Customers ordering a Primary Rate ISDN service will be charged for each signaling channel.
Therefore an NFAS configuration can be cheaper than Facility Associated Signaling, due to historical reasons.
North American switches such as the Lucent 5ESS, and the Nortel DMS-100 did not handle common channel signaling such as ISDN on the same line card that terminated the T1 circuit.
So, the telephone company needs to buy and maintain a separate signaling card for every D channel.
The line cards in switches designed for the E1 system already include processing for the signaling timeslot.