H.225: Handles call setup and teardown between H.323 devices on a packet-based network, terminal to gatekeeper signaling using Registration, Admission, and Status Protocol (RAS), and call signaling. H.323 can use ISDN Q.931 signals, formatted as H.225 messages, to interoperate with legacy voice networks.

H.235: Specifies security for messages between the gateway and gatekeeper.

H.245: Controls the traffic flow, performs DTMF Relay, limits media transmission rates, negotiates capability, and controls opening and closing channels for media streams. Uses TCP.

H.261/ H.263: Specify video conferencing standards.

H.450: Controls supplementary services between H.323 entities. Examples of supplementary services include call waiting, hold, transfer, park, and pickup.

T.120: Used for real-time multipoint data transfer during videoconferences. Allows application sharing, whiteboarding, and file transfer. Uses TCP.

H.320: Defines the standard for video conferencing over ISDN networks. H.320 uses H.221 frames for media. It requires a gateway to interwork with H.323 conferencing over IP because H.221 frames must be translated into RTP packets, and vice versa.