In this research, we derive the system parameters or performance measures, specifically throughput, of open queueing network by using a corresponding deterministic network. In an open unrestricted queueing networks, a mathematical model for optimal routing with maximum throughput is considered. In an open restricted queueing network, we propose a new decomposition method yielding the upper bound on throughput. This method is then applied to determine optimal buffers or service rates which maximize the system throughput.