Series of physical protocols (OC1, OC2, OC3 and so on), defined for SONET optical signal transmission. OC signal levels put STS frames onto multmode fiber optic lines at a variety of speeds. The base rate is 51.84 Mbps or OC1. Each signal level thereafter operates at a speed divisible by that number. OC3 runs at 155.52 Mbps. PCR (Peak Cell Rate) In cells/sec, the cell rate that the source may never exceed.