H.323 is one of the oldest standards which are generally used for VoIP telephony and video conferencing. It is a system of various protocols and elements that allows transferring media data over packet networks. This standard recommendation structure opens up different multimedia communication options, including telephony, video conferencing and media transfer.
One of the main benefits of H.323 standard is interoperability. With this internet protocol, solutions produced by different vendors can connect and operate with each other seamlessly.
Many years have passed since the introduction of H.323, and the standard was gradually developing. 7 versions have been released since 1996, when H.323 standard was approved by the International Telecommunication Union (ITU) to promote interoperability and consistency in video conference transmissions over IP networks.
Before H.323 protocol, all VoIP solutions were based on their own signaling protocols and lacked interoperability. However, in 1996, with H.323 first version, the standard became widely available.
The first version was rather poor, because its only goal was to establish communication between endpoints of different vendors. This version did not add to reliability, security and good quality of communications, while the endpoints which had been incompatible previously could now "communicate" only within the corporate network.
The second version which was released two years later was the real breakthrough. It was aimed at the active use of VoIP telephony in multipoint conferencing. This time the key purpose was the standard’s reliability and higher security, including such elements as authentication of the endpoints (conference participants), fixed packet data transfer and unauthorized data access protection in networks. Oddly enough, in this version users could not reject incoming calls. Additionally, this version boosted connection between endpoints and added call redirection option.
The third version added support for multiple signalling sessions over single TCP connection. It was particularly useful for network gateway equipment allowing for thousands of simultaneous calls.
The fourth version featured increasing H.323 endpoints capacity. The launch of the fifth version was aimed at general standard stabilization. TrueConf solutions are based on the fourth version of H.323 protocol.
In June 2006, the sixth version of the standard was approved including a set of changes in H.225 and H.245 transport protocols. The sixth version added support for Assigned Gatekeeper, where the endpoint from the list of alternative gatekeepers is registered. Additionally, a number of applications and documents were supported to allow use of GSM and H.264 codecs in H.323 solutions. The seventh H.323 version was released in November 2009. Among many other updates, there are two important features worth mentioning:
H.323 standard is based on four components used to run video conferencing, e.g. point-to-point or multipoint conferences:
Endpoint is a tool used to manage H.323 device (sort of user interface). Endpoints can connect with each other using VoIP telephony or video conferencing mode.
Gateways are used to connect endpoints from various networks, e.g. H.323 and ISDN. By the way the gateway performs the following functions:
If the endpoints are in the same H.323 network, the gateways are not used.
Zone controller or gatekeeper — is the central point of the H.323 network. It is responsible for addressing calls, manages bandwidth and defines authenticity of endpoints and gateways during connection. Although H.323 recommendation does not set gatekeeper as a required element, it is impossible to use many modern features implemented in VoIP applications and video conferencing solutions without gatekeeper.
Multipoint Control Unit (MCU) server is used to connect three or more endpoints during single session. All the endpoints participating in video conferencing are first connected to the MCU server, and then MCU distributes video streams to all the endpoints. MCU performs video streams transcoding in order to mix and downscale other participants video in a unique layout for every participants. This process require a lot of computational elements and resources which makes MCU a very expensive device. MCU device usually combines MCU, gateway and gatekeeper roles.
Each endpoint or device supporting H.323 protocol has its own IP address. This internet protocol packets routing within networks is established via H.323 endpoint. UDP protocols are used to connect endpoints to gateways and gatekeeper and transmit media traffic. TCP transport protocols are only used to establish a call between endpoints and exchange additional features, so called signalling.
According to H.323 recommendation, media data transfer is divided into five basic steps:
Detection process is required for endpoints to find a gatekeeper by its network address and to register on it. This procedure can be performed automatically (if there are several gatekeepers, the endpoint selects gatekeeper to register on by means of multicasting messaging over the networks) or manually (the gatekeeper's network address is known in advance and set during device configuration). The first variant of gatekeeper detection is more preferable, as the endpoint will be able to switch to another gatekeeper automatically in case of gatekeeper breakdown.
The registration procedure is required to allow endpoints send their addresses to gatekeeper and join its control zone.
To establish connection between the endpoints and to exchange media traffic in networks, the following protocols are used:
TCP:
UDP:
To terminate connection, the endpoints send a message to the gatekeeper. After that, the channel closes and the connection is terminated.
Audio exchange function is considered the main function of H.323 standards (as they had originally been used in VoIP telephony). Thus, each endpoint should support at least one codec from the G.7XX family.
Video communications were positioned as secondary in respect to this standard. As a consequence, video codec support was not obligatory. However, today, as video conferencing is the main task for every endpoint supporting the standard, video codecs are required. H.323 uses video codecs from H.26X codec family for video encoding.
There is a list of technical requirements for audio codecs in H.323, since sound quality is essential in VoIP telephony. The requirements are as follows:
G.7XX family codecs meet these requirements. However, bandwidth requirement is met by few G.7XX codecs.
By default, H.323 uses the G.711 codec with high bandwidth rate (about 64 kbit/s). However G.711 is considered to be outdated. Its sampling frequency (converting analog signal into digital signal) is only 8kHz, while G.722.1 has twice as high frequency (16kHz). Low frequency codecs such as G.723 (5.3/6.3kbps) and G.729 (8kbps) were used for the Internet connections initially. As for video codecs, every modern H.323 endpoint should support H.264, which become a standard. There is new H.265 video codec a successor for H.264, however it is supported by few devices on the market so far and we don’t expect it will become a default codec earlier than in 2022.
In this article we have compared H.323 and SIP protocols.
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