In Bluetooth classic (BR/EDR) systems, a Serial Port Profile (SPP) is an adopted profile defined by the Bluetooth Special Interest Group (SIG) used to emulate a serial port connection over a Bluetooth wireless connection. For BLE systems, an adopted SPP profile over BLE is not defined, thus emulation of a serial port must be implemented as a vendor-specific custom profile.
This reference design consists of two Demos, the ble spp server and ble spp client that run on their respective endpoints. These devices connect and exchange data wirelessly with each other. This capability creates a virtual serial link over the air. Each byte input can be sent and received by both the server and client. The spp server is implemented as the [ble_spp_server](../ble_spp_server) demo while the spp client is implemented as the [ble_spp_client](../ble_spp_client) demo. Espressif designed the BLE SPP applications to use the UART transport layer but you could adapt this design to work with other serial protocols, such as SPI.
This vendor-specific custom profile is implemented in [spp_client_demo.c](../ble_spp_client/main/spp_client_demo.c) and [spp_server_demo.c](../ble_spp_server/main/ble_spp_server_demo.c).
Both the server and client will first initialize the uart and ble. The server demo will set up the serial port service with standard GATT and GAP services in the attribute server. The client demo will scan the ble broadcast over the air to find the spp server.
*`spp_cmd_task` - process command messages, the commands and processing were defined by customer
*`spp_heartbeat_task` - if heartbeat is supported, the task will send a heatbeat packet to the remote device
### Packet Structure
After the Uart received data, the data will be posted to Uart task. Then, in the UART_DATA event, the raw data may be retrieved. The max length is 120 bytes every time.
If you run the ble spp demo with two ESP32 chips, the MTU size will be exchanged for 200 bytes after the ble connection is established, so every packet can be send directly.
If you only run the ble_spp_server demo, and it was connected by a phone, the MTU size may be less than 123 bytes. In such a case the data will be split into fragments and send in turn.
In every packet, we add 4 bytes to indicate that this is a fragment packet. The first two bytes contain "##" if this is a fragment packet, the third byte is the total number of the packets, the fourth byte is the current number of this packet.
The phone APP need to check the structure of the packet if it want to communicate with the ble_spp_server demo.
The client will be sending WriteNoRsp packets to the server. The server side sends data through notifications. When the Uart receives data, the Uart task places it in the buffer. If the size of the data is larger than (MTU size - 3), the data will be split into packets and send in turn.
