MAC address
Nowadays most Wi-Fi router or wireless access point vendors provide an API for listening the device's MAC address of the signals to identify smartphones, this is the base of solution vendors of Wi-Fi tracking and analytics systems, but almost new smartphones emit more than one MAC address when they are not connected to the Wi-Fi. An iPhone can produce a lot of different and/or false MAC addresses when visit a venue during 30-40 minutes, because every time you touch the screen and awake from sleep mode the MAC address changes.