he explosion of mobile data communications, the emergence of multitechnology environments with diverse capabilities, the integration of such environments at both terminal and network sides, and the great variety of offered end-user services have completely changed the role of handover management, which nowadays faces the challenge of adaptation to such heterogeneous and multiparametric environments. Traditionally, handover management in cellular networks is carried out by technology-specific mechanisms since it has only involved intratechnology handovers, given the singletier network landscape and the capabilities of single-mode terminals. By intratechnology handovers we mean handovers between equipment of the same technology [1], such as Universal Mobile Telecommunications System (UMTS) to UMTS or Global System for Mobile Communications (GSM) to GSM. Another important aspect is that such handovers, even after the integration of the IP and telecom worlds that brought the IP layer to the terminal side, are transparent to the IP layer and still only involve layer 2 mechanisms (i.e., radio-technology-specific mechanisms). Therefore, intratechnology handover in such networks is in some way synonymous with the notion of layer 2 handover.In all cases, session continuity and minimal handover disruption time has always been the primary goal of handover management. This concept of handover seamlessness is, however, very much dependent on the service being provided. For example, in pure voice networks such as GSM, seamlessness is perceived as delivering the voice service with bounded handover latency in order not to disturb voice conversation. In General Packet Radio Service (GPRS)/UMTS networks offering data services as well, handover seamlessness of a Web session is interpreted as minimizing packet loss without further concerns on additional delays. This target is more easily reached by certain technologies, such as UMTS, that support macrodiversity, that is, the capability of a terminal to send/receive radio frames to/from more than one base stations (BSs) at the same time (a layer 2 capability). Therefore, a mobile terminal can be serviced in parallel by more than one BSs and thus perform soft handover, where no break in radio communication occurs. For other technologies, such as wireless LANs (WLANs), no such capability is supported. The mobile terminal cannot be serviced in parallel by more than one access point (AP) -the BS equivalent in WLAN terminology -and therefore has to break its communication with its current AP before establishing a connection with a new one. This break in communication is from a layer 2 perspective.The need for integration of this heterogeneous network environment and the emergence of multimode terminals have placed extra requirements on handover management. It is common sense that this functionality needs to be "pushed" to the IP layer, which is generic enough and serves as the rendezvous point for all underlying technologies. Therefore, handovers between APs of different types (intertech...
