The Next Generation Network (NGN) architecture, including elements of the IP Multimedia Subsystem (IMS) concept, is a proposition of a telecommunication network dedicated to the needs of current and future information society. The main goal of NGN is to provide Quality of Service (QoS), for which proper network design is necessary with respect to among others standardized call processing performance parameters, including expected values of Call Set-up Delay (E(CSD)) and Call Disengagement Delay (E(CDD)). These aspects are the subject of the paper, in which analytical and simulation traffic models of IMS/NGN are described, allowing assessment of E(CSD) and E(CDD). As the simulation model reflects the phenomena taking place in real IMS/NGN network, it is used as a reference for evaluation of quality of the analytical results, obtained using different queuing system models. Consequently, the queuing system models most appropriate for engineering applications are indicated. Moreover, application of the proposed analytical traffic model for design of IMS/NGN service stratum resources is presented and discussed.