Integration of different networks in real-time scenario can provide more benefits related to speed, bandwidth, and connectivity. UMTS and WLAN are two different types of networks with varying specifications. Integrating both the networks will be helpful in making use of all the features simultaneously WLAN networks provide higher data rate but cover smaller areas, while UMTS offers better connectivity with low-speed data rates and also covers larger areas at a time. It means integration of these services can provide higher speed wireless data services with ubiquitous connectivity. This paper focuses on implementing integrated UMTS–WLAN architecture using IP multimedia subsystem (IMS) as an intermediate network. IMS enables the user to switch from one network to another without interrupting a call. The main purpose of using IMS is to minimize the delay incurred during the hand off process and also the authentication, authorization, and accounting (AAA) process servers are inbuilt in its architecture. In this paper, integrating architecture has been implemented by two methods named as loose coupling and tight coupling. To compare the effect of using IMS, different scenarios with and without IMS have been simulated in OPNET Modeler 14. Comparison has been made on the basis of response time and download time for web browsing and FTP applications.