on-FA cility ignaling; an AG ISDN stack mode in which a single D channel on a T1 trunk carries the signaling for all B channels on more than one trunk. This leaves the 24th channels on the trunks free for use as B channels. NFAS is not supported on E1 trunks. See also ACU stack mode, B channel, D channel, LAPD stack mode, Channelized stack mode, T1.