1 Introduction
There are two strategies in the network architecture of the next generation network: one is intelligent endpoints and edges, simple network facilities; the other is simple endpoints and edges, intelligent network facilities. For the former, because the driving force of end-to-end multimedia convergence services comes from endpoints and edges, the booming innovation and prosperity of next-generation network services is relying on this. The success of the Internet also demonstrates the importance of this strategy to multimedia services. For the latter, only a simple unified endpoint and edge equipment is conducive to large-scale operation, management and control. The voice services provided by the PSTN network have proved the reliability of this strategy business practice. ITU's H.323protocoland IETF SIPprotocolIt is to some extent that the technical system of the first strategy is reflected. In order to achieve the telecommunications operation goals of end-to-end IP phones to a greater extent, soft switch technology mechanisms have been designed for telecommunications operators. IP phone voice service is the most important and fundamental business function in soft switches.
2 Soft Switch Technology
The concept of soft switch was proposed by Lucent's Bell Laboratory in 1997. The original intention was to integrate the existing traditional PSTN circuit switching network with IP/ATM data network.
Softswitch is the control function entity of the next generation network, providing call control and connection control functions for services with real-time requirements of the next generation network (NGN). It is the core of next generation network call and control. Soft switch has the following three basic characteristics: integrated packet network and circuit switching network; possesses the ability to connect and end offices; and separate call control from the media layer and service layer.
The soft switch device is located in the control layer and provides connection control for a variety of services.routing, network resource management, billing, authentication and other functions. Standards are adopted between soft switch equipment and various media gateways, terminals, application servers, and other soft switch equipmentprotocolCommunication with each other.
3 H.323protocol
H.323 is the recommendation of ITU-T Working Group 16, by a groupprotocolThe structure includes the encoding, decoding and packaging of audio and video signals, the signaling responsible for call signaling and transmission and reception, and the signaling responsible for capability exchange. The fourth version of H.323 has the characteristics of making a telecommunications-grade large network. The IP telephone network built with it as the standard can be easily compatible with traditional PSTN telephone networks. From this point of view, H.323 is more suitable for building telecommunications-grade large networks from telephone to telephone.
H.323protocolfamily stipulates components for providing multimedia communication on packet switching-based networks, mainly including IP networks,protocoland procedures. H.323 defines four components in total: terminal, gateway, gatekeeper and multipoint control unit. Using them, H.323 can support point-to-point or point-to-multipoint communication of audio, video and data. H.323protocolFamily includes H.225.0 for setting up calls, H.245 for control, H.332 for large conferences, and H. for supplemental services, etc. H.323protocolIt contains 3 signaling control channels: RAS signaling channel, call signaling channel and H.245 control channel. The coordination of 3 channels allows H.323 calls to proceed.
The components and channels of H.323 are described below: 3.1 Parts
H.323 defines four main components to build a network-based communication system: terminal (Terminal), gateway (Gateway), gatekeeper (Gatekeeper) and multi-point control unit (MCU).
3.1.1 Terminal
On an IP-based network is a client point. It needs to support the following 3 functions: support signaling and control, i.e. support H.245 (complexity about channel usage and channel capabilities)protocol) and H.225 (a Q.931-like call signaling sending and receiving and establishingprotocol) and RAS (defined in H.225, used for terminal communication with gatekeepersprotocol); supports real-time communication, that is, supports RTP/RTCP (a sequential processing of audio and video packetsprotocol); supports encoding, that is, compression before transmission and decompress after collection. For interoperability, each H.323 terminal needs to support Q.711, and other encoding methods are options.
3.1.2 Gateway
Provides a connection between a packet switched network and a circuit switched network (SCN, Switch Circuit Network).
3.1.3 Gatekeeper
It is optional in the H.323 system, but if it appears, they have certain mandatory functions, and the gatekeeper completes four necessary functions: address translation, acceptance 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.
3.1.4 Multi-point control unit (MCU)
The multi-point control unit supports more than 3 end users to conduct sessions. A typical MCU includes a multipoint controller (MC) and several (or may not) multipoint processors (MP). MC provides control functions such as negotiation between terminals to decide on the ability to handle voice or video. MP completes the processing of media streams in the session, such as voice mixing, voice/video exchange.
3.2 Channel
H.323 uses a channel to structure information exchange between two communication entities. A channel is a connection to a transport layer.
3.2.1 RAS channel
This channel allows endpoint users to communicate with their gatekeepers, defined in H.225.0, through the RAS channel, the endpoint user logs on to the gatekeeper and requests that it be allowed to call with another end user. If the request is approved, the gatekeeper sends back a transmission address (including the IP address and port number) as the call signaling channel of the called point.
3.2.2 Call signaling channel
This channel carries control information for calls and supplementary services, and this channel uses similar to Q.931protocol，protocolDescription In H.225.0 and H., when the call is established, the transmission address of the H.245 control channel will be specified in this channel.
3.2.3 H.245 control channel
This channel carries H.245protocolinformation for media control with capability to exchange support. After the parties participating in the call complete the capability exchange, a logical channel of media is created through this channel.
3.2.4 Media logical channel
This channel carries voice, video and other media information, and each media type carries on a pair of one-way channels, using RTP and RTCP in each direction.
H.323 stipulates that RAS channels and media logic channels carry on a very reliable transmissionprotocolOn the H.245 control channel specified in reliable transmissionprotocolOn, starting from the third edition, you can choose to carry in unreliable transmissionprotocolsuperior.
4 Comparison between H.323 and SIP
SIP was proposed by the IETF's MMUSIC team, with the ITU's H.323protocolCompared with family, it has the characteristics of simplicity, scalability and scalability, and is closely combined with existing Internet applications.
Both propose complete solutions to IP telephone system signaling. They all have the ability to establish, manage and cancel calls, and haveNetwork ManagementFunctions enable endpoint users to have the ability to establish and interact with QoS, and are easy to expand new functions, supporting different types of interoperability. Of course, these twoprotocolBoth contain requirements for decompression of voice encoding, but this part is for twoprotocolIt's the same in terms of it.
Currently, H.323 and SIP are two major competing forces within the packet network.protocol, of which H.323protocolIt is the standard set by ITU-T, SIPprotocolIt is a standard set by the IETF, and both can complete functions such as call establishment, release, supplementary services and capability exchange. H.323protocolAdopted the design idea of ISDN, adopting Q.931protocolComplete the establishment and release of calls, which clearly has the characteristics of manageability and centralization of the telecommunications network. Currently, H.323protocolIt has been widely used online. Compared with SIP, H.323 is more mature, so now all IP telephone networks of all operators in my country use thisprotocol. And SIPprotocolIt has the characteristics of simplicity, good scalability and close integration with existing Internet applications, which many people believe isprotocolIt is relatively easy to implement and has recently developed rapidly in the United States. At the same time, SIP will be widely used in the third-generation mobile communication core network and smart services. Supports multipleprotocolIt is one of the main features of soft switch, so soft switch can support these two types ofprotocol。
5 Conclusion
H.323'sprotocolThe correction process shows that when building VoIP networks, the scalability of the network is always an issue that cannot be ignored. With the construction of broadband networks, streaming media services have become more and more important, and the trend of broadband packet networks gradually replacing PSTN has begun to emerge, which provides a good platform for the development of VoIP. H.323protocolAs a support for the next generation of networksprotocolOne will play an increasingly important role.
The second version of H.323 is very close to the functions and services supported by SIP, and for supplementary services, H.323 is defined more strictly, so in terms of implementation, H.323 rarely has interoperability problems. twoprotocolThey are comparable in their QoS support (similar to call establishment delay, no resource reservations or CoS (Class of Server) settings), but the third version of H.323 will allow signaling required by CoS. Therefore, the third version of H.323 is more suitable for giant telecom operators.
Soft switching technology is only a means of implementation, which can be used in an IP telephone system based on H.323, or in an IP telephone system based on SIP.
H.323 provides a centralized processing and management working model, which is consistent with the management method of the telecommunications network. It has the ability to make IP phone systems of any size and is a good working mode.