In the basic user/network interface of N-ISDN (ITU-T Recommendation I.430), the D channel is shared by up to 8 terminals for signal and data packets. An analytical model is proposed to reveal the performance characteristics of the access control scheme for the D channel. Numerical and simulation results are shown to demonstrate the performance differentiation of the terminals with different priorities. It is observed that the mean signal delay at a low load become large because of the long service time for packets and that the priority mechanism may not work properly when the loads at the terminals are very asymmetric.