ndover 7.1. Introduction Handover is the mechanism that maintains the current session (the voice or the conversational video communication) when the mobile changes the radio cell. During the inter-system handover, cell change takes place without mobile network changes. During the intra-system handover, serving gateway (SG) and PDN gateway (PGW) entities play the role of an anchor point, which will hide the terminal mobility from the IMS network. There are two types of intra-system handover: - X2-based handover, from the name of the interface between the eNB entities; - S1-based handover, from the name of the interface between the MME (Mobility Management Entity) and eNB entities. X2-based handover occurs if both eNB entities, the source and the target, belong to the same group (pool) and the X2-AP interface has been enabled. VoLTE and ViLTE: Voice and Conversational Video Services over the 4G Mobile Network, First Edition. André Perez. C ISTE Ltd 2016. Published by ISTE Ltd and John Wiley & Sons, Inc. VoLTE and ViLTE During X2-based handover, the MME entity does not change, which is not the case of SGW entity that can be relocated or not. S1-based handover occurs for the following four cases: - X2-AP interface between two eNB entities of the same group was not activated, the MME must not be relocated and the SGW entity may be relocated (scenario described in section 7.3.1); - both eNB entities belong to two different groups, the MME entity must be relocated and the SGW entity may be relocated (scenario described in section 7.3.2) or not. During the inter-system handover, cell change is accompanied by a change of network, the mobile being transferred from 4G network to 2G or 3G networks. There are two types of inter-system handover: - packet-switched (PS-PS) handover, for which the mobile does not change the mode when changing the network; - circuit-switched (PS-CS) handover, for which the mobile is also changing the mode. In the case of a voice or conversational video communication, PS-PS inter-system handover oc