Insteon tries to combine the best of both worlds which means it supports powerline communication as well as radio frequency communication. Considering it only as a protocol would be an understatement. It is rather a complete technology stack which also consists of separate protocols for the two different types of communication.
Browsing: Wireless Connectivity Protocols
Universal Powerline Bus or short UPB is a communication protocol developed for the needs of home automation. As the name suggests it uses the existing powerlines of homes for sending data. It was developed in 1999 by PCS Powerline Systems and was inspired by the already ubiquitous X10 protocol.
ZigBee is a home automation protocol which was originally invented for the use in Personal Area Networks (PAN). Like Z-Wave it uses low power radio frequency communication which results in long battery lives for ZigBee using devices. The protocal is based on the IEEE 802.15.4 standard and is widely considered as an alternative to Bluetooth and Wi-Fi.