With Facility Associated Signaling (FAS), an ISDN-PRI T1/E1 interface D-channel can carry signaling information for all the bearer (B) channels on its associated spans.

With Non-Facility Associated Signaling (NFAS), an ISDN-PRI T1/E1 interface D-channel can carry signaling information for as many as 300 bearer (B) channels on its associated spans.