The process of establishing Bluetooth is a complicated process. Even people who have had a considerable amount of work and experience will not understand it if they don't carefully understand it.
Usually we listen to music with a Bluetooth headset, share files with different devices, make calls, etc., all have a pairing-connection--transfer data process.
The Bluetooth pairing process is actually a process of authentication.
Why can't I establish a connection without pairing?
Any wireless communication technology has the possibility of being intercepted and cracked. In order to ensure the security of Bluetooth communication, the Bluetooth SIG uses the authentication method for data interaction. At the same time, in order to ensure the convenience of use, the first communication authentication between the two Bluetooth devices is completed in the form of pairing, and after the pairing, the subsequent communication connection does not have to be confirmed every time. Therefore, the authentication code is generated from the pairing. After pairing, the link key established by the device with the PIN code is used to generate the initial authentication code for the connection established later.
Therefore, if there is no pairing, the authentication relationship between the two devices cannot be established, and the connection and subsequent operations cannot be performed. Therefore, the pairing ensures the security of the Bluetooth communication to a certain extent. Of course, this security guarantee mechanism is relatively easy to be cracked because Many personal devices now have no human-machine interface, so the PIN codes are fixed and mostly set to the common 0000 or 1234, so it is easy to guess and establish pairing and connection.
Bluetooth connection process
Today's Bluetooth chip vendors offer a very strong technical support capability with complete hardware and software solutions. For the application, firmware is provided to implement the underlying protocol stack, and the profile library and source code are provided to standardize various applications. The developer only needs to focus on application development. Some things about the underlying Bluetooth are often not very good. I used to do this. I recently did an automatic search to achieve automatic connection. I found that I still need to understand some underlying mechanisms.
We can easily make a connection between a mobile phone and a hands-free device, so how is this connection established?
First, the master (the device that initiates the connection) will page the slave (slave, receive the connected device), the master will have a frequency hopping method to page the slave, and the slave will scan at regular intervals ( Scan) External paging, ie page scan, will respond to the response page when scan to the external page, so that the connection between the two devices will be established, that is, the connection of the ACL link. After the ACL link is established, the master device initiates a connection request for the channel, that is, the L2CAP connection. After the L2CAP connection is established, the master device uses the SDP to query the hands-free service of the slave device, obtains the channel number of the rfcomm, and then the master. The device initiates a rfcomm connection request to establish a rfcomm connection. Then the connection to the application is established.
That is, link establish-"channel establish-"rfcomm establish-"connecTIon
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