1 VNC protocol features VNC is composed of VNC Server and VNC Client. The VNC Server generates a local display and realizes the communication between the VNC Server and the VNC Client through the VNC protocol. The VNC protocol implements a remote graphical user interface access protocol RFB [2] (Remote Frame Buffer) protocol. The RFB protocol is based on the frame buffer (Frame Buffer) level, so it is suitable for all operating system platforms and applications, such as X Windows, MS Windows and Macintosh etc. VNC displays the real-time desktop desktop data on the remote client through a simple graphical interface transmission protocol based on the TCP / IP protocol.
The display aspect of the protocol is based on a very simple principle, which is to place the pixels of a rectangular area at a specific location. By adopting different image coding methods, a balance can be reached between transmission speed, image quality, server processing speed, terminal display speed, and network bandwidth. Through each window update, you can get the latest window content. In some ways, this looks more like a stream of video images. Each window update message is sent to the server through the client, and each time a message is received, the server responds to it, so that the protocol itself can control its own refresh rate. If the client or the network is slow, the refresh rate will also be lower, which can complete the speed adaptation.
The input protocol is based on the keyboard response message of the standard workstation and the mouse device. When the client receives these input messages, it is simply transmitted to the server in the form of events and corresponding actions are generated.
2 Problems with the application of VNC in a low-bandwidth network environment In a large-scale video conference, when it is necessary to share the desktop or an application of one of the conference members, the computing performance and network access conditions of the VNC server Both are relatively poor, which puts high requirements on the sharing mechanism, which must have high efficiency, small delay, and require light architecture, occupy less system resources, and save bandwidth. Due to the sufficient network bandwidth in foreign countries, the multi-point application sharing using the VNC mechanism can run smoothly, but under the current low-bandwidth network conditions in China, the VNC mechanism uses a star-shaped direct connection structure. If the VNC default star is directly used -Type direct connection mechanism, it will be very difficult to achieve application sharing in multi-point remote video conference. VNC Server communicates with each VNC Client in a direct connection mode. In this case, the export bandwidth of VNC Server is limited, and the data on VNC Server The transmission pressure will increase linearly. Under the condition of 2 MHz ADSL exit bandwidth, when the number of conference members reaches 10, it will cause serious network congestion, which will greatly reduce the real-time nature of application sharing, so it does not have a remote video conference system. Actual availability.
3 Improvement scheme of VNC sharing mechanism
3.1 VNC shared communication mechanism By default, the VNC mechanism communicates with the VNC Client in a direct connection. This article will propose a new implementation scheme, which uses the relay server forwarding mode to achieve sharing in the remote video conference system [4]. The specific structure is shown in the figure 1 shown. At this time, the VNC Server only needs to upload data to the transit server. The VNC Client only establishes a connection with the transit server. The transit server sends data to each client. Generally, the transit server's network exit bandwidth is relatively large, so the network bandwidth will not become Performance bottleneck. This greatly reduces the network transmission pressure on the VNC Server side, and the system's response speed will be improved when the application is shared.
3.2 Screen change detection algorithm The performance of the screen change detection algorithm will directly affect the amount of data sent by the VNC Server. An efficient screen detection algorithm will greatly reduce the amount of data transmitted and save network bandwidth, thereby improving the real-time effect of the system . The default screen change detection mechanism of VNC is to obtain the coordinate information of the area through the message hook mechanism, and then send the screen pixel data of the coordinate area to each VNC Client. This detection method allows a large amount of screen data that has not changed to be sent, which increases the amount of data sent by the network and easily causes network congestion in a low-speed network environment, affecting system performance. Therefore, on this basis, this paper proposes a new screen change area detection algorithm.
The VNC mechanism uses the message hook mechanism to intercept the system screen redrawing area information, split the obtained area into a rectangular linked list (traversing the linked list) to store the area coordinate information, and record it as array_list, define a change area and record it as vnc_rgn. Re-send the location information of the area, call this detection algorithm for each member rectangle of array_list to detect the changed area. The specific algorithm flow chart is shown in Figure 2.
(1) Assuming that one of the member rectangles is arri, if the length and width of arri are less than 32 × 32, it is directly used as the change area to be retransmitted and stored in vnc_rgn, otherwise arri is analyzed and detected.
(2) First define a new rectangle arrj (arri.left, 0, arri.right, 0), find the changed line in arri, record the y coordinate of the line, record arrj.top = y, and at the same time y + = 16 Incrementally find the line that has not changed, and temporarily record arrj.bottom = y; then compare from the bottom of arrj y- = 1, find the line with the content changed, and set the y coordinate of this line to arrj.bottom = y after finding it. At this point, the final top and bottom coordinates of the changing rectangle arrj are determined. At the same time, the value of arri is set to arri (arri.left, arrj.bottom, arri.right, arri.bottom). After the analysis of arrj is completed, the analysis of arri is continued until the end.
(3) After determining the rectangle arrj, analyze the rectangle arrj. First define a rectangle arrn (0,0,0,0), find the column that changes first in arrj, record the left and top coordinates of the changed area after finding, arrn.left = x, arrn.top = y , Analyze the x and y axis cycle traversal separately (denoted as x + = 32, y + = 32), if the screen pixel data of this column changes, then x + = 32 continue to compare; otherwise, record arrn.right = x, y Make the same comparison in the axis direction, y + = 32, and record arrn.bottom = y, and finally get the rectangular coordinates of the change area that needs to be sent, and save this rectangle in the finally sent rectangular linked list, and record arrk (arrn.right , Arrn.top, arrj.right, arrn.bottom), arrm (arrn.left, arrn.bottom, arrj.right, arrj.bottom), assign arrk and arrm to arrj for similar analysis and detection, and loop through the whole in turn arrj until the end.
Through this screen change area detection algorithm, after traversing the member rectangles of the entire array_list linked list, the coordinate information of the change area that needs to be resent is obtained.
4 Experimental results This improved solution has been applied to VcomOffice remote video conference system. The experimental environment is: In the local area network, the relay server uses the Linux operating system, and the VNC Server and VNC Client both use the Windows XP operating system. The specific software and hardware configurations are shown in Table 1.
In order to verify the change of the amount of data generated by the same operation before and after calling the screen change area detection algorithm, the experiment was conducted in a 100 M local area network. VNC Server uses the VNC default TIght encoding [5], which is compressed by zlib after encoding [5] Later sent to the transit server, mainly for the following scenarios:
(1) Open a Word document, cut, copy, paste a piece of text, save the picture to Word, and enter some content in Word;
(2) Open the IE browser to log in to the homepage;
(3) After opening a PDF document, drag the scroll bar to the bottom;
(4) Open the resource manager, do the drag operation on the screen, and then maximize it.
The amount of data sent by the VNC Server before and after calling the detection algorithm is shown in Figure 3. It can be seen from the comparison that before and after calling the screen area change detection algorithm, the amount of data sent by the VNC Server will decrease to some extent. When performing the same Word operation, the amount of data sent by the VNC Server decreased by 12.21%, and the amount of data sent during the login to the Sina.com homepage only decreased by 4.82%. Open the PDF document and drag the scroll bar to the bottom The total amount of data sent decreased by 14.63%. During the scaling process of the resource manager, the amount of data sent decreased by 22.19%.
In the test result table, when the VNC system calls the screen area change detection algorithm, the amount of data sent by the VNC Server will decrease by about 10% on average. This will greatly reduce the sending pressure on the VNC Server end and reduce the probability of network congestion in remote multi-point application sharing, thereby increasing the response speed of the remote client when sharing applications.
This paper introduces the problems that exist when sharing applications through the VNC protocol in a remote video conference system in a low-speed network environment. A new improved scheme is proposed, and a new screen area change detection algorithm is proposed in conjunction with the new scheme. In addition, the system was tested and compared before and after calling the new screen area change detection algorithm. This solution has been applied to the VcomOffice remote video conference system of Dalian Haoshi Digital Technology Co., Ltd. Through the test of multiple experimental scenarios, the experimental results show that after using the new screen change area detection algorithm, the amount of data sent by the VNC Server has dropped to a large extent, which saves network bandwidth for the remote video conference system in low-speed network environments. Improves the real-time performance of the application sharing system, thereby realizing the application of VNC in a low-speed network environment. Application sharing enables geographically dispersed users to share an application through the connection of the computer network, in the Internet office, remote video conference system , Remote assistance and other aspects have an irreplaceable role and have a very broad development prospect.
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