This article proposes active-ATM, a flexible, simple and cost-effective ATM-WAN architecture that can handle multiple user-customized ATM-layer protocols, such as ABR and ABT, by using a simple universal ATM transit network. The proposed active-ATM architecture enables the construction of flexible networks that can evolve easily. With active-ATM and the ATM multi-protocol emulation network architecture called ALPEN, it is easy to implement new ATM-layer protocols by using user-created programs called active-program capsules that modify only the edge nodes. Because these user-sent program capsules can be used to quickly customize the edge nodes, there is no waiting for standardization and implementation of new services. The ATM-layer protocols are emulated only at the edge nodes, making the transit network independent of customer ATM-layer protocols. The active-ATM edge node is based on the flexible programmable node architecture called PUN(programmable unified node). The PUN is a platform for user-programmable ATM-layer services; it is achieved by using programmable devices, such as FPGAs and DSPs. An prototype system has demonstrated the flexibility of the resulting ATM network. The active-ATM architecture is an efficient approach to implementing multimedia, multi-protocol ATM services in an ATM WAN.