The VSC8112 is a SONET/SDH rate compatible 4-bit STS-3/STM-1 to STS-12/STM-4 Mux/Demux with integrated high speed clock generation. The high speed clock is generated using an on-chip PLL with a 155 MHz reference clock. To facilitate testing the part has a test clock input which can be used in place of the internally generated 622 MHz clock. In addition the deivce provides both facility and equipment lookback modes.