GR-303-CORE 1.3.2 Generic IDLC Interface Requirements Summary. The Generic IDLC Interface uses a separate 64-kb/s data link (DS0-channel) to perform per-call time-slot assignments. igMaxActiveCalls refers to the maximum number of calls available to this Interface Group. This is used in conjunction with igCurrActiveCalls, the current number of calls, to provide a metric for describing utilization. Does not imply that a call has completed, but rather that the DS0 time-slot has been allocated for a call by the GR-303 interface. The DS1 frame is composed of 24 consecutively numbered (1-24) time-slots (channels or DS0s). Maximum number of active calls is the maximum number of DS1s assigned to the IG times the maximum number of channels or DS0s per DS1 less 2 or 4 channels for primary only EOC/TCM and primary/secondary EOC/TCM = ({1..28} * 24 - {2,4}) = {22..670} 28*24 equals maximum number of DS0's on a DS3, with 0 signaling channels. If you have one 28-T1 IG with no redundancy you can have 672 - 2 signaling DS0's , or 670, or 670 DS0's available for active calls. If you have a single-T1 IG, you have only 24 - 2 signaling DS0's, or 22, DS0's available for active calls.