in a specific VLAN .

Outdoor DSLAM

Outdoor DSLAM (rear) the size of a garage for 2500 residents. For comparison, cable distributor at the front .

In areas in which the connection line was implemented with fiber optic cables right up to the customer ( OPAL , HYTAS ), the DSL termination must take place where the transition from fiber optic lines to copper cables takes place, e.g. in the building itself or in the cable distributor . This is done with the help of devices called outdoor DSLAM or mini DSLAM . These are considerably smaller than the DSLAMs in the exchanges because they have considerably fewer connections.

The great advantage of this arrangement is that the very short copper lines between DSLAM and subscriber allow very high bandwidths and, for example, VDSL can also be used. The disadvantage is the significantly higher cost per subscriber, which means that DSL coverage has progressed relatively slowly in large parts of the east of Germany - the main area of application for OPAL technology - and in areas far away from the switching centers. As an alternative, the fiber optic cabling was extended into every single building in the affected area within a GPON pilot project.

Outdoor DSLAMs are also used in areas connected by conventional copper connection lines in which adequate DSL coverage is not possible due to the distance to the exchange ( i.e. excessive line attenuation of the DSL signal). ( FTTC )

In Austria, since the introduction of VDSL2 in 2009, A1 Telekom Austria has used outdoor DSLAMs called Access Remote Units ( ARU , upstream unit ). They are usually about 2 m high boxes with a unique number in the form "ARxxxx". There is therefore VDSL @ CO (Central Office) at the location of the exchange (VSt) and VDSL @ ARU at the converted or newly built cable distributor (KVz). The crosstalk of the VDSL signals from the subscriber to the ARU has a disadvantageous effect on the longer classic DSL connections in the exchange, resulting in a competitive disadvantage for the providers who use unbundled lines from A1 Telekom Austria.

Technical regulation

If DSL connections supplied from the exchange are switched in the catchment area of ​​an outdoor DSLAM at the same time, the outdoor DSLAM must limit the transmission power to parts of the frequency range in order to avoid interference with the exchange-supplied connections through crosstalk . The procedure is called Downstream Power Back-Off (DPBO) and is standardized in the ITU standard G.997.1. When installing an outdoor DSLAM, the attenuation from the exchange to the KVz to be supplied is first measured (main cable attenuation). A value is determined from this, which can then be inserted into the formulas for calculation. The network compatibility reports "Test report 5" for ADSL and "Test report 3" for VDSL are decisive here. Corresponding profiles must then be set up on the outdoor DSLAM. The DPBO profile active by the outdoor DSLAM at the port leads to a spectrally adapted power distribution, which, starting from the low frequencies, increases the attenuation up to an upper limit frequency (MUF; maximum usable frequency ) causes. Above this upper limit frequency, a usable DSL signal can no longer be expected from the main distributor . It can be seen that this frequency depends on the length and nature of the main cable (cable from the exchange to the KVz). For this reason, DSL 6000 and slower connections in the form of ADSL2 + are typically also switched on the outdoor DSLAM and can therefore switch to the frequency range between 1.1 and 2.2 MHz only used by ADSL2 if DPBO has to be used. In the same way, a VDSL2 DSLAM can take into account ADSL2 connections supplied by a main distributor by transmitting in the frequency range up to 2.2 MHz with reduced power. DPBO often has the effect that ADSL lines connected to the outdoor DSLAM achieve low bit rates despite the short copper line length.

Significance in the broadband market

DSLAMs are the most common network element in most telecommunications markets for broadband connection of subscribers to the network of Internet providers and are used by both established operators and competitors .

For this purpose, competitors get access to the subscriber lines at the main distributor and rent collocation areas in the local exchanges of the established operators, where they place their own DSLAMs and connect them to their concentrator network. This access of competitors called unbundling (Engl. Unbundled local loop ) or in the particular case and line sharing .

A corresponding method ( sub loop unbundling ) is not yet available for outdoor DSLAMs . In Germany, the Federal Network Agency is currently (mid-2007) creating the conditions for this.

Alternatively, Internet providers can use the so-called bitstream access to reach their customers via the DSLAMs of the established operators, whereby they can usually specify the DSLAM switching profiles, which in Germany, contrary to the relevant position of the European Regulators Group (ERG), is not implemented by the Federal Network Agency .

In Germany, however, the use of in-house technology is made possible by the collocation variants on the cable distributor or on the switching distributor. The alternative provider builds its own system technology (mostly in an outdoor housing) close to the customer, connects it to the telecom switchboard or switchboard via cross-cable, and rents the subscriber connection line to the customer either exclusively or (since mid-2011) in the line -Sharing procedure.

See also

Web links

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

Individual evidence

  1. Martin Stepanek: Glass fiber: TA connects Klagenfurt. In: , November 5, 2010
  2. VwGH Gz. 2010/03/0168 , November 28, 2013