The main difference between wireless and wired surveillance cameras is the transmission method. The wireless transmission part is mainly to complete the front-end system signal conversion, transmission, relay, and reception, until the signal is connected to the monitoring center system. The transmission part of wireless monitoring is the same as the optical fiber and coaxial cable of wired monitoring. It is a video transmission channel. What kind of video can be transmitted by this channel is mainly based on the bandwidth of the transmission channel and the amount of data that the transmission channel can transmit. The second is what kind of modulation method to choose. As for the peripheral surveillance cameras and control systems, they are not much different from wired systems.
Currently, wireless transmission methods mainly include satellites, microwaves, and telecom operator network systems. Satellites are expensive in signal transmission and have blind spots under dense buildings and obscured objects. Therefore, they have not become mainstream in the wireless surveillance civilian market.
Industry-specific microwave monitoring, small-range WiFi monitoring, and wide-range wireless monitoring by operators are currently more commonly used wireless transmission methods.
Microwave monitoring
It can be divided into two ways: analog microwave and digital microwave.
1. Analog microwave surveillance camera
This method of transmission is to directly modulate the video signal on the microwave channel and transmit it through the antenna. The monitoring center receives the microwave signal through the antenna, and then demodulates the original video signal through the microwave receiver. According to Hyde Security Company, this monitoring method has no compression loss and almost no delay, so it can guarantee the video quality, but it is only suitable for point-to-point single-channel transmission and is not suitable for large-scale deployment. In addition, because there is no modulation calibration process, it is anti-interference The performance is poor, and it can hardly be used in the case of complex wireless signal environment. The lower the frequency of the analog microwave, the longer the wavelength, and the stronger the diffraction ability, but it is very easy to interfere with other communications. Therefore, this method was used more in the 1990s, and it is rarely used at present.
2. Digital microwave surveillance camera
Digital microwave first encodes and compresses the video signal, modulates it through the digital microwave channel, and then transmits it by the antenna; on the other hand, at the receiving end, the signal is received by the antenna, followed by microwave despreading and video decompression, and finally restored to analog video signal transmission Going out, this method is also more commonly used in the domestic market. Digital microwave has large scalability, at least a dozen channels can be used for communication capacity, and it is relatively easy to construct, with high communication efficiency and flexible use. Digital microwave has the incomparable advantages of analog microwave, such as more monitoring points, many situations where relays are needed, complicated situations and many interference sources.
In summary, digital microwave has large capacity, strong anti-interference ability, and good confidentiality. The same transmission power has a longer transmission distance, is less affected by terrain or obstacles, has rich interfaces, and has strong expansion capabilities. On the contrary, analog microwave does not have these advantages, but the cost is a bit cheaper.
WiFi surveillance camera
The IEEE802.11 standard defines the physical layer and media access control (MAC) specifications. The physical layer defines the signal characteristics and modulation of data transmission. It operates in the 2.4000-2.4835GHz frequency band. IEEE 802.11 is a wireless local area network standard originally formulated by IEEE. It is mainly used for wireless access of computers in difficult-to-wire environments or mobile environments. Since the transmission rate can only reach 2Mbps, the business is mainly used for data access .
This series mainly includes IEEE802.11a/b/g/n wireless local area network standards, of which the IEEE802.11b standard, namely WiFi, is currently used more. This standard stipulates that the working frequency band of wireless local area network is 2.4GHZ-2.4835GHZ, and the transmission rate reaches 11Mbps, which is a supplement to IEEE802.11. According to Shi Zhaozhao from Xieyuan Tiancheng, WiFi products have relatively good advantages in bandwidth, anti-interference, encryption, etc., and have powerful network management functions, which provide powerful means for large-scale networking under various applications, and are truly suitable High-bandwidth video transmission is currently the most widely used wireless transmission technology. It is understood that the signal radius of WiFi can reach about 100 meters, which can be used in offices or even the entire building, and the transmission speed is also very fast.
However, someone in the industry pointed out that because WiFi is a standard protocol, users only need to take a laptop or PDA that supports wireless to the area where WiFi is available to access the Internet at a high speed, which determines that its security is not very good. If the more sensitive surveillance camera images can not be used to prevent others from stealing; in addition, its transmission distance is short and flexibility is poor, so it does not have the capability of wide-area video surveillance.
Therefore, based on the needs of operation level, the 802.11 standard has developed an intelligent distributed technology architecture of base station controller + WiFi base station, which can realize large-scale networking of 100,000 WiFi base stations and millions of terminals, and can be used in a wide range of areas. . 802.11n is currently the highest wireless bandwidth standard. A single 802.11n base station can achieve a wireless bandwidth of 300Mbps, an effective bandwidth of more than 200Mbps, and 200 channels of D1/1Mbps image transmission, which is better than the most common 100M optical fiber transmission on the market.
In fact, 802.11 has the highest level of WAPI/802.11i Chinese and international encryption standards, which can fully meet the high-density communication transmission requirements of the military, finance, and government; it has stricter anti-interference based on the physical layer and MAC layer and reduces error codes. Rate-related protocol standards are the strongest anti-interference standard system among current wireless standards.
Products based on the IEEE 802.11 standard have achieved large-scale industrialization and have been recognized worldwide, and their prices have dropped; in addition, they can also transmit other data while transmitting video. But it is only a channel responsible for transmission, and a codec must be set at the front and back ends before the video image can be transmitted.
2G/3G surveillance camera
The transmission mode of 2G mainly includes CDMA, GSM two kinds of modes. These two modes have lower cost, larger coverage area, and faster transmission speed. The theoretical value of CDMA transmission rate is 153.6Kbps, which can basically reach 60-80Kbps in actual use. Favored by fewer manufacturers. And GPRS based on GSM mode, although the coverage rate is higher than CDMA, but the transmission rate is slightly slower, so it is still at a disadvantage in use.
The 3G technology access methods adopted by mobile (TD-SCDMA), telecommunications (CDMA2000 EVDO), and China Unicom (WCDMA) operators have been vigorously promoted by various operators since 2009, and many monitoring manufacturers have developed research and development in this area. Related products. The outstanding advantage of 3G is the high-speed download capability. The ideal value can reach a transmission rate of 3Kbps-1G, but it is still in the promotion stage, so further research is needed in terms of transmission rate. 3G currently has limitations such as limited wireless bandwidth capacity, limited access users, and time extension. In the case of multi-user sharing, it is difficult to guarantee the rate and delay requirements of wireless video surveillance.
Of course, different wireless transmission technologies have their own applicable places. For example, in centralized office areas, there are more wireless monitoring using WiFi; and in special projects in remote areas such as border defenses and forests, it is more appropriate to use specially built microwave transmission methods; with the improvement of various aspects of 3G performance, the network environment Maturity, as well as the general demand for large-scale wireless monitoring, will also promote the further use of 3G.
