Brief introduction of video telephone system in circuit domain and packet domain

With the development of modern communication technology, mobile multimedia communication has become a new business requirement. Among them, the mobile multimedia videophone service realizes the comprehensive processing of multiple media such as audio, video, data (text, graphics, etc.), which enables real-time audio and video communication between users in different locations. People are concerned. According to the actual development of the 3G mobile network, the conditions for developing a videophone service are already in place. Hong Kong and Huang have launched the WCDMA WCDMA circuit domain videophone service, and South Korea SKT has launched the EV-DO packet domain videophone service.

1. WCDMA / TD-SCDMA circuit domain videophone

The mobile videophone service provides any combination of real-time video, audio, or data media formats on the WCDMA / TD-SCDMA circuit-switched wireless network. It mainly uses the WCDMA / TD-SCDMA network to realize wireless interworking of videophones on mobile devices , So that mobile users can interact with real-time audio and video anytime, anywhere. At this stage, it may be limited to the interworking between mobile terminals, and in the future, it may be extended to the interworking between mobile terminals and various network devices such as PSTN and ISDN.

In the WCDMA / TD-SCDMA system, since the current videophone service is implemented as a bearer service in the circuit domain, the rate can only reach 56kb / s / 64kb / s in the RDI / UDI mode. The lower transmission rate affects its editing and decoding of audio and video

The choice of coding protocol determines that it is impossible for mobile videophone systems to use large-rate coding methods.

1.1 Business functions

The videophone shall include the following modules: video input / output and codec module, audio input / output and codec module, user data application function module, multiplexing function module, system control module, etc. The functions of each module are as follows:

â—† Video input and output and codec module: encode and decode the input video stream data according to a specific protocol. Encode the data collected from video input devices such as cameras and compress it to a bit rate suitable for transmission at low speed; at the same time, decode the video data decomposed by the H.223 demultiplexing layer and then transmit it to the corresponding video Processing equipment for display or other processing. The video coding protocols used for WCDMA / TD-SCDMA circuit-domain videophone services mainly include H.263 and MPEG-4.

â—† Audio input and output and codec module: The main function of this module is to encode and decode the input audio data. Encode and compress audio signals from audio collection devices such as microphones, and then transmit the encoded stream data to the AL2 layer of H.223. At the same time, the audio data decoded by the H.223 demultiplexing module is decoded, and the decoded data is transmitted to the corresponding sound processing equipment, such as earpieces and speakers. The audio coding protocols used for the WCDMA / TD-SCDMA circuit-domain videophone service mainly include AMR and G723.1.

â—† User data application function module: The typical user data application is T.120. This protocol supports multi-point data conferences including data and image transmission. In addition, data applications such as shared whiteboards and applications can also be implemented.

â—† Multiplexing function module: multiplex the video stream, audio stream, data information and control information represented by the encoded logical channel into a single output stream, and decompose the received stream into various multimedia The code stream, the multiplexing protocol used for the WCDMA / TD-SCDMA circuit-domain videophone service, adopts the ITU-T H.223 protocol.

â—† System control module: Mainly to ensure the normal establishment and release of the videophone connection and to provide information control during the videophone conversation, such as master-slave decision between terminals, capability exchange, and opening and closing of logical channels. WCDMA / TD-SCDMA circuit domain videophone service uses ITU-T H.245 as the control protocol.

The specific position of each module in the entire system and the structure of the entire WCDMA / TD-SCDMA circuit domain videophone system are shown in Figure 1:

Figure 1 Schematic diagram of the functional structure of the WCDMA / TD-SCDMA circuit domain videophone system

1.2 Business Model

The following takes two WCDMA / TD-SCDMA circuit domain video phone terminals in the PLMN coverage area as an example to briefly introduce the business model of the video phone service. The videophone service is implemented as a bearer service in the circuit domain. Two mobile phones that have selected the video phone service mode establish a CS connection through the sending UTRAN, GSTN, and receiving UTRAN. The video phone service data is transparently transmitted between the 3G-324M terminals. The business model is shown in Figure 2:

Figure 2 WCDMA / TD-SCDMA circuit domain videophone service model

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