The determination of the performance characteristics, dependability, and availability is one of main issues when designing, analyzing, and tuning systems of any kind. There are a variety of modeling techniques for evaluating existing or even not yet existing systems using analytical and simulative methods. In addition, numerous software tools have been developed to support different methods,each with its own language for describing the model to be evaluated. We consider in this book a Non-homogeneous multi-server system with Markovian arrival and service process. Firstly,we deal with the case with only one queue and different servers and a job assigns to one server. Secondly, we developed the model to have more than one queue,in which the jobs are assigned to different queues with a fixed size. We assume that our system has different servers with different service times (i.e.Non-Homogeneous Multi-server system) and when the job arrives to the system, it assigns to the server using the following two strategies:either Randomly or Fastest free server (FFS). We consider a general queuing system (M/M/n) with K is a finite number of jobs. The implementation is done using MOSEL language.