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H.323 uses a binary method based on ASN.1 and compression coding rules to express its message. ASN.1 usually requires special code generators for lexical and grammar analysis. And SIP's text-based protocol is similar to HTTP. Text-based encoding means that the meaning of the header field is clear, such as From, To, Subject and other domain names. This kind of distributed, standard specification style with almost no complicated documentation, its superiority has been fully demonstrated in past practice (the popular mail protocol SMTP is now an example). The message body part of SIP is described by SDP. Each item in SDP has a format of = and is relatively simple.
In terms of supporting conference calls, H.323 centralized execution of conference control functions by the multipoint control unit (MCU), all participating conference terminals send control messages to the MCU, and the MCU may become a neck, especially for large conferences with additional features. And 323 does not support the signaling multicast function. Its single function limits scalability and reduces reliability. The SIP design is a distributed call model with distributed multicast functions. The multicast function not only facilitates conference control, but also simplifies user positioning, group invitation, etc., and can save bandwidth. However, H.323 centralized control facilitates billing and management of bandwidth is relatively simple and effective.
H.323 defines special protocols for supplementary services such as H.450.1, H.450.2 and H.450.3. SIP does not specifically define the protocol for this purpose, but it conveniently supports supplementary services or smart services. As long as you make full use of SIP's defined header fields (such as the Contact header field) and simply extend the SIP (for example, adding several domains), you can implement these services. For example, for call transfer, adding a Contact header field to a BYE request message and adding a third party address to which the user intends to transfer can implement this service. For some intelligent services that are difficult to implement by extending the header domain, service agents may be added to the architecture, and some supplementary services or interfaces with the intelligent network devices may be provided.
In H.323, the call setup procedure involves a third signaling channel: the RAS signaling channel, the call signaling channel, and the H.245 control channel. Through the coordination of these three channels, the H.323 call can be made and the call setup time is long. In SIP, the session request process and the media negotiation process are performed together. Although H.323v2 has made improvements to the call setup process, it still requires no more than 1.5 loop delays compared to SIP to establish a call. H.323 call signaling channel and H.245 control channel require a reliable transmission protocol. SIP is independent of low-level protocols. Generally, SIP uses unreachable protocols such as UDP and uses its own signaling layer's reliability mechanism to ensure the reliable transmission of messages.
In short, H.323 follows the traditional implementation of the telephone signaling model. It is relatively mature and many H.323 products have emerged. H.323 conforms to the traditional design concept in the field of communications, performs centralized and hierarchical control, and uses the H.323 protocol to facilitate connection with a traditional telephone network. The SIP protocol draws on the design concepts of other Internet standards and protocols, and follows the principles of conciseness, openness, compatibility, and extensibility that the Internet has consistently adhered to. It is relatively simple.
Talking about the difference between H.323 and SIP protocols
At present, VoIP communication protocols are mainly divided into two camps: SIP and H.323. H.323 attempts to treat IP telephony as a well-known traditional telephone, except that the transmission mode has changed, from circuit switching to packet switching. The SIP protocol focuses on the use of IP phones as an application on the Internet. Compared with the actual applications (such as FTP, E-mail, etc.), the requirements for signaling and QoS are increased. The services they support are basically the same, and all use RTP as media transmission. Agreement. However, H.323 is a relatively complex protocol.