ISDN stands for Integrated Services Digital Network and is a telephone standard based on a fiber optic cable. This made it possible for the first time in telecommunications to operate a line with two channels simultaneously. In the mid-1990s, Germany was equipped with an ISDN network almost nationwide. However, to use an ISDN line, an ISDN-capable terminal or an ISDN telephone system is required. The systems allowed the use of up to 10 telephone numbers per line. This revolutionized communication, especially in small and medium-sized companies. Digital transmission improved voice transmission and fax transmission was faster.