DSL modem
A DSL modem or N etwork t erminationspunkt B riding b and a onnection (English N etwork T ermination B road B and A ccess), short NTBBA is a device for transmitting data over a subscriber line via Digital Subscriber Line (DSL). It forms the network termination (NT) for the DSL part at the subscriber and represents the counterpart to the digital subscriber line access multiplexer (DSLAM) at the office.
Connection to the public telecommunications network
Via the normal subscriber connection line , the subscriber initially received a mixed signal from telephony and DSL, which is split into a pure telephony and a pure DSL frequency range with the help of a DSL splitter (broadband connection unit, BBAE) and output on two separate lines.
The DSL modem is connected to the line for DSL . A two-wire cable is sufficient to connect the splitter and modem, either two wires from a normal telephone installation cable or an RJ-45 cable with pins 4 and 5 in the middle.
In the case of SDSL and unbundled DSL / IP connection , there is no splitter and the modem is connected directly to the connection line, outside of Germany also with Annex A ADSL modems with integrated DC blocking.
Modulation method
The ADSL standards use for communication over as copper wire pair running connecting line between the DSLAM in the local exchange and the DSL modem a modulation method having different carrier frequencies ( Discrete Multitone ); with conventional ADSL z. B. 256 carrier frequencies at a distance of 4.3125 kHz each. With SDSL , TC - PAM is used instead.
Types and forms
The classic external DSL modem is either connected directly to a PC (for example via USB ) or to a network (for example by means of a router ). According to the OSI model , the modem is an Ethernet bridge and is located on layer 2 . However, the Ethernet traffic is first placed transparently on an ATM layer for the local port and converted back to Ethernet in the DSL-AC . For the PPPoE client, it looks like the DSL-AC is in its own network. He can be reached under his own MAC address as well as under the broadcast address. The latter is necessary to find it at all. The data traffic is also transparent for the Internet Protocol (IP) based on the Ethernet . Modems that are limited to pure bridge functionality and function on the principle of the black box are becoming increasingly rare today. In addition to the design as an external device offers high speed modems as PCI - plug-in card .
DSL modems often have more or less well-documented access to the configuration (for example via Telnet or web interface), via which the user can query the measured line values ( line attenuation , signal-to-noise ratio ) and make settings down to the finer points of DSL signaling can.
Combinations of (A) DSL modems and ( WLAN ) routers - so-called (A) DSL routers are now common . Some devices sold as pure DSL modems are in reality also such DSL routers, but are limited by the marketer - mostly identical to the DSL connection provider - to the modem function in order to keep the support effort within limits. These devices sometimes have an amazing hack value . With the emergence of VoIP telephony and the migration of fixed networks to NGN , analog telephone adapter modules are increasingly being integrated and the devices identified as integrated access devices .
1TR112 - the U-R2 interface from Deutsche Telekom
In Germany, Deutsche Telekom has disclosed the interface between its network technology (DSLAM) and the DSL modem, which customers have been purchasing separately since January 2002; the appropriate policy is 1TR112 ( T echnical R line of sight). U-R2 has become established as the “civil” name of this standard . Strictly speaking, this is not entirely correct, as U-R2 is actually just the technical name of the splitter output (the “R” stands for “Remote”, i.e. the customer side, “U” for “User”. The splitter output on the DSLAM means U-C2 for “Central Office”). More precisely, the 1TR112 specifies the signaling at this interface, which a U-R2-compliant DSL modem has to adhere to. Essentially, the ITU-T standards G.99x.x were used as a basis, but some Telekom-specific changes were included. 1TR112 now contains regulations for ADSL, ADSL2, ADSL2 +, VDSL2 and SDSL. Some of the essential contents of the 1TR112 standard are as follows:
- If the modem is to be used on an outdoor DSLAM, DPBO (called Downstream Power Back-Off and standardized in the ITU standard G.997.1) must be supported. DPBO reduces the transmission power in the downstream direction in order to protect the heavily attenuated signals from DSL connections supplied by the exchange from crosstalk.
- The modem must be able to transmit certain data rates over defined distances (for "normal" T-DSL 768 this is 864/160 kbit over 2,800 m standardized cable) and with attenuation losses of a certain amount (<40 dB at frequencies of 30 kHz to 1.104 MHz).
- The modem must provide the DSLAM with manufacturer ID, firmware version and serial number on request (this can be used to better adjust the DSLAM to the modem on the other side). With U-R2 there is no longer any possibility of uploading new firmware to the modem from the DSLAM, as was common in the pre-U-R2 era.
- Another large section of the standard is devoted to the ATM implementation that the modem must provide and what the ATM cells should look like.
All DSL modems sold in Germany since October 2001 are U-R2 capable. Caution is advised with modems made by Siemens and ECI-Inovia, which are still from Telekom's DSL field test. Externally, the most recently produced U-R2-capable modems often hardly differ from the older devices that do not adhere to the U-R2 standard.
DSL modems, different DSL standards and rate-adaptive DSL switching
While narrow-band transmissions are problem-free even over long lines, the high-frequency DSL signals are strongly attenuated by the line. For this reason, the range of DSL around the exchange of the telephone company is limited to a few kilometers, depending on the cross-section of the connection line, the DSL method used ( Annex B- ADSL as used in Germany approx. 5 km line length, Annex A -ADSL on analog connections outside of Germany additionally about ½-1 km, RE-ADSL2 and G.SHDSL up to 8 km) and the quality of the DSL modem technology used. The rate-adaptive ADSL connection negotiation, which is also increasingly established in Germany, puts the quality of the DSL modem technology back in the foreground, especially for medium-sized and longer connection lines, whereas the previously widespread fixed negotiation with the high signal-to-noise margin safety margins that are common there is competition from DSL end devices mainly take place via the additionally integrated functions (see above) and the quality of the actual DSL modem technology is neglected.
Cross-country compatibility
In principle, there is no guarantee that modems of the same ADSL standard can be used across countries, as the manufacturers e.g. Make country-specific adjustments to the ADSL line code in the firmware.
Energy demand
Usual DSL modems have a power requirement of around 5 watts . Most of them run in continuous operation, so that 5 W · 24 h / d · 365 d = 43.8 kWh of electrical energy are required annually. At a price of 0.30 euros / kWh (as of 2014), the operating costs are around 12 euros per year.
Web links
- Current and older DSL modems sent by Deutsche Telekom
- Deutsche Telekom AG - Public interface description page
- DSL routers / modems with downstream power backoff support
Individual evidence
- ↑ wiki.mhilfe.de: DSL-Modem-Wiki with DSL-Modem-Interna ( Memento of the original from February 27, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ 1TR112 Technical Specification of the U-Interfaces of xDSL Systems in the network of Deutsche Telekom, in English, ZIP archive, Version 12.2, as of February 2014 ( page no longer available , search in web archives ) Info: The link was automatically saved as a marked defective. Please check the link according to the instructions and then remove this notice.
- ↑ Heise.de: DSL modems and ADSL negotiation c't 8/2007: Effects of the Telekom switching rules for T-DSL and T-DSL resale for longer connection lines ( Memento from April 6, 2007 in the Internet Archive ) in more detail in der c't 8/2007 p. 86 ff and p. 92 ff
- ↑ DrayTek Australia, Frequently Asked Questions ( Memento of the original from February 4, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
