In a single chassis APS both circuits of an APS group are terminated on the same node.
The working and protect lines of a single chassis APS group can be:
Two ports on the same MDA
Two ports on different MDAs but on the same IOM
Two ports on different MDAs on two different IOMs (installed in different slots)
Two ports on two TDM satellites
If the working and protection circuits are on the same MDA, protection is limited to the physical port and the media connecting the two devices. If the working and protection circuits are on different IOMs then protection extends to MDA or IOM failure. Figure: SC-APS group with MDA and IOM protection shows a configuration that provides protection against circuit, port, MDA, or IOM failure on the 7750 SR connected to an Add-Drop-Multiplexer (ADM).
