SONET continued

SONET was developed to aggregate (multiplex) and carry circuit switched traffic such as T-1, E-1, T-3 and E-3 as well as slower rates from multiple sources on fiber-optic networks. SONET transports traffic at high speeds called OC (optical carrier). The international version of SONET is synchronous digital hierarchy (SDH). SDH carries traffic at synchronous transport mode (STM) rates. See the table below for optical carrier and synchronous transport mode speeds. Interfaces in the equipment make SONET and SDH speeds compatible with each other. The same SONET equipment can be used for both OC and SDH speeds.

Europe's time division hierarchy is based on E1 (2-megabit) and E3 (34-megabit) signals. E1 circuits carry 30 channels at 64 kilobits per channel. E3 circuits carry 512 channels at 64 kilobits per channel. Traffic that is carried between cities in Europe or in undersea cables is often referred to as being carried at STM-1 or STM-16 rates.