The conventional Unidirectional Path Switched Ring（UPSR） is software-based

but for high capacity devices it cannot satisfy the switching-time requirement by ITU-T

especially for the low order path protection of STS-1 or tributary-based traffic.Message Assisted Protection Switching（MAPS） encodes the path alarm and inserts the code into its idle overhead bytes for automatic hardware protection switching

which can not only satisfy the system switching-time requirement of 50 ms

but also take much load off the microprocessors.