itously installed IP infrastructure which is widely deployed and has high flexibility and scalability; (2) capitalizes on the IETF standardization process; (3) brings flexibility to an operator in choosing a layer 2 transport technology; 2.3 Evolution of UMTS (4) profits from the low cost IP equipments and IP bandwidths based on a cheap layer 2 transport technology, e.g. Ethernet, which can significantly reduce the overall infrastructure costs; (5) benefits from both existing and emerging IP-related technologies and services; (6) and facilitates the integration of different radio access technologies operating over a common IP backbone and therefore provide a flexible and reliable way for enabling heterogeneous network access. In this context, the concept of an All-IP Network (AIPN) is promoted in UMTS. It requires establishing a single unified UMTS network that is fully using IP as the underlying network architecture. That means all services, not only packet data but also control data and circuit-switched data like traditional speech services, will be delivered over IP. The evolution of the UMTS into all IP-based system is considered in the recent releases of the 3GPP specifications. The feasibility study for an All-IP network within 3GPP is given in 3GPP TR 22.978 [3GP05a]. The architecture of an All-IP network is specified in 3GPP TR 23.922 [3GP01b]. With the scope of building an All-IP UMTS network, the UTRAN is certainly expected evolve towards an IP-based network. The following subsections introduce different alternative approaches to use IP as the transport technology in the UTRAN. 2.3.4.1 IP-Based UTRAN The most straightforward approach is to completely replace the legacy ATM transport networks with the IP networks in the UTRAN. The IP-based transport should meet the delay requirements of the UTRAN in a cost effective way in terms of efficiency and maximal resource utilization. To achieve this, multiplexing schemes and UDP/IP header compression are adopted to reduce the IP layer overhead. Multiplexing sche