Zigbee standard has been designed for low data rate, low cost and limited power applications for short-range wireless communication. The successful implementation of Zigbee based network depends on the suitable selection of physical (PHY) layer and medium access control (MAC) layer parameters. In this work the PHY layer parameters have been optimized for star, tree and mesh topologies. The performance investigations have been carried out for different frequency band and data rate and different bandwidth (BW) in each of standard topologies. Through extensive simulations, QoS parameters like throughput, network load and delay have been evaluated to achieve optimal performance of physical layer.