The paper describes the design and implementation of L/sup 4/ATM, an ATM (asynchronous transfer mode) based networking server. While ATM emphasizes deterministic high speed communication, applications can not yet fully utilize its potential. We demonstrate an architecture-and a corresponding implementation-to resolve this dilemma by developing implementable resource quantification techniques and QoS (Quality Of Service) management algorithms for host resources. L/sup 4/ATM has been built in the context of DROPS (Dresden Real-Time Operating System). DROPS supports coexisting real time and time sharing applications in a /spl mu/kernel environment. Evaluating L/sup 4/ATM's implementation in a real world environment, we show that: (i) performance guarantees are maintained under heavy time sharing load, and (ii) the implementation outperforms a standard OS significantly.