More information about UART service (AT Command), Pls refer to below link
Nordic UART Service implementation.
The Nordic UART Service is a simple GATT-based service with TX and RX characteristics. Data received from the peer is passed to the application, and the data received from the application of this service is sent to the peer as Handle Value Notifications. The service is used by the application to send and receive ASCII text strings to and from the peer.
Although UART is a quick and easy access to build up the BLE products, we would like to provide some kind reminding listed in below for all developers.
- Packet size: 20 byte
- Interval: Suggested set as 50m Sec
- For Data <400 byte/sec (baud rate set as 9600), there is no need to support flow control function
- For Data>400 byte/sec (baud rate>9600, says as 19200 or 115200), we are strongly suggest to check if MCU support flow control function to avoid data loss.