Changing MTU size using data length extension

[Kasper Leonhardt]

Hi

I am having trouble changing the MTU size. I am using SDK 15.3 on nRF52832. I am developing a combined peripheral/central application, but I only need Data Length Extension on the peripheral part, which holds 3 custom services and 2 standard services.

In sdk_config.h I changed:
#define NRF_SDH_BLE_GAP_DATA_LENGTH 67
#define NRF_SDH_BLE_GAP_EVENT_LENGTH 200
#define NRF_SDH_BLE_GATT_MAX_MTU_SIZE 64

And bonding is enabled.

Log snippet:

[00:00:08.493,000] <debug> nrf_ble_gatt: Updating data length to 67 on connection 0x1. [00:00:08.494,000] <info> app: Peripheral connected [00:00:08.772,000] <debug> nrf_ble_gatt: ATT MTU updated to 64 bytes on connection 0x1 (response). [00:00:08.822,000] <debug> nrf_ble_gatt: Data length updated to 67 on connection 0x1. [00:00:08.822,000] <debug> nrf_ble_gatt: max_rx_octets: 27 [00:00:08.822,000] <debug> nrf_ble_gatt: max_tx_octets: 67 [00:00:08.822,000] <debug> nrf_ble_gatt: max_rx_time: 328 [00:00:08.822,000] <debug> nrf_ble_gatt: max_tx_time: 2120 [00:00:09.761,000] <info> peer_manager_handler: Connection secured: role: Peripheral, conn_handle: 1, procedure: Encryption [00:00:09.761,000] <info> app: Link status. Role: 1. conn_handle: 1, Procedure: 0 [00:00:09.859,000] <debug> nrf_ble_gatt: Peer on connection 0x1 requested a data length of 27 bytes. [00:00:09.859,000] <debug> nrf_ble_gatt: Updating data length to 27 on connection 0x1. [00:00:09.866,000] <debug> nrf_ble_gatt: Data length updated to 27 on connection 0x1. [00:00:09.866,000] <debug> nrf_ble_gatt: max_rx_octets: 27 [00:00:09.866,000] <debug> nrf_ble_gatt: max_tx_octets: 27 [00:00:09.866,000] <debug> nrf_ble_gatt: max_rx_time: 328 [00:00:09.866,000] <debug> nrf_ble_gatt: max_tx_time: 2120

I do not understand why the central first acknowledges with the 27/67 bytes data length after a request on 67 bytes. And why does the central lower the acceptable data length?

I am using the "BLE scanner" App on Android phone for testing. It should support data length > 20.

Any one have some experience?

I have read some guides that tells that besides defining your needed data length, you also needs to enable data length extension. But how? Is this done automatically by the softdevice?

BR

Kasper

[Marjeris Romero]

Hi Kasper,

Sorry for the late answer. The Softdevice only supports symmetric RX/TX data length settings, from the logs you can see RX setting was different from TX so that's why TX sets to 27. Can you try to setup the BLE sniffer to see what is happening between your device and your phone? What kind of phone are you using? Are you sure it supports higher data lengths? It needs to support DLE for both TX  and RX.

I have read some guides that tells that besides defining your needed data length, you also needs to enable data length extension. But how? Is this done automatically by the softdevice?

This is done automatically by the GATT module when you define a higher data length.

Best regards,

Marjeris

[Kasper Leonhardt]

Hi Marjeris

Thanks for your reply.

I have tried with the following devices.

Samsung S10+ (Android 9) (Supports 2Mbps, and apparently only RX/TX:>67/27)
Alcatel 8082 1T EEA (Android 8.1) (Will soon be tested)
Sony Z5 Compact (LinageOS, Android 7.1.2) (Supports only 1Mbps, and apparently only RX/TX:>67/27, reduced later to 27/27 by phone) (Used for the trace in the original post)
Lenovo Tablet TB-8304F1 (Android 7.0) (Supports only 1Mbps, and apparently only RX/TX:27/27)

The Samsung phone seems to do the same as the Sony phone, except that the data length is NOT reduced to 27 even that it is not symmetric:

[00:36:18.813,000] <info> app: Peripheral connected
[00:36:18.813,000] <info> app: Sending PHY Update.
[00:36:18.901,000] <debug> nrf_ble_gatt: ATT MTU updated to 64 bytes on connection 0x1 (response).
[00:36:18.901,000] <info> app: ATT MTU exchange completed. MTU set to 64 bytes.
[00:36:18.904,000] <warning> app: Backends flushed
[00:36:19.291,000] <debug> nrf_ble_gatt: Data length updated to 67 on connection 0x1.
[00:36:19.291,000] <debug> nrf_ble_gatt: max_rx_octets: 27
[00:36:19.291,000] <debug> nrf_ble_gatt: max_tx_octets: 67
[00:36:19.291,000] <debug> nrf_ble_gatt: max_rx_time: 328
[00:36:19.291,000] <debug> nrf_ble_gatt: max_tx_time: 2120
[00:36:19.291,000] <info> app: Data length updated to 67 bytes.
[00:36:19.760,000] <info> app: PHY update accepted. PHY set to 2 Mbps.
[00:36:43.604,000] <debug> app: BLE_GAP_EVT_SEC_PARAMS_REQUEST
[00:36:45.588,000] <info> peer_manager_handler: Connection secured: role: Peripheral, conn_handle: 1, procedure: Bonding
[00:36:45.588,000] <info> app: Link status. Role: 1. conn_handle: 1, Procedure: 1
[00:36:45.588,000] <info> app: BLE_GAP_EVT_AUTH_STATUS: status=0x0 bond=0x1 lv4: 0 kdist_own:0x3 kdist_peer:0x3
[00:36:46.088,000] <info> app: on_write: handle: 22 Len: 4
[00:37:02.077,000] <info> app: Peripheral disconnected. conn_handle: 0x1, reason: 0x13

Can you recommend any other app for browsing services and reading/writing characteristics. I cannot find any other than BLE Scanner. Are you aware of apps that can display the capabilities for DataLengtExtensien? Bluetooth version implementation?

I will try to set up the BLE Sniffer later. (Not a trivial task, last time I gave up. I miss the old BLE Sniffer three years which was much easier to setup)

BR
Kasper

[Marjeris Romero]

Hi,

With BLE scanner do you mean "nRF Connect app"? You should be using nRF Connect app for Android.

And we recommend using nRF Sniffer v2.

Best regards,

Marjeris

[Kasper Leonhardt]

Hi,

I was referring to the third party app "BLE scanner" from BluePixel. I have tried "nFR Connect" It seems to better handle the long writes. And my peripheral device application works fine.

[00:00:13.122,000] <info> app: Peripheral connected
[00:00:13.122,000] <info> app: Sending PHY Update.
[00:00:13.223,000] <warning> app: Backends flushed
[00:00:13.234,000] <debug> nrf_ble_gatt: Data length updated to 27 on connection 0x1.
[00:00:13.234,000] <debug> nrf_ble_gatt: max_rx_octets: 27
[00:00:13.234,000] <debug> nrf_ble_gatt: max_tx_octets: 27
[00:00:13.234,000] <debug> nrf_ble_gatt: max_rx_time: 328
[00:00:13.234,000] <debug> nrf_ble_gatt: max_tx_time: 328
[00:00:13.234,000] <info> app: Data length updated to 27 bytes.

[00:00:13.483,000] <debug> nrf_ble_gatt: ATT MTU updated to 64 bytes on connection 0x1 (response).
[00:00:13.483,000] <info> app: ATT MTU exchange completed. MTU set to 64 bytes.
[00:00:13.634,000] <info> app: PHY update rejected. PHY set to 1 Mbps.

[00:01:37.643,000] <info> app: on_write: patient handle: 37 Len: 30

I do not understand the first part of the log with the data length and max_?_octets. But "ATT MTU exchange completed. MTU set to 64 bytes." and "app: on_write: patient handle: 37 Len: 30" says it all. :-)

Thanks for your guidance.

Kasper

[Marjeris Romero]

Hi,

Happy to help :) first part of the log output says the packet data length is being updated to 27 (both for RX and TX). The ATT packet will then be sent over several on-air packets, see Bjørn's answer in this ticket.

Best regards,

Marjeris