Understanding H.323�Part III: Signaling
The communicationssuch as on/off-hook, dialtone, and busy signalthat set up and terminate calls on the PSTN are replaced by a complex set of protocols and procedures in the packet-based H.323 world.
In our previous two tutorials on the H.323 standard, developed the International Telecommunications UnionTelecommunications Standard Sector (ITU-T) to support multimedia communication, we explored the history and architecture of that standard, and the various protocols that are required to support the terminal functions . This tutorial will further examine the roles of two ancillary protocols, H.225 and H.245, which are used to establish and disconnect a H.323 call between two endpoints.
The term signaling is used to describe the processes that initiate or terminate a communication session between two parties. If those parties are using the traditional telephone network, the signaling consists of on-hook, off-hook, ringing tones, busy tones, and so on, that communicate the status of one of the parties, or the status of the network.
H.323 endpointswhich could be Terminals, Gateways, Gatekeepers, or Multipoint Control Unitsare packet-based, and as such, rely upon signaling messages, instead of signaling tones, to convey required control information to the desired destination. Note that control information is distinguished from data information, as the former is used to supervise and manage the connection between the two endpoints, and must be successfully completed before the data can be sent and received.
Two standards are involved with H.323 signaling: H.225.0, Call Signaling Protocols and Media Stream Packetization for Packet-based Multimedia Communication Systems; and H.245, Control Protocol for Multimedia Communications, both of which are available from the ITU website, www.itu.int. Note the distinction in the titles: H.225.0 defines call signaling protocols, while H.245 defines a control protocol. Invariably, the call signaling function, which establishes the connection, precedes the control protocol, which conveys operational parameters associated with that connection.
Recall from our previous tutorial that the Gatekeeper, which may or may not be present in the network, provides call control services to H.323 endpoints. These services include address translation, admissions control, and bandwidth control. The portion of the network under the watchful eye of a Gatekeeper is called a zone, and large networks may have multiple Gatekeepers, and therefore multiple zones.
When the Gatekeeper is not present, such as with an IP-phone-to-IP-phone connection, the signaling messages are passed directly between the two endpoints. When the Gatekeeper is present, the endpoints register with the Gatekeeper within their zone upon startup, using a process defined in H.225.0 called RAS, which stands for Registration, Admission and Status. For example, the Registration function occurs when the endpoint sends a Registration Request (RRQ) message to the Gatekeeper, and either a Registration Confirmation (RCF) or a Registration Reject (RRJ) message is returned. Other endpoint messages include the Admission Request (ARQ) asking for admission to the network, Bandwidth Change Request (BRQ), to request a specific amount of network bandwidth.
Once the endpoint has been registered with the Gatekeeper, an H.225.0 call signaling Setup message can be sent to the remote terminal, and H.225.0 Call Proceeding, Alerting, and Connect messages returned. At this point, a connection exists between the calling and called endpoints. Next, the endpoints must exchange their capabilities using the TerminalCapabilitySet [sic] signaling messages defined by the H.245 standard. A very broad set of parameters may be passed with these capability messages. For audio connections, the capabilities include the type of codec (G.711, G.728, G.729, etc.) plus parameters specific to that codec, such as the sampling rate, and the number of audio channels. For data connections, the capabilities include the data protocol in use (such as T.38 for fax or T.120 for whiteboarding applications), plus parameters specific to that data protocol, such as the transmission rate or data compression algorithm to be used. For video connections, the capabilities include the video codec type (such as H.261, H.262 or H.263), and plus parameters specific to that video codec, such as the number of samples per line, lines per frame, and so on. Similar procedures are defined which govern the call disconnection phase, but do not involve as many steps.
As you can see, the operation of both H.225.0 and H.245 are both quite complex, with many different possible scenarios, depending upon the type of terminals to be connected, the presence or absence of a Gatekeeper, the presence or absence of Gateways to other networks, and so on. Those readers wanting to dig deeper are referred to the standards themselves, where these scenarios are described in exacting detail. But be prepared to spend some time by the fire, as the H.225.0 document (188 pages in length) and the H.245 document (329 pages in length) can take some time to sort through.
In our next tutorial, we will consider implementation and interoperability testing of H.323 systems.
Copyright Acknowledgement: © 2005 DigiNet ® Corporation, All Rights Reserved
Mark A. Miller, P.E. is President of DigiNet ® Corporation, a Denver-based consulting engineering firm. He is the author of many books on networking technologies, including Voice over IP Technologies, and Internet Technologies Handbook, both published by John Wiley & Sons.