What is aDSL?
ADSL (Asymmetric Digital Subscriber Line) is technology from the xDSL family serving for ensuring an access to tele-computer lines, so the most frequently – the Internet. Practical observations of the typical Internet users are fundamental in ADSL. For these users the amount of data downloaded to a local computer (e.g. browsing web pages, downloading files, reading one’s e-mails, etc.) is much bigger than the amount of data uploading to the Internet (sending e-mails, chatting on communicators, filling forms in online shops, etc.).
The already existing copper telephone cabling (twisted-pair cable) is enough to use the aforementioned technology. For setting working connection it is necessary to install two ADSL modems, one of each end of the transmission line, which then have to be normally connected to computers or routers. In majority of cases installing one more device, separating a telephone signal from a “computer” one will also be necessary. It has a form of a simple analogue filter which works based on signal frequencies in cabling.
A possibility of a simultaneous telephone conversation and data transmission in both ways is a particularly essential advantage of ADSL. It results directly from frequency bands used by particular services:
A scope of 300 – 3500 Hz: a telephone conversation
A scope of 30 – 125 kHz: a band earmarked for uploaded data (narrow)
A scope of 165 – 1.1 MHz: a band earmarked for downloaded data (wide)
The above-mentioned division is quite symbolic, especially delimitation of digital bands – enlarging the bandwidth for uploaded data can be gained by widening the frequency band at the expense of the band of downloaded data. Obviously, the opposite situation also happens.
At present, there are three variants of ADSL technology available on the market, although a more appropriate name would be two generations, from which the second one in two versions:
ADSL1 – the first version of technology, enabling downloading files with a speed up to 8 Mb/s and sending up to 1.5 Mb/s. The maximum distance for which a standard predicts obtaining the aforementioned bandwidths is 5.4 kilometers.
ADSL2 – standard introduced as development the ADSL1 version. Its main feature is a band of working frequencies, widened to 2.2 MHz, what has let obtain bandwidth of 12 Mb/s one way and 3.5 Mb/s the other way with the maximum length of cabling segment – 3.7 kilometers.
ADSL2+ - is modification of the ADSL2 standard in which new methods of signal modulation were introduced. The algorithm of creating packets (frames) and the algorithm of data converting have been changed. It results in smaller amount of controlling data translating directly into the increase of the ultimate bandwidth. Thanks to these modifications, on lines using the ADSL2+ standard it is possible to obtain bandwidth up to 24 Mb/s one way and 3.5 Mb/s the other with the maximum length of the cabling segment coming to 2.2 kilometers.
Lines using the ADSL technologies are currently the most popular method of gaining access to the Internet in Poland.