Is it possible to set up different MTU sizes for multiple concurrent links?

[Andy]

Hi,

I'm developing an application with SDK 15 that requires multiple concurrent links. One peripheral link and several central links.

I want the peripheral link to have as much throughput as possible, and therefore I want it to have an MTU of 247. The central links are secondary and they have a low priority. They can be left with an MTU of 23, and use long connection intervals.

How can I set up this? I can't have the central links with an MTU of 247 due to RAM restrictions (see https://devzone.nordicsemi.com/f/nordic-q-a/40663/ram-requirements-per-central-link).

The closest I got with my implementation is all peripheral and central links with MTU = 23.

Any help with this is appreciated.

[Vidar Berg]

Hi,

The softdevice allows you to define connection specific configurations which are identified by the "conn_cfg_tag" tags used when you enable the softdevice. One option is to create one tag for the peripheral role where you want high bandwidth, and another one for the central role. You can then pass the "peripheral" tag to  sd_ble_gap_adv_start() to use the high bandwidth configuration for peripheral links and the "central" tag to sd_ble_gap_connect() to use the low bandwidth configuration for central links. 

[Andy]

I did that actually. And I think it works. But I'm having an issue with scanning simultaneously when the peripheral link is connected:

https://devzone.nordicsemi.com/f/nordic-q-a/41267/multi-role-in-sdk-15---scanning-for-devices-only-working-when-peripheral-is-not-connected

tesc suggested that it might be because of the softdevice configuration, so I thought that maybe since the peripheral link connected first, maybe the softdevice started using the "peripheral" configuration and prevented it from scanning.

[Vidar Berg]

Did it work when you used the same configuration for all links? I don't think the configuration is related to the issue you're seeing now. Scan start should have returned an error code if it for some reason was prevented from starting it. Do you have a scan timeout, or is it supposed to scan until it finds a peripheral to connect to?  

[Andy]

The scan start function does not return an error at all. It actually starts scanning, but it stops almost immediately after the peripheral connects.

I know that the scanning procedure is stopped when a connection happens, but I'm restarting the scan on BLE_GAP_EVT_CONNECTED. It returns success and it receives a couple of adv reports, but nothing afterward.

I do have a scan timeout of 60 seconds.

[Vidar Berg]

Excerpt from the scan API documentation:

The scanner will automatically stop in the following cases:

• sd_ble_gap_scan_stop is called.
• sd_ble_gap_connect is called.
• A BLE_GAP_EVT_TIMEOUT with source set to BLE_GAP_TIMEOUT_SRC_SCAN is received.
• When a BLE_GAP_EVT_ADV_REPORT event is received and ble_gap_adv_report_type_t::status is not set to BLE_GAP_ADV_DATA_STATUS_INCOMPLETE_MORE_DATA. 

Are you calling scan start from the ADV report event when ble_gap_adv_report_type_t::status is _not_ set?