When using a specific datalink layer technology, for example Ethernet, do we have limitations of the used medium (i.e. Ethernet). Can we use radio connections to connect two LANs over Ethernet?
Ethernet (IEEE 802.3) is a series of layer-1/2 protocols used on various media. I don't believe there is currently an approved standard for ethernet over radio. One obstacle to that is that radio is a shared medium, and ethernet must be able to simultaneously send and listen for collisions on a shared medium, but the limitations of radio prevent that.
The IEEE has defined several other standards for radio LAN communications (802.11, 802.15, 802.16, etc.). These standards are not ethernet, but completely different LAN protocols. The best known are Wi-Fi (802.11) and Bluetooth (802.15.2).