In Sections 6.3 through 6.6, we looked at the ongoing work in the area of cross-layer design. In doing so, we came face to face with the general motivations behind cross-layer design, the several different interpretations of cross-layer design, many examples of cross-layer design, some initial ideas on how cross-layer interactions may be implemented, and a quick snapshot on how cross-layer design ideas are getting into ongoing standardizations and commercial deployments. Having taken stock of the ongoing work, we now raise and discuss what we call the 'open challenges' for cross-layer design. Broadly speaking, we include two kinds of issues in the open challenges: questions about cross-layer design that, in our opinion, are important but are not getting sufficient attention in the research literature; and some questions regarding the fundamental nature of the wireless medium, questions whose answers will influence how communication architectures for the wireless networks should look like, and hence are pertinent to the cross-layer design effort. We note that some of these issues have been raised elsewhere in the literature too. As before, our purpose here is to consolidate the different issues and discuss their significance with respect to the cross-layer design activity.
The following are some open challenges for the designers proposing cross-layer design ideas:
How do the different cross-layer design proposals coexist with one another?
Will a given cross-layer ...