# Server Operation and Virtualization to Save Energy and Cost in Future Sustainable Computing
## 2 Background
### EPC and vEPC (Virtual EPC)  
The recent emergence of Distributed Cloud Environment, Network Function Virtualization (NFV), and Software Defined Networking (SDN) has affected the evolution of EPC (Evolved Packet Core), a mobile core network. In other words, it has evolved into a mobile core network structure through EPC function virtualization not only to reduce CAPEX/OPEX but also to facilitate innovative service development and application. EPC (Evolved Packet Core) [14] is an integrated framework for packet-based real-time and non-real-time service and is a high-performance, high-capacity new all-IP mobile core network for LTE (Long Term Evolution) as defined in the 3 GPP (3 rd-Generation Partnership Project) Release 8 standard. It integrates the mobile core function separated in the existing mobile network (2G/3G) as Circuit-Switched (CS) for voice and Packet-Switched for data. In other words, the 4G network is defined as End-to-End (user mobile device having IP function--IP-based LTE station (eNB)--EPC--application domain IMS (IP Multimedia Subsystem)--and non-IMS, all-IP-based flat architecture Figure 3. Here, EPC is an essential component for the End-to-End IP service that enables introducing innovative services or applications such as new business model development. In terms of mobile network structure evolution, EPC separates control, data, and plane, and improves network performance through flat IP structure (e.g., data connection from station passes the EPC gateway only) that simplifies the hierarchical structure between mobile data components. The precise definition of EPC component varies by network equipment vendor, but common components include S-GW (Serving Gateway), P-GW (Packet Data Network Gateway), and MME (Mobility Management Entity). Sometimes, PCRF (Policy and Charging Rules Function) is added. To examine the functions of each component briefly, S-GW is a data flat component that manages user mobility as a local mobility anchor playing the role of a border point between RAN (Radio Access Network) and core network and maintaining the data path between station and P-GW. P-GW is a data flat component performing the role of IP anchor point. In other words, it serves the external network (e.g., Internet, IMS core, and other data network) and data transmission between user terminals where the high-speed packet processing of service edge of the mobile network service provider takes place. It performs bearer creation and closing, packet inspection/filtering, policy enforcement, accounting, and reporting. Although S/P-GW is a data flat component, it works as a control flat component such as mobility control. As a control flat component, MME allocates appropriate S-GW to each  
Figure 2: OPNFV Development Scope. user terminal, coordinates bearer channel setting in the network, tracking user terminal and network resource allocation/optimization as the user terminal moves and HSS (Home Subscriber Server), and provides security management between the user terminal and the network. As a newly defined network node in the 3 GPP Release 7 standard, PCRF is an evolved form of PDF (Policy Decision Function) and CRF (Charging Rules Function). In the 3 GPP Release 8 standard, it expands and improves PCC (Policy Charging and Control) to non-3 GPP access such as Wi-Fi (Wireless Fidelity) and wired IP bandwidth network. Due to the recent requirements of various multimedia services, QoE (Quality of Experience), VoLTE (Voice-over LTE), and M2M (Machine-to-Machine), mobile network service providers want to develop a network that handles unpredictable user requirements more efficiently without service quality downgrade with available low cost and which supports quick service development/innovation and expansion. Accordingly, mobile network service providers pay much attention to Network Function Virtualization as a technology that responds to the dynamic and unpredictable characteristics of mobile bandwidth service more efficiently [15] and reviews the applicability in various aspects.