This paper investigates the potential impacts caused by Call Admission Control (CAC) mechanism of the air interface on simulations carried out for the transport network domain of UMTS Radio Access Network. Independent from the TNL CAC mechanism of the transport network which is in charge of Iub link bandwidth, the Radio CAC function deals with the highly non-linear resources needed for an attempted connection depending on its specific propagation and interference situation. The consideration of the air interface acting as bottleneck results into a less challenging amount of traffic for the transport domain. In this work, a simulation model with Radio CAC mechanism is implemented to study this correlation, and evaluate the UTRAN transport network performance given a limited radio capacity. The analysis on the impact of the Radio CAC is derived from qualitative simulations.