Early CAS systems were developed independently by individual equipment manufacturers, and exist in many varieties. Later CAS systems, notably those developed after the Second World War, show the increasing influence of national and international standards. This section describes three important CAS systems, and their use on frequency-division multiplexed (FDM) analog trunks and time-division multiplexed (TDM) digital trunks (1.4.5 and 1.5.2).