Wireless systems have been developed in an evolutionary way generation by generation over the last twenty years or so. First generation (1G) systems are of diminishing importance. The dominant generations today are 2G and 2.5G with 3G coming into use and are represented in Europe by GSM (Global System for Mobile communications), GPRS (General Packet Radio Services) and UMTS (Universal Mobile Telecommunications System), respectively. In a wider context, besides these terrestrial wide area cellular systems there are a number of other wireless systems, such as the global area satellite systems, the wireless local area networks (WLANs), and the personal area networks (PANs) using, e.g., Bluetooth technology. These systems are optimised for different ranges and this gives the potential for them to co-operate in a complementary way. Next-generation (NG) wireless system (3G and beyond) will provide multimedia mobile communications anytime and anywhere. It would be cost-effective to achieve this aim by converging various wireless networks. However, mobility management (MM) across such a heterogeneous system is not ready.

Mobility management in 2G includes two tasks: one is location management (enabling incoming calls delivery for idle mobile hosts), and the other is handoff (or handover) management (maintaining ongoing calls for active mobile hosts). Accordingly, the traditional MM definition can be referred to as terminal mobility. In NG, for a user roaming across heterogeneous networks, a complete MM scenario can include but maybe not be limited to the following aspects:

Mobile terminals and users authenticate and identify themselves by reporting their locations to their Home Location Registers (HLRs) through the MAP (Mobile Application Part) messages. The Visited Location Registers (VLRs) store caches of necessary user information locally. A mobile terminal updates its location when crossing location areas (LAs) which are sets of cells.

Mobile terminals report their locations to their HLRs through SGSNs (Serving GPRS Support Nodes). The location area unit that GPRS uses is Routing Area (RA), which is typically a subset of one, and only one, GSM LA. This smaller granularity allows for signalling and paging over smaller areas, and thereby achieves a better optimisation of radio resources. GPRS co-operates with the GSM LA-based location management, resulting in a more efficient paging mechanism for mobile terminals that use GSM and GPRS simultaneously. Although IP networks were introduced between SGSNs and GGSNs (Gateway GPRS Support Node), GPRS is only a step preparing for other than using IP mobility by tunnelling IP, and in principle, the mobility management of 2G and 2.5G are both Link-Layer based and for terminal mobility only.

In UMTS, mobile terminals report their locations to their HLRs through combined SGSNs and GGSNs. In a later phase, Mobile IP would be introduced for IP mobility. By then, SGSNs and GGSNs would have been integrated and the integrated node would act as the home agent of the mobile terminal. In contrast to the monopoly role in GPRS, UMTS SGSN shares mobility management with the UMTS Terrestrial Radio Access Network (UTRAN). For further thinning location management requirements, RAs are in turn partitioned into URAs (UTRAN Registration Areas) to better serve pico-cells. In UMTS, service mobility is facilitated with concepts of Customised Applications for Mobile network Enhanced Logic (CAMEL) and Virtual Home Environment (VHE) defined.

In 3G, global roaming becomes more practical with GSM, GPRS and UMTS co-existing to cover a global area. The evolution approach of cellular generations, cumbersome as it is in a sense, facilitates the mobility management of the hybrid system. However, some modifications are entailed.

From a protocol stack perspective, Network Layer is the lowest possible layer where convergence of the heterogeneous wireless systems can be developed since the major difference between them lies in the two lower layers. Besides, the desire to extend the great success of IP in the wired world to wireless leads to an all IP vision. Usually, Network-Layer mobility management protocols, together with traditional Link-Layer schemes, are used to deal with terminal mobility. So far, only Mobile IP has reached the RFC standard level, however, its basic form only suits macro (inter-domain) mobility.

Thus, a number of micro mobility management proposals (including some Mobile IP variants) such as Cellular IP, HAWAII, TeleMIP (IDMP) and Hierarchical MIP have been proposed. The basic idea is to handle local (intra-domain) mobility locally. The primary difference between these schemes lies in the local routing schemes in a domain. Most of them, if not all, rely on Mobile IP for inter-domain mobility management.  

Other Mobile IP variants, such as the MIPMANET architecture, may facilitate ad hoc mobility and mode mobility (including network mobility).

SIP has been chosen by the 3GPP for call signalling but also could be used for Application-Layer mobility management although no standardised specifications for this purpose have been worked out. In principle, SIP is an Application-Layer multimedia signalling protocol. With the help of SIP infrastructure, SIP provides a framework and has the potential capabilities to support personal, session and service mobility by augmented signalling. Extensions of SIP (or SIP-based solutions) are a promising approach. Notably, SIP can also be extended for terminal mobility.

Alternative approaches such as Mobile People, ICEBERG and IPMoA prefer to tackle part of this problem from their own points of view and take different angles, which could contribute to building parts of a complete MM solution. In addition, schemes reside in other layers (other than Network and Application layers) may also well support some mobility scenarios. The Migrate architecture is an example.

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