We have shown the interworking architecture and scenarios. The functionalities of each scenario are also discussed. We also introduce standardized protocols and
compare the provided functionalities of them. There are
still weakness and limitations in these protocols. Some
improvements or enhancements can be found in current
issues. For example, billing mechanism is not supported
in standardized protocols. Plus, non-repudiation should
be provided in a billing mechanism. Thus, the improved
protocol is proposed in [39]. Although these proposals
improve the shortage of standardized protocol, there are
still some drawbacks in these proposals. Hence, we discuss some future works in order to improve these protocols
such as the reduction of the computational cost and the
smooth transmission of a transferred service. The future
works are discussed below.