This document describes the protection switching protocols and algorithms for optical interfaces referred to as SONET. The SONET optical interface standard is specified in American National Standard for Telecommunications - Digital hierarchy ?C Synchronous Optical Network (SONET) - Basic description including multiplex structures@ rates@ and formats@ T1.105-1995. In its initial version@ this standard addresses protection switching for linear 1 + 1 and 1:n topologies@ and bidirectional line-switched ring network topologies. Nested protection is for further study. SONET automatic protection switching schemes utilize two bytes@ K1 and K2@ to implement a bit-oriented protocol for time critical switching operation. Other operations channels@ including the Section and Line Data Communications Channels@ may also provide non-time critical (e.g.@ > 50 ms) protection switching functions. Independent protection of overhead channels@ such as the DCC@ are beyond the scope of this standard. Any special considerations pertaining to the use of wave-division multiplexing or directional couplers for transmitting multiple signals over the same fiber are beyond the current scope of this standard.