ubscribe. An operator's network is called a Public Land Mobile Network (PLMN). Consequently, the network to which the subscriber belongs is called a Home PLMN (HPLMN), and a network where the subscriber is roaming is a Visited PLMN (VPLMN). There are two types of roaming: domestic and international. Domestic roaming can be used in a country where some operators cannot provide adequate national coverage, and therefore may augment their coverage through roaming partners' networks. The inter- national roaming is used to provide network service to subscribers when they are in a different country. The operators have their own roaming partners governed by roaming contracts. Therefore, it is not always possible to roam to every network in a foreign country. As the 3GPP network technology is the most used cellular network technology, and most major operators have practically global roaming partner networks, it is now possible to find a network that provides service almost anywhere in the world. 2.4.3 Mobility Management Beyond 3GPP 3GPP also specifies interworking and mobility management with other access technologies, such as Wireless Local Area Network (WLAN), or 3GPP2 networks. The mobility management provided can be either seamless or non-seamless. Non-seamless mobility management refers to mobility where the network attachment and authentication is performed automatically, but the UE is connected to a different IP network, and thus, Basics of the 3GPP Technologies its IP address changes. Practically, the handover is non-seamless, and the UE's IP stack has to reconfigure itself, and the applications running on the UE will notice the change. Seamless mobility management is either performed using GTP or PMIP network based mobility management technologies, or the mobility management can be host based using technologies such as Dual-Stack Mobile IPv6 (DSMIPv6 [44]). 2.5 Central Concepts for IP Connectivity We have now illustrated what the different 3GPP specified architectures look like, and how they do session managemen