Mobile video monitoring system based on WLAN
Introduces WLAN-based mobile video surveillance system, and describes the principles and working process of the component modules such as video capture, video reproduction, and wireless video transmission. The appendix discusses three methods, such as bandwidth adaptive flow control, ring buffer processing, and daemon thread, to solve the QoS of wireless video transmission.
Keywords: mobile video surveillance system; video communication; QoS?
Keywords: mobile video survcilance system; video communicaTIon; QoS
The system supports one-to-multipoint IP multicast protocol (MulTIcast). In the network, a multipoint IP multicast address in the range of 224.0.0.0 to 239.255.255.255 can be defined. The advantage of multipoint multicast is that it can provide multiple images to many end users at the same time when the network scale is large. However, it should be noted that the reduction in the burden of the video source comes at the cost of increasing the burden on the router, which can be limited by the video distribution server.
? (1) Use Osprey-100 image acquisition card to realize multi-channel video capture. Under the condition that the performance of the encoder allows, multi-channel video transmission can be realized. (2) The video codec adopts MPEG4 coding standard, and cooperates with the network to process QoS. (3) The video flows through the WLAN based on IEEE802.11b and is transmitted through the UDP channel; due to the characteristics of the WLAN wireless channel, necessary flow control and QoS processing must be performed. (4) TCP feedback control and other data interaction through TCP channel interaction.
The system in Figure 2 consists of a video source, WLAN, and embedded mobile terminal. The video source can be divided into three modules: video capture, MPEG4 encoding, and wireless network transmission; embedded mobile terminals can be divided into: embedded mobile terminal, MPEG4 decoding, video reproduction three modules; WLAN and wireless network transmission module implementation information of both parties Interaction. ?
The IPP library functions have multi-layer processing functions. For weaker functions, you can choose the bottom function of the IPP library; for stronger functions, you can choose a suitable high-level IPP function; the combination of the underlying "atomic" function can also replace the high-level function, and you can make changes if the algorithm requires This allows great flexibility in optimizing codec algorithms.
The specific optimization process of the system for coding and decoding is shown in Figure 3. According to the MPEG4 coding standard, the algorithm optimization is made with IPP at the Micro Block layer and Block layer, which achieves the purpose of greatly improving the efficiency of coding and decoding and improving the real-time performance of the system. ?
4.2 CF wireless network card driver The focus of this work is on the rewriting and transplantation of wireless network card drivers. The ET? CWB1000 wireless CF network card based on Intersil Prism chipset (Version 3.0) can be selected and the Linux Spectum24 PCMCIA driver is used. As shown in Figure 5, the Linux network driver structure can be divided into four layers: device driver / network device layer, network protocol layer, and user application layer. The CF card is compatible with PCMCIA. The PCMCIA controller connects the card to the system bus, maps the card memory to the main I / O and memory window, and routes the interrupt generated by the card to the free processor interrupt line.
Rate control is a technology to determine the transmission rate of video communication based on the available bandwidth of the network. During the system's adaptive network transmission process, the user end continuously detects the quality factors of the code stream transmission, the number of data packets sent, the number of lost data packets, and other statistical data, and then passes the information to the decision controller after processing; decision control The device controls the relevant components according to the corresponding strategy, completes the dynamic integration of the transmission code stream, and delivers the UDP transmission component of the sending end to complete the transmission of the multimedia stream; the browsing user end UDP receives the multimedia data and hands it to the subsequent network player for decoding and playing.
In order to realize bandwidth-based adaptive transmission, it is first necessary to perform dynamic detection and evaluation of the network bandwidth, and then adopt appropriate adjustment strategies to integrate the code stream. In the assessment of network status, the quality factors can be parameters such as packet loss rate, packet delay, and buffer space. The system uses a packet loss rate indicator. The receiving user terminal continuously measures the packet loss rate and sends it back to the sender via TCP. The decision controller compares the packet loss rate with the specified threshold to determine whether to increase or decrease the code rate. When the packet loss rate is greater than the upper threshold, the code stream rate is reduced; when the packet loss rate is less than the lower threshold, the code stream rate is increased. ?
Linear address mapping is used in the memory area to open up continuous memory blocks, which ensures the access speed of threads and effectively improves the decoding efficiency. ?
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