This paper describes design and implementation of L ATM, an ATM (Asynchronous Transfer Mode) based networking server. While ATM emphasizes deterministic highspeed 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 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 kernel environment. Evaluating L 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.