Node comfortable earphones in bluetooth communication

How do two nodes exchange data in Bluetooth communication?
This is the most confusing place for beginners in Bluetooth communication. In most communications, especially TCP/TP, the matchmaking that does not save data is data packets, but in Bluetooth communication, engineers cannot find a way to access data packets, so they have doubts. In fact, the bottom layer of Bluetooth is also based on wireless data packet exchange, but through layer-by-layer encapsulation, the API interface delivered to the engineer becomes the way for the Client to access the Server. www.bjbjaudio.com

How are Client and Server nodes defined?
In layman's terms, the Server (server) is the Cient (client) in the data that accesses the data. In particular, it is an independent concept from the master/slave device: a master device can act as both a Server and a Client; the same is true for a slave device. www.bjbjaudio.com 

How does the server provide data?
Server first organizes a service in the format of "attribute/handle/value/description", and then calls the API function GATTServAppReqisterService to register the service data. As an example, suppose a battery power service byte is provided, which allows the Client to read, the data is an 8-bit unsigned number (0~100%), and its organization is as follows: 022500192A, these 5 data (little endian format) ) are: 002=read-only attribute, 0x0025=handle: 0x2A19=service UUID  www.bjbjaudio.com

Don't understand the UUID in the service provided by the Server?
UUID (UniversalUniqueIdentifier) ​​is a globally unique identifier, which was originally an identifier assigned to a specific Bluetooth service by the SIC organization. For example, assign 0x2A25 as the UUID of the serial number of the device, so that any Bluetooth device can get the serial number of another device through it; by analogy, it is like the title of a book, such as "Modern Operating System", everyone knows it is A book written by computer guru AndrewS.Tanenbaum. www.bjbjaudio.com

What is the handle in the service provided by the server?
The handle (Handle) is the address of the service data in the data center. When all the service data is organized, it must have a sequence, and the location of a service is its handle. Or the above analogy, if you want to borrow "Modern Operating System" from the library, you need to find out which floor and which room the book is on. This is the Handle of the book. www.bjbjaudio.com