H.323 and SIP are the recommendations launched by the two camps in the communications field and the Internet, respectively. H.323 attempts to treat IP phones as well-known traditional phones, but the transmission method has changed from circuit switching to packet switching. The SIP protocol focuses on the use of IP phones as an application on the Internet, which increases signaling and QoS requirements compared to other applications (such as FTP, E-mail, etc.). The services they support are basically the same, and they all use RTP as media transmission. Agreement. But H.323 is a relatively complicated protocol.

H.323 uses a binary method based on ASN.1 and compression coding rules to express its messages. ASN.1 usually requires special code generators for lexical and grammatical analysis. The text-based protocol of SIP 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. The standard specification style of this distributed, almost no need for complex documentation, its superiority has been fully proved in the past practice (now the popular mail protocol SMTP is such an example). The message body part of SIP is described by SDP. The format of each item in SDP is =, which is relatively simple.

In support of conference calls, H.323 has a multipoint control unit (MCU) that centrally executes conference control functions. All participating conference terminals send control messages to the MCU. The MCU may become a neck, especially for large conferences with additional features. ; And H.323 does not support the multicast function of signaling, its single function limits scalability and reduces reliability. The SIP design is a distributed call model, with distributed multicast function, which not only facilitates conference control, but also simplifies user positioning, group invitation, etc., and can save bandwidth. However, the centralized control of H.323 is convenient for billing, and the 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 a protocol for this purpose, but it conveniently supports supplementary or intelligent services. As long as the header fields defined by SIP (such as the Contact header field) are fully utilized, and SIP is simply extended (such as adding several fields), these services can be realized. For example, for call forwarding, as long as the Contact header field is added to the BYE request message, and the third-party address to which the intention is transferred is added, this service can be realized. For some smart services that are more difficult to achieve by extending the header domain, service agents can be added to the architecture to provide some supplementary services or interfaces with smart network devices.

In H.323, the call setup process involves the third signaling signal: RAS signaling channel, call signaling signal and H.245 control channel. Only through the coordination of these three channels can H.323 calls be carried out, and the call establishment time is very long. In SIP, the session request process and the media negotiation process are performed together. Although H.323v2 has improved the call establishment process, compared with SIP, it only requires 1.5 loop delays to establish the call, which is still incomparable. H.323 call signaling channel and H.245 control channel need reliable transmission protocol. SIP is independent of lower-layer protocols, and generally uses unconnectable protocols such as UDP, and uses its own credit-layer reliability mechanism to ensure reliable transmission of messages.

In short, H.323 continues to use the traditional telephone signaling mode, which is relatively mature, and many H.323 products have appeared. H.323 conforms to the traditional design ideas in the field of communication, performs centralized and hierarchical control, and adopts the H.323 protocol to facilitate connection with the traditional telephone network. The SIP protocol borrows from other Internet standards and protocol design ideas, and follows the simple, open, compatible, and extensible principles that the Internet has always adhered to in style, which is relatively simple.

The following is a simple analysis of their application goals, standard structure, system composition, and system implementation difficulty.

Standard application goals

The h.323 standard was established by the itu-t organization in 1996 on the basis of h.320 / h.324, and its application goal is to achieve reliable real-time applications for audio, video, and data in a network environment based on IP . After years of technical development and continuous improvement of standards, h.323 has become a mature standard family accepted by the majority of itu members and customers.

The sip standard was proposed by the ITEF organization in 1999, and its application goal is to achieve real-time data, audio and video communication based on the Internet environment, especially to popularize the application of video communication through the Internet and introduce it to thousands of households. Compared with h.323, the sip protocol is relatively simple and free, and manufacturers can construct a system that meets the application at a relatively low cost. For example, just using MSN based on the SIP protocol and RTC can construct a simple video communication environment based on the Internet application environment. In this way, network operators can use existing network resources to expand video and audio communication services on the basis of as little cost as possible.

Standard architecture

h.323 is not a single standard, but a complete standard family of real-time multimedia applications in the ip environment. It has perfect and strict regulations on call establishment, management, and transmitted media formats. A multimedia system established in compliance with the h.323 standard can ensure the realization of stable and complete multimedia communication applications for customers.

The sip standard is strictly a signaling standard for real-time multimedia applications. Because it uses text-based encoding, it has great flexibility in applications, especially in point-to-point application environments. Expansibility and cross-platform compatibility, this allows operators to easily use the existing network environment to achieve large-scale promotion and application.

However, the SIP protocol itself does not support multi-point conference functions and management and control functions, but depends on other protocol implementations, which affects the completeness of the system. Especially for the requirements of multi-point communication, it is difficult to apply a simple SIP system. achieve. In response to these shortcomings, the itu-t sg16 group led by radvison proposed the application specification of sip, and realized the interconnection and interconnection between sip and h.323, and successfully solved the problem of sip application in multi-point environment.

The composition of the system

First of all, an analogy is made in terms of the functionality of the main members of the system. Sip's ua is equivalent to a h.323 terminal, enabling call origination and reception, and completing the codec application of the transmitted media; sip proxy server, redirect The functions of the server and the registration server are equivalent to the gatekeeper of h.323, which realizes terminal registration, call address resolution and routing.

Secondly, although the specific implementation of call signaling and control is different, a sip-based call flow is similar to h.323's q931, and the session description protocol (sdp) used by sip is similar to the call in h.323 Control protocol h.245.

Ease of implementation

The signaling information of the h.323 standard adopts binary coding that conforms to asn.1 per, and must be strictly defined in the whole process of connection implementation, and the degree of freedom of the system is small. For large-scale applications, the entire network needs to be implemented. Planning for all aspects of

The signaling information of the sip standard is text-based, using utf-8 encoding that conforms to iso10646, and the structure of the entire system is relatively flexible. The implementation of terminals and servers is relatively easy and the cost is low. From the perspective of the network operator, To construct a large-scale video communication network, the cost of using the SIP system is much cheaper, and it is also more achievable.

to sum up

By comparing the sip and h.323 protocols, it is not difficult to see that the relationship between h.323 and sip is not a confrontational relationship, but complements each other in different application environments. SIP, as a communication standard with internet application as the background, is an effective and practical means to popularize video communication and introduce it to thousands of households. The organic combination of the h.323 system and the sip system ensures that users can realize diverse functions such as multi-party conferences on the basis of constructing a relatively inexpensive and flexible sip video system, and reliably implement the sip system and the h.323 system. The intercommunication between them meets the user's requirements for future real-time multimedia communication to the greatest extent.

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