 the transport stratum applies QoS requirements (coming from the service stratum) by using certain Internet QoS techniques in network nodes (e.g. switches, routers). The main entity for QoS in NGN is the resource and admission control function. 3.4.1 Resource and Admission Control Function QoS functions in NGN are defined through the standardization of the RACF [13]. The RACF is the central functional entity in NGN regarding QOS, with further communi- cation with other functional entities dependent upon the type of access network (e.g. Ethernet, cable, xDSL, Fiber To The X - FTTx, mobile access networks). The RACF is a "link" between the SCFs and transport functions in NGN. Its main goal is to provide admission control regarding given calls/sessions to/from end-users. So, a user connection can be admitted in the transport network or rejected (or modified) according to transport subscription information, service level agreements, different policies and service priorities, etc. The RACF hides network topology from the service stratum, thus providing an abstract view of transport architecture to SCFs. The RACF is not mandatory in NGN, such as in cases in which admission control is not needed (e.g. best-effort Internet traffic, such as the Web). On the other side, the RACF interacts with SCFs for applications that require certain QoS support, which results in a need for admission control (e.g. QoS-enabled VoIP or IPTV). Besides that, RACF also provides network address and port translation (NAPT) traversal functions which are necessary when an NGN operator uses private IP addresses for addressing end-user terminals. Finally, RACF interacts with NACF for exchange of user subscription information, as well as with RACF in other NGN for delivering services over multiple network or service providers. RACF policy decisions are based on transport subscription information, SLA, network policy rules, admission control, and service priority [14], as well as transport resource status and policy information. Functional architectu