H.323 and SIP are recommendations for the two major camps in the communications field and the Internet. H.323 Attempts to put the IP phone as a well-known traditional phone, just a change in transmission, and the circuit exchange becomes a packet exchange. The SIP protocol focuses on an application on the Internet as an application of the Internet, which adds signaling and QoS, which supports the QoS, which supports the business, and RTP is used as media transmission. Agreement. But H.323 is a relatively complex protocol.
H.323 uses a binary method based on ASN.1 and compressed coding rules to represent its message. ASN.1 typically requires special code generators to perform lexics and syntax analysis. And SIP's text-based protocol is similar to HTTP. The text-based coding means that the meaning of the head domain is an atmosphere, such as from, to, subject and other domain names. This distribution, almost no complicated document description of the standard specification style, its superiority has been fully proof in the past practice (now a popular email protocol SMTP is an example). SIP's message body part uses SDP to describe that each format in the SDP is =, it is also relatively simple.
In supporting the conference call, H.323 is implemented by the conference control function by the multi-point control unit (MCU), all participating in the conference terminal sends a control message to the MCU, and the MCU may become neck, especially for large conferences with additional characteristics. And H.323 does not support the multicast function of signaling, and its single function limits scalability and reduces reliability. The SIP design is a distributed call model, which has a distributed multicast function. The multicast feature is not only for conference control, but also simplifies user positioning, group invitation, etc., and can save bandwidth. However, the centralized control of H.323 facilitates charging, and the management of bandwidth is also relatively simple and effective.
Special protocols are defined in H.323 for supplementary services, such as H.450.1, H.450.2 and H.450.3, and the like. SIP is not specifically defined for this purpose, but it is convenient to support supplemental business or intelligent services. These services can be implemented as long as the SIP-defined head domain (such as a Contact Schera) is made to make simple extensions for SIP (such as adding several domains). For example, for call forwarding, this service can be implemented as long as the Contact headnament intended in the BYE request message. For some intelligent services that are difficult to implement by extended head domains, a business agent can be added to the architecture, providing some supplementary services or interfaces with intelligent network devices.
In H.323, the call setup process involves a third signaling letter to: RAS signaling channel, call signaling, and H.245 control channel. The coordination of these three channels allows the H.323's call to be carried out, and the call setup time is very long. In the SIP, the session requesting process and the media consultation process are taken together. Although H.323V2 has improved the call setup process, it is still not possible to extend the call when the SIP only needs 1.5 loops. The H.323 call signaling channel and the H.245 control channel require a reliable transmission protocol. The SIP is independent of the low-level protocol, generally using the unacceptable protocols such as UDP, with the reliability of the message with the reliability mechanism of their own credit layers.
In summary, H.323 is used as a traditional implementation of telephone signaling mode, mature, has appeared a lot of H.323 products. H.323 is in line with traditional design ideas in communication, focusing, hierarchical control, using H.323 protocol to connect to traditional telephone networks. The SIP protocol draws on the design idea of other Internet standards and protocols, and comply with the principle of concretion, opening, compatibility, and scalables such as the Internet.
The following is simple to analyze the following aspects of their application objectives, standard structures, system composition, and system implementation.
Standard application goal
The H.323 standard is established by ITU-T organization in 1996 in H.320 / H.324, and its application goal is to achieve reliable video and data real-time applications in the network environment of base IP. . After years of technology development and standards have been continuously improved, H.323 has become a mature standard family that is accepted by customers and customers.
The SIP standard is proposed by ITEF in 1999. Its application goal is based on Internet environment, realizing data, audio and video real-time communication, especially through the Internet, and introduces thousands of households. Since the SIP protocol is relatively simple, free, manufacturers can use relatively small costs to construct a system that satisfies the application. For example, using Microsoft's SIP-based MSN, and RTC can construct a simple, Internet access environment based on the Internet application environment. Such network operators can use existing network resources to expand the expansion of video and audio communication services on the basis of as little costs.
Standard architecture
H.323 is not a single standard, but a complete standard family in real-time multimedia applications in the IP environment, which has improved and strict provisions for all aspects of calling, management, and transported media formats. A multimedia system that complies with the H.323 standard, ensures that the customer's stable and improvement of multimedia communication applications.
SIP standard is a signaling standard for real-time multimedia applications. Since it uses text-based encoding mode, it has great flexibility in applications, especially point-to-point applications, The expansion and the compatibility of cross-platform use make the operators can easily use the existing network environment to achieve large-scale promotion applications.
However, the SIP protocol itself does not support multi-point conference functionality and management and control functions, but to rely on other protocol implementations, affect the system's completeness, especially for multi-purpose communication requirements, application simple SIP system is difficult accomplish. In response to these shortcomings, the ITU-T SG16 team headed by Radvison has proposed SIP application specification and implements interoperability between SIP and H.323, and successfully solves the application challenge of SIP in multi-point environment.
System composition structure
First, in terms of the functional aspect of the system, the UA equivalent to a H.323 is equivalent to the functionality of the system, and the initiation and reception of the call is implemented, and the codec application of the transmitted medium is completed; the SIP proxy server, redirect The server and the function of the registration server are equivalent to the GateKeeper of H.323, and the terminal registration, the resolution of the call address, and the route.
Second, although the specific implementation of call signaling and control is different, a SIP-based call flow is similar to the H.323 Q931, and the session description protocol (SDP) used by SIP is similar to calls in H.323. Control protocol H.245.
System realization is difficult
The signaling information of the H.323 standard is a binary code that conforms to ASN.1 Per, and is defined in the definition of strict standards in connection implementation, and the system has a small degree of freedom. To achieve large-scale applications, you need to the entire network. Each link is planned.
The SIP standard signaling information is based on the text. It is used in accordance with the ISO10646 UTF-8 encoding, and the constructive structure of the whole system is relatively flexible. The implementation of the terminal and the server is relatively easy to cost, and from the perspective of network operators, Constructing a large-scale video communication network, using the cost of the SIP system is much cheap, and it is also more realistic.
Summarize
By comparing between SIP and H.323 protocols, we are not difficult to see that between H.323 and SIP is not an opposite relationship, but complement each other in different application environments. As a communication standard for the background of the Internet, SIP is a means of effective and realistic means of popularizing video communications, introducing thousands of households. The H.323 system and SIP system organically combined, but also ensures that users can realize diversified functions such as multi-party conferences on the basis of constructing relatively low-cost SIP video systems, and reliably implement SIP systems and H.323 systems. Interworking interoperability, to maximize the user's requirements for future real-time multimedia communications. Technology area
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