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Standards for LAN-based Voice and Video Applications Standards for voice and video over the LAN fall into two categories: those designed for native ATM protocols, and those intended for general-purpose LAN protocols, particularly IP. Standards for native ATM protocols, such as the ATM Forums Voice Telephony over ATM (VTOA) are appropriate only for ATM-connected desktops, or desktops running Cell-in-Frame over Ethernet. Standards for applications that run over IP are applicable both to ATM-connected desktops as well as desktops in general Ethernet or Token Ring environments. The most important standard in this space is H.323, which was developed by the International Telecommunications Union. While H.323 is designed to be independent of the underlying networking protocol, it will most often be deployed running over IP. H.323 references other existing standards for the digital encoding and compression of voice and video signals, and describes how audio and video streams are carried in the payload of IP packets with the aid of the Real Time Protocol (RTP), which provides timing and synchronization information. H.323 also covers the handling of data streams for application sharing, shared whiteboarding, and real-time file transfer (referencing the T.120 standard), and includes signaling based on ISDN messaging protocols for call setup and teardown. The H.323 standard is flexible and accommodates any combination of real-time voice, video, and data as part of a single point-to-point or multipoint conference call. It may be used with a voice stream alone as the basis of a LAN telephony solution. H.323 enjoys the broadest support in the industry as a proposed standard for Internet telephony. Additional Components: Gateways and Gatekeepers Creating a LAN infrastructure that can consistently deliver voice and video streams with sufficiently low delay is an absolute pre-requisite for integrating voice and video on the LAN, but it is by no means the complete answer to the problem. There are two other key components of a complete voice and video solution, which in H.323 parlance are known as the Gateway and the Gatekeeper. An H.323 Gateway provides interconnection between voice and video services on the LAN, and external voice and video services typically provided over circuit-switched networks such as ISDN and the public telephone network. The Gateway terminates the IP and RTP protocols carrying the voice and video streams, and converts them to appropriate formats for external networks. For videoconferencing, the conversion is most likely to be to H.320, another ITU standard that specifies how voice and video are carried over ISDN connections. For voice-only connections, the conversion will be to the G.711 standard for digital telephony. This allows voice interworking with any phone on a public network or connected to a PBX. An H.323 Gatekeeper is a pure software function that provides central call control services. While it is possible to run H.323 voice and video communications over the LAN without a Gatekeeper, in practice this function is extremely useful. At the most basic level, the Gatekeeper provides directory services and policy-based controls applied to the use of voice and video communications. For example, the Gatekeeper can bar stations from accessing certain types of external phone numbers at certain times of day. The Gatekeeper can be thought of as the server in a client/server model of LAN-based telephony and videoconferencing. At a more sophisticated level, the Gatekeeper may be able to support supplementary services, including call transfer, hold and divert, hunt groups, pick-up groups, attendant operation and so on-features that are typically found in high-end PBXs for controlling and managing voice calls. While the H.323 standard does not explicitly describe how supplementary call control features may be supported, the standard does provide a framework for the addition of these advanced capabilities. SUMMARY This chapter has explained the value of voice and video integration on the LAN in terms of both application-driven needs and the desire for infrastructure efficiencies. It has looked at the technology issues surrounding the transport of real-time voice and video streams over LAN infrastructures, and concluded that ATM backbones provide a solution in the near term, with the possibility later of a solution based entirely on switched Ethernet or Token Ring. Finally, we have described some additional functional elements, such as Gateways and Gatekeepers, that are an essential part of a complete solution for voice and video over the LAN. Over the last decade, the open standards-based environment typified by PCs and LANs has revolutionized the way data is handled and processed in enterprise environments. Now this open and standards-based approach is set to tackle the challenge of voice and video, formerly the exclusive domain of the PBX. The history of LAN evolution is set to repeat itself, and we can expect the traditional proprietary mainframe PBX to diminish in importance to the enterprise, giving way to client/server telephony and videoconferencing, just as the mainframe computer has been pushed into the background by client/server techniques for data processing.
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