H.225.0, a key protocol in the H.323 VOIP architecture defined by ITU-T, is a standard to cover narrow-band visual telephone services defined in H.200/AV.120-Series Recommendations. It specifically deals with those situations where the transmission path includes one or more packet based networks, each of which is configured and managed to provide a non-guaranteed QoS, which is not equivalent to that of N-ISDN, such that additional protection or recovery mechanisms beyond those mandated by Rec. H.320 are necessary in the terminals. H.225.0 describes how audio, video, data and control information on a packet based network can be managed to provide conversational services in H.323 equipment. H.225.0 has two major parts: Call signaling and RAS (Registration, Admission and Status).
H.225.0 call control signaling is used to setup connections between H.323 endpoints. This is achieved by exchanging H.225 protocol messages on the call-signaling channel. The call-signaling channel is opened between two H.323 endpoints or between an endpoint and the gatekeeper. The ITU H.225.0 recommendation specifies the use and support of Q.931 signaling messages. A reliable (TCP) call control channel is created across an IP network on TCP port 1720. This port initiates the Q.931 call control messages for the purpose of connecting, maintaining, and disconnecting calls. When a gateway is present in the network zone, H.225.0 call setup messages are exchanged either via Direct Call Signaling or Gatekeeper-Routed Call Signaling (GKRCS). The gatekeeper decides the method chosen during the RAS admission message exchange. If no gatekeeper is present, H.225 messages are exchanged directly between the endpoints.
H.225.0/RAS (Registration, Admission and Status) is the protocol between endpoints (terminals and gateways) and gatekeepers. The RAS is used to perform registration, admission control, bandwidth changes, status, and disengage procedures between endpoints and gatekeepers. An RAS channel is used to exchange RAS messages. This signaling channel is opened between an endpoint and a gatekeeper prior to the establishment of any other channels.
Protocol Structure
| 1 bit | 2 bits | 3 bits | 4 bits | 8 bits |
| Protocol Discriminator | ||||
| 0 | 0 | 0 | 0 | Length of call reference bits |
| Call reference value | ||||
| 0 | Message type | |||
| Information Elements | ||||
- Protocol discriminator - Distinguishes messages for user-network call control from other messages.
- Length of call ref - The length of the call reference value.
- Call reference value - Identifies the call or facility registration/cancellation request at the local user-network interface to which the particular message applies. May be up to 2 octets in length.
- Message type - Identifies the function of the message sent.
- Information elements - Two categories of information elements are defined: single octet information elements and variable length information elements, as shown in the following illustrations.
| 1 bit | 4 bits | 8 bits |
| 1 | IEI | Contents of IE |
| 1 bit | 8 bits |
| 1 | IE Identifier |
| 1 bit | 8 bits |
| 1 | IEI |
| Length of contents of IE | |
| Contents of IE (variable) | |
Key RAS messages:
| Message | Function |
| RegistrationRequest (RRQ) | Request from a terminal or gateway to register with a gatekeeper. Gatekeeper either confirms or rejects (RCF or RRJ). |
| AdmissionRequest (ARQ) | Request for access to packet network from terminal to gatekeeper. Gatekeeper either confirms or rejects (ACF or ARJ). |
| BandwidthRequest (BRQ) | Request for changed bandwidth allocation, from terminal to gatekeeper. Gatekeeper either confirms or rejects (BCF or BRJ). |
| DisengageRequest (DRQ) | If sent from endpoint to gatekeeper, DRQ informs gatekeeper that endpoint is being dropped; if sent from gatekeeper to endpoint, DRQ forces call to be dropped. Gatekeeper either confirms or rejects (DCF or DRJ). If DRQ sent by gatekeeper, endpoint must reply with DCF. |
| InfoRequest (IRQ) | Request for status information from gatekeeper to terminal. |
| InfoRequestResponse (IRR) | Response to IRQ. May be sent unsolicited by terminal to gatekeeper at predetermined intervals. |
| RAS timers and Request in Progress (RIP) | Recommended default timeout values for response to RAS messages and subsequent retry counts if response is not received. |
Related protocols: RTSP, SIP, RTP, RTCP, SDP, Megaco/H.248, Q.931, H.323, H.245
Sponsor Source: H.225.0 is an ITU-T (http://www.itu.int/ITU-T/) standard.
Reference:
http://www.javvin.com/protocol/H225v5.pdf: Call signalling protocols and media stream packetization for packet-based multimedia communication systems Version 5.
http://www.h323forum.org/papers/: H.323 papers and documents