The development of mobile Internet and wireless communication technologies promote heterogeneous wireless networks (HWNs)

in which a variety of wireless networks coexist and coverage overlap. The differences between various wireless access technologies and single network cannot meet all the requirements of users. Consequently

it is inevitable for the integration of HWNs. The access selection as one of the key technologies of HWNs has become a research hotspot. Based on analyzing the infrastructure of HWNs and the concepts of the access selection

this paper focuses on the classification and detailed comparative analysis of the access selection algorithm. Simultaneously

the existent problems are summarized

and the future research directions and challenges are presented.