What is DSL
The notion DSL (Digital Subscriber Line) hides in itself a family of technologies enabling transmission of digital data of a maximum bandwidth up to 40 Mb/s, with the use of classic analogue telephone lines. From the practical side this procedure looks as follows:
· The analogue telephone line serves as a transmission medium between two distant from each other computers or devices.
· The DSL devices (colloquially named modems) are connected to both of the line ends, whereas computers are connected to modems, with the help of a typical polycore network cable (twisted pair cable).
· The DSL modem receives information from a computer in a digital form, then converts it into an analogue form and sends via a telephone line to the second of modems.
· The receiver’s modem performs a conversion again, this time into information in a digital form, and transfers it to the computer.
· For sending information the DSL modems use frequency bands above 5 KHz which are not used during ordinary telephone conversations.
· There has to be a device called splitter between a modem and a telephone line. It is simply an analogue filter – its task is to divide a signal of frequency below 5 KHz, and direct it to a telephone and to a higher-frequency signal which should get to the DSL modem.
There is currently a variety of technologies of the DSL family on the market (however, xDSL is a more appropriate and more common name). The most popular are the following:
· ADSL (Asymmetric Digital Subscriber Line) – asymmetric line. It is used for creating client-side connections with the Internet. It is characterized by bigger maximum bandwidth into a client’s side than into a server (the Internet). At the bottom of this solution there lies a fact that an average user of the Internet downloads considerably more to their personal computer than uploads to the Web. Therefore, the technology of asymmetric line will allow increasing comfort of working with simultaneous reducing or preserving costs of its installation and maintenance. The most common ADSL bandwidths are from 8 to 24 Mb/s to a user, and from 128 kb/s to 1 Mb/s from a user.
· SDSL (Symmetric Digital Subscriber Line) – symmetric line. It is relatively seldom used during creating an access network to the Internet. It is characterized by equal bandwidth to both sides, most frequently amounting to 1.5 or 2 Mb/s.
· HDSL (High data rate Digital Subscriber Line) – one of the newest technology from the xDSL family, using copper telephone wires as a transmission medium, and simultaneously using much wider range of frequencies – from 5 to 300 KHz. Thanks to this solution it is possible to obtain bandwidth up to 784 kb/s on one pair of copper wires, and the maximum line bandwidth is directly dependent only on a number of wire pairs. The disadvantage of HDSL technology is a notable deterioration of quality and line bandwidth with the increase of cabling length, the maximum range of one segment can amount to 4000 meters.
· VDSL (Very High Speed Digital Subscriber Line) – technology enabling to obtain bandwidth up to 52 Mb/s one-way or 26 Mb/s to both, with using only one pair of copper wires. A fundamental disadvantage of VDSL is the maximum range of a single cabling segment which cannot exceed 300 meters. Thus, the VDSL technology is used most frequently for building the local infrastructure of an access to the network.
· Apart from the aforementioned, there is also a variety of other technologies from the xDSL family, e.g. IDSL (a digital subscriber’s line, similar to ISDN), skyDSL (a technology using satellite lines), etc. However, they are used on a much smaller scale so they are definitely less important.