  Network Function Virtualization (NFV) is considered one of the key
technologies for the 5G mobile networks. In NFV, network functions are
implemented in software components denominated Virtual Network Functions (VNFs)
running on commodity hardware. In this paper, we propose an analytical model
based on an open queuing network of G/G/m queues to model VNFs with single- or
multi-tier designs, and chains of VNFs. Our model is flexible and generic
enough to capture the behavior of such systems. We validate our model by
simulation. Specifically, we validate it for an LTE virtualized Mobility
Management Entity with a three-tiered architecture use case. We also compare
our model with the estate of the art, in terms of computational complexity and
estimation error. The results show that our model has a computational
complexity similar to the method for analyzing Jackson's networks.
Additionally, our model exhibits an estimation error, measured as the relative
error for the estimation of the mean response time, approximately equal to 10%,
whereas for the considered baseline systems it ranges roughly from 60% to 90%.
