H.225
H.245
H.323
Half Duplex

H.225
Protocols and message formats for H.323 .

H.245
A protocol for capability negotiation, messages for opening and closing channels for media streams, etc. [+ info]

H.323
A standard approved by the International Telecommunication Union (ITU) in 1996 to promote compatibility in videoconference transmissions over IP networks. [+ info]

Half Duplex
The opposite of full duplex, just one party can hear or speak at the same time .Example: walki-talkies.