Asymmetric Digital Subscriber Line
With Asymmetric Digital Subscriber Line (ADSL, English for asymmetric digital subscriber line ), the currently most common connection technology of broadband for consumers is called. It was a variant of DSL technology developed with the proviso to work through your existing telephone connection cable, without affecting the telephony via the fixed line, and at the same time the most asymmetric ( unequal ) data rate needs of private clients comply receive and transmit data rate for higher.
Technology
ADSL can be used to connect analog lines (POTS ) and digital ( ISDN) telephone lines or unbundled data connection ( unbundled DSL).
POTS / ISDN lines, there are frequency ranges that are not used for telephony and therefore lie fallow. These higher frequency ranges are used for ADSL. Basically, the line attenuation increases with increasing distance to the exchange point, what the available data rates and cover with ADSL limited geographically. Outdoor DSLAMs offer here a workaround, see, however, due to the higher cost per port, at least for ADSL little spread.
Operating Principles of ADSL technology is frequency-division multiplex method, Fourier Transform and Discrete Multitone Transmission ( DMT). An ADSL modem contains as essential ingredients a rapid DMT-Modulator/Demodulator and a digital signal processor to calculate the Fourier transforms of the individual frequencies.
In order not to disturb the two types of use of the telephone line, the two frequency ranges in both the participants as well as in the main distribution by a crossover, the so-called splitter, separately. Basically, as evidenced by the ADSL use no voice channel, so that you - unlike a conventional Internet access via telephone modem - can simultaneously use the Internet on an analog connection and make calls by means of traditional landline telephony.
The data transmission when used in Germany ADSL-over- ISDN runs in 4,312.5 kHz wide bands with a symbol rate of 4 per KiBd ( Kibibaud ) in the range 138-275 kHz for the upstream and 275-1104 kHz for the downstream. Because of the poor line quality - after all the phone lines were not designed for the transmission of signals with a bandwidth of about 1 MHz - the line from the terminal to switch " measured " and individual bands, this may be hidden, if the damping is too large or reflections occur.
With ADSL2 standard used in an increasing extent by the ADSL provider an extension of the frequency range used up to 2.2 MHz is associated, which allows for short connection lines significantly higher data rates: In general, up to 25 Mbit / s ( Megabit per second) in the receive direction and up to 3.5 Mbit / s upstream; be in Germany by means of the data rates weak ADSL-over- ISDN circuit, however, only up to 16 Mbit / s offered in reception and up to 1,125 kbit / s ( kilobits per second) upstream.
ADSL on analogue and ISDN connections
Normally ADSL is connected to analog lines and pure data connections to the ADSL-over- POTS standards while used on ISDN connections ADSL-over -ISDN. These standards are hardly different; only the signaling as well as the frequency bands used are different.
The up to now (2008) in German landline installed ADSL equipment technology used, however, as most extraordinary features of ADSL-over- ISDN to all ADSL connections, which has the advantage that only one instead of two technical standards must be supported by the ADSL provider and the subscriber can change without changing the installed ADSL technology between analog connection, ISDN connection and pure data connection. ( At the same time there is the decision to use the technology of ADSL-over- ISDN even with analog telephony ports, bandwidth and range drawbacks. ) Telekom but plans eg until 2018 to complete the so-called IP-based connections ( "Annex J" ) switch. This then means the end for analog telephone lines and digital telephone ports of ISDN (and ADSL-over - ISDN, "Annex B") in the network of Deutsche Telekom.
As adverse consequences but the ADSL availability is thus clearly limited to long connecting cables over ADSL-over- POTS by recessing the damping poorest and most far-reaching ADSL-over- POTS frequency range 26-138 kHz, the maximum possible data rate on all ADSL connections decreases. Newer versions of ADSL, such as the most far-reaching and since the spring of 2006 the country set up by France Telecom Reach Extended ADSL2 (ITU G.992.3 Annex L ), which the suppliable by an exchange surface opposite ADSL-over-POTS/Annex A again to about 40 % larger or ITU G.992.5 ADSL2 Annex M with dramatically to 3.5 Mbit / s data rate also increased transmission are not ADSL-over -ISDN available.
Negotiation
When setting up the ADSL connection to the ADSL modem at the subscriber side and the DSLAM in the central office agree first on the used ADSL standard ( see below) and then negotiate the connection parameters of the ADSL connection: the transmission capacity of the individual DMT - frequency carrier of the copper pair is measured, the downstream and upstream transmission rate is negotiated according to the specifications of the configured for connection to the DSLAM profile and distributed to the individual carrier. After completion of the connection negotiation the DSL connection remains ( ie connected ) to the termination of the DSL link is synchronized.
When negotiating the transfer rate is the rate-adaptive (English rate adaptive ) negotiating distinguished (also rate adaptive mode, RAM) of the fixed or solid negotiation.
In the fixed negotiation of the DSLAM is the ( Upstream/Downstream- ) Transfer rate to be learned. May be due to the current Leitungsausmessung ( for example, due to temporary interference ) the predetermined transmission rate can not be reached, fails the DSL connection.
In the rate-adaptive negotiation on the other hand only the maximum transmission rate (or data rate corridor) is set by the DSLAM; does not succeed, the connection negotiation with the specified maximum transfer rate from the DSLAM, the connection with the highest possible data rate that can allow the current line conditions ( within the data rate corridor ), alternatively synchronized. That the negotiated when establishing a connection transmission rate adapts to line conditions. Thus configured ADSL connections are thus connected ratenadaptiv.
The ADSL2 possible Seamless Rate Adaptation allows additionally to customize the transmission speed when connected to the transmission quality of the cable connection without losing the synchronization ( ie without separating the DSL connection ); This feature is currently (2010) by the German ADSL2 providers, however, largely (except HanseNet, QSC and M-net DSLAMs ) not yet implemented.
Because of the necessary for the safe operation of ADSL connections with fixed rate circuit high SNR margin of safety can be made available than that which would be possible with adaptive negotiation of data rate is usually at these ports only a much lower data rate. Because of this drawback, set both nationally and internationally, most ADSL provider for several years, the rate-adaptive circuit a.
In Germany, the fixed rate circuit is almost exclusively even in technically realized by the German Telekom DSL connections to 6 Mbit / s (T- DSL, T- DSL resale, bitstream access ) use. The autumn 2007 of their intention extending the rate-adaptive ADSL circuit to the entire product portfolio at the end of 2008 has since been repeatedly postponed. The announced in October 2009 gradual introduction in February 2010 was suspended after a few days because of problems in the CRM T- booking system.
While the fixed negotiation the SNR margin of safety is usually so high that just about all on the market ADSL modems and routers to ensure trouble -free operation, has in the rate-adaptive circuit in the technical boundary to the quality of the modem used rather longer connecting cables, a significant influence on the achievable data rate and the line stability of the ADSL connection.
Protocols
To establish a connection to the Internet more protocols are used as the PPP -based: PPPoE ( for example in Germany and Switzerland, and to the based on T- DSL technology Telekom DSL connections (also T- DSL resale ) Multiple PPPoE connections to different Internet access providers exist simultaneously), 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 address).
Interleaving
In order to improve error correction of the ADSL connection by some providers (eg T -DSL Telekom ) interleaving is employed. This means that the packets are not simply sent in their original order, but nested. This leads to that the reception of a packet requires a longer time, thus increasing the latency of the connection, as, for example, after the packet 10 is not the same package 11, but the packets 11 to 18 intertwined. If the line conditions allow it, from these sellers can be switched off partially by means of generally requiring extra option ( FastPath ) interleaving.