**

Datagram **

(It is defined by the gap layer starting from advdata in the payload, and it was defined by the ll layer before)

pdu type Data type transmitted

0 Normal broadcast data
1 Directional broadcast data (data is transmitted to a specified device)
2 Unconnectable broadcast data

Payload format analysis

adva indicates the address of the broadcaster
advdata broadcaster data
Indicates data type in data
0 limited discovery mode
1 general discovery mode

From the above table, the physical layer table can be seen
The broadcaster must have the sending attribute (which can be understood as having a transmitter)
The observer must have the receiving attribute (which can be understood as having a receiver)
The central device and the peripheral device must have the sending attribute and the receiving attribute

Broadcast event
Non-connectable and non-scannable undirected event
Non-connectable, unscannable, undirected event
Procedure
There are 2 methods for observation procedure
1 Active scan:
Observer notifies broadcaster first, then scans broadcaster
2 Passive scan:
Observers just scan for broadcasters

Modes and Procedures

Divided into
1 Undiscoverable mode
2 Restricted discoverable mode
3 Normal discoverable mode

Connection modes and procedures

1. Non-Connectable Mode
   Non-Connectable Mode

2. Directed Connectable Mode
   Directed Connectable Mode, which
   peripherals can use if they want to quickly connect to the central device.
   In this mode, it sends out the ADV_DIRECT_ADV broadcast message directly to the designated central device.
   Therefore, the peripheral device should have established a connection with the central device.
   The sending frequency of ADV_DIRECT_ADV broadcast messages is very fast. This mode lasts for 1.28S at most, after which the controller automatically stops broadcasting.

3. Undirected Connectable Mode
   Undirected Connectable Mode.
   Use this mode if a peripheral device does not need to establish a connection quickly, or if you intend to save as much power as possible.
   It sends out the ADV_IND broadcast message.

4. Auto Connection Establishment Procedure
   Auto Connection Establishment Procedure.
   This procedure is used by the central device to set up the controller to automatically establish a connection to one or more peripheral devices.The Host needs to write the device to be connected to the "whitelist" in advance.
   It has a disadvantage: only one basic set of connection parameters can be used for all devices.

5. General Connection Establishment Procedure
   The general connection establishment procedure.
   This is our commonly used procedure, first scan out the device, select a certain one, and then use the "Direct Connection Establishment Procedure" to connect it.

6. Selective Connection Establishment Procedure
   Selective Connection Establishment Procedure.
   The Host first puts the device to be connected into the "whitelist" of the Controller.Devices that are broadcasting, only those on the whitelist will be submitted to the host.
   The Host decides whether to connect the device and uses different parameters to connect it.Compared to the "automatic connection establishment procedure", the "selective connection establishment procedure" can use different parameters to connect each device.

7. Direct Connection Establishment Procedure
   Direct Connection Establishment Procedure, direct connection to the specified device.

8. Connection Parameter Update Procedure
   Connection parameter update procedure.

9. Terminate Connection Procedure
   Terminate the connection procedure.
   Central equipment or peripheral equipment can execute this procedure, just send LL_TERMINATE_IND PDU to each other through LL layer.