There are various standardised ADSL systems. The ADSL2 + standard often used in Europe uses a frequency range of up to 2.2 megahertz and, thanks to modern frequency multiplexing, achieves download data rates up to 25 megabits per second and upload rates of up to 3.5 megabits per second. The maximum bandwidth is limited by many providers at 16 megabits per second downstream and about one megabit per second upstream. In addition, many so-called rate-adaptive methods are used. The DSLAM and modem dynamically control the maximum transmission rate when making the DSL connection.
ADSL is transmitted on the same copper twin wires as analogue or ISDN telephony. To separate the frequency ranges for telephony and ADSL a crossover, so-called splitters, are used. With the introduction of Voice over IP telephony more and more DSL connections are installed that do not require a splitter. These are often referred to as Annex-J connectors and they transfer calls directly into the IP data stream. The use of cloud PBX requires no separate frequency range for analogue or ISDN telephony. The available upload and download bandwidth is important. They limit, among other factors, the maximum number of parallel calls.