A good telecommunication networks can be represented as queueing networks with state-dependent arrival rates and service rates. We provide necessary and sufficient conditions for such queueing networks to be sense that the steady-state distribution depends on the service time distribution at each queue through the mean only. The property is key to the development of simple engineering rules that do not require the knowledge of fine traffic statistics. A computer communication network with window flow control is modeled by queueing network. The severe computational limitation of previous solution algorithms is overcome with a heuristic derived from the recently found mean value analysis arrival time and departing time of first in first customer. A large numerical example is given. also The type of queueing systems of interest to us in this thesis are the types encountered in a communication system. In such a system, the item could be a packet arriving at a router and switchand needs its headers read so it can be forwarded to the appropriate next router or the final destination. In this introductory chapter we will use all the similies of queueing system characteristcs interchangeably so as to emphasize that we are dealing with a phenomenon that occurs in diverse walks of life.The type of queueing systems of interest to us in this thesis are the types encountered in a communication system.