CN103117833B - Shielding device and shielding method of wireless local area network - Google Patents
Shielding device and shielding method of wireless local area network Download PDFInfo
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- CN103117833B CN103117833B CN201210517154.9A CN201210517154A CN103117833B CN 103117833 B CN103117833 B CN 103117833B CN 201210517154 A CN201210517154 A CN 201210517154A CN 103117833 B CN103117833 B CN 103117833B
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
- Y02D30/00—Reducing energy consumption in communication networks
- Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks
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Abstract
The invention discloses a shielding device of a wireless local area network, and simultaneously discloses a method for shielding interference of the shielding device of the wireless local area network. The shielding device of the wireless local area network comprises antennas, a receiving radio frequency module, a sweep frequency module, a coarse synchronization module, an interference data generation module, an emitting radio frequency module and a power amplifier. After frequency points, bandwidth and a standard which are used in the present wireless local network are determined by the sweep frequency module through frequency sweeping, the coarse synchronization module triggers interference signals to be generated to interfere long training sequences and SIGNAL fields in data frames, normal receiving of a receiving end is affected, and the purposes of shielding and interfering are achieved. The long training sequences and the SIGNAL fields in the data frames are affected by double interference, so that the shielding device and the shielding method of the wireless local area network have the advantages of being good in interference effects, large in actuating range and small in power consumption.
Description
Technical field
The present invention relates to a kind of WLAN (wireless local area network) shielding device and screen method, particularly relate to a kind of by disturbing the long training sequence of Frame and SIGNAL field, realize WLAN (wireless local area network) shielding device and the screen method of shield effectiveness, belong to wireless communication technology field.
Background technology
WLAN (wireless local area network) (Wireless Local Area Network, WLAN) is supplementing of fixed broadband data network and extends.User only need install wireless network card on computers, can realize high speed Internet access in WLAN (wireless local area network) coverage.Current, the development of WLAN (wireless local area network) is very rapid, not only as the network access of office, family's first-selection, and on airport, conference and exhibition center, the public arena such as hotel complete part and cover.It and 3G, 4G network supplement mutually, can provide service on net easily at a high speed.
Radio communication is bringing easily simultaneously, proposes new challenge also to communication security and security work.In recent years, occurred a lot ofly to utilize WLAN (wireless local area network) to carry out eavesdropping, divulged a secret, the malfeasance such as examination hall cheating in China, its extent of injury and seriousness have caused the great attention of relevant department.Therefore, in suitable place, legal and necessary shielding is implemented to the proper communication of WLAN (wireless local area network), has become exigence in practice.
Be in the Chinese patent application of 201210059557.3 at application number, disclose a kind of Full-band examination place radio signal shielding device.This shielding device comprises fundamental frequency signal generator, modulation signal generator, modulator, frequency hopping generator, digital frequency converter and radio frequency unit.In this technical scheme, provide low side interference signal by fundamental frequency source, obtain different frequency range signal by saltus step difference frequency, thus realize the shielding to all frequency ranges.This technical scheme, for the mode of current radio communication and technical conditions, can be implemented comprehensive, full frequency band, seamless blanket type cover jamming to the frequency range within the scope of 50MHz ~ 2500MHz, effectively stop all cheatings utilizing wireless means.
In addition, be in the Chinese patent application of 201010604926.3 at application number, disclose a kind of energy-conservation WLAN (wireless local area network) all-channel screen method, comprising: the channel starting shielding device Wlan scanning, is recorded in the maximum of each working channel RSSI; Calculate the radio transmission loss PL of the communication equipment end that RSSI is maximum on shielding device end to each channel; Determine that the shielding device be operated on i channel makes the throughput of j channel reduce to the minimum emissive power Y of zero
ij; Find out Y when shielding device is operated in different channels
ijmaximum, and therefrom find out minimum value Y
k, corresponding channel k is the best effort channel of shielding device; The working channel of shielding device is set as k, with Y
kpower emission shielded signal.This technical scheme is disturbed whole channel simultaneously, ensures that channel throughput reduces simultaneously, avoids channel hopping and causes former channel throughput to go up and can not launch the problem of interference signal during redirect, improve jamming effectiveness.
At present, realize the mode of WLAN (wireless local area network) shielding in prior art, be utilize radio frequency in suitable frequency range sequential filming interference signal substantially, the signal interference ratio of WLAN (wireless local area network) receiving terminal is reduced, improve frame error rate when receiving; And reduce the probability of WLAN (wireless local area network) transmitting terminal successful execution clear channel assessment (CCA), make the rate reduction of its outside transmission frame.Comparatively speaking, the power dissipation ratio continuing to launch interference signal in prior art is comparatively large, and interference effect has much room for improvement.
Summary of the invention
For the deficiencies in the prior art, technical problem to be solved by this invention is to provide a kind of WLAN (wireless local area network) shielding device and screen method.This technical scheme can make WAP (wireless access point) in predetermined coverage and website can not proper communication, realizes the effect of shielding and interference.
For realizing the object of foregoing invention, the technical solution adopted in the present invention is as follows:
A kind of WLAN (wireless local area network) shielding device, is characterized in that:
Described WLAN (wireless local area network) shielding device comprises antenna, received RF module, scan module, thick synchronization module, interfering data generation module, launches radio-frequency module and power amplifier;
Described received RF module connects described antenna on the one hand, connects described scan module and described thick synchronization module respectively on the other hand;
The output of described thick synchronization module connects described interfering data generation module, described transmitting radio-frequency module and described power amplifier respectively; Described interfering data generation module is connected with described power amplifier by described transmitting radio-frequency module; The output of described power amplifier is connected with described antenna.
Wherein more preferably, perform frequency sweep at described scan module, after finding the information of current wireless Local Area Network, the frequency of the resident frequency sweep of received RF module, described thick synchronization module search short training sequence; Once described thick synchronization module searches described short training sequence, trigger described interfering data generation module, described transmitting radio-frequency module and described power amplifier, launch interference signal.
Wherein more preferably, described interference signal implements interference for the long training sequence of frame head in Frame and SIGNAL field (comprising SIGNAL/L-SIG field, HT-SIG field and VHT-SIG A field).
A kind of WLAN (wireless local area network) screen method, realizes based on above-mentioned WLAN (wireless local area network) shielding device, it is characterized in that:
Scan module carries out frequency sweep to current wireless Local Area Network, after determining the information of current wireless Local Area Network, and the frequency of the resident scanning of received RF module;
Described thick synchronization module is at current resident frequency, Frame is carried out to the identification of short training sequence, once described thick synchronization module searches described short training sequence, trigger described interfering data generation module, described transmitting radio-frequency module and described power amplifier, launch interference signal;
Described interference signal implements interference for the long training sequence of frame head in Frame and SIGNAL field, makes receiving terminal can not identify the information comprised in long training sequence and SIGNAL field, realizes disturbing the shielding of current wireless Local Area Network.
Wherein more preferably, when described short training sequence is not by described thick synchronization module identification, described interfering data generation module, described transmitting radio-frequency module and described power amplifier are in holding state, do not have interference signal to launch.
Wherein more preferably, described interference signal has identical bandwidth with disturbed signal.
Wherein more preferably, for the Frame of IEEE 802.11n standard, described interference signal implements interference for long training sequence wherein, L-SIG field and HT-SIG field.
Wherein more preferably, for the Frame of IEEE 802.11ac standard, described interference signal implements interference for long training sequence wherein, L-SIG field and VHT-SIG-A field.
Wherein more preferably, length launch time of described interference signal adjusts according to communication standard and signal bandwidth.
The present invention has following technical characterstic:
1. the long training sequence of pair Frame and SIGNAL field carry out dual interference, have the advantage that interference effect is good, sphere of action is large;
2. do not need to continue to launch interference signal, there is the advantage that power consumption is little;
3. screen method is simple, and the cost of facilities and equipments is lower.
Accompanying drawing explanation
Fig. 1 is the module composition schematic diagram of WLAN (wireless local area network) shielding device provided by the present invention;
Fig. 2 is the substandard data frame format schematic diagram of IEEE 802.11a/g;
Fig. 3 is SIGNAl(L-SIG) bit of field distributes form schematic diagram;
Fig. 4 is the substandard data frame format schematic diagram of IEEE 802.11n;
Fig. 5 is the substandard data frame format schematic diagram of IEEE 802.11ac;
Fig. 6 is that the bit of Medium Access Control (MAC) Protocol Data Unit distributes form schematic diagram;
Fig. 7 is the Frame transmitting-receiving schematic flow sheet of MAC layer;
Fig. 8 is under IEEE 802.11a/g standard, the schematic diagram of interference signal;
Fig. 9 is under IEEE 802.11n standard, the schematic diagram of interference signal;
Figure 10 is under IEEE 802.11ac standard, the schematic diagram of interference signal.
Embodiment
Below in conjunction with the drawings and specific embodiments, the present invention is described in further detail.
The invention provides a kind of WLAN (wireless local area network) shielding device, also provide the method that this WLAN (wireless local area network) shielding device realizes net mask simultaneously.This equipment and method are applicable to the WLAN (wireless local area network) adopting IEEE802.11a/g/n/ac standard, therefore cover current all popular WLAN standard.In the present invention, for the wireless local net data frame adopting I EEE 802.11a/g/n/ac standard, by after identifying the short training sequence (short trainingsymbols) of frame head and locating, further interference is implemented to long training sequence (long trainingsymbols) and SIGNAL field, make receiving terminal can not identify the information comprised in long training sequence and SIGNAL field, thus make WAP (wireless access point) (the Access Point in WLAN (wireless local area network) shielding device coverage, be abbreviated as AP) and website (Station) can not proper communication, realize the object carrying out shielding interference to the proper communication in WLAN (wireless local area network).
Fig. 1 is the module composition schematic diagram of WLAN (wireless local area network) shielding device provided by the present invention.This WLAN (wireless local area network) shielding device mainly comprises antenna, received RF module, scan module, thick synchronization module, interfering data generation module, launches radio-frequency module and power amplifier.Wherein, antenna connects the input of received RF module, and the output of received RF module is connected with thick synchronization module respectively with scan module.The output of thick synchronization module then respectively connected interfering data generation module, launches radio-frequency module and power amplifier.Interfering data generation module is connected with power amplifier by launching radio-frequency module.The output of power amplifier is connected to antenna.The interference signal of launching is gone out by antenna transmission.Above-mentioned received RF module, scan module, thick synchronization module, interfering data generation module, launch radio-frequency module and power amplifier can adopt existing ripe integrated circuit (IC) chip to realize, also can at FPGA(field programmable gate array) basis is upper is achieved by programming.As the routine techniques that those skilled in the art can grasp, just do not describe in detail at this.
In the WLAN (wireless local area network) shielding device shown in Fig. 1, received RF module receives the wireless signal of place WLAN (wireless local area network) by antenna.Scan module carries out frequency sweep to the WLAN (wireless local area network) receiving the current in-scope that radio-frequency module searches.In frequency sweep process, scan module needs network standard, the information such as frequency and bandwidth of determining current wireless Local Area Network.If current network standard, frequency and bandwidth information are determined, WLAN (wireless local area network) shielding device directly can carry out the setting of relevant information to the WLAN (wireless local area network) of required interference, without the need to carrying out frequency sweep to determine the relevant information of WLAN (wireless local area network) through scan module again.For the network of the unknown, scan module carries out frequency sweep analysis by the current wireless Local Area Network signal received reception radio-frequency module, determines the information such as the network standard of current wireless Local Area Network, frequency and bandwidth.After the information of current wireless Local Area Network is determined, received RF module is carried out resident at current scanning frequency.
Shielding jamming program of the present invention carries out mainly for the physical layer of WLAN (wireless local area network).Be described as follows:
In IEEE 802.11a/g standard, physical layer adopts OFDM technology, can use 5MHz, 10MHz and 20MHz bandwidth.First the physical layer of transmitting terminal carries out scrambling, coding, punching, intertexture, interpolation pilot bits and inverse fast fourier transform (IFFT) to Medium Access Control (MAC) Protocol Data Unit (MPDU), before data, then add the known training sequence of receiving terminal and SIGNAL field composition data frame.The frame format of this Frame as shown in Figure 2, is first 10 short training sequence t1 ~ t10, and being next 2 long training sequence T1 and T 2, being then SIGNAL field, is finally data division.Cyclic Prefix (Cyclic Prefix) is all comprised in long training sequence, SIGNAL field and data division.SIGNAL field has 24 bits, contains the information such as modulation system and data length of these frame data part.Its concrete bit distributes form as shown in Figure 3.
IEEE802.11n standard adopts OFDM technology equally, and can use 20MHz and 40MHz bandwidth.As shown in Figure 4, frame format in IEEE 802.11n standard is similar to the frame format in IEEE 802.11a/g standard, and wherein the form of short training sequence, long training sequence and SIGNAL field (referred to herein as L-SIG field) is identical with IEEE 802.11a/g standard with effect.But add HT-SIG(high-throughput SIGNAL in this Frame) field.In the HT-SIG field increased, further comprises the information such as modulation system, bandwidth, length, diversity mode, have 48 bits.
IEEE 802.11ac standard also adopts OFDM technology, uses 20MHz, 40MHz, 80MHz, 80+80MHz and 160MHz bandwidth.As shown in Figure 5, frame format in IEEE 802.11ac standard is similar to the frame format in IEEE 802.11a/g standard, and wherein the form of short training sequence, long training sequence and SIGNAL field (referred to herein as L-SIG field) is identical with IEEE802.11a/g standard with effect.But add VHT-SIG A(ultra-high throughput SIGNAL field A in this Frame) and VHT-SIGB(ultra-high throughput SIGNAL B) field.In the VHT-SIG A field increased, further comprises the information such as spatial flow number of bandwidth, diversity mode, sets of numbers, each user, have 48 bits.
Therefore, for the shielding interference of IEEE 802.11n standard and IEEE 802.11ac standard, except aforesaid long training sequence and SIGNAL(L-SIG) except field, also need to comprise HT-SIG field and VHT-SIG A field.
In the physical layer of receiving terminal, complete automatic growth control (Automatic Gain Control), thick time synchronized and Frequency Synchronization by the short training sequence of Frame; By long training sequence deadline, frequency precise synchronization and channel estimating.Then SIGNAL(L-SIG is separated according to time, Frequency Synchronization value and channel estimation value) field is (in IEEE 802.11n and IEEE802.11ac, also comprise HT-SIG field and VHT-SIG A, B field), obtain the information such as modulation system and data length of follow-up data, resolve follow-up data according to all these instructions again, the MPDU parsed is delivered to MAC layer.As the information in SIGNAL field can not correctly be parsed, then receiving terminal correctly can not parse follow-up data, and these frame data will be dropped.
In a wireless local area network, MAC layer delivers to physical layer after forming MPDU after MSDU packet is added MAC head and tail.The form of MPDU as shown in Figure 6, wherein comprises the information such as frame control information, length, source address, destination address in MAC head, judges whether this frame is the data being sent to this receiving terminal for receiving terminal; Whether MAC tail comprises 32 bit frame checking datas (Frame CheckSequence, FCS), detect these frame data and be correctly received, if correctly do not received, then abandon this frame for receiving terminal.For the Frame that transmitting terminal sends, after the MAC layer of receiving terminal correctly receives parsing, need after data receiver terminates, wait for SIFS(ShortInter-Frame Space) time, then send ACK frame to confirm the correct reception of this Frame to transmitting terminal.After transmitting terminal receives ACK frame, namely confirm that this Frame correctly receives, start the transmission of next Frame, detailed process as shown in Figure 7.If transmitting terminal is after sending the SIFS time terminated, correctly do not receive ACK frame, so transmitting terminal thinks that the Frame sent there occurs with the Frame that other website sends and conflicts, be not correctly received, therefore in the scope of competition window, Stochastic choice Backoff time, resends this Frame when reaching the value of Backoff time etc. channel idle again.If after Frame number of retransmissions has exceeded maximum retransmission, still do not send successfully, so this Frame will be dropped.This communication mechanism is utilized the shielding realized WLAN (wireless local area network) proper communication to disturb by the present invention.
In the WLAN (wireless local area network) shielding device shown in Fig. 1, thick synchronization module is mainly used in identification to frame head short training sequence, monitoring.For the RM of frame head, can identify according to the correlation of each pattern according to existing technology.Although the frame format of heterogeneous networks standard is different, wherein the form of short training sequence and shared bit identical, the mode therefore identified and time are also identical.At the current frequency that scan module is resident, the short training sequence of frame head in thick synchronization module search current wireless Local Area Network Frame.Once thick synchronization module searches short training sequence, namely thinking has dataframe, triggers interfering data generation module at once, launches radio-frequency module and power amplifier, launch interference signal corresponding with it.When the short training sequence of frame head is not by thick synchronization module identification in a data frame, interfering data generation module, transmitting radio-frequency module and power amplifier are in standby state, do not have interference signal to launch.Continue to launch the WLAN (wireless local area network) shielding device of interference signal for needing, be adaptive to the transmitting of interference signal in the present invention, power consumption is relatively low.
After interfering data generation module is triggered, produce the interference signal identical with disturbed signal bandwidth immediately.Launch radio-frequency module and the interference signal of generation is changed into wireless signal, go out through power amplifier and antenna transmission.This interference signal is mainly for the long training sequence of frame head in former Frame and SIGNAL(L-SIG) field.For the Frame of IEEE 802.11n and IEEE 802.11ac standard, interference signal for also comprise HT-SIG field and VHT-SIG-A field.After long training sequence is disturbed, time of receiving terminal, frequency precise synchronization and channel estimation process receive interference, and make receiving terminal can not get correct parameter, SIGNAL field and follow-up data can not correctly be parsed.After SIGNAL field is disturbed, receiving terminal correctly can not obtain the information such as modulation system and length of Frame, just can not be correctly decoded follow-up data.Like this, the physical layer of receiving terminal correctly can not parse MPDU, and MAC layer will not send ACK frame to transmitting terminal, and therefore transmitting terminal will retransmit these frame data.After transmitting terminal reaches maximum retransmission, these frame data will be dropped, thus make WAP (wireless access point) in interference signal coverage and website can not proper communication, realize the effect of shielding and interference.
In the present invention, WLAN (wireless local area network) shielding device only needs to disturb the long training sequence in Frame and SIGNAL field, and long training sequence and the SIGNAL field proportion shared by whole Frame is very little, does not therefore need to continue to launch interference signal.Emission needle is to the interference signal of long training sequence and SIGNAL field, and relative to emission needle concerning the interference signal of whole Frame, power consumption significantly reduces.Such as in IEEE 802.11a standard, under the condition of 20MHz bandwidth, for every frame data, only launch 16 μ s interference signals.When transmission rate is 6Mbps, when a frame data length is 1000 bytes, only account for launch time of interference signal 1.18% of frame data launch time; When a frame data length is 2000 byte, only account for launch time of interference signal 0.59% of frame data launch time.Significantly can reduce WLAN (wireless local area network) shielding device power consumption operationally like this, reduce the operating cost of integral device.
In interfering data frame when the long training sequence of frame head and SIGNAL field, for different communication standards and signal bandwidth, the time span that interference signal is launched is different.Such as, in the network of IEEE 802.11a/g standard, corresponding with it interference signal is 16 μ s, 32 μ s or 64 μ s(along with signal bandwidth is different and different launch time); In the network of IEEE802.11n/ac standard, corresponding with it interference signal launch time is 24 μ s.Interference signal corresponding to distinct communication standards and signal bandwidth is as shown in table 1 for launch time.
Interference signal launch time under table 1 IEEE 802.11a/g/n/ac series standard
In IEEE 802.11a/g, IEEE 802.11n and IEEE 802.11ac standard, the time range that interference signal is launched is respectively as shown in Fig. 8, Fig. 9 and Figure 10.
It should be noted that to make the present invention be easier to understand, it is more known ins and outs for the person of ordinary skill of the art that description above eliminates.
Above WLAN (wireless local area network) shielding device provided by the present invention and screen method are described in detail.To those skilled in the art, to any apparent change that it does under the prerequisite not deviating from connotation of the present invention, all by formation to infringement of patent right of the present invention, through bearing corresponding legal liabilities.
Claims (8)
1. a WLAN (wireless local area network) shielding device, is characterized in that:
Described WLAN (wireless local area network) shielding device comprises antenna, received RF module, scan module, thick synchronization module, interfering data generation module, launches radio-frequency module and power amplifier;
Described received RF module connects described antenna on the one hand, connects described scan module and described thick synchronization module respectively on the other hand;
The output of described thick synchronization module connects described interfering data generation module, described transmitting radio-frequency module and described power amplifier respectively; Described interfering data generation module is connected with described power amplifier by described transmitting radio-frequency module; The output of described power amplifier is connected with described antenna;
At the frequency that described received RF module is resident, described thick synchronization module search short training sequence; Once described thick synchronization module searches described short training sequence, trigger described interfering data generation module, described transmitting radio-frequency module and described power amplifier, launch interference signal.
2. WLAN (wireless local area network) shielding device as claimed in claim 1, is characterized in that:
Described interference signal implements interference for the long training sequence of frame head in Frame and SIGNAL field.
3. a WLAN (wireless local area network) screen method, realizes based on WLAN (wireless local area network) shielding device as claimed in claim 1, it is characterized in that:
Scan module carries out frequency sweep to current wireless Local Area Network, after determining the information of current wireless Local Area Network, and the frequency of the resident scanning of received RF module;
Described thick synchronization module is at current resident frequency, Frame is carried out to the identification of short training sequence, once described thick synchronization module searches described short training sequence, trigger described interfering data generation module, described transmitting radio-frequency module and described power amplifier, launch interference signal;
Described interference signal implements interference for the long training sequence of frame head in Frame and SIGNAL field, makes receiving terminal can not identify the information comprised in long training sequence and SIGNAL field, realizes disturbing the shielding of current wireless Local Area Network.
4. WLAN (wireless local area network) screen method as claimed in claim 3, is characterized in that:
When described short training sequence is not by described thick synchronization module identification, described interfering data generation module, described transmitting radio-frequency module and described power amplifier are in holding state, do not have interference signal to launch.
5. WLAN (wireless local area network) screen method as claimed in claim 3, is characterized in that:
Described interference signal has identical bandwidth with disturbed signal.
6. WLAN (wireless local area network) screen method as claimed in claim 3, is characterized in that:
For the Frame of IEEE 802.11n standard, described interference signal implements interference for long training sequence wherein, L-SIG field and HT-SIG field.
7. WLAN (wireless local area network) screen method as claimed in claim 3, is characterized in that:
For the Frame of IEEE 802.11ac standard, described interference signal implements interference for long training sequence wherein, L-SIG field and VHT-SIG-A field.
8. WLAN (wireless local area network) screen method as claimed in claim 3, is characterized in that:
The launch time of described interference signal adjusts according to communication standard and signal bandwidth.
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| CN104333858B (en) * | 2013-07-22 | 2018-03-16 | 中国科学院信息工程研究所 | It is a kind of based on the channel resource control method for going association/de-authentication frames |
| CN104333857B (en) * | 2013-07-22 | 2017-12-15 | 中国科学院信息工程研究所 | A kind of combined type channel management-control method based on assessment preferentially |
| US9474073B2 (en) * | 2013-10-31 | 2016-10-18 | Qualcomm Incorporated | Methods and apparatus for multiple user uplink bandwidth allocation |
| US10044635B2 (en) * | 2014-12-09 | 2018-08-07 | Qualcomm Incorporated | Frame formats for channel bonding and MIMO transmissions |
| CN106375059B (en) * | 2016-08-31 | 2018-08-28 | 广州市瀚云信息技术有限公司 | A kind of TD-LTE cellular phone signal shielding devices synchronization realizing method |
| CN113794531B (en) * | 2021-09-13 | 2023-12-29 | 东南大学 | Preamble-based WiFi channel interference method and system for preventing eavesdropping |
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| US20090053996A1 (en) * | 2007-08-20 | 2009-02-26 | Jean Pierre Enguent | Active Signal Interference |
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| CN101527574A (en) * | 2009-03-27 | 2009-09-09 | 王方松 | Shielding system for seamless grading management |
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