X-layer in wireless networks mean, when you are considering all layers together in designing networks, like PHY, MAC etc. One example will be considering rate from PHY layer and Queue sizes from MAC layer and designing scheduling algorithms for the network.
* Why we go for multi hop wireless networks?
* Deference between long-term and short-term end-end rate in multi hop networks
* difference between session and link in adhoc networks.
* How higher modulation level and coding rates has lead to higher SINR.