"1 Introduction
In the 3G network, the videophone service based on circuit domain can be provided, that is, 3G-324M videophone, which allows mobile users to interact with video and voice anytime and anywhere, which greatly facilitates the needs of customers. In fixed networks, video communication and video applications based on IP Bearer have also been widely used, such as videophone, video conference, video surveillance, etc. these video applications are mainly based on H.323 and sip protocols.
The integration of 3G network and IP network [1] unification can not only enable operators to improve the utilization of network resources, but also mobile terminals can easily access and enjoy various information and services on the Internet through the unified IP network. However, because 3G network and fixed network are different in video communication and video application service session and transmission protocol, video interworking gateway (Vig) needs to be added between the two networks to realize video interworking based on CS domain, so as to realize point-to-point video telephone, video conference and other services. In this paper, the multimedia interworking between 3G-324M terminal and h.323 terminal is deeply studied and discussed.
2. Introduction and interworking analysis of 3G-324M and H.323 standards
2.1 introduction to 3G-324M standard
All along, people's pursuit of videophone has not stopped.
From the invention of the first telephone to the present more than 100 years, people have long been tired of the traditional telephone that only hears its sound without seeing its face. However, for a long time, video phone has not become as popular as traditional phone and mobile phone. He has always been the proprietary equipment of some large companies or special groups. The reasons can be summarized as follows:
(1) The visual fixed telephone formed by adding display equipment to the traditional fixed telephone has not been recognized by the public since its formation because of its high price and small demand compared with ordinary telephone.
(2) Although the color screen and camera have almost become standard on mobile phones, due to the low bit rate and high bit error rate of GSM network, the two-way video transmission can not achieve acceptable image quality.
But now, with the full spread of 3G network, the bottleneck of mobile video phone has finally been solved. 3G-324M video telephony protocol recommended by 3GPP is a protocol to realize real-time video call on 3G network. It is based on H.324 protocol. In order to adapt to the characteristics of 3G network, it has made corresponding changes. It can realize real-time video call protocol on 3G networks such as WCDMA and TD-SCDMA.
In order to meet the real-time requirements of audio telephone, h324m protocol needs to be established on circuit switched wireless network rather than IP packet switched network.
H. 324 [2] is a series of PSTN multimedia videophone standards formulated by ITU in May 1996, that is, H.324 framework recommendations. The protocol specifies the implementation standard of multimedia communication with telephone switching network as the transmission medium and embedded equipment or PC as the terminal. H. 324 Annex C defines the multimedia communication standard in error prone transmission environment, so that the protocol can be applied to wireless mobile environment. The H.324 protocol applying Annex C is also called H.324M protocol.
H. 324m protocol standard has been accepted by 3GPP and 3GPP2 mobile communication organizations and has become the international standard of mobile videophone. Among them, 3GPP reinterprets and modifies some details in the standard according to its own needs, which is called 3G-324M [3] standard. The 3G-324M standard is very similar to H.324M in technology, but it specifies H.263 as the mandatory video coding standard and MPEG-4 as the recommended video coding standard. AMR is a mandatory standard for audio coding. H. 223 formulated the multiplexing application standard of multiple audio and video signals in a single mobile communication channel, and H.245 formulated the message control exchange standard in each stage. In addition, level 2 (also formulated in Appendix B of H.223) is designated as a mandatory multiplexing protocol layer, which can provide enhanced fault-tolerant control.
2.2 introduction to H.323 standard
H. 323 standard is a multimedia communication system based on data packet. It describes the multimedia communication system and its constituent units for packet switching network, and specifies the communication process between units.
On the whole, H.323 [4] is a framework protocol, which involves terminal equipment, video, audio and data transmission, communication control, network interface, etc. it also includes multipoint control unit (MCU), multipoint controller (MC), multipoint processor (MP), gateway (MGW), gatekeeper (GK) and other equipment constituting multipoint conference. Its basic component unit is "domain". In H.323 system, the so-called domain refers to a collection managed by gatekeeper, including multipoint control unit (MCU), multipoint controller (MC), multipoint processor (MP) and terminal. A domain contains at least one terminal, and there must be only one gatekeeper. H. Various logic components in H.323 system include: terminal, gateway, multipoint control unit (MCU), multipoint controller (MC) and multipoint processor (MP). Among them, terminal, gateway and multipoint control unit (MCU) are the terminal equipment of H.323 and the logic unit in the network.
H. 323 standard protocol is not a simple protocol, but a collection of a series of protocols including gatekeeper Ras protocol, call signaling H.225 protocol, media control protocol H.245 protocol, media transmission RTCP / TCP protocol, audio and video codec protocol and data sharing protocol T.120 protocol.
In audio coding, there are mainly G series standards (g.7ll, g.722, G.728, G.729, G.723.1 and MPEG-1 of ITU-T. For video coding, it is mainly h.260 series standards (H.261 and H.263). Data application mainly refers to T.120 series multimedia conference data protocol, including nine standards t.120-t.128, which supports real-time and multipoint data communication and application. In order to meet the real-time communication needs of audio and video, H.323 system uses RTP packaging to transmit audio and video signals, and H.225.0 defines how to use RTP to package these signals.
System control and management protocol includes RTCP, part of H.225.0 protocol and multimedia communication control H.245 protocol. RTCP is the control protocol corresponding to RTP protocol. It provides monitoring means of data transmission QoS and obtains the information of communication parties. H. 225. O protocol is divided into Ras protocol and call signaling protocol. Ras protocol mainly completes the management between endpoint and gatekeeper, including gatekeeper discovery, endpoint registration, endpoint location, call permission, call exit, bandwidth management, status query and gateway resource query. The call signaling protocol completes the tasks of call establishment, release and H.245 control channel establishment.
2.3 interworking analysis of 3G-324M standard and H.323 standard
According to the previous introduction, we know that 3G_ 324m is different from H.323 protocol in packet network in composition, so it is necessary to add interworking gateway equipment Vig to complete 3G communication between H.323 video terminal in NGN and UMTS CS domain_ Interworking of 324m terminal. It mainly includes the following aspects:
(1) Interworking at call control level: in 3G_ In 324m, the establishment of call and handshake between communication parties are completed by traditional call signaling (TUP / ISUP / BICC / MCC); In H.323, these functions are completed by H.225 protocol.
(2) Interworking at system control level: in 3G_ In 324m and H.323, system control signaling is completed by H.245 protocol, but in 3G_ In 324m, H.245 messages are exchanged at al1 layer of H.223 multiplexing protocol and transmitted through logical channel 0 specified in the protocol; In H.323, through the interaction process of H.225, the communication parties open another TCP port to exchange H.245 messages, so mapping is also required between the two.
(3) Interworking at the media level: in mobile wireless channels, AMR voice coding format is generally used for audio coding and decoding, and H.263 or MPEG4 is used for video. In packet broadband networks, there are many audio coding and decoding formats, such as G.723.1 with high compression rate, G.729 with low delay, g.722 and G.711 with high quality, and there are also many video coding and decoding formats to choose from. If there is no media stream of the same coding standard between two interworking network nodes, the media processor equipment is required to convert the media data compression format.
(4) Interworking at media bearing level: 3G_ 324m uses H.223 protocol to package and multiplex the bearer control H.245 message and media stream compressed data, and mix them into data suitable for circuit switched network channel transmission for transmission. In the packet network, the H.323 node does not need to multiplex the media data, but directly transmits it through RTP (real-time transmission protocol) / RTCP (real-time transmission control protocol), and various media are transmitted through different RTP session ports., Technology Zone
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