1 Introduction
In the 3G network, the video phone service based on the circuit domain can be provided, that is, the 3G-324M video phone, which allows mobile users to interact with video, voice, etc. anytime, anywhere, which greatly facilitates the customer's demand for video calls. In fixed networks, video communication and video applications based on IP are also widely used, such as video phones, video conferencing, 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 not only enables operators to improve the utilization rate of network resources, but also mobile terminals can easily access and enjoy various information and services on the Internet through a unified IP network. However, since the 3G network and the fixed network are different in terms of video communication and video application service sessions and transmission protocols, it is necessary to add a video interworking gateway (VIG) between the two networks to achieve video interworking based on the CS domain to achieve point-to-point Video telephony, video conferencing and other services. This article conducts in-depth research and discussion on the multimedia interworking between 3G-324M terminal and H.323 terminal.
2 Introduction to 3G-324M and H.323 standards and analysis of interworking
2.1 Introduction to 3G-324M standard
All along, people's pursuit of video phones has not stopped.
In the more than 100 years since the invention of the first telephone, people have long been tired of traditional telephones that only hear their voices but never meet them. However, for a long time, video telephony has not formed such a popular level as traditional phones and mobile phones. He has always been the proprietary equipment of certain large companies or special groups. The reasons for chasing it can be summarized as follows:
(1) The visual fixed telephone formed by adding a display device to a traditional fixed telephone has not been recognized by the public since its formation, due to its high price and low demand.
(2) Although color screens and cameras are almost standard on current mobile phones, the low bit rate and high bit error rate of the GSM network prevent bidirectional video transmission from achieving acceptable image quality.
But now, with the full rollout of 3G networks, the bottleneck of mobile video telephony has finally been solved. The 3G-324M video telephony protocol recommended by 3GPP is a protocol for real-time video calling on a 3G network. He is based on the H.324 protocol. In order to apply the characteristics of the 3G network, corresponding changes have been made to enable real-time video call protocols on 3G networks such as WCDMA and TD-SCDMA.
In order to meet the real-time requirements of audio telephony, the H324M protocol needs to be built on a circuit-switched wireless network, not an IP packet-switched network.
H.324 [2] is a series of PSTN multimedia videophone standards developed by ITU in May 1996, that is, H.324 framework recommendations. The agreement stipulates the implementation standard of multimedia communication with the telephone exchange network as the transmission medium and the embedded device or PC as the terminal. H.324 Annex C defines the multimedia communication standard in the error-prone transmission environment, so that the protocol can be applied to the wireless mobile environment. The H.324 protocol applying Annex C is also called the H.324M protocol.
The H.324M protocol standard is accepted by two major mobile communication organizations, 3GPP and 3GPP2, and has become an international standard for mobile video phones. Among them, 3GPP re-explained and modified some details in the standard according to its own needs, which is called the 3G-324M [3] standard. The 3G-324M standard is technically very similar to H.324M, 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 has established multiplexing application standards for multiple audio and video signals on a single mobile communication channel, and H.245 has established message control switching standards at various stages. In addition, level 2 (formulated in H.223 Appendix B) is designated as a mandatory multiplexing protocol layer, which can provide enhanced fault tolerance control.
2.2 Introduction to H.323 standard
The H.323 standard is a multimedia communication system based on data packets. It describes the multimedia communication system and its components used in packet-switched networks, and specifies the communication process between the units.
On the whole, H.323 [4] is a framework protocol, which involves the content of terminal equipment, video, audio and data transmission, communication control, network interface, etc., and also includes the multipoint control unit that forms the multipoint conference (MCU), multipoint controller (MC), multipoint processor (MP), gateway (MGW) and gatekeeper (GK) and other equipment. Its basic unit is the "domain". In the H.323 system, the so-called domain refers to a multi-point control unit (MCU), multi-point controller (MC), multi-point processor managed by the gatekeeper ( MP) and terminals. A domain contains at least one terminal, and there must be only one gatekeeper. The various types of logic components in the H.323 system are: terminal, gateway, multipoint control unit (MCU), multipoint controller (MC), multipoint processor (MP), of which terminal, gateway, multipoint control unit ( MCU) is the H.323 terminal equipment and the logical unit in the network.
The H.323 standard protocol is not just a protocol, but includes the 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 is a collection of protocols.
In audio coding, there are mainly the ITU-T G series standards (G.7ll, G.722, G.728, G.729, G.723.1 and MPEG-1 audio part. For video coding, mainly H. The 260 series of standards (H.261 and H.263). Data applications mainly refer to the T.120 series of multimedia conference data protocols, including T.120-T.128 nine standards, supporting real-time, multi-point data communication and applications. In order to meet the real-time communication needs of audio and video, the H.323 system uses RTP packaging to transmit audio and video signals, and H.225.0 defines how to use RTP to encapsulate these signals.
System control and management protocols include RTCP, H.225.0 protocol part and multimedia communication control H.245 protocol. Among them, RTCP is the control protocol corresponding to the RTP protocol. It provides a monitoring method for data transmission QoS and learns the information of all parties to the communication. The H.225.O protocol is divided into two parts: RAS protocol and call signaling protocol. The RAS protocol mainly completes the management work between the endpoint and the 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 3G-324M standard and H.323 standard interworking analysis
According to the previous introduction, we know that 3G_324M is different from the H.323 protocol in the packet network in terms of composition, so it is necessary to add the interworking gateway device VIG to complete the H.323 video terminal and UMTS CS domain in NGN during communication. 3G_324M terminal interworking. Mainly includes the following aspects of intercommunication:
(1) Interworking at the call control level: In 3G_324M, the establishment of the call and the handshake between the two parties of communication are completed by traditional call signaling (TUP / ISUP / BICC / MCC); in H.323, these functions are caused by H.225 agreement completed.
(2) Interoperability at the system control level: In 3G_324M and H.323, system control signaling is done by the H.245 protocol, but in 3G_324M, the exchange of H.245 messages is in the H.223 multiplexing protocol AL1 layer, and through the logical channel 0 specified in the protocol to transmit; and in H.323, through the interaction process of H.225, the communication parties opened another TCP port to exchange H.245 messages, so There is also a need to map between these two.
(3) Intercommunication at the media level: In mobile wireless channels, audio codec generally uses AMR voice coding format, video uses H.263 or MPEG4, and there are many audio codec formats in packet broadband networks, such as: higher compression ratio G.723.1, G.729 with small delay or G.722, G.711 with higher quality, etc., there are many video codec formats to choose from. If there is no media stream with the same encoding standard between two interworking network nodes, the media processor device needs to convert the media data compression format.
(4) Intercommunication at the media bearer level: 3G_324M uses the H.223 protocol to package and multiplex the bearer control H.245 messages and media stream compressed data, and mix them into data suitable for circuit-switched network channel transmission. In a packet network, H.323 nodes do not need to multiplex media data, and directly transmit through RTP (real-time transmission protocol) / RTCP (real-time transmission control protocol), and various media are transmitted through different RTP session ports. .
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