L2CAP connection refused with BLE_L2CAP_CH_STATUS_CODE_NO_RESOURCES

[Marco]

I'm trying to get iOS to open an L2CAP Connection Oriented Channel on the nRF52840.

I'm stuck at the nRF always rejecting the connection with result 0x0004 (Connection refused – no resources available).
On the firmware, an event BLE_L2CAP_EVT_CH_SETUP_REFUSED is raised with Source=Local, Status=BLE_L2CAP_CH_STATUS_CODE_NO_RESOURCES.

It sounds like I need to configure the stack to allocate resources for the L2CAP connection.
I've tried to set:

#define NRF_SDH_BLE_PERIPHERAL_LINK_COUNT 3
#define NRF_SDH_BLE_TOTAL_LINK_COUNT 3

thinking that this could be related to the number of L2CAP channels but it made no difference.
What does the SD need to accept L2CAP connection?

Also, if I break the program in the debugger and the resume it, it tends to crash with a SD assert for PC 0x00012cfa.
I'm not sure if it's relevant, but it looks odd.

[Marco]

Any Nordic engineer able to offer some hints? I've tried a few different configuration options but I'm stuck on this.

[Mttrinh]

Hi,

Which softdevice are you using? and what is the L2CAP configuration? The parameters that you are using when calling sd_ble_cfg_set().

[Marco]

Hi Mttrinh,

I'm using S140 nRF52 6.0.0.

I've started the project from the ble_app_template sample project, which calls nrf_sdh_ble_default_cfg_set().
I thought that function would fully configure the stack as specified in sdk_config.h but now that you mentioned it I've checked and noticed it calls sd_ble_cfg_set() to configure various things but not L2CAP.
I've added the L2CAP configuration, with ch_count=1 as the default is 0, and now it works.

Thank you for pointing me in the right direction 

[JoeBakalor]

Hi Marco,

Have you done any throughput testing between iOS and the NRF52 using L2CAP CoC? Just curious. I was able to get around 600kbps with the 2M phy between iPhone X and NRF52 using notifications. Curious how much of an increase using L2CAP CoC might add 

[Marco]

Hello Joe,

The project I was working on has been put on hold. I can't find any notes about my tests but I'm pretty sure I couldn't get to the 40 kB/s (about 400 kbps?) that were the requirement. My tests were done with an iPhone 6s and an iPad 5th gen. I can't tell what the maximum performance would be as I didn't fully investigate all the options to tune the configuration.

FYI, a couple of related points that you might want to consider:

• At the time, Android support was a concern as the API for CoC seemed to only be available on latest OS versions.
• Performance varies significantly depending on the iOS device

Sorry if this is not particularly helpful.