The S3052 IC implements all of the necessary performance monitoring functions on the SDH/SONET transport overhead bytes at OC-3, OC-12 and OC-48 data rates. It also has a mode of operation permitting it to monitor a Gigabit Ethernet data stream for loss of synchronization, 8B/10B code violations, and disparity errors. Monitor and insertion applications involve data being received from the fiber and passed through a multi-rate clock/data-recovery device and a de-multiplexer device to the S3052. The S3052 optionally carries out performance monitoring for error checking, overhead data extraction, and insertion of the transport overhead. Detected errors and accumulated error counts can be accessed by the user either through a processor interface, or in a number of cases by means of signal pins on the I/O. The data stream is then transmitted out onto the fiber by means of a high-speed multiplexer and an optics device.