Digital telephony

The Public Switched Telephone Network (PSTN) has gradually evolved towards digital telephony which has improved the capacity and quality of the network. End-to-end analog telephone networks were first modified in the early 1960s by upgrading transmission networks with T1 carrier systems. Later methods such as SONET and fiber optic transmission further advanced digital transmission. Although analog carrier systems existed, digital transmission allowed lower cost and more channels multiplexed on a single transmission medium. Today the end instrument remains analog but the analog signals are typically converted to digital signals at the (Serving Area Interface (SAI), central office (CO), or other aggregation point. Digital loop carriers (DLC) place the digital network ever closer to the customer premises, relegating the analog local loop to legacy status.