Based on asynchronous communication protocol of geographic space information service access mechanism research

: At present, the traditional way of accessing to classified network in geographic spatial information services is using network gatekeeper and firewall etc. to ensure public and classified network communications links. However, the physical isolation between classified network and public network is crossed, which is bound to cause classified network potential security hazard. In Yunnan province space Land dynamic monitoring integration project, it proposed the point to point text message communication protocol and asynchronous transmission mechanism. Using geo-spatial information encryption processing and data compression processing method, it reduced the risk of data sensitivity and monitored, namely to ensure data security, which realized geographic spatial information services data communication effectively between classified network and public network in the rigid field conditions.

At present, the traditional way of accessing to classified network in geographic spatial information services is using network gatekeeper and firewall etc. to ensure public and classified network communications links.
However, the physical isolation between classified network and public network is crossed, which is bound to cause classified network potential security hazard.In Yunnan province space Land dynamic monitoring integration project, it proposed the point to point text message communication protocol and asynchronous transmission mechanism.Using geo-spatial information encryption processing and data compression processing method, it reduced the risk of data sensitivity and monitored, namely to ensure data security, which realized geographic spatial information services data communication effectively between classified network and public network in the rigid field conditions.

INTRODUCTION:
In the process of land and resources law enforcement is commonly in the field, but it needs analyze and the query a large number of geo-spatial data which saved in classified network.Provided it accessing directly, it needs break through the physical isolation between the classified network and the network.Traditional access method using GPRS and 3G network, which expose the server to the Internet environment.Although network gatekeeper and firewall are used, the physical isolation between classified network and public network is crossed, which is bound to cause classified network potential security hazard.
Besides, land and resources Law enforcement is mainly in the suburbs and field, where 3G network is not covered completely, some location, even GPRS network communication, cannot be guaranteed.Land and resources law enforcement urgently need a new kind of methodology to tackle the problem of accessing geo-spatial data between the classified network and the network.SMS (short message service) is a store and forward service, first of all, the initiator terminal sends data to the server transitorily; After that the server store and forward data; Finally, the receiver terminal passive receiving data from the server with unpersistent connections way, the receiver terminal does not send feedback information to the initiator.This method is not easy to be sniffed and monitored by the third party.In addition, the requirement of this way for network environment is not strict, as long as to meet the requirements of digital voice communication, it can conduct the service.And communication cost is low.But the SMS service has a data transmission size limit every time that means every time transfer shall not exceed 70 characters or 140 bytes.There are massive number The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XL-4, 2014   ISPRS Technical Commission IV Symposium, 14 -16 May 2014, Suzhou, China   This contribution has been peer-reviewed.

transmission optimization
Considering limitation length of data for the message communication protocol each time, we design the coordination offset and data compression to reduce the length of the data transmission .The two optimization scheme improve the transmission efficiency and reduce the transport cost.The principle of LZW compression algorithm [5] is to extract different characters of the original text file data and to create a table on the basis of these characters.Then use the index of the characters compile table instead of the corresponding characters in original text file data, which reduces original data size.But the compile table is not to create in advance, it will be created according to the original file data dynamically.When decoding it restore the original text file data from the encoded data compilation tables.The description is shown in Figure 4.

Figure 4 LZW compression algorithm
As to GeoData object, coordinate data is transmitted , content is digital character (" 0 ", "9") and the space character (", "and"; "), the scope of the characters is fixed, so using a LZ77 algorithm [6] of fixed dictionary tables compress the data .The example is shown in Figure 5.The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XL-4, 2014   ISPRS Technical Commission IV Symposium, 14 -16 May 2014, Suzhou, China   This contribution has been peer-reviewed.doi:10.5194/isprsarchives-XL-4-35-2014 doi:10.5194/isprsarchives-XL-4-35-2014 of research on small amount of transmission monitoring indicators such as low, pressure, etc. in domestic.However, as to large amount of data transmission investigation, like geographical spatial information query, is rare.The author taking Yunnan province space Land dynamic monitoring integration project as instance, proposed asynchronous communication protocol based on SMS service, using coordinate offset and data compression method to fulfill geo-spatial information accessing service in the stringent conditions. 1 DATA COMMUNICATION PROTOCOL Through the investigation of land and resources law enforcement business process, we found that the wild enforcement is mainly though the inspectors to carry terminal equipment with data acquisition functions collecting geo-spatial information.Then overlapping collecting geo-spatial information with existed graphic data that is classified.After that inspectors analyze various index to make a judgment whether collecting geo-spatial data from the reality environment just now satisfy existed graphic data requirement.Due to security requirements, field inspectors cannot carry large amounts of confidential graphic data.Field inspectors submit the collected geographic spatial information data to the server, the server receive the data and analyze it.Then the analysis results are returned to the field inspectors, field inspectors acquire information of if the land use is illegal.In order to guarantee the security, terminal equipment and the server shall encipher the information before sending.When the receiver can decipher the encode information the data dictionary which promissory in advance.Program flow as Figure 1.

Figure
Figure 1.Program flow 1.1 data structure Through the analysis of data transmission, transmitted data content two types, one is geographical spatial coordinate information which terminal equipment submitted to the server, the data structure is regular, format-fix; the other is the analysis results that the server returned to the terminal equipment.This type data is text information, the length of data is flexible.The difference of the two types of data structure is large, it is necessary to design a common data structure to carry the query information.The primary data structure design of the whole system is shown inFigure 2.

Figure 2
Figure 2 data structure -spatial data transmission is mainly coordinate point transmission, considering information transmission security, with longitude and latitude method for data transmission.Because of higher accuracy of latitude and longitude coordinates, the meters-level accuracy data needs seven decimal places generally.If it store the information with character type, a number will reach 11 byte, a coordinate point account at least 22 byte, so a text message can carry little information.Considering the geographic space information query is usually no more than several kilometers, we adopt coordinates offset to shorten the length of coordinates: if needed transmitted data contain multiple points (for a single point, this way is no value), it amplify all latitude and longitude coordinates to integer.Completing data unit conversion, it selects minimum longitude and latitude in the all points as a relative coordinate system origin, so all of the data converted into positive integer, which achieve the purpose of using short integer to indicate the floating point coordinates.The example is shown in figure 3.

Figure 3
Figure 3 coordinate offset

Figure
Figure 5 LZ77 algorithm of fixed dictionary tables compress algorithm