Detailed explanation of network time synchronization (NTP network timing) of video monitoring system

Network time synchronization of video monitoring system （NTP Network time service ） Problem explanation
Network time synchronization of video monitoring system （NTP Network time service ） Problem explanation
This paper is written by Anhui Jingzhun company Provide , Reprint requires authorization @@

Video monitoring system refers to the integrated application of video and audio monitoring 、 signal communication 、 Computer network and other technologies monitor the fortified area , And real-time display 、 An electronic system or network for recording live images . The system can be used in case of emergency , Time will be superimposed in time 、 Record the site conditions of information such as location , So as to analyze and investigate during playback , And as an important evidence with legal effect , This not only improves the accuracy of the security personnel in dealing with the police , It can also provide strong evidence for public security personnel to solve cases quickly . However, the video monitoring system often has the problem of incorrect display time , The digital evidence provided by the system is greatly reduced , It doesn't even have legal effect and can't be used , This paper analyzes the causes of the problems from many aspects and gives effective solutions .
Due to the working relationship, the author has participated in a municipal government department for nearly four years 、 enterprises and institutions 、 The project design of more than 50 video monitoring systems, such as new residential areas 、 Bidding review and project acceptance . Participate in the security monitoring and acceptance project , The display time of the video monitoring system is often incorrect , Approximate ratio 60%, These small problems make the digital evidence provided by the video surveillance system greatly reduced , It doesn't even have legal effect and can't be used .
The video monitoring system generally consists of front-end monitoring equipment 、 Transmission equipment 、 Back end storage servers 、 Control and display equipment are composed of five parts , The most closely related to time is the front-end monitoring equipment （ Webcam IPC） And control equipment （ Network hard disk video recorder NVR） And data storage server , Let's start with the first two devices to analyze the reasons for the frequent incorrect display time ：
Fault one ： The time displayed by the video monitoring system is “1970 year X month X Japan ” or “2000 year X month X Japan ”, Pictured 1 Shown in the upper left corner .

chart 1
The original computer operating system was 32 position , And time is also used 32 Who said , in other words 32 The longest time a bit can express is 68 year . The first UNIX The operating system is designed , Considering the age of the computer and the time limit of its application , Integrated setup 1970 year 1 month 1 Japanese act UNIX TIME The era of （ Starting time ）, and Java、 database 、 Many network devices follow this constraint , So the default time for many devices is 1970 year 1 month 1 Japan , However, some devices set the default time to 2000 year 1 month 1 Japan . There are two possible reasons for the failure ：
1、 Front end monitoring device problems ： In many environments without network hard disk recorders （ For example, home webcam ）, Due to network line failure, etc , Or the device is in a closed network （ Not connected Internet） in , Failed to communicate with Internet Time server synchronization in , The system default time is used . At present, the time servers commonly used in the network are 210.72.145.44（ National time service center server IP Address ）、time.windows.com（ Microsoft official time server ）、time.nist.gov etc. , It is recommended to use the domestic time server , Because the time servers abroad are far away , Access congestion is widespread 、 Large time delay and other problems , Plus DNS Some network problems such as the failure to resolve the domain name , So it is not very stable .
2、 Network hard disk video recorder problem ： In an environment with a network hard disk video recorder , In addition to the failure to synchronize with the time server due to network line failure , It is also possible that the button battery on the main board of the network hard disk video recorder is low , Under normal circumstances, it can be solved by replacing a new battery . If the time still cannot be synchronized after a new battery is replaced , That is the failure of the network hard disk video recorder motherboard , It may be that there is a short-circuit problem on the main board, causing leakage （ for example CMOS Circuit fault ）、 The crystal oscillator of the motherboard （ Some are also called clock crystals ） Broken wait , It needs to be returned to the factory for maintenance .
Fault two ： The display time of the video monitoring system differs from the standard time by about ten minutes , Each webcam has different display time , The biggest difference is about ten seconds , Pictured 2 Shown .

chart 2
For the video monitoring system, the display time differs from the standard time by about ten minutes , The main reason is the webcam 、 Network devices such as network hard disk video recorders have integrated real-time clocks , But the timing accuracy of the clock is limited , It depends on the accuracy and stability of the crystal oscillator , There are great differences between different types of crystal oscillators , Even if the same type of equipment runs in different environments for a long time , Deviations may also occur . According to statistics , The deviation between the computer or network equipment time and the international standard time is within 1 Minutes or more 90% above , This is because the clock signal of the computer or equipment comes from the simple crystal oscillator , But this kind of crystal oscillator is not very punctual , After adjusting the time , There are usually a few seconds of time drift every day , Therefore, calibration is required after a certain period of time . There is a difference of about ten minutes between fault two , The most common solutions are as follows ：
1、 For access Internet Your webcam or webcam , Can pass NTP（Network Time Protocol） The agreement is aligned .NTP The protocol is an internationally accepted network time service protocol , Its principle is that the client sends a time poll to the server every once in a while , According to a certain filtering algorithm, the time deviation between the server and the client and the propagation delay caused by network transmission are calculated , To adjust the local time of the client , Make it consistent with the server . Compare protocols for other calibration times ,NTP The protocol can eliminate the influence caused by network propagation delay , Therefore, it can provide reliable time service , Provide time accuracy in 1-50ms Between . On the webcam （ Pictured 3 Shown ） Or hard disk video recorder configuration interface （ Pictured 4 Shown ）, Access by filling in the network clock server address Internet You can calibrate the clock .

chart 3

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Be careful ： Because of the video monitoring network and Internet Network NTP The network between time servers is complex , Set up NTP Time server can complete time synchronization of video monitoring network , High reliability , But the accuracy is poor , Because of the delay 、 Network congestion and the location of external authoritative clock sources , It is also possible that the clock calibration of the equipment in the security video monitoring network may be inaccurate .
2、 If it is a LAN application （ The webcam cannot be accessed Internet） Or private webcams and network video recorders , You have to set up the configuration inside the network first NTP Clock server , And then NTP The address of the clock server is filled in the configuration interface of each webcam or network hard disk video recorder , In order to guarantee the time synchronization .
Be careful ： In this case, it is necessary to ensure the clock accuracy of local clock server , Generally, the use of high-precision local clock source requires high cost , You can also choose to NTP The clock server is regularly connected to the Internet , Synchronize the local clock source with the external authoritative clock source , Or use GPS Positioning and calibration , Connect devices with standard protocols that support time calibration 、 Ensure that the platform and equipment comply with the clock synchronization agreement in the standard protocol , Under the condition of low cost, the time synchronization of video monitoring network is guaranteed , Reduce system clock disorder .
In fault 2, the display time of each webcam is different , The biggest difference is about ten seconds , The main problem is the network switching delay . It takes time for webcam video capture and encoding output , At the same time, the digital signal encoded by the network camera is transmitted through the network , According to the bandwidth capacity and performance of network equipment , To generate a certain network delay , The back-end device receives the encoded digital signal , It also takes time to decode it . The webcam is in different locations on the network , Affected by many factors , The delay time of each device may be different , It mainly depends on the indicators of network equipment such as switches 、 performance 、 Quality and network architecture , If there is a decoder on the back end , There will also be decoding delay , If the preview device performance is poor , There will also be preview delay . The data transmission delay of various network cameras and network video recorders in the network is certain , And it's different , The webcam recently produced by Hikvision can achieve 120ms, But there is still a long way to go to achieve no delay , commonly 0.5S Left and right are acceptable .
Fault three ： The time displayed by the video monitoring system differs from the standard time by more than ten hours , The display time of each camera is different , Some have a difference of about ten seconds , Some have a difference of more than ten hours , Pictured 5 Shown .

chart 5
The problem is more complicated , The author analyzes two possible reasons ：
1、 Different time zones are used ： Many types of cameras are used in the network , Webcam or network hard disk recorder may use different time zones , Some use Greenwich mean time GMT（Greenwich Mean Time）, Some use world coordinated time UTC（Coordinated Universal Time）, There is also the possibility of using summer to save time DST（Daylight Saving Time）, Pictured 6 Shown . When calling different time formats in different devices , Fail to identify or convert accurately , Therefore, there is a difference of more than ten hours between some network devices , The same time zone can be set and used in different camera or hard disk recorder configuration interface in advance

chart 6
2、 Webcam compatibility issues ： Due to the multiple webcams used in the same monitoring network , For example, figure 5 Huawei cameras are used in the original video surveillance network , The new camera used in the upgrade is Dahua , These different brands of webcam video monitoring network have some devices based on Linux、 Some are based on AIX、Solaris, Some are even based on Windows platform , There are some compatibility problems between these different brands of webcams and different platforms , Or the time zone time format of these terminals is inconsistent , So there is a big time error .
In some situations where precise time synchronization is required , Such as power communication 、 Communication billing 、 Distributed network computing 、 The weather forecast 、 Police video surveillance, etc , It is not enough to rely on the clock signal provided by the computer or the equipment itself , Therefore, various means are needed to synchronize time .
It is very important to synchronize the time of video surveillance system , At present, video surveillance system has entered the era of intelligent network video surveillance . In the intelligent video surveillance system based on network , The accuracy and reliability of equipment in time directly affect the working efficiency of video monitoring system . However, for each device working in the video surveillance system network , It is obviously unrealistic to modify the calibration clock only by manually inputting commands by operators , Because not only the workload of manual command input is too huge , And manual operation can not guarantee the accuracy and reliability of the clock . therefore , Only through clock synchronization technology , Quickly synchronize the clock of each device in the video monitoring network , At the same time, accuracy and reliability can be guaranteed .
Except for the above NTP Beyond the agreement , There are other protocols and technologies that support time synchronization in video surveillance systems ：
1、 It can be used in some civil application scenarios that do not require high time accuracy SNTP（Simple Network Time Protocol） agreement , For example, parking lot management system , Only second accuracy is required .SNTP By simplifying NTP agreement , On the premise of ensuring time accuracy , It makes the development and application of network time easier .SNTP Mainly for NTP The protocol deals with access security 、 The automatic migration part of the server has been reduced , It can work with NTP The protocol is interoperable , namely SNTP Customers can communicate with NTP Servers work together , Again NTP Customers can also receive SNTP The time service information sent by the server . Pay attention to the following items in daily use ：（1） Try to deploy in the local LAN SNTP The server , Instead of using Internet Public on the Internet SNTP The server , because Internet Network delay uncertainty , The quality of service is not guaranteed , Will have a great impact on the accuracy of timing ;（2）SNTP The time service request cycle of the client to the server should be greater than 1 minute , So as not to cause SNTP Server resources are rapidly consumed , And can not respond to the customer's request in time ;（3） When the number of clients in the network is greater than 500 When the machine , There should be more than one SNTP The server , To achieve the required timing accuracy ,SNTP Up to one response per second 500 A request , Once this number is exceeded , The accuracy of timing cannot be guaranteed ;（4） In applications that require highly reliable timing , It is better to equip multiple sets SNTP The server , utilize DNS The system implements load balancing and clustering .
2、 In order to solve the problem of clock synchronization in closed networks , Front end devices such as webcams can also use other frequency monitoring networking standard protocols to support timing , For example, national standards GB/T28181 agreement 《 Security video monitoring network system information transmission 、 In exchange for 、 Control technical requirements 》 Pass the rules SIP（Session Initiation Protocol） Signaling for clock synchronization , The front-end device must be registered according to SIP Server header Date The time information carried by the domain is used to synchronize the local time . In addition, the monitoring networking protocols of some large security manufacturers generally have time service interfaces , For example, Hikvision's open device development kit （SDK） With time service interface , At the same time, some manufacturers' network hard disk cameras also have the same IP The internal clock synchronization function of the front end of the camera （ More private protocols are used to support ）.
3、 You can also use a professional time server to provide time sources , Some video surveillance networks have very strict time requirements , For example, highway section speed measurement system 、 Public Security Video Operational investigation system, etc . Many companies at home and abroad have launched their own professional time servers , Professional time servers are generally configured with high accuracy 、 High reliable constant temperature crystal oscillator as a punctual system , From Beidou or GPS The satellite gets the time service signal , Accurate analysis of core servers and Applications 、 Reliable timing , All terminals everywhere （ The server 、PC、 Switch 、IPC、NVR Other equipment ） It can synchronize with the time server . Beidou or GPS The time reference contained in the satellite signal is synchronized with the global coordination time （UTC）, The long-term frequency stability reaches the level of cesium atomic clock 10-13 The order of magnitude , amount to 30 Ten thousand years is slow 1 second . Use this signal as a time reference to adjust the local time , It can eliminate the time accumulation deviation caused by the low accuracy of the local clock , Greatly improve the timing accuracy of the server , At the same time, the professional time server selects the professional Beidou or GPS Time service receiver , The speed of receiving stars is fast , Reliable locking .
Take Dahua Company DH-NTP8010-DR For example , Pictured 7 Shown , Establish a time synchronization service system at level 1 , Receive Beidou and GPS Clock signal source , As a long-term stable time service source , stay GPS And Beidou lost its lock , The constant temperature crystal oscillator is used as the time source , Meet the requirements of core switches in Level 2 、 The server 、 Requirements for clock synchronization of clients such as trunk network transmission gateway , Level 3 subordinate platform servers and other client devices can perform time calibration to level 2 , Time synchronization .

chart 7
Although the national standard GB/T28181 The protocol does not make clear requirements for the time error of the video monitoring system , But when we design various security video surveillance systems , According to the characteristics of each system , Fully consider the problem of time synchronization between various devices . In practical applications , If there is no professional time server , You can also calibrate the platform manually on a regular basis （NVR） The clock of 、 Connect devices with standard protocols that support time calibration 、 Ensure that the platform and equipment comply with the clock synchronization agreement in the standard protocol , It is a necessary means to reduce the problem of system clock disorder .