With an analogue telephone line, the information about the dial-up connection, as well as the voice information, are transferred down telephone line in parallel, so special dialling procedures are used. The older method is so-called pulse dialling, which dates from the time of switching centres with rotary dials. The individually transmitted pulses turn the dial into a certain position that ultimately establishes the connection. The pause between the pulses ensures the correct control of the various rotary dials.
Current analogue telephone lines use the system known as multi-frequency dialling (DTMF). Each key tone produces two defined frequencies (DTMF - Dual Tone Multi Frequency), which initiate the dialling process at the telephone switching station.
Since analogue telephone lines only allow the transmission of analogue signals, to transfer data, it is necessary to convert digital data into analogue information. This task is performed by a modem (modulator-demodulator). At the transmission end it turns the digital signal into an analogue one and the, at the receiving end, back into digital again. Owing to the limited frequency spectrum in the analogue telephone network the maximum data transmission bandwidth is limited and not comparable to other transmission methods, such as DSL.
Even though modern cloud-based telephone systems that use Internet Protocol (IP) and function completely digitally by transmitting the voice information using VoIP, it is still possible to connect conventional analogue devices to them. To enable this, adapters are used, which are connected to the network and translate the analogue signals into VoIP data. Often, these adapters are integrated into access routers, allowing multiple analogue devices to be connected at the same time.