Current situation of video surveillance in rail transit

The construction of urban rail transit in China has been fully popularized from first tier cities to second tier cities in the past decade. Not only has the operating mileage of urban rail transit in Beijing, Shanghai, Guangzhou, and Shenzhen exceeded 1500 kilometers, but all eastern provincial capitals have already built subways. There are also dozens of planned and under construction urban rail transit lines in eastern prefecture level cities and western provincial capitals. It is expected that by 2020, more than 70 new rail transit lines will be built, and the total number of rail transit lines in the country will exceed 100, with an operating mileage of over 10000 kilometers.

The video surveillance system in urban rail transit is an important guarantee system for operation and public security, and has shown new characteristics in recent years.

The system is showing a trend towards high-definition digitization. Since the operation of the first subway in China, video surveillance technology has gone through three development cycles: from analog networking, to digital networking based on DVR, and today's all digital networking. Many cities' rail transit systems start directly from digital high-definition. The advantages of digitalization are reflected in the overall system's high interference, convenient networking, and open interfaces. The advantages of high-definition are reflected in the fact that more details and information provide great possibilities and imaginative space for visual management. More importantly, the rapid decline in prices of IP cameras and network storage devices has largely eliminated the concerns of homeowners;

The video surveillance system is divided into two main vertical systems based on business: operation and security. CCTV needs to meet the needs of subway operation and the security needs of the public security for the security environment of subway stations and subway carriages. For the vertical division of this business, different cities have different approaches, mainly the construction mode of unified construction and separate use, and the two modes of separate construction and resource sharing;

The integration of video surveillance and integrated management system for rail transit. Unlike early simulation systems, video surveillance systems based on digital network technology can exchange data and integrate business with other computer software platforms based on communication protocols. Therefore, in some urban rail transit systems, CCTV systems exist as a component module of the Integrated Surveillance System (ISCS), providing basic video resources to other ISCS systems such as SCADA, environmental control, platform screen doors, fire protection, security, etc., without setting up independent monitoring terminals. It can be expected that with the development of intelligent technology, the use of video in various business systems will become more common, and this system level integration will become more common;

The CCTV system of urban rail transit will be incorporated into the social monitoring system. At present, video surveillance technology has not only become a basic requirement for safe and smart cities, but also the lifeline of urban rail systems. Therefore, rail transit will definitely become an important component of urban video surveillance systems. This raises a technical question for us: can the CCTV system of rail transit adapt to the requirements of being included in the social surveillance system in the future?

Requirements for the construction of urban rail transit system. Since the first generation of urban rail operation, the technological system has been constantly evolving and the management system has undergone significant changes. Now, city level rail operation centers, traffic management centers, and emergency command centers require different rail lines to be managed as a whole. This requires different rail lines, regardless of their construction years, to consider the integration of the overall system, rather than creating a one line system and chimney style system integration.

This article mainly discusses the standardization selection of video surveillance for urban rail transit based on the last two development trends.

The trend of standardization in video surveillance networking is a continuous development process of digital video surveillance, and it is also a process of corresponding standards constantly being introduced and improved. However, compared with broadcasting and television systems, although digital surveillance systems also focus on standards for audio and video compression and transmission, and share some technological achievements, for video surveillance, the standards for network control protocols proposed by large-scale networking requirements are still a recent phenomenon. The following briefly explains the standardization work involved in video surveillance network control from three aspects.

Video and Audio Encoding and Decoding Standards

The standard for video and audio encoding and decoding is the most basic "descriptive language" for all video information. Without a unified language, it requires layers of translation, and long-term maintenance of various languages, especially early non-standard languages, is very difficult and unsustainable. Since the development of rail transit, the video and audio encoding and decoding standards have gone through several stages such as MPEGII, MPEGIV, and H.264. However, due to the lack of early specifications for transmission and packaging standards, there are often situations where lines cannot encode and decode each other. The operation center managing each line must stack many devices in order to access each line, which brings great difficulties to system operation and maintenance.

H. 264 has been introduced for nearly a decade and is currently the most widely used and advanced video encoding technology (H.265 was officially released in March 2013, but for large-scale commercial use, it is estimated to take 3-5 years). With the same image quality, its bitrate is less than half of MPEG-4, which can greatly save storage space and bandwidth occupation. This is crucial for networked video surveillance systems with a large demand for video transmission and storage.

H. The 264 standard specifies multiple profiles such as baseline, main, and high in terms of encoder processing and quality, and includes numerous options in the specific algorithm of H.264. Therefore, based on the requirements of rail transit for video quality and compatibility implementation, specific profile and level levels can be flexibly selected, as well as specific parameter specifications for encoding and decoding processes, to meet different needs in different scenarios. For example, high compression ratio HighProfile streams can be selected for video storage to ensure the capture of details, while low latency Constrained Baseline can be selected to meet the real-time browsing requirements of low latency.

Due to the mature development of H.264, it has gained more support from manufacturers compared to standards such as SVAC and AVS. Therefore, it has been chosen by numerous standardization alliance organizations both internationally and domestically, and has strong vitality. Moreover, from current practice, the implementation of H.264 tends to be standardized, and there is a good guarantee of interoperability among different vendors.

Based on the above factors, we believe that H.264 should be chosen as the basic encoding framework for video encoding in rail transit, and on this basis, relevant standards for signaling, transmission, and file encapsulation should be improved.

System Control Protocol

With the continuous trend of networked development in the security industry, there is a lack of communication standards for IP security equipment from different manufacturers. In September 2008, a research group for ONVIF was jointly initiated by companies such as Hikvision, AXIS, BOSCH, SONY, etc., gradually forming an international standardization organization with a large number of major manufacturers participating, and the ONVIF 2.2 version specification has been launched. In addition, more than 60 manufacturers such as Cisco and Honeywell have initiated PSIA organizations.

The ONVIF standard defines a universal protocol for information exchange between network video devices, including device search, real-time video, audio, metadata, and control information. Online video products can provide multiple possibilities for end users, integrators, consultants, and manufacturers to expand their functionality more flexibly and openly. By utilizing service-oriented interface technology, they can reduce system management costs, obtain cost-effective, more flexible solutions, opportunities for market expansion, and lower risks.

ONVIF has gained widespread support from mainstream video equipment manufacturers and is increasingly becoming the de facto interface standard for front-end devices, storage devices, and back-end devices. At the same time, SDK access methods based on manufacturers' proprietary technologies will be increasingly resisted by users.

However, the drawbacks of ONVIF are also quite obvious. As ONVIF focuses more on the interface and management between devices and less on the management and interface between systems, it does not have a good solution for multi-level system management, permission allocation, video call and other functions in China.

The development status of domestic video surveillance standards: The standardization of domestic video surveillance systems is relatively lagging behind, especially for the networked control of digital video surveillance, which lacks national standards.

From the perspective of its development process, the first step was the formulation of local standards, which gradually evolved from focusing on video image quality, coverage of key points, and mandatory functional regulations to specifying the overall structure and paying attention to system level networking standards.

On June 1, 2012, the "Technical Requirements for Information Transmission and Exchange Control of Security and Prevention Video Surveillance Networking Systems" (GB/T28181-2011) was officially promulgated and implemented. It can be said that GB/T28181, based on the GA/T669.1-2008 series of standards, planned an industry-specific networking system that can be promoted to a national scale. It provides a detailed description of the networking control protocol and system architecture, and has strong operability. However, 28181 has been simplified compared to the 669 series standards, and it is believed that some subsequent standards are needed to describe it. Can the relevant content of ONVIF or GB28181 be used to standardize the communication interface between network video devices and the protocol between systems in the rail transit video surveillance system? We have found that relying on a single standard cannot effectively adapt to the diversity of rail transit business, and a unique standardized architecture must be adopted.