Provision of quality-of-service (QoS) guarantees is an important and challenging issue in the design of asynchronous transfer mode (ATM) networks. call admission control (CAC) is an integral part of the challenge and is closely related to other aspects of network designs such as traffic characterization and QoS specification. Since the usage parameter control (UPC) parameters are the only standardized traffic characterization, developing efficient CAC schemes based on UPC parameters is significant for the implementation of CAC on ATM switches. We develop a CAC algorithm called TAP (derived from tagged probability) as well as two other CAC algorithms using the UPC parameters. These CAC algorithms are based on our observation that the loss-probability-to- overflow-probability ratio tends to decrease as the number of sources increases. By introducing the loss-probability-to-overflow-probability ratio K, we find that this ratio sheds light on increasing resource utilization while still guaranteeing QoS. Analysis, simulation, and numerical results have shown that the TAP algorithm is simple and efficient.