GPS establishes network connection

　　Before the connection in the piconet is established, all devices are in standby state. In this mode, the unconnected unit periodically "listens" for information every 1.28 seconds. Every time a device is activated, it monitors 32 frequency hopping points planned for the unit. The number of frequency hopping points varies according to different geographical regions. The number 32 only applies to most countries except Japan, France and Spain. The master device first initializes the connection program. If the address is known, the connection is established through a page message. If the address is unknown, the connection is established through an inquiry message followed by a page message. In the initial paging state, the master unit will send a string of 16 identical page messages on the 16 frequency hopping points allocated to the paged unit. If there is no response, the master will continue paging on the remaining 16 frequency points in the activation order. The maximum delay time for Slave to receive messages from the master is twice the activation cycle (2.56 seconds), and the average delay time is half of the activation cycle (0.6 seconds). Inquiry messages are mainly used to find Bluetooth devices, such as shared printers, fax machines and other similar devices with unknown addresses. The inquiry message is very similar to the page message, but the inquiry message requires an additional data string cycle to collect all responses.

　　If the connected device in the piconet does not transmit data for a long time, Bluetooth also supports the energy-saving mode. The master can set the slave to hold mode. In this mode, only one internal counter is working. Slave can also actively request to be set to hold mode. Once the cell in hold mode is activated, the data transfer starts again immediately. Hold mode is generally used for connecting several piconets or low energy consuming devices, such as temperature sensors. In addition to the hold mode, Bluetooth also supports two other energy-saving working modes: sniff mode and park mode. In sniff mode, slave reduces the rate of "listening" messages from the piconet, and the "breathing" interval can be adjusted appropriately according to application requirements. In park mode, the device is still synchronized with the piconet without data transmission. The device working in park mode gives up the MAC address, occasionally listens to the master message, resumes synchronization, and checks the broadcast message. If we rank these working modes in ascending order according to energy saving efficiency, then they are: breathing mode, holding mode and pause mode.

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The basic principle of GPS navigation system is briefly introduced