In the original meaning of bandwidth the units of measurement was the hertz (Hz) and multiples thereof, such as kilohertz or megahertz. In terms of data transfer rate, bandwidth is a measure of how much data can be transmitted via a given medium, during a particular unit of time, with the unit being bits per second. Due to the high data transmission rates in modern networks, however, kilobits, megabits and gigabits per second are more commonly used units.
The bandwidth of an analogue telephone is located between the lower limit frequency of 300 Hz and the upper limit frequency of 3,400 hertz (a bandwidth of 3.1 kilohertz), which is high enough quality to transmit a voice that can be understood. However, high-quality HD telephony uses a wider range. In digital telephony, the typical data rate of an uncompressed call is 64 kilobits per second. With Voice-over-IP telephony further data is needed for the communication protocols and the overhead, so around 100 kilobits per second, in both directions of transmission, should be expected for a VoIP call. Thanks to modern compression methods and codecs, the data rate needed can be reduced considerably. This means that modern systems are able to transfer calls with a bandwidth requirement of just eight kilobits per second or less.
If a telephone system is to be operated as a cloud service through an Internet connection, the bandwidth of the connection is the main factor in determining the maximum possible number of parallel calls. It is important that sufficient bandwidth is available for the total number of possible simultaneous voice channels, in both directions of transmission. With ADSL connections, which have low upload data rates compared to the download data rates, the upload bandwidth is the limiting factor of such a system.