orking platform for all types of services, the existing ones as well as future ones. However, one may note that traditional telecommunication networks have influenced the NGN network concept, particularly for QoS-enabled end-to-end VoIP and IPTV. Traditional telecommunication networks (before the appearance of NGNs) were designed primarily to carry circuit-switched digital voice by using unified 64 kbit/s bitstream in each direction for a single voice connection (ITU-TG.711 voice codec standard). Such networks were based on simple telephone devices (without any com- putational capabilities), circuit-switched transport networks based on time division multiplexing (TDM) with 64 kbit/s "capacity slots" (e.g. SDH/SONET (synchronous optical networking) transport networks), complex and expensive digital exchanges located in telecom operators' networks, and usage of globally unified Signaling System No. 7 (SS7, standardized by ITU-T). As shown in Figure 3.1, the evolution from PSTN and PLMN toward NGNs is performed by gradual replacement of TDM-based transport systems and interfaces, circuit-switching network nodes (e.g. telephone exchanges), and SS7 signaling (which is packet-based, separated from the user traffic, but is not IP-based) with IP/MPLS transport networks, media gateways, and IP-based signaling (e.g. SIP), respectively. NGN specifies IP on network layer end-to-end, and such requirement defines the all-IP network concept. That means that all access, core (or backbone), and transit net- works in NGN are IP based. On one side, IP hides the lower protocol layers (i.e. OSI-1 and OSI-2) from the upper layers, SO an NGN can have single core and transport network infrastructure for different fixed and wireless access networks. In the all-IP network principle, the interconnection between networks is also done via IP links established between pairs of gateway routers or controllers. 3.2 Transport and Service Stratum of NGNs Telecom/ICT world TV broadcast Call/session TV media Call control control control (SS7) (IMS) 