Alongside the new transmission and switching technologies, the beginning of the 80s saw the arrival of the ANSI SONET system and its international version ITU SDH during the 90s. These transmission technologies offered higher bandwidths at lower prices, while microprocessors allowed an accurate management of telecommunications equipment that is becoming more and more sophisticated all the time. The main advantages of the SDH system are outlined in the following sections.

Advantages of the SDH network   Direct internetworking   High speeds   Direct add/drop   Back-up circuits   Management capacity   Versatility of applications   Centralization   Integration   Compatibility