VoIP can communicate with other systems operating on a packet-switch network. To be able to communicate, a standard communication system that is compatible with each other is needed. One of the communication standards in VoIP according to the recommendations of the International Telecommunications Union-Telecommunications (ITU-T) is H.323 (1995-1996). The H.323 standard consists of components, protocols, and procedures that provide multimedia communication over a packet-based network. Forms of packet-based networks that can be traversed include the internet, Internet Packet Exchange (IPX)-based, Local Area Network (LAN), and Wide Area Network (WAN). H.323 can be used for multimedia services such as voice communication (IP telephony), video communication with voice (video telephony), and a combination of voice, video and data.
Figure 5.34. Packet Network Terminal
The design and development goal of H.323 is to enable interoperability with other types of multimedia terminals. Terminals with the H.323 standard can communicate with H.320 terminals on N-ISDN, H.321 terminals on ATM, and H.324 terminals on the Public Switched Telephone Network (PSTN). H.323 terminals enable real-time two-way communication of voice, video, and data.
1. H.323 Architecture
The H.323 standard consists of 4 physical components that are used when connecting point-to-point and point-to-multipoint multimedia communications on various types of networks:
a. Terminal
Used for two-way real-time multimedia communications. H.323 terminals can be personal computers (PCs) or other stand-alone devices capable of running multimedia applications.
b. Gateway
Used to connect two different networks, namely between H.323 networks and non-H.323 networks, for example a gateway can connect and provide communication between H.233 terminals and telephone networks, for example: PSTN. In connecting two different forms of networks, this is done by translating protocols for call setup and release and sending information between networks connected to the gateway. However, a gateway is not needed for communication between two H.323 terminals.
c. Gatekeeper
It can be considered as the brain of the H.323 network because it is the critical node in the H.323 network.
d. Multipoint Control Unit (MCU)
Used for H.323 conference services of three or more terminals. All terminals wishing to participate in the conference can establish a connection with the MCU which manages the materials for the conference, negotiating between the terminals to determine the audio or video coder/decoder (CODEC). According to the H.323 standard, an MCU consists of a Multipoint Controller (MC) and several Multipoint Processors (MPs). The MC handles the H.245 negotiation (signaling) between the terminals to determine the audio and video processing capabilities. The MC also controls and determines the audio and video sequences to be multicast. The MC does not directly deal with the media sequence. This task is assigned to the MPs which mix, switch, and process the audio, video, or data bits. The gatekeeper, gateway, and MCU are logically separate components in the H.323 standard but can be implemented as one physical device.
Figure 5.35. H.323 architecture
2. H.323 Protocol
In H.323 there are several protocols in data delivery that support real-time data delivery. Below are explained several protocols in the network and transport layers.
a. Standard codec
H.323 establishes standards for compression and decompression of video and audio data streams, to ensure that different equipment still supports the same technical features.
b. Interoperability
Users who want to conduct a conference do not have to worry about compatibility on the receiving end. In addition to ensuring that the receiver can decompress the information sent, H.323 also develops a method for receiving clients to communicate, which is as capable as the sender.
c. Network Independence
H.323 is designed to run on the upper layers of common network architectures. As network technologies evolve, and bandwidth management techniques improve, H.323-based solutions are expected to keep up with these developments.
d. Platform and Application Independence
H.323 is not tied to any particular hardware or operating system. H.323-compliant platforms will come in a variety of shapes and sizes, including video-enabled PCs, dedicated platforms, IP-enabled telephony equipment, and cable TV.
e. Support for multipoint
Although H.323 can support conferences of up to three or more endpoints without the need for a social multipoint control unit, MCUs actually provide a flexible and powerful architecture for multipoint conferencing. Multipoint capabilities can be included in every component of an H.323 system.
f. Bandwidth management
Video and audio traffic is a traffic that requires large bandwidth and can mostly make data communication networks hampered. H.323 tries to find a solution to this problem by preparing bandwidth management. The network manager can limit the number of users connected to the H.323 network simultaneously, according to the bandwidth available for the H.323 application. This limitation ensures that the critical point of the traffic will not be passed.
g. Support for multicast
H.323 supports multicast transport in multipoint conferences. Multicast sends a single packet to a subset of destinations in the network without replication. Unicast sends multiple point-to-point transmissions, and broadcast sends a packet to all destinations. In unicast or broadcast, the network is used inefficiently because many packets are replicated throughout the network. Multicast transmission uses bandwidth more efficiently because all terminals in the multicast group only read a single data stream.
h. Flexible
An H.323 conference can include multiple endpoints with different capabilities. For example, a terminal capable of voice only can participate in a conference with terminals capable of video and data. Furthermore, an H.323 multimedia terminal can share the data portion for a video conference with a terminal capable of T.120 (data) only, while sharing voice, data and video with other H.323 terminals.
i. Inter-Network Conferencing
Many users want to conduct conferences from a LAN to a long distance. Therefore, H.323 tries to establish a connection between a LAN-based desktop system and an ISDN-based group system. H.323 uses common codec technology for each different video conference standard to reduce transcoding delay and to provide optimal performance.