In 1988, the United States launched the first of a digital transmission standard, the entire network clock synchronization levels are from a very precise master clock (using expensive cesium atomic clock, accuracy better than plus or minus 10 -11 th).

SONET fiber optic transmission system defines a line rate hierarchical structure of synchronous transmission, the transmission rate to 51.84Mb / s based on approximately corresponding to the transmission rate T3 / E3, and the rate of the electrical signal called Level 1 synchronous transport signal, namely STS -1; optical signal level has become the first optical carrier (Optical Carrier, OC), i.e., OC-1. It has been defined from the OC-1 -51.84Mb / s up to OC-3072 - about 160 Gbit / s standard. Usually expressed as OC-n.

Sometimes we will see in the literature "POS port supports transfer rates STM-1 / OC-3 (155.52Mbit / s) ......" words. For STM (synchronous transport module, used in SDH: Synchronous Digital Hierarchy, synchronous digital hierarchy), its different levels of synchronous transport modules, STM-1 to level synchronization module, which corresponds to the basic rate of STM 155.52Mb / s. The OC-3 optical carrier level 3 is the rate of SONET, 51.83 * 3 = 155.49Mb / s, so that the STM-1 and SONET OC-3 rate equivalent, compared appeared earlier.

It is standard to connect fiber optic transmission system, is American National Standards Institute in the mid-1980s developed. It is a global physical network, much like the LAN Ethernet twisted-pair cable. SONET can use more than 1Gbps speed transmission data, and can transmit data, voice and images.