Research Of Wireless Network Application In The Coal Mine Gas Monitoring System

802.11 family includes over the air modulation techniques, using the same basic protocol family. The most popular are those defined in the 802.11b and 802.11g protocols, which is standard on the original amendment. 802.11-1997 was the first wireless networking standard, but 802.11b was the first widely accepted, followed by 802.11g and 802.11n. The original security is weak in some countries as export destinations request of the Government, the Government and legislative changes and later adopted. 802.11i amendment to increase security. 802.11n is a new multi-stream modulation. In the 802.11 family (F, H, j) the revision and expansion of other standard services, or to change the extension to the previous specifications.802.11b and 802.11g use the 2.4 GHz ISM band, according to the U.S. Federal Communications Commission rules and regulations of Article 15. Because of this frequency band, 802.11b and g equipment may occasionally suffer from the choice of microwave ovens, cordless phones and Bluetooth devices interference. 802.11b and 802.11g interference and control the use of direct sequence spread spectrum (DSSS) and orthogonal frequency division multiplexing (OFDM) signal means, respectively, sensitive to disturbance. 802.11a uses the 5 GHz in the U-NII band, which, in many parts of the world, rather than providing 2.4 GHz ISM band, at least 23 non-overlapping channels overload in all channels. Overload or worse, higher performance or lower frequency (channel), can be realized, depending on the environment.The segment of the radio frequency spectrum used by 802.11 varies between countries. In the US, 802.11a and 802.11g devices may be operated without a license, as allowed in Part 15 of the FCC Rules and Regulations. Frequencies used by channels one through six of 802.11b and 802.11g fall within the 2.4 GHz amateur radio band. Licensed amateur radio operators may operate 802.11b/g devices under Part 97 of the FCC Rules and Regulations, allowing increased power output but not commercial content or encryption.Frames are divided into very specific and standardized sections. Each frame has a MAC header, payload and frame check sequence (FCS). Some frames may not have the payload portion. First 2 bytes of MAC header is a frame control field that provides detailed information about the frame. The sub fields of the frame control field is presented in order.Protocol Version: It is two bits in size and represents the protocol version. Currently used protocol version is zero. Other values are reserved for future use.Type: It is two bits in size and helps to identify the type of WLAN frame. Control, Data and Management are various frame types defined in IEEE 802.11.Sub Type: It is four bits in size. Type and Sub type are combined together to identify the exact frame.ToDS and FromDS: Each is one bit in size. They indicate whether a data frame is headed for a distributed system. Control and management frames set these values to zero. All the data frames will have one of these bits set. However communication within an IBSS network always set these bits to zero.More Fragment: The More Fragmentation bit is set most notably when higher level packets have been partitioned and will be set for all non-final sections. Some management frames may require partitioning as well.Retry: Sometimes frames require retransmission, and for this there is a Retry bit which is set to one when a frame is resent. This aids in the elimination of duplicate frames.Power Management: The Power Management bit indicates the power management state of the sender after the completion of a frame exchange. Access points are required to manage the connection and will never set the power saver bit.More Data: The More Data bit is used to buffer frames received in a distributed system. The access point uses this bit to facilitate stations in power saver mode. It indicates that at least one frame is available and addresses all stations connected.WEP: The WEP bit is modified after processing a frame. It is toggled to one after a frame has been decrypted or if no encryption is set it will have already been one.Order: This bit is only set when the "strict ordering" delivery method is employed. Frames and fragments are not always sent in order as it causes a transmission performance penalty.The next two bytes are reserved for the Duration ID field. This field can take one of three forms: Duration, Contention-Free Period (CFP), and Association ID (AID).An 802.11 frame can have up to four address fields. Each field can carry a MAC address. Address 1 is the receiver, Address 2 is the transmitter, Address 3 is used for filtering purposes by the receiver.The Sequence Control field is a two-byte section used for identifying message order as well as eliminating duplicate frames. The first 4 bits are used for the fragmentation number and the last 12 bits are the sequence number.An optional two-byte Quality of Service control field which was added with 802.11e.The Frame Body field is variable in size, from 0 to 2304 bytes plus any overhead from security encapsulation and contains information from higher layers.The Frame Check Sequence (FCS) is the last four bytes in the standard 802.11 frame. Often referred to as the Cyclic Redundancy Check (CRC), it allows for integrity check of retrieved frames. As frames are about to be sent the FCS is calculated and appended. When a station receives a frame it can calculate the FCS of the frame and compare it to the one received. If they match, it is assumed that the frame was not distorted during transmission. Management Frames allow for the maintenance of communication. Some common 802.11 subtypes include:Authentication frame: 802.11 authentication begins with the WNIC sending an authentication frame to the access point containing its identity. With an open system authentication the WNIC only sends a single authentication frame and the access point responds with an authentication frame of its own indicating acceptance or rejection. With shared key authentication, after the WNIC sends its initial authentication request it will receive an authentication frame from the access point containing challenge text. The WNIC sends an authentication frame containing the encrypted version of the challenge text to the access point. The access point ensures the text was encrypted with the correct key by decrypting it with its own key. The result of this process determines the WNIC's authentication status.Association request frame: sent from a station it enables the access point to allocate resources and synchronize. The frame carries information about the WNIC including supported data rates and the SSID of the network the station wishes to associate with. If the request is accepted, the access point reserves memory and establishes an association ID for the WNIC.Association response frame: sent from an access point to a station containing the acceptance or rejection to an association request. If it is an acceptance, the frame will contain information such an association ID and supported data rates.Beacon frame: Sent periodically from an access point to announce its presence and provide the SSID, and other parameters for WNICs within range. Deauthentication frame: Sent from a station wishing to terminate connection from another station.Disassociation frame: Sent from a station wishing to terminate connection. It's an elegant way to allow the access point to relinquish memory allocation and remove the WNIC from the association table.Coal mine production safety is the most important part of it relates to the safety of all production personnel. With the recent development of industrial level. Increasing coal production, coal production safety accidents there have been year on year growth. The harmonious development of society caused by a very bad influence.Gas explosion, the gas and air mixture under certain conditions, occur when the temperature heat source heat - chain oxidation, accompanied by high temperature and pressure (pressure) increases markedly. Gas explosion in its essence, is a certain concentration of methane and oxygen in the air under the action of a certain temperature, the intense oxidation.Function of coal mine gas monitoring system is to gauge the concentration of gas is set to alert the value, when the underground gas concentration exceeds the value of the time alert: Underground Gas gas monitoring system in the host computer display sound and light alarm. Underground gas can also be monitored by the management procedures set action, when the power off when a gas gauge, increase air flow and so on.