Now superseded by the high-speed fiber, and still a broadband connection ubiquitous among internet users. Here's how it works and Find out how to test the performance of your data line.

In the last article, we talk about WLAN Repeater and how it works. We, therefore, continue our journey within the great Network by examining one of the most widespread and known types of connection: ADSL.

Broadband on a twisted pair

For years the term ADSL has been synonymous with the Internet. We all know very well that it is a technology we use to connect to the big network both from home and from the office, but few know what it consists of. Let's start from the beginning, the ADSL, acronym of Asymmetric Digital Subscriber Line, is a data transmission technology that allows access to broadband Internet; indeed it can be considered the forefather of broadband connections. It uses the regular telephone twisted pair or the fixed line cable that arrives in most of our homes. The advantage, therefore, compared to other technologies, and that to activate it is not necessary to have a dedicated line brought home or install an additional network system. It is sufficient only to have a special ADSL modem / router, thus facing a fairly contained expense (often the router is supplied on loan or used by the telephone company). In our country it became widespread, for domestic users, in the early 2000s, supplanting both the classic dial-up connections which, through traditional analog modems, allowed maximum connection speeds of 56 KB/s (in download) and 48 KB/s (in upload), both ISDN lines instead of up to 128 KB/s. For some years ADSL has in turn been replaced in the home by optical fiber, which guarantees superior performance. However, the primary type of broadband connectivity offered by providers in areas that are not yet reached by the fiber remains.

Broadband on a twisted pair

Initially, the telephone twisted pair, which makes up a large part of the planet's telecommunications network, was created for the analog transmission of human voice using frequencies ranging from 300 Hz to 3.400Hz. In reality on the twisted pair, it is possible to use a range of frequencies much more significant than that necessary to convey the voice, so it is possible to receive data. Therefore, by transmitting on the twisted pair frequencies that are not used by the human voice, for example, ultrasounds, in an interval greater than 25.8 KHz, other digital information can be carried. The good range between 4 KHz and that of 25.8 KHz is used to ensure that the two frequency bands (voice and data) do not interfere with each other. The frequencies available for the data are subdivided into sub-channels of about 4 KHz and then grouped into bands of varying sizes depending on the DSL technology adopted, so there are an upload and a download band. It so happens that voice and data are conveyed through the twisted pair and travel together from our home to the telephone exchange (and vice versa) that serves our area. Here a device called splitter separates the frequencies used by the telephone line from those of the data line which then sent to an electronic device called DSLAM (Digital Subscriber Line Access Multiplexer), which conveys the data to the network of our telephone operator.

An asymmetrical connection

ADSL is by definition an asymmetric data transmission technology because different upload and download speeds characterize it. In fact, in residential connections, the bandwidth dedicated to the upload flow is less than the download bandwidth, because usually, web browsing activities require more resources (for example to download large files). The first ADSL contracts provided for a maximum speed of 640 KB / s, today insufficient to navigate easily in Web 2.0. Only in a second time, it reached a maximum download of 12 Mb / s, and ADSL2 +, which allowed to reach a maximum theoretical download speed of 24 MB / s and 3.5 Mb / s in an upload. An evolution of the ADSL / 2 + and the VDSL2 that allows connections with an excellent transmission speed of 300 MB / s on a telephone pair.

What is the ADSL filter for?

To eliminate the possibility of interference and overlapping of signals, it is necessary that in your home (or office) each telephone is connected to the telephone socket through a special ADSL filter. It can avoid background rustling and possible loss of line whenever you answer the phone. Furthermore, the filter, which must also be applied to the modem/router, allows you to limit the disturbances that may not allow perfect alignment of the device when it is connected to the control panel to negotiate the upload or download speed. The ADSL filters essentially consist of a low pass filter that allows the phones to continue to operate without interference even in the presence of an ADSL connection and are of two types, with a three-pin plug or RJ-11, depending on the wall socket. Installation is straightforward: plug it into the wall socket and connect to it the telephone cable and the ADSL modem cable.

The performance of the ADSL line

Nowadays most telephone operators offer an ADSL connection that can reach the maximum download speed of 20 MB / s. The performance of the ADSL line depends above all on the distance from the telephone exchange due to an effect known as "signal attenuation." Other factors that can adversely affect the performance of our ADSL connection are the quality of the cables (in particular the purity and their state of preservation and oxidation, let us think, for example, of the systems present in period or country houses) and any electromagnetic interference due, for example, to high voltage power lines nearby. On the Internet, numerous tools allow you to measure the real speed of your ADSL connection regardless of the nominal one indicated on the contract signed with the provider. The main ones are SpeedTest by Ookla and SpeedOf.me.
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