The IP Multimedia Subsystem (IMS) is envisioned as the solution for the next generation multimedia rich communication. Based on an open IP infrastructure, it enables convergence of data, speech, video and mobile network technology. The Session Initiation Protocol (SIP) is the signaling protocol chosen by the 3GPP consortium for IMS. We propose to analyze the IMS network based on the SIP signaling delay and predict performance trends of the network, that allow us to choose parameter values optimally. The paper focuses on the formulation of queuing models for the IMS network and characterization of the SIP server workload, and on a methodology for the design of such networks for optimal performance. Our analysis is based on a careful study of real-life SIP network traffic.