H.323 is an ITU-T standard for packet-based audio, video, and data communications across IP-based networks.
The ITU-T H.323 standard is a foundation for audio, video, and data communications across IP-based networks, including the Internet. By complying with the H.323 standard, multimedia products and applications from multiple vendors can interoperate, thereby allowing users to communicate without concern for compatibility.
The H.323 standard is broad in scope and includes standalone devices (such as IP telephones and voice gateways), embedded personal computer technology (such as PCs with Microsoft's NetMeeting), and point-to-point and multipoint conferencing. H.323 includes call control (including session setup, monitoring, and termination), multimedia management, bandwidth management, and multicast support in multipoint conferences.
Communications under H.323 are a mix of audio, video, data, and control signals. To establish a voice call, H.323 refers to other standards, including H.225 and H.245. The H.225 standard is based on the Q.931 protocol. It describes call signaling and the Registration, Admission, and Status (RAS) signaling used for H.323 session establishment and packetization between two H.323 devices. For example, the H.225 setup message has information elements that include the calling party number and the called party number H.245 is a control standard for multimedia communication that describes the messages and procedures used for opening and closing logical channels for audio, video, and data, capability exchange, control, and indications.
An H.323 conference can include endpoints with different capabilities. For example, a terminal with audio-only capabilities can participate in a conference with terminals that have video and data capabilities. An H.323 multimedia terminal can share the data portion of a videoconference with a data-only terminal while sharing voice, video, and data with other H.323 terminals.
H.323 defines four major components for a network-based communications system: terminals, gateways, gatekeepers, and multipoint control units (MCU).
Terminals are client endpoints that provide real-time two-way H.323 communications with other endpoints, such as H.323 terminals, gateways, or MCUs. All terminals must support standard 64-kbps PCM-encoded voice communications; video and data are optional. Examples of H.323 terminals are IP telephones and PCs with Microsoft NetMeeting software.
An H.323 gateway is an optional element in the voice network; it can be a voice-enabled router or switch. Gateways provide many services, such as translation between H.323 endpoints and non-H.323 devices, which allows H.323 endpoints and non-H.323 devices to communicate. In addition, the gateway also translates between audio, video, and data formats; converts call setup signals and procedures; and converts communication control signals and procedures.
Gateways are not required between two H.323 terminals because these endpoints can communicate with each other directly.
Terminals use the H.245 and Q.931 protocols to communicate with H.323 gateways. An example of a gateway is a voice-enabled router providing a connection to the PSTN, a PBX, or an analog phone. An interface on a voice gateway that carries voice data is a voice port. A voice port is a physical port on a voice module; this is what makes a router voice-enabled.
A voice module enables connectivity with traditional circuit-switched voice devices and networks. It converts voice into IP packets and vice versa. Specialized processors called digital signal processors (DSP) are located on the voice module and perform the coding and compressing of voice data. The following are some of the voice modules available on Cisco voice gateways:
■ ISDN PRI on an E1 or T1 voice module
■ E1-R2 signaling on an E1 voice module
■ T1-CAS signaling on a T1 voice module
■ FXS on a low-capacity voice module
■ FXO on a low-capacity voice module
■ ISDN BRI on a low-capacity voice module
An H.323 gatekeeper is another optional element that manages H.323 endpoints. The terminals, gateways, and MCUs managed by a single gatekeeper are known as an H.323 zone; there is a one-to-one relationship between a zone and a gatekeeper.
A gatekeeper is typically used in larger, more complex networks; the gatekeeper function can be performed by a Cisco IOS router or by third-party software. A gatekeeper serves as the central point for all calls within its zone and provides call control services to registered H.323 endpoints. All H.323 devices in the zone register with the gatekeeper so that the gatekeeper can perform its basic functions, such as H.323 address translation, admission control, bandwidth control, and zone management. Optionally the gatekeeper provides call control signaling, call authorization, bandwidth management, and call management.
The gatekeeper can balance calls among multiple gateways, either by integrating their addressing into the Domain Name System or via Cisco IOS configuration options. For instance, if a call is routed through a gatekeeper, that gatekeeper can forward the call to a corresponding gateway based on some routing logic. When an H.323 gatekeeper acts as a virtual voice switch, its function is known as gatekeeper-routed call signaling.
NOTE The Cisco Unified Communications Manager does not support the gatekeeper-routed call signaling capability.
The Importance of a Gatekeeper
Figure 8-16 illustrates some different voice design options and emphasizes the importance of a gatekeeper, especially in large voice network designs.
Voice network design depends primarily on the number of voice gateways and, consequentially, the number of logical connections between them. The maximum number of logical connections between voice gateways, and, as a result, the network's complexity, can be calculated by the formula (N * (N-1))/2, where N is the number of voice gateways in the system. For example, the maximum number of logical connections between three voice gateways is three, between five voice gateways is ten, and between eight voice gateways is 28. The complexity of the network grows quickly with the number of gateways; adding one more voice gateway to an existing network means reconfiguring all other voice gateways, making network maintenance quite difficult. A solution for this issue is the use of a gatekeeper.
Figure 8-16 The Importance of a Gatekeeper in Voice Networks
Small Voice Network Scenario Medium Voice Network Scenario
Complete Dialing Plan on Every Gateway
Large Voice Network Scenario
The gatekeeper stores the dialing plan of the entire zone. Gateways only have to register with the gatekeeper; the gatekeeper provides all call control services to the gateways.
Therefore, the configuration of a voice gateway becomes simpler and does not require modification when a new voice gateway is added to the system.
An MCU is an H.323 endpoint that enables three or more endpoints to participate in a multipoint H.323 conference. An MCU incorporates a multipoint controller (MC) and optionally one or more multipoint processors (MP).
The MC is the conference controller that handles H.245 capability negotiations between the endpoints and controls conference resources. An MC is not a standalone unit and can be located within an endpoint, terminal, gateway, gatekeeper, or MCU.
The MP handles the conference's data streams. It receives multiple streams of multimedia input, switches and mixes the streams, and retransmits the result to the conference members. An MP resides in an MCU.
Figure 8-17 illustrates the components typically involved in an H.323 call and the interactions between them.
Figure 8-17 Interactions of H.323 Components Traditional Phone Voice Trunk
Voice-to-IP and IP-to-Voice Conversions in Voice Gateways
No Conversion Required for H.323-Capable Devices
No Conversion for H.323-Capable Devices
If traditional telephones are used and an IP network must transport calls, a voice gateway is required on both sides of the IP network. In this example, the gateway is a voice-enabled router that performs voice-to-IP and IP-to-voice conversions in DSPs. After the gateway router converts voice into IP packets, it transmits the packets across the IP network. The receiving router performs the same function in the reverse order: It converts IP packets to voice signals and forwards them through the PBX to the destination telephone.
A voice gateway is not required when H.323-capable devices (terminals) communicate over an IP network; the router forwards IP packets it receives from an H.323 device to the appropriate outgoing interface. A voice gateway is required, however, to convert between an IP network and the PSTN.
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