For systems that support more than one DS1, protection switching for the Embedded Operations Channel (EOC) and a Time-slot Management Channel (TMC) is mandatory. This object refers to the ifIndex for the DS1 line supporting the active EOC and TMC path protection switching. The value for this object is equal to the value of ifIndex from the Interfaces table of MIB II (RFC 1213). Per-call time-slot assignment is accomplished over a 64 kb/s TMC that carries mesages between the LDS and RDT. These messages are used to make and break time-slot assignments between line units and DS0s on a per-call basis. A separate DS0 data link, called the Embedded Operations Channel (EOC), is used to transmit operations messages between RDT and LDS/OSs. The initial protection channel for the EOC is channel number 12 of a DS1 and channel number 24 on the same DS1 is reserved for TMC protection. Note that GR-303-CORE states that protection switching for DS1 lines is optional. GR-303-CORE R5-263 [1835] An RDT and an IDT shall support EOC and TMC/CSC path protection switching on systems supporting more than one DS1. R5-264 [505] An RDT and an IDT shall receive from a remote managing system and process retrieval of time slots for the second EOC (i.e., the initial standby) and the second TMC or CSC. R5-266 [507] An RDT and an IDT shall receive from a remote managing system and process requests to assign and to retrieve the DS1 line for the standby EOC and the standby TMC/CSC. The value will be zero if the Interface Group is not installed on a specific card.