SONET 4h byte receiver and filter

摘要

A filtering algorithm for the SONET H4 byte is implemented with circuitry including an internal H4 counter which is incremented each frame and is locked to a received H4 byte once every 24 frames if a proper bit sequence is detected in a designated frame. If the designated bit sequence is not detected, the counter is not reset and continues to be incremented. If parity errors are detected, the counter is not reset and is allowed to free run to simulate the appropriate H4 byte input.