e HeNBs to MME. The HeNB GW Figure 3.26 Control plane protocol stack. Reproduced with permission from C 2009 3GPP [14] 14 NAS is a functional layer in the wireless telecom protocol stack between Core Network and User Equipment. The layer supports signalling and traffic between those two elements. Next Generation Mobile Communications Ecosystem MME/S-GW MME/S-GW ((o)) ((o)) HeNB GW E-UTRAN ((o)) ((o)) ((o)) ((o)) Figure 3.27 EPS HeNB architecture. Reproduced with permission from C 2009 3GPP [14] serves as a concentrator for the C-Plane, specifically the S1-MME interface. The S1-U interface from the HeNB may be terminated at the HeNB GW, or a direct logical U-Plane connection between HeNB and S-GW may be used. The HeNB GW appears to the MME as an eNB whereas the HeNB GW appears to the HeNB as an MME. The S1 interface between the HeNB and the EPC is the same whether the HeNB is connected to the EPC via a HeNB GW or not. The HeNB GW connects to the EPC in a way that inbound and outbound mobility to cells served by the HeNB GW do not necessarily require inter MME handovers. One HeNB serves only one cell. The functions supported by the HeNB remain the same as those supported by an eNodeB (with the possible exception of NAS Node Selection Function) and the procedures that run between a HeNB and the EPC are also same as those between an eNodeB and the EPC. Key Attributes of E-UTRA In this section we will describe the key attributes of LTE including: QoS, RRM (radio resource manage- ment), Multi-Antenna Transmission, Interference Coordination and Cancellation, Spectrum Flexibility, Security, MBMS, and One Voice. 3.9.1 The EPS provides IP connectivity between a UE and a PLMN external packet data network which is referred to as PDN Connectivity Service. The PDN Connectivity Service supports the transport of traffic flow aggregate(s), consisting of one or more Service Data Flows (SDFs). For E-UTRAN access to the EPC the PDN connectivity service is provided by an EPS bearer in case of GTP¹5 - based 15 GPRS Tunneling Protocol 