Driven by concepts such as "service anywhere and anytime" and "always best connected", it is well accepted that next generation wireless networks will be heterogeneous, integrating different radio access technologies in order to provide all types of service for mobile users with multi-mode access capability. IEEE 802.21 standard aims at providing the means to facilitate and improve the intelligence for handover procedures. This paper points out the necessity of integrating Media Independent Handover (MIH) entities in an overall next generation handover-enabling framework in order to improve its real market applicability. Several schemes and mechanisms scatteringly found in the literature are presented and being mapped to the objectivges of the ongoing 802.21 task groups providing thus directions for future research.