Gigabit Ethernet (GbE) backbone network with wireless extension is gaining much popularity in campus, small businesses, and corporate network environments worldwide. A good understanding on the impact of wireless extension to a wired backbone network is required for proper design and deployment of such systems. In this paper we investigate the effect of wireless extension (i.e. increasing wireless nodes) on the performance of a GbE backbone network by extensive simulation. We develop a simulation model (using OPNET simulation tool) to study the system performance with and without wireless extension under FTP, VoIP and Video Conferencing traffics for various network scenarios. Results show that FTP download response time and Video packet delays increased about 61% and 94%, respectively as a result of a wireless extension to the GbE backbone network. The findings reported in this paper provide some insights into the design and deployment of GbE-Wireless networks that may help network planners and engineers to contribute further towards developing next generation wireless networks.