How to calculate the required traffic for transmitting a video signal?
Last time we stopped at discussing the fact that very often cameras with IP output load the network much less than standard systems based on cameras with analog output and video recorders. This is due to the fact that cameras with IP output have a built-in recording buffer, which stores current information.
Only when the motion detector is turned on and a signal is received about the need for pre-recording and recording by the motion detector, this information begins to be transmitted to the central video server. This can be very important for building systems with a large number of video cameras without creating powerful broadcast networks. What is the real network load of such a camera?
Let's consider this issue using the example of a camera with an IP output from one of the well-known manufacturers.
Operated in real-time mode using a proprietary compression format, this camera takes up 1–1.5 Mbps of bandwidth when transmitting good-quality (megapixel) images at full frame rate. However, this can double when images are being stored. To reduce network congestion, the cameras offer the option of selecting a different frame rate during recording than for real-time imaging.
The traffic load of 15-20 cameras operating in operational mode is 30-40 Mbit/s. To avoid display delays, the practical network load should not exceed 30-40%, i.e. 30-40 Mbit/s in a 100-Mbit/s network. This corresponds to 15-20 operational cameras at full frame rate. For this reason, it is advisable to use a switch that provides greater network bandwidth at two of its terminals, at the connection to the PC display and, possibly, at the connection to the PC memory.
Now let us give an example of traffic calculation for a standard video recorder:
We consider that – 25 frames/s – real-time mode – frame transfer rate in real-time mode.
The approximate size of one frame during compression is given in Table 1.
When compiling this table, it was assumed that the Wavelet compression format is used, the frame size is given in MB. Of course, the calculations provide an approximate frame size for an approximate estimate. Since the stream obtained after compression contains reference frames and the difference between them, it is fair to talk about bitrates.
In addition, the stream size changes depending on the degree of information in the frame and the presence of successive events on the scene. For the calculation, we will select a color image with a resolution of 704 x 576 and a size of 50 KB according to Table 1.
It is also clear to everyone that the traffic size is usually described in bits (we remind you that usually 1 byte = 8 bits).
As a result, we get:
50 KB x 8 x 25 fps = 10,000 kbps = 9.77 Mbps – the traffic size for broadcasting a signal from one high-resolution video camera in the “live video” mode of 25 fps.
Thus, the difference in the traffic size between cameras with an IP output with a built-in buffer for information and cameras with an analog output broadcasting signals through a video recorder is almost an order of magnitude in favor of cameras with an IP output.
If several video cameras need to be connected, the traffic volume increases by the same amount. Thus, video surveillance systems are highly informative, the volume of information and data transmitted in them is significantly higher than, for example, in a fire alarm system.
Let's not forget that in all networks operating on TCP/IP protocols, a certain volume is occupied by service information (about 50 bytes).
Therefore, in order to obtain the result from the operation of video surveillance systems and control what is happening with their help, operators who are at the facility's security duty station or at the centralized information collection point must build a data transmission environment that ensures reliability and quality not only when creating and processing video and alarm signals, but also at the stage of transmitting the received signals over a certain distance. The data transmission networks used must ensure bidirectional transmission of video images, sound, information from alarm inputs, requests from several users, and counter control signals. In order to transmit this information of sufficient quality and at high speed, it is necessary to use broadband and high-speed networks.
This is determined by the amount of equipment at the facility, the parameters of the information being transmitted and, of course, the type of security systems used. The creation of data transmission networks is regulated by the international standard ISO/ISE 11801. In it, the capacity of networks that are assigned a certain category is linked to the quality and type of cable (twisted pair or optical cable), the quality of connectors, switching cabinets, the quality and methods of installing detachable connections. Another important part of data transmission networks is signal amplifiers for twisted pair, active IT switches, fiber-optic connectors, etc.
In order to ensure guaranteed data transmission over networks, they must be built using categories of structured cabling systems.
SCS is a low-current telecommunications cable system that services all engineering systems of an enterprise located in its buildings and on its territory. SCS has a standardized structure and topology, uses only standardized components (cables, distribution devices, connectors, etc.). SCS provides standardized electromagnetic parameters (attenuation, bandwidth, etc.) of communication lines organized with its help.
Managed (administered) by standardized methods.
Structured cabling systems categories:
category 5: up to 100 MHz, local area networks with data transfer rates up to 1000 Mbit/s (class D);
category 6: up to 250 MHz, local area networks with data transfer rates up to 1000 Mbit/s (class E);
category 7: up to 600 MHz, local area networks with data transfer rates up to 1000 Mbit/s (class F).
To transmit a video signal over very long distances, equipment that converts information into a continuous high-speed E1 stream is usually used after the end of the SCS lines. The E1 stream with a cyclic structure provides for division into 32 OCC channels of 64 kbit/s in the form of division into channel intervals (Time Slot – TS) from 0 to 31.
In addition to determining the traffic volume, it is a very important and difficult task to determine the number of power sources for video cameras, their capacity, and the locations where they can and should be installed most optimally.
An example of calculating the power supply organization over the network for a camera with an IP output. The network power source is 30 V, power is supplied via 7 and 8 twisted pair wires. The camera consumes 24 V (6) V, current 100 mA.
According to the international standard ISO/IEC11801, twisted pair category 5 (class D) 100 MHz with a data transfer rate of 1 GGb/s has a resistance of no more than 20 Ohm per 100 m (in reality, about 2 Ohm per 100 m). At 300 m of twisted pair, no more than 6 V of voltage drops. Therefore, the power supply can be connected at a distance of about 300 m from the camera. For more accurate calculations, it is necessary to test the structured cabling system.
Installation of a video surveillance system
Let's consider some aspects of installing video surveillance systems.
Installation includes mounting cameras, installation, adjustment and programming of the video recorder or video server, adjustment of the entire system as a whole, which is carried out in the following sequence:
lens adjustment;
checking the quality of the video signal in low light;
checking the quality of the video signal in normal light;
checking the absence of backlighting.
According to existing regulatory documents, the video surveillance system is created with the ability to connect a backup power supply, which is necessary when the main power source voltage fails. A backup AC network or DC power sources can be used as a backup power source.
The nominal voltage of the backup DC power supply is 12.24 V. The transition to the backup power supply occurs automatically without disrupting the established operating modes and functional state of the system. When switching to the backup power supply, a light and/or sound signal is issued. The backup power supply ensures the performance of the main functions of the system specified in the TU, when the voltage in the network is lost for at least 0.5 hours.
Let's give an example of the construction and design of a video surveillance system.
It should be noted that digital video recorders are currently the most commonly used devices for creating digital video surveillance systems. They switch the signal from several video cameras, digitize it, provide image multiplexing, compress the image and transmit it to the network, making it available to a certain number of workstations, and also provide interaction of sensors through alarm inputs with the video surveillance system.
One of the main functions of video recorders is to record images, create an archive and search for incidents in the archive. Video recorders also provide the ability to connect and control PTZ cameras. Matrix switches are used simultaneously with video recorders when building very large video surveillance systems. In such systems, to increase the reliability of large video surveillance systems, video recorders are assigned the function of digitization, broadcasting to the network and creating an archive.
Control functions are left to matrix switches. The practice of using such partially duplicate products (a high-end video recorder with control functions and a matrix switch) is well known worldwide and is used not to increase the cost of the system, as it may seem at first glance, but to increase its reliability and increase the period of uninterrupted operation.
To create large video systems with real-time video acquisition and video transmission to a remote network, expensive matrix microprocessor switches are usually used.
Example: 16 video cameras are used to protect a factory, and a 16-channel video recorder is used to record and store the archive.
The image from the camera located at the checkpoint is recorded continuously during working hours (from 6.00 to 21.00 Monday through Friday) and by motion detector at other times, and images in the server room are recorded only by motion detector.
The storage and recording duration of information in the video recorder archive is 100 hours with a resolution of 720 x 576 px. To provide such an archive volume, the internal storage capacity is required — 1 TB.
Main observation postlocated in the administrative building, equipped with analog monitors and a workstation with monitors.
The post operates around the clock, the workstation monitor displays a multi-picture (a combined image from 16 cameras), and 4 monitors are designed to display images from alarm cameras or any selected ones. The selection of cameras for individual viewing on analog monitors is carried out from the matrix switcher keyboard. The workstation user has the rights to control PTZ cameras, search for images in the video recorder archive, send images by e-mail, and display images from cameras on monitors.
The auxiliary observation post is located in the checkpoint room. It is equipped with an analog monitor and keyboard. An image from any of the 16 cameras is displayed on the monitor upon command from the keyboard.
Video surveillance is also carried out on analog monitors located in the service offices of the head of the complex security service and the security system administrator (technical specialist).
The selection of images to be displayed is carried out from the corresponding keyboards. These offices also contain workstations for managing video recorders. The standard software supplied with the equipment is installed on the workstations.
Matrix switches have a modular design, which is determined by the number of cameras and the structure of surveillance posts (keyboards, workstations, monitors).
In conclusion, an example from foreign experience.
Thanks to a multi-million dollar grant from the US Department of Homeland Security, a perimeter control system equipped with radar video surveillance based on matrix switches was installed at the Houston airport.
The cameras are tied to a coordinate grid to ensure recognition of targets detected by both ground radars and a video analytics system.
The combination of ground radar and analytical video surveillance systems, linked into a single security management system, gives the airport the ability to fully control and evaluate the actions of all security systems and services, from takeoff and landing to the outermost points of the perimeter, all within a single graphical interface.
The system uses the most sophisticated fixed and panoramic infrared video cameras.