H.225.0 is a sub-standard of H.323.

The main objective of H.225.0 is the definition of messages:

• To establish, control and end an H.323 call. This set of messages is known as Call Signaling. The call signaling of H.225.0 is based on Q.931, the standard that defines the call signaling in ISDN.

• To register at and utilize the various services of an H.323 Gatekeeper. This set of messages is called Registration, Admission, Status or in short - RAS.