## 2 Related Work  
Our review of the work related to the analysis and design of IMS/NGN networks indicates that standards organizations (e.g., ITU-T, ETSI, 3 GPP, etc.) do not provide any guidelines for traffic modeling and traffic engineering which can be used to achieve appropriate values of CPP parameters. Their scope focuses on functionality [1, 2], architecture [2, 3] and communication protocols [3]. Although the keywords \"IMS\" and \"NGN\" lead to many scientific papers, only a small number of them concern the subject of traffic modeling and engineering in an IMS/NGN network. Examples of such papers in which an analytical approach to network analysis and design is used are described in the next part of this section. In Ref. [12], a trade-off between delay and throughput in IMS session setup is discussed. A scheme for decreasing the excessive signaling in IMS session setup is proposed. To assess this proposition, an analytical traffic model is used. The model focuses only on IMS elements and does not consider reservation of the transport resources necessary for the services. In particular, there is no Resource and Admission Control Function (RACF) unit proposed by the ITU-T for controlling transport resources. An analytical traffic model allowing estimation of packet delay variation is proposed in Ref. [13]. Despite the authors' declarations, the proposed model is general and not closely related to the IMS and NGN architecture. The paper [14] proposes an analytical model for IMS/NGN with G/G/1/N queuing models based on diffusion approximation. The traffic model allows evaluation of average time for establishing multimedia sessions. The authors do not, however, specify the detailed network architecture, number of domains and considered service scenarios. Moreover, no information about verification of the analytical model is provided. A similar study with the same types of queuing models was performed in Ref. [15]. The described analytical traffic model considers only one IMS domain and does not include resource control mechanisms defined by the ITU-T (RACF unit). In Ref. [16], an analytical traffic model is presented that allows assessment of bandwidth requirements for IMS architecture. The model contains only the basic elements and a single domain of IMS. The application of MPLS technology in an NGN is discussed in Ref. [17]. However, this paper is an overview and does not contain any propositions for IMS/NGN performance analysis and resource design. Papers [18, 19] concern the subject of performability of IMS in virtualized containerized environments. They assess availability of a single IMS domain with respect to an extensive set of input parameters. Moreover, an automated procedure aimed at supporting the performability management of IMS deployments that must satisfy the optimal trade-offamong high availability requirements, capacity load and deployment costs is proposed in Ref. [19]. Paper [20] proposes an analytical model based on an open queuing network of G/G/m queues to estimate mean response time of a chain of Virtualized Network Functions (VNFs). The presented analytical model is verified using simulations. The proposition is very general and can be applied to model various systems, including IMS/NGN networks. The described papers demonstrate that the available analytical approaches to IMS/NGN network design and analysis do not cover these aspects completely. They are either very general (not related closely to the IMS/NGN architecture) or consider only the elements of the service stratum (in most cases, it is a single domain of pure IMS without taking into account elements of the transport stratum, including RACF) and only selected network functionalities (services and elements). Moreover, in the majority of cases, there is no information about verification of the abovementioned analytical models using simulations or real networks.