"H.323 protocol specifies the components, protocols and procedures for providing multimedia communication on packet switched networks mainly including IP networks. ITU's H.323 protocol and IETF's SIP protocol reflect the technical system of the first strategy to some extent. In order to achieve the telecom operation goal of end-to-end IP phone to a greater extent, people have designed soft switch technology mechanism for telecom operators. IP telephone voice service is the most important and fundamental service function in softswitch.
Softswitch technology
The concept of soft switch technology was proposed by Lucent Bell Laboratories in 1997. Its original intention is to integrate the existing traditional PSTN circuit switching network with IP / ATM data network. Softswitch technology is the control function entity of the next generation network. It provides call control and connection control functions for the real-time services of the next generation network (NGN). It is the core of the call and control of the next generation network. Softswitch technology has the following three basic characteristics: integrated packet network and circuit switching network; Have the capacity of tandem office and terminal office; Call control is separated from media layer and service layer.
The soft switch equipment is located in the control layer and provides the functions of connection control, routing, network resource management, billing, authentication and so on. The soft switch equipment communicates with various media gateways, terminals, application servers and other soft switch equipment using standard protocols. H. 323 is the recommendation of ITU-T working group 16. It is composed of a group of protocols, including the signaling responsible for the encoding, decoding and packaging of audio and video signals, the signaling responsible for the sending, receiving and control of call signaling, and the signaling responsible for capability exchange. H. The fourth version of H.323 has the characteristics of being a carrier level large network. The IP telephone network built based on it can be easily compatible with the traditional PSTN telephone network. From this point of view, H.323 is more suitable for building a telephone to telephone carrier level large network.
H. The 323 protocol family specifies the components, protocols and procedures for providing multimedia communication on packet switched networks, mainly including IP networks. H. 323 defines four components: terminal, gateway, gatekeeper and multipoint control unit. With them, H.323 can support point-to-point or point-to-multipoint communication of audio, video and data. H. The 323 protocol family includes H.225.0 for call establishment, H.245 for control, h.332 for large conferences, H.450. X for supplementary services, etc. H. 323 protocol contains three signaling control channels: Ras signaling channel, call signaling channel and H.245 control channel. The coordination of three channels enables the call of H.323. The components and channels of H.323 are introduced below: H.323 defines four main components to build a network-based communication system: terminal, gateway, gatekeeper and multipoint control unit (MCU).
It is a client point on an IP based network. It needs to support the following three functions: signaling and control, i.e. H.245 (complex protocol related to channel use and channel capability), H.225 (a call signaling transceiver and establishment protocol similar to Q.931) and RAS (defined in H.225 for communication protocol between terminal and gatekeeper); Support real-time communication, that is, support RTP / RTCP (a protocol for sequential processing of audio and video packets); Support coding, that is, compression before transmission and decompression after reception. For interoperability, each h.323 terminal needs to support q.711, and other coding methods are used as options. Provides a connection between a packet switched network and a circuit switched network (SCN).
It is optional in H.323 system, but if they appear, they have some mandatory functions. The gatekeeper completes four necessary functions: address translation, admission control, bandwidth control and domain management. The gatekeeper also supports four optional functions: call control signaling, call authentication, bandwidth management and call management. When there is a gatekeeper in an H.323 system, all types of end users need to log in to the gatekeeper and obtain its permission before establishing a call.
The multipoint control unit supports more than 3 end users to have a session. A typical MCU includes a multipoint controller (MC) and several (or no) multipoint processors (MPS). MC provides control functions, such as negotiation between terminals to determine the common ability to process voice or video. MP completes the processing of media stream in the session, such as voice mixing and voice / video exchange. H. 323 uses channels to exchange information between two communication entities. A channel is a transport layer connection.
H. Comparison between 323 and SIP
SIP was proposed by mmusic group of IETF. Compared with H.323 protocol family proposed by ITU, SIP has the characteristics of simplicity, good scalability and scalability, and close combination with existing Internet applications. Both of them put forward a complete solution to the signaling of IP telephone system. They have the ability to establish, manage and revoke call connections, and have network management functions, so that endpoint users have the ability to establish and interact QoS, and it is easy to expand new functions and support different types of interoperability. Of course, these two protocols contain the requirements for speech coding decompression, but this part is the same for the two protocols.
At present, H.323 and sip are two competing protocols in packet networks. Among them, H.323 protocol is the standard formulated by ITU-T and SIP protocol is the standard formulated by IETF. Both can complete the functions of call establishment, release, supplementary service and capability exchange. H. 323 protocol adopts the design idea of ISDN and Q.931 protocol to complete the establishment and release of calls. It obviously has the characteristics of manageability and centralization of telecom network. At present, H.323 protocol has been widely used on the Internet. Compared with SIP, H.323 is more mature. Therefore, at present, the IP telephone networks of various operators in China use this protocol. SIP protocol has the characteristics of simplicity, good scalability and close combination with existing Internet applications. Many people think that it is relatively easy to implement. Recently, it has developed rapidly in the United States. At the same time, SIP will be widely used in the third generation mobile communication core network and intelligent services. Supporting multiple protocols is one of the main characteristics of softswitch, so softswitch technology can support both protocols at the same time.
conclusion
H. 323 protocol modification process shows that network scalability is always a problem that can not be ignored when building VoIP network. With the construction of broadband network, streaming media service becomes more and more important. The trend of broadband packet network gradually replacing PSTN begins to appear, which provides a good platform for the development of VoIP. H. As one of the supporting protocols of the next generation network, 323 protocol plays a more and more important role.
H. The second version of H.323 is very close to the functions and services supported by SIP. For supplementary services, H.323 is more strictly defined. Therefore, there are few interoperability problems in the implementation of H.323. The two protocols can be compared in their QoS support (similar to the delay of call establishment, and do not support resource reservation or cos (class of server) setting), but the third version of H.323 will allow the signaling required by cos. Therefore, the third version of H.323 is more suitable for giant telecom operators. Softswitch technology is only an implementation means, which can be used in IP phone system based on H.323 or sip. H. 323 provides a working mode of centralized processing and management, which matches the management mode of telecom network. It has the ability to make IP telephone system of any scale, which is a good working mode., Technology Zone
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