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The Illustrated Network- P79

The Illustrated Network- P79:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 30 Voice over Internet Protocol 749 H.225 RAS H.225 Call Status H.245 Control UDP TCP IP Data Link Physical Media H.323 Signaling Stack MGCP Megaco H.248 Signaling Stack FIGURE 30.9 Three VoIP signaling architectures. H.323 the International Standard The H.323 signaling protocol framework is the international telephony standard for all telephony signaling over the packet network not just the Internet . When work on H.323 began the packet network most commonly mentioned for H.323 was X.25 then ATM and not the Internet. In a sense H.323 doesn t care it s just an umbrella term for what needs to be done. Like RTP H.323 was designed for audio and video conferencing not just point-to-point voice conversations. A LAN with devices that support H.323 capabilities H.323 terminals which have many different subtypes also has an H.323 multipoint control unit MCU for conference coordination. The LAN includes an H.323 gateway to send bits to other H.323 zones and an H.323 gatekeeper. The gatekeeper is optional and is needed only if the terminals are so underpowered they cannot generate or understand H.323 messages on their own. Most can although H.323 is not trivial. The H.323 gateway is essentially a router but with the ability to support packetized voice to PSTN connections and the terminals are computers of course . The main H.323 signaling protocols used with VoIP are H.225 RAS Registration Admission and Status which is used to register the VoIP device with the gatekeeper and H.255 CS call status which is used to track the progress of the call. The structure 750 PART VII Media FIGURE 30.10 H.323 zone components. Optional components are shown in italic. of a typical H.323 zone is shown in Figure 30.10. H.323 signaling uses both UDP and TCP when run on an IP network and uses RTP and RTCP for transport. Components that are not strictly needed for VoIP are shown in italics. H.323 supports not only audio and video conferencing but also data conferencing where users can all