What is MTU (Maximum Transmission Unit)?

MTU is the maximum transmission unit during the BLE data transmission. MTU is set to limit the maximum data length of BLE devices. The MTU of BLE4.0 is 23 bytes and BLE5.0 is 251 bytes.

For BLE4.0, the maximum data package should be (MTU-3) bytes, that is, the data length should be 20 bytes at most.

For BLE4.2, the communication rate is increased according to the raise of the MTU.

For BLE5.0, MTU will be different from the SDK of different manufacturers. 

• Nordic nRF52 series: 247 bytes; 
• RF-star RS02Ax series: 251 bytes; 
• SiliconLabs EFR32BG22 series: 250 bytes; 
• TI CC26XX series: 251 bytes.

Different mobile phone systems have different MTU. Android is 251 bytes, while iOS is 185 bytes. Each BLE packet is (MTU-3) bytes.

For RF-star serial port module, the transparent transmission rate is one of the most important factors that the users will take into consideration. So, how can we achieve the biggest transparent transmission rate?

The connection status of the BLE serial port module is the periodic operation of sleep events and connection events. The time between two events is the connection interval. The data only can be sent out when the connection event comes. There is no chance to send out the data during the sleep event. The smaller the connection interval, the closer the connection events. Then, more opportunities to send data and more data is sent. At most 6~7 frames of data can be sent during each connection event. So, when more data can be sent in one frame of data, more data can be transmitted during one connection event. One frame of data means the MTU. The bigger MTU, the higher the transparent transmission rate.

When we test the limit transparent transmission rate, we usually shorten the connection interval and increase the MTU. What’s more, there are so many other factors that can influence the rate, including baud rate, single sending interval of serial port data.