## 3 Traffic Model  
Figure 1 presents the structure of the considered multidomain IMS/NGN network, which is divided into two domains belonging to two operators (the terms \"operator\" and \"domain\" will be used interchangeably with similar meaning). To distinguish between elements of IMS/NGN service and the transport stratum administered by different operators, we designate them with the numbers 1 and 2, respectively. The service stratum of each domain controls service requests sent by user terminals (User Equipment, UE) and includes elements derived from the IMS concept [3; 4; 5]:  
* Call Session Control Function (CSCF) servers:
* Proxy-CSCF, P-CSCF--the first contact point for UE;
* Serving-CSCF, S-CSCF--the main server handling all calls in its domain;
* Interrogating-CSCF, I-CSCF--the server handling messages coming from other domains. * Service User Profile Functional Entity/Service Authentication and Authorization Functional Entity, SUP-FE/SAA-FE--the element storing user profiles and performing AAA functions. In each domain, the transport stratum includes one access network with several access areas and one MPLS core network. The technology of access networks in domains 1 and 2 is not determined--it may be, for example, xDSL, fiber, Wi-Fi or 4G/5G/6G mobile networks. The resources of all access and core networks are controlled by dedicated Resource and Admission Control Function (RACF) units (RACF A1, RACF C1, RACF A2 and RACF C2; Figure 1). There are two main assumptions for the operation of MPLS core networks: a static bandwidth reservation mode [8] and a quota-based approach [21] are used for Label Switched Paths (LSPs, logical channels transporting aggregated data). LSPs are created based on routing tables, which are determined by a routing algorithm working in the background. Consequently, routing has a negligible impact on the performance of the IMS/NGN service stratum, which is described by CPP parameters. The assumption of a static bandwidth reservation mode means that LSPs are initially allocated with some bandwidth, which can be modified by RACF through the Label Edge Router (LER) beginning the LSP. The quota-based approach indicates that during network operation, LSP bandwidth is allocated with some reserves so that a part of resource allocation requests generated from the service stratum results only in updating the resource state database of RACF without changing the physical LSP bandwidth. A similar situation takes place for resource release requests generated from the service stratum--they are partially handled by updating the RACF database, and the LSP bandwidth is decreased only when its utilization is low. The set of service scenarios considered in the modeled network includes UE registration in domains 1 and 2 (scenarios a1 and a2) as well as various type of calls, whose parameters are presented in Table 1. Figure 2 depicts an example of a successful call scenario (inter-operator call originating in domain 2--scenario f2). The most important events for this scenario are as follows:  
* Originating UE (UE 2) sends call set-up request (SIP INVITE) to terminating domain (domain 1) (messages 1-11). * P-CSCF 1 starts transport resources reservation in access and core networks on behalf of UE 1 (messages 14-15) and informs P-CSCF 2 about initiating this process (messages 13 and 16-18). * P-CSCF 2 starts transport resources reservation in access and core networks on behalf of UE 2 (messages 19-20) and confirms receiving message 18 (messages 21-28). * P-CSCF 2 informs P-CSCF 1 about successful resource reservation (messages 29-32), and this information is confirmed (messages 33, 35-37). * After resources are reserved in both domains, SIP INVITE message (34) is sent to UE 1. * UE 1 starts ringing (messages 39-44), it is confirmed by P-CSCF 2 (messages 45-52). * UE 1 answers (53-62), which involves updates in packet filtration in access networks (messages 54-55 and 60-61) and ACK confirmation (messages 63-68).