Physical layer is another way of saying hardware layer. This is the wire, cable and electronics that connect the devices and networks to each other. Physical also implies "touchable" or real. For the AirDrop module, the physical layer consists of RF waves. RF isn't "touchable", but it exists and thus, is real. On a wired network, the physical network consists of an Ethernet hub/ router, possibly an AP (most of the APs aimed at home wireless networks have a wired switch functionality as well as wireless capability) and any other cabling or electronic devices that tie the physical network together using wire. The bottom line is that the physical layer always sees the entire packet whether it is receiving it or transmitting it and never adds or subtracts to a packet's contents.