For a user roaming among these networks, it is desirable that his or her mobile device can have anytime connectivity, and therefore vertical handoff becomes an important issue.

• 对于漫游在这些网路之间的用户而言，他们希望不管漫游到哪个网路中，都能够随时随地连接上线。因此，垂直无缝交递成为一个重要的问题。