Future generations of mobile networks are expected to serve a multitude of applications with different requirements and traffic characterizations. Through network slices, the applications shall be even served on the same physical infrastructure. However, this requires a sophisticated network management, for an efficient sharing of the common resources and for meeting the demanding requirements of applications with very stringent requirements, e.g., ultra-high reliability, low-latency. In this regard, mathematical models can help in understanding and optimizing the network performance. By introducing queuing models for general systems and scheduling policies for heterogeneous traffic, this paper is an important step for accurate modeling of general systems and towards slice configuration and optimization.