The H.323 standard defines the protocols to provide audio-visual communication sessions on any packet network1 The H.323 standard uses a number of protocols to establish and control multimedia sessions, such as voice and video calls. Two of the protocols used in the H.323 standard are H.245 and Q.931.
H.245 is a protocol that is used for media control and logical channel signaling2 H.245 allows the endpoints to negotiate the media capabilities, such as the codec, the bit rate, and the format, and to open and close the logical channels for media transmission. H.245 also provides functions for conference control, such as floor control, chair control, and multipoint mode. H.245 messages are exchanged between the endpoints using TCP or UDP.
Q.931 is a protocol that is used for call signaling and connection control3 Q.931 is based on the ISDN protocol and is adapted for packet networks. Q.931 allows the endpoints to set up, maintain, and terminate the calls, and to exchange information such as the call identity, the call type, the call state, and the reason for call release. Q.931 messages are exchanged between the endpoints using TCP.
The other options are not protocols used in the H.323 standard. SDP is a protocol that is used to describe the media session parameters, such as the media type, the codec, the port number, and the encryption method. SDP is used in conjunction with SIP, but not with H.323. SIP is another protocol that is used to establish and control multimedia sessions on packet networks, but it is not part of the H.323 standard. SIP is an alternative to H.323 and has a different architecture, syntax, and functionality. 802.1 is a standard that defines the LAN protocols, such as the MAC address, the VLAN, the spanning tree, and the link aggregation. 802.1 is not related to the H.323 standard, which operates at the application layer. References: H.323 - Wikipedia, H.245 - Wikipedia, Q.931 - Wikipedia
