csOc3PosRcvBytes is a read-only 64-bit counter of the total number of bytes written into this OC3 port's receive FIFO buffer while running Packet over SONET. Tracking this counter's rate gives an admin the actual inbound throughput on the link, the baseline figure used to detect both unexpected traffic surges and unexplained drops in received data. For example, if csOc3PosRcvBytes suddenly stops incrementing while the optical signal still reports as up, an admin suspects a framing or encapsulation mismatch rather than a physical fiber cut, since bytes would still be arriving at the receiver but not registering.