Throughput ATT_MTU

[Ridhi]

We are using nrf52832 soc for our product development and using that we are trying to increase the throughput of our device using the information from the following forum:
Novel Bits throughput guide 
In this forum it states that the Maximum GATT MTU SIZE possible is 247 bytes in which 244 bytes of data is the actual payload size and if the data is greater than the limit, then it will split into multiple packets. So we tried to send more that 247 bytes of data just to check how the data split in multiple packet is working. We tried to sent 256 bytes of data and we were able to get 255 bytes of data without any splitting, which is more than 247 bytes. So we are wondering what is the Maximum GATT MTU SIZE possible in nrf52832 soc. 
We also looked into the throughput example provided by the nordic (ble_app_att_mtu) and in that MTU size is set to 247 and data length is set to 251 which is less that what we are able to send from nrf52832 to phone, is there any possible explanation for this?

[Simonr]

I'm not sure you understand the Wireshark data correctly. According to the file you uploaded, none of the packets transmit more than 27 bytes each: 

And the final notification adds the fragments together to present the one 252 byte package on the receiving end. Please also share what mobile device you're using, as it might not support larger packages than 27 bytes at a time, depending on the model.

Best regards,

Simon

[Ridhi]

Hello,

can you tell me who has dedicde these 27 bytes. actually i am aiming to calculate throughput.

i am able to send 252 bytes data getting one notification. From study of LE Uncoded packet i have found that pnly 244 bytes of ATT_Payload is possible to transmit at one packet. i am not getting that how i am able to send 252 bytes of data as i have shared you the sniffer trace. how shall i calculate the throughput 

[Simonr]

Hi

The 27 bytes of data are the minimum data length that can be set in the BLE spec. and older devices (older than BLE v5.0) generally don't support payloads larger than 27 bytes. Can you share some information on what phone you're using.

Again, as I tried explaining below. Your notification is indeed 252 bytes, but each data packet (L2CAP fragment) is 27 bytes. One packet and one notification is not the same, and your device sends multiple packets that the central puts together in the notification.

Best regards,

Simon

[Ridhi]

i have data length 12 bytes

#define MIN_CONN_INTERVAL MSEC_TO_UNITS(1000, UNIT_1_25_MS) /*

#define MAX_CONN_INTERVAL MSEC_TO_UNITS(1000, UNIT_1_25_MS) /*

case 1:

27 bytes L2CAP.pcapng

NRF_SDH_BLE_GAP_DATA_LENGTH 27

NRF_SDH_BLE_GATT_MAX_MTU_SIZE 247

i am able to get notification and able to get 244 bytes of data after reassembly by L2CAP.

and in case 2:

NRF_SDH_BLE_GAP_DATA_LENGTH 251

NRF_SDH_BLE_GATT_MAX_MTU_SIZE 247

i am using  nrf connect mobile app on  iphone 11

251 bytes.pcapng

i want to know that what will be beneficial in terms of throughput if i use NRF_SDH_BLE_GAP_DATA_LENGTH 27 or NRF_SDH_BLE_GAP_DATA_LENGTH 251 because in that case no L2CAP fragmentation will occur. so which is better ?

i have to determine the throughput based on number of packets per connection interval ?

i want to know how to get how many packets i am getting per connection interval?

[Simonr]

Hi

In regards of throughput, you can see from the maximum throughput guide you linked to, you get the highest throughput with ATT MTU set to 247 bytes, and SDH_BLE_GAP_DATA_LENGTH to 251 bytes. Throughput is decided not by packets per connection interval, but by data (bits) per second. 

I would strongly recommend using NRF_SDH_BLE_GAP_DATA_LENGTH to 251 for the best possible throughput in your case, as well as following the sugggestions to "Optimize for maximum data throughput" in the throughput guide.

Best regards,

Simon