NUS Max data size

[eleven-x_devteam]

Hello,

What is the max data size for NUS?

According to BLE data throughput in the S140 document, it looks like 247 but the data was sent successfully with  NRF_SDH_BLE_GATT_MAX_MTU_SIZE 435 by using NUS and if I set more than that, it caused crashes.

Can anyone clarify it if NUS MAX MTU size is bigger than BLE spec or my code just worked luckily but I am supposed to do that?

Thank you in advance.

[Hieu]

Hi eleven-x_devteam,

247 is the maximum number of bytes that can be transmitted in an GATT write/notification in one Link Layer packet. Achieving that give you the least header/total data ratio, which gives you the best throughput.

Higher MTU is still possible. That just means that your GATT operations get broken down into more than one Link Layer packet.

Could you please give some more details about the kind of crashes that you encounter when setting NRF_SDH_BLE_GATT_MAX_MTU_SIZE to higher than 435?

Hieu

[eleven-x_devteam]

Hello Hieu,

Thanks for the quick reply.

I see. Then I should be able to use even more than 435.

I'll get back with some more information about crash but can it be related to the Softdevice ram size?

Thanks!

[Hieu]

Hi eleven-x_devteam,

I don't remember the details around a GATT service initialization for sure, but I am rather certain that if it is related to SoftDevice RAM size, then there would be some errors clearly indicate so.

P.s: Also, BLE Attribute size is 512 max, so while you should be able to use more, there is still a hard ceiling.

[eleven-x_devteam]

We decided to use around 230 bytes. Thanks!