This paper describes the protocol stack for an ATM air interface that is able to efficiently support all ATM service categories. The multiplexing of ATM cells on the radio link is controlled by a service strategy that optimizes the resource Ž . allocation based on the negotiated quality of service. A medium access control MAC protocol realizes the transmission order of ATM cells given by the service strategy. The paper focuses on a strategy for transmission of acknowledgments of Ž . an automatic repeat request ARQ protocol and on a collision resolution algorithm for transmission of capacity requests over the uplink. The performance of the overall protocol stack is evaluated under realistic traffic and channel models by means of an integrated stochastic simulation model. q 1999 Published by Elsevier Science B.V. All rights reserved.