 often referred to as a two-node architecture because, log- ically, only two nodes are involved-in the user and control plane paths-between the user equipment and the core network. These two nodes are (1) the base station, called eNode B, and (2) the serving gateway (S-GW) in the user plane and the mobility management entity (MME) in the control plane, respectively. The MME and the GW belong to the core network, called evolved packet core (EPC) in 3GPP terminology. The GW executes generic packet processing functions similar to router functions, including packet filtering and classification. The MME terminates the © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology RLC/MAC RLC/MAC UDP/IP UDP/IP S1-UP IP transport IIIII S1-CP network eNode B eNode B S1-AP S1-AP RLC/MAC RLC/MAC Figure 4.1 The 3GPP long term evolution (LTE) RAN architecture. so-called nonaccess stratum signaling protocols with the user equipment (UE) and maintains the UE context, including the established bearers, the security context, and the location of the UE. In order to provide the required services to the UE, the MME talks to the eNode B to request resources for the UE. It is important to note, however, that the radio resources are owned and controlled solely by the eNode B and that the MME has no control over the eNode B radio resources. Although the MME and the GW are LTE-specific nodes, they are radio agnostic. The LTE architecture is depicted in Figure 4.1, which shows the control plane and user plane protocol stacks between the UE and the network. As it can be seen in the figure, the radio link specific protocols, including radio link control (RLC) [2] and medium access control (MAC) [3] protocols, are terminated in the eNode B. The packet data convergence protocol (PDCP) layer [4], which is responsible for header compression and ciphering, is also located in the eNode B. In the control plane, the eNode B uses the radio resource control (RRC) protocol [21] to execute the longer time-scale radio