4 Mobility and Roaming One of the main functions of the mobile, cellular networks is mobility management. Simply put, mobility management is making sure that the network always knows precisely enough where a UE is, in order to be able to send packets to it, or to receive packets from it. This is still relatively simple. On the other hand, managing mobility of potentially Basics of the 3GPP Technologies millions of UEs while ensuring the network does not become overloaded with signaling load, and the UE's battery is conserved, is not that simple anymore. However, it is not the aim of this book to go through the secrets of mobility management. Thus, we will not overload the reader with the details. In this section, we try to give a very short overview of the mobility management. This should be enough to understand the tasks of the different nodes in the 3GPP network architecture, and understand how the UE's IP stack sees the 3GPP networks. 2.4.1 Mobility Management Mobility management makes sure that a UE is connected to the mobile network while it moves within the network's coverage. To ensure this, the UE and the network exchange signaling messages including radio measurement information to decide when is the right time to move from one BTS, NodeB, or eNodeB to another. The UE's movement from one network element's control to another network element's control is called a handover. The handovers, and thus the mobility, are controlled by the 3GPP radio access specific handover procedures between the UE and the radio access network. Inside the network, the mobility management is provided by the mobility management protocols, GTP in GPRS and in the GTP variant of EPS architecture, and by a combination of GTP and PMIP in the PMIP variant. (During a heavy debate on the merits of PMIP versus GTP an intelligent man stated that he does not know of any case where GTP-based network would have lost a UE forever. This nicely crystalizes what the main purpose of a mobility management protocol is.) When the UE moves far enough f