H.225.0 is part of the H.323 family of telecommunication protocols.
The main objective of H.225.0 is the definition of messages and procedures for:
- Call Signaling: establish, control and end an H.323 call. The call signaling of H.225.0 is based on the call setup procedures for ISDN, Recommendation Q.931.
- RAS Signaling Function: perform registration, admission, bandwidth changes, status and disengage procedures between endpoints and an H.323 Gatekeeper. The RAS signaling function uses a separate channel (RAS channel). This set of messages is called Registration, Admission and Status (RAS).
- Additionally, H.225.0 defines media packetization using RTP for traditional codecs like G.711, H.261 and H.263. Newer ITU recommendations for media compression/signaling refer to RFCs for packetization (e.g. H.264 utilizes the single NAL unit mode of RFC 3984 in H.323 as defined in H.241).
The Message Encoding is performed by utilizing Q.931 User-to-User information elements to convey H.225 messages. The messages themselves are encoded according to the Packed Encoding Rules (PER) of ASN.1.
The structure of H.225 follows the Q.931 standard as seen in the table below.
8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | Octet |
| Protocol Discriminator | 1 | |||||||
0 | 0 | 0 | 0 | Length of call reference bits | 2 | |||
Call reference value | 3 (-4) | |||||||
0 | Message type | |||||||
| Information Elements | ||||||||
H.225 structure | ||||||||