This chapter contains information about the H.323 standard and its architecture, and discusses how Microsoft® Windows® NetMeeting® supports H.323 for audio and video conferencing. It also explains H.323 protocols and the interoperability of H.323-based products.

What is the H.323 Standard?

H.323 is an International Telecommunications Union (ITU) standard that provides specification for computers, equipment, and services for multimedia communication over networks that do not provide a guaranteed quality of service. H.323 computers and equipment can carry real-time video, audio, and data, or any combination of these elements. This standard is based on the Internet Engineering Task Force (IETF) Real-Time Protocol (RTP) and Real-Time Control Protocol (RTCP), with additional protocols for call signaling, and data and audiovisual communications.

Users can connect with other people over the Internet and use varying products that support H.323, just as people using different makes and models of

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telephones can communicate over Public Switched Telephone Network (PSTN) lines. H.323 defines how audio and video information is formatted and packaged for transmission over the network. Standard audio and video codecs encode and decode input/output from audio and video sources for communication between nodes. A codec (coder/decoder) converts audio or video signals between analog and digital forms.

Also, H.323 specifies T.120 services for data communications and conferencing within and next to an H.323 session. Most importantly, this T.120 support means that data handling can occur either in conjunction with H.323 audio and video, or separately.

Microsoft and more than 120 other leading companies have announced their intent to support and implement H.323 in their products and services. This broad support establishes H.323 as the standard for audio and video conferencing over the Internet.

Benefits

H.323 products and services offer the following benefits to users:

• Products and services developed by multiple manufacturers under the H.323 standard can interoperate without platform limitations. H.323 conferencing clients, bridges, servers, and gateways support this interoperability.
• H.323 provides multiple audio and video codecs that format data according to the requirements of various networks, using different bit rates, delays, and quality options. Users can choose the codecs that best support their computer and network selections.
• The addition of T.120 data conferencing support to the H.323 specification means that products developed under H.323 can offer a full range of multimedia functions, with both data and audiovisual conferencing support.

H.323 Interoperability and Testing

The interoperability of H.323 products is measured on the following three levels:

• Call signaling and control

  Test cases verify that NetMeeting can establish a conference over Transmission Control Protocol/Internet Protocol (TCP/IP) connections with the appropriate data flow and sequencing. Testing identifies whether third-party products interoperate based on the H.323 specifications for the H.245 and Q.931 protocols.

  Call signaling and control tests attempt to negotiate these capabilities in the following ways:

  • Verifying that a third-party product can accept a NetMeeting call using the same default codecs, or that NetMeeting can negotiate a suitable set of codecs.
  • Determining whether the products can open channels and pass data after the call is established.
  • Verifying that all control sequencing runs correctly.

  Typically, call control interoperability testing fails when a call is out of sequence or the call is not accomplished in the allotted amount of time.

• Audio and video streaming

  Test cases verify that NetMeeting and third-party products can manage the streaming of audio and video packets over User Datagram Protocol (UDP) connections. The streaming mechanisms for H.323-compliant products are Real-Time Protocol (RTP) and Real-Time Control Protocol (RTCP). Interoperability problems might occur within RTP and RTCP; for example, the packet header might be incorrect or a bit could be missing.

• Audio and video codec compatibility

  Test cases determine whether a third-party product provides compatible codecs for formatting and transmitting the audio and video input/output. NetMeeting runs best using G.723 and H.263, but can negotiate other codecs, such as H.261 and G.711, as necessary. Testers verify that the third-party product can exchange real-time audio and video successfully with NetMeeting. Typically, codec problems evolve from subtle differences in the algorithms used by NetMeeting and the third-party product.

H.323 Architecture

The following illustration shows the H.323 architecture. This architecture defines a set of specific functions for framing and call control, audio and video codecs, and T.120 data communications. The illustration also shows interfaces for the network, and audio and video equipment interfaces.

H.323 terminal architecture, shown in the illustration, is the most common implementation of the H.323 specification. This same architecture can also be implemented for an H.323 Multipoint Control Unit (MCU), gateway, and gatekeeper. For more information about these H.323 components, see "H.323 MCUs, Gateways, and Gatekeepers" later in this chapter.

Framing and Call Control

The following standards make up the System Control Unit, which provides call control and framing capabilities:

• H.225.0

  This standard defines a layer that formats the transmitted video, audio, data, and control streams for output to the network, and retrieves the corresponding streams from the network. As part of audio and video transmissions, H.225.0 uses the packet format specified by Internet Engineering Task Force (IETF), RTP, and RTCP specifications for the following tasks:

  • Logical framing

    Defines how the protocol frames (packages) the audio and video data into bits (packets) for transport over a selected communications channel.

  • Sequence numbering

    Determines the order of data packets transported over a communications channel.

  • Error detection

    After initiating a call, one or more RTP or RTCP connections are established. Multiple streams allow H.225.0 to send and receive different media types simultaneously, each with their own frame sequence numbers and quality of service options. With RTP and RTCP support, the receiving node synchronizes the received packets in the proper order, so the user hears or sees the information correctly.

    The H.225.0 standard also includes registration, admission, and status (RAS) control, which is used to communicate with the gatekeeper. A RAS signaling channel makes the connections between the gatekeeper and H.323 components available. The gatekeeper controls H.323 terminal, gateway, and MCU access to the local area network (LAN) by granting or denying permission to H.323 connections. For more information about gatekeepers, see "H.323 MCUs, Gateways, and Gatekeepers" later in this chapter.

• Q.931

  This protocol defines how each H.323 layer interacts with peer layers, so that participants can interoperate with agreed upon formats. The Q.931 protocol resides within H.225.0. As part of H.323 call control, Q.931 is a link layer protocol for establishing connections and framing data. Q.931 provides a method for defining logical channels inside of a larger channel. Q.931 messages contain a protocol discriminator that identifies each unique message with a call reference value and a message type. The H.225.0 layer then specifies how these Q.931 messages are received and processed.

• H.245

  This standard provides the call control mechanism that allows H.323-compatible terminals to connect to each other. H.245 provides a standard means for establishing audio and video connections — the series of commands and requests that must be followed for one component to connect and communicate with another. This standard specifies the signaling, flow control, and channeling for messages, requests, and commands.

  The built-in framework of H.245 enables codec selection and capability negotiation within H.323. Bit rate, frame rate, picture format, and algorithm choices are some of the elements negotiated by H.245.

Audio and Video Codecs

Codecs define the format of audio and video information and represent the way audio and video are compressed and transmitted over the network. H.323 provides a variety of options for audio and video coding. Two codecs, G.711 for audio and H.261 for video, are required by the H.323 specification. H.323 terminals must be able to send and receive A-law and µ-law coding algorithms (also known as G.711), as determined by the International Telecommunications Union, Telecommunication Standardization Sector (ITU-T). Additional audio and video codecs provide a variety of standard bit rate, delay, and quality options that are suitable for a range of network selections. Using H.323, products can negotiate nonstandard audio and video codecs.

The following paragraphs describe the required audio and video codecs (G.711 and H.261), as well as the two default codecs preferred for NetMeeting connections (G.723 and H.263), which offer the low-bit rate connections necessary for audio and video transmission over the Internet.

• G.711

  This codec transmits audio at 48, 56, and 64 kilobits per second (Kbps). This high-bit-rate codec is appropriate for audio over higher speed connections.

• G.723

  This codec specifies the format and algorithm used to send and receive voice communications over the network. G.723 is a high-speed codec that transmits audio at 5.3 and 6.3 Kbps, which reduces bandwidth usage.

• H.261

  This codec transmits video images at 64 Kbps (VHS quality). This high bit-rate codec is appropriate for video over higher speed connections.

• H.263

  This codec specifies the format and algorithm used to send and receive video images over the network. This codec supports common interchange format (CIF), quarter common interchange format (QCIF), and sub-quarter common interchange format (SQCIF) picture formats and is superior for Internet transmission over low-bit-rate connections, such as a 28.8 Kbps modem.

T.120 Data Communications

H.323 makes a provision for using T.120 as the mechanism for packaging and sending data. T.120 can use the H.225.0 layer to send and receive data packets or simply create an association with the H.323 session and use its own transport capabilities to transmit data directly to the network. Data from conferencing programs, such as file transfer and program sharing, use T.120 support to operate in conjunction with H.323 connections. Also, H.323-compatible products interoperate with data conferencing products developed under the T.120 specification.

For more information about the T.120 architecture, see the International Multimedia Teleconferencing Consortium (IMTC) Web site: http://www.imtc.org/h323.htm

H.323 MCUs, Gateways, and Gatekeepers

In addition to the H.323 terminal architecture, additional components can be implemented as described below:

• Multipoint Control Unit

  A Multipoint Control Unit (MCU) in an H.323 conference, also called conferencing servers or conferencing bridges, allows three or more H.323 terminals to connect and participate in a multipoint conference. An MCU includes both multipoint controllers, which manage the H.323 terminal functions and capabilities in a multipoint conference, and multipoint processors, which process the audio, video, and data streams between H.323 terminals.

• Gateway device

  H.323 conference gateways make H.323 terminals on a LAN available to H.323 terminals on a wide area network (WAN) or another H.323 gateway. Gateways are the translation mechanism for call signaling, data transmission, and audio and video transcoding. Gateways satisfy part of the interoperability vision of H.32x products due to the ability to connect to each other.

  Gateways can serve the following purposes:

  • To bridge an H.323 call to another type of call, such as a telephone. Potentially, NetMeeting could call any telephone in the world.
  • To bridge H.323 calls to H.320, which is audio and video transmission over Integrated Services Digital Network (ISDN) .
  • To bridge H.323 calls to H.324, which is audio and video transmission over standard telephone lines.
  • To bridge different networks; an organization could put a bridge on a firewall to connect an internal corporate network with external networks to accept incoming calls.

    In this case, gateway functions are similar to an MCU for connecting people over networks. Typically, though, the gateway is the translation mechanism in a point-to-point connection, where only one endpoint is an H.323 device. On the other hand, an MCU typically connects many H.323 devices in a multipoint conference.

• Gatekeeper device

  Gatekeepers provide network services to H.323 terminals, MCUs, and gateways. H.323 devices register with gatekeepers to send and receive H.323 calls. Gatekeepers give permission to make or accept a call based on a variety of factors.

  Gatekeepers can provide network services such as:

  • Controlling the number and type of connections allowed across the network.
  • Helping to route a call to the correct destination.
  • Determining and maintaining the network address for incoming calls.

How NetMeeting Uses the H.323 Standard

Microsoft developed NetMeeting audio and video conferencing features based on the H.323 infrastructure, which allows NetMeeting to interoperate with other H.323 standards-based products. H.323 codecs and protocols are the building blocks for the following NetMeeting functions:

• The ability to establish and maintain audio and video connections

  Two participants can establish a NetMeeting conference with audio and video over a Transmission Control Protocol/Internet Protocol (TCP/IP) connection. With H.225.0, multiple audio and video streams transport inbound and outbound NetMeeting information. With H.323 protocols, NetMeeting users can communicate with and transmit data to other compatible audio or video clients. In addition, an MCU allows multiuser audio and video conferences between multiple NetMeeting clients, as well as other H.323-compatible products.

• Audio and video codecs that optimize Internet connections

  NetMeeting provides a suite of codecs operating between 4.8 Kbps and 64 Kbps that support various computer and connection types. For optimal performance over the Internet, NetMeeting specifies H.263 and G.723 as the default codecs. NetMeeting can negotiate other codecs, such as H.261 or G.711, depending on the requirements of other H.323-compatible products. Also, NetMeeting creates appropriate payload formats and handlers for custom codecs.

• Support for T.120 data communications

  NetMeeting creates the association between T.120 and H.323 during a NetMeeting conference. This association allows the NetMeeting call to be completed in two phases, one each for T.120 and H.323, but appear as a single call.

Improved security and interoperability with streaming media servers are possible future additions to the H.323 standard.

H.323 Conferencing Products and Services

NetMeeting can potentially operate with any H.323 conferencing product or service over TCP/IP connections. These products and services can be separated into the following categories:

• Audio and video conferencing clients

  NetMeeting is capable of connecting to any audio or video conferencing client that supports the H.323 specification. Depending on the availability of compatible audio and video codecs, H.323 products can potentially interoperate and exchange audio and video information. For example, NetMeeting is fully interoperable with the Intel® Internet Video Phone, providing high-quality, face-to-face video and audio conferencing from a local desktop computer.

• Conferencing servers or bridges

  Conference servers and bridges can host multipoint audio and video conferencing for H.323-compatible products. Some servers and bridges may also host audio and video conferencing in conjunction with data conferencing, such as program sharing and file transfer. Service providers can offer an advanced network of audio, video, and data conferencing bridges, professional conference administrators, and special features, such as password security. Connecting multiple sites in the United States or worldwide, these services assist with equipment, reservation, scheduling, and customized reporting needs.

• Gateway devices

  Several companies have developed gateways that allow NetMeeting users to bridge to other types of networks, such as an H.320 ISDN video conference or a regular telephone on a public network.

• Gatekeeper devices

  Several companies have developed H.323 gatekeepers that can provide network services to NetMeeting, like routing calls through corporate firewalls and to the PSTN. Gatekeepers can also limit the number of calls that can be placed at any given time.