BLE Throughput too low (for large characteristic)

Hi pvmellor,

Thanks for your screenshot. Let me check it and I will get back to you as soon as possible. Could you please clarify how you send data? Are you using notifications? Also, a sniffer log would be very helpful in order to understand how tha packets are exchanged over the air.

Thanks, PM_Dialog

Hi pvmellor,

Could you please check I much is the maximum MTU size? Please set the max_mtu item of user_gapm_conf structure in user_config.h to 256.

Thanks, PM_Dialog

Hi pvmellor,

Let me check your questions and try to replicate your issue and I will get back to you as soon as possible.

Thanks, PM_Dialog

Hi pvmellor,

期间设备发送的数据量connection interval depends on the packets that the central allows to a peripheral to send. The payload for each packet with the standard MTU selection is 20 bytes. So each data that you send can carry up to 20 bytes. You can't control the how many packets the BLE will send during a connection interval because is up to the master of the connection, if he doesn't want to accept the data he just won’t accept it even if you indicate that you have more data to send. If you would like to send more than 20 bytes of data you will have to increase your MTU size, then the L2CAP will take of the rest, chop the data and fit them in more than one packets.

Specifically, the number of bytes that a device can send over the air is limited by the MTU (Maximum Transfer Unit), the MTU by default is limited in 23 bytes including the ATT layer overhead, so the payload is 20 bytes. By increasing the MTU size that means that you can send more bytes over the air. In your case the maximum transfer unit should be the number of bytes you would like to send + 3 extra bytes. You should change the .max_mtu of the user_gapm_conf structure of the user_config.h header file. After doing that in order to perform the exchange with the central you should send the GATTC_EXC_MTU_CMD when you have a connection (in the user_on_connection) and the 580 will perform the exchange. There is no API implemented for the MTU exchange functionality but you can use the snippet below:

static void user_gattc_exc_mtu_cmd(uint8_t conidx)

{

struct gattc_exc_mtu_cmd *cmd = KE_MSG_ALLOC(GATTC_EXC_MTU_CMD,

KE_BUILD_ID(TASK_GATTC, conidx), TASK_APP,

gattc_exc_mtu_cmd);

cmd->req_type = GATTC_MTU_EXCH;

ke_msg_send(cmd);

}

Please check the attached screenshots. I have implemented a 256bytes - “Write with no response” characteristic. Also, the .max_mtu=259 and I used the above code snippet in the user_on_connection. For your information, I worked on the ble_app_peripheral example of SDK5.0.4. As you can see in the attached snapshot, the 256bytes are divided in smaller packets. The 256bytes are sent from a BLE generic application to the peripheral in 1 connection interval. You could see that while sending the data over the air only the Channel-30 was used, which means that was only 1 connection interval. The payload through is 7400 bits/sec. Possible reason why you got a smaller throughput would be that you might not have done the MTU exchange, so for sending 256bytes more than 1 connection interval was needed. Be aware that re-transmitions might be happened. You are able to use API uint16_t gattc_get_mtu(uint8_t idx) to poll the negotiated MTU. Regarding you question for the sniffer; I used the Frontline sniffer products for debugging BLE protocol.

Thanks, PM_Dialog