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H.323 is a standard for real-time audio and video conferencing among end systems on the Internet. As shown in Figure 6.13, the standard also covers how end systems attached to the Internet communicate with telephones attached to ordinary circuit-switched telephone networks. In principle, if manufacturers of Internet telephony and video conferencing all conform to H.323, then all their products should be able to interoperate, and should be able to communicate with ordinary telephones. We discuss H.323 in this section, as it provides an application context for RTP. Indeed, we'll see below that RTP is an integral part of the H.323 standard.
H.323 end points (terminals) can be standalone devices (for example, Web phones and Web TVs) or applications in a PC (for example, Internet phone or video conferencing software). H.323 equipment also includes gateways and gatekeepers. Gateways permit communication among H.323 end points and ordinary telephones in a circuit-switched telephone network. Gatekeepers, which are optional, provide address translation, authorization, bandwidth management, accounting, and billing. We will discuss gatekeepers in more detail at the end of this section.
The H.323 standard is an umbrella specification that includes:
Figure 6.14 shows the H.323 protocol architecture.
Minimally, each H.323 endpoint must support the G.711 speech compression standard. G.711 uses PCM to generate digitized speech at either 56 Kbps or 64 Kbps. Although H.323 requires every endpoint to be voice capable (through G.711), video capabilities are optional. Because video support is optional, manufacturers of terminals can sell simpler speech terminals as well as more complex terminals that support both audio and video.
As shown in Figure 6.14, H.323 also requires that all H.323 end points use the following protocols:
Audio and Video Compression
The H.323 standard supports a specific set of audio and video compression techniques. Let's first consider audio. As we just mentioned, all H.323 end points must support the G.711 speech encoding standard. Because of this requirement, two H.323 end points will always be able to default to G.711 and communicate. But H.323 allows terminals to support a variety of other speech compression standards, including G.723.1, G.722, G.728, and G.729. Many of these standards compress speech to rates that are suitable for 28.8 Kbps dial-up modems. For example, G.723.1 compresses speech to either 5.3 Kbps or 6.3 Kbps, with sound quality that is comparable to G.711.
As we mentioned earlier, video capabilities for an H.323 endpoint are optional. However, if an endpoint does support video, then it must (at the very least) support the QCIF H.261 (176 x144 pixels) video standard. A video-capable endpoint may optionally support other H.261 schemes, including CIF, 4CIF, 16CIF, and the H.263 standard. As the H.323 standard evolves, it will likely support a longer list of audio and video compression schemes.
H.323 Channels
When an end point participates in an H.323 session, it maintains several channels, as shown in Figure 6.15. Examining Figure 6.15, we see that an end point can support many simultaneous RTP media channels. For each media type, there will typically be one send media channel and one receive media channel; thus, if audio and video are sent in separate RTP streams, there will typically be four media channels. Accompanying the RTP media channels, there is one RTCP media control channel, as discussed in Section 6.4.3. All of the RTP and RTCP channels run over UDP. In addition to the RTP/RTCP channels, two other channels are required: the call control channel and the call signaling channel. The H.245 call control channel is a TCP connection that carries H.245 control messages. Its principal tasks are (1) opening and closing media channels, and (2) capability exchange, that is, before sending media, endpoints agree on an encoding algorithm. H.245, being a control protocol for real-time interactive applications, is analogous to RTSP, the control protocol for streaming of stored multimedia that we studied in Section 6.2.3. Finally, the Q.931 call signaling channel provides classical telephone functionality, such as dial tone and ringing.
Gatekeepers
The gatekeeper is an optional H.323 device. Each gatekeeper is responsible for an H.323 zone. A typical deployment scenario is shown in Figure 6.16. In this scenario, the H.323 terminals and the gatekeeper are all attached to the same LAN, and the H.323 zone is the LAN itself. If a zone has a gatekeeper, then all H.323 terminals in the zone are required to communicate with it using the RAS protocol, which runs over TCP. Address translation is one of the more important gatekeeper services. Each terminal can have an alias address, such as the name of the person at the terminal, the e-mail address of the person at the terminal, and so on. The gateway translates these alias addresses to IP addresses. This address translation service is similar to the DNS service, covered in Section 2.5. Another gatekeeper service is bandwidth management: The gatekeeper can limit the number of simultaneous real-time conferences in order to save some bandwidth for other applications running over the LAN. Optionally, H.323 calls can be routed through gatekeeper, which is useful for billing.
The H.323 terminal must register itself with the gatekeeper in its zone. When the H.323 application is invoked at the terminal, the terminal uses RAS to send its IP address and alias (provided by user) to the gatekeeper. If the gatekeeper is present in a zone, each terminal in the zone must contact the gatekeeper to ask permission to make a call. Once it has permission, the terminal can send the gatekeeper an e-mail address, alias string, or phone extension for the terminal it wants to call, which may be in another zone. If necessary, a gatekeeper will poll other gatekeepers in other zones to resolve an IP address.
An excellent tutorial on H.323 is provided by [WebProForum 1999]. The
reader is also encouraged to see [Rosenberg 1999] for an alternative
architecture to H.323 for providing telephone service in the Internet.
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