I have query related to BLE Connection Parameters

[VKT]

Hi,

I am working on BLE 5, I have few queries related to connection parameters

1. Can you tell what is the min PDU size does nrf52805 BLE stack supports with LE secured (that is considering LTK)?

As we know Range for connection interval for nrf52805 is from 7.5ms to 4000ms

2. Can you tell me what is the minimum connection interval I can set so that I can connect to 4 devices at once and can receive transmitted data from peripheral device in almost every 10ms from each device. also, I need to maintain a sequence in periodic manner from each device like (1234, 1234 and so on) it should not come in burst like (1123334,111233444, and so on)

3. Do you have any guard Time Parameter which can provide fix scan time for each connection ? 

4. if present than what is the value of that guard time and is that guard time is editable or it is fixed guard time?

5. I wanted to know does using LE security or Legacy security affect data transmission over the air ?

as I have observed that using LE security Frequency is 98.9 Hz and and with Legacy it is 94.5 Hz, so you can see the reduction in the frequency that is data loss.

Thank you

[Heidi]

Hi!

1. What is the minimum PDU size that the nRF52805 BLE stack (S112, S113) supports with LE secured?

There is no minimum PDU size for packets used for data channel packets, so you can leave that field empty.

When it comes to advertising channel packets, the minimum PDU depends on what kind of advertising is being done. In your ticket, it looks like you are looking to do connectable advertising, in which case the Flags AD (type 0x01) field is required. 

2. What is the minimum connection interval I can use to connect to 4 devices at once, and receive transmitted data from peripherals almost every 10 ms.

This depends on how much data you want to receive from each device. If you select a low connection event length, then you should be able to communicate with all four in 10 ms. 

As for maintaining a sequence in a periodic manner from each device, that can't be guaranteed. Packets can be lost or corrupted (if packets are lost, then they will be retransmitted during the next connection event), and then the data won't come in the correct sequence. Why do you need the packets to come in a specific sequence? You can check which connection handle the data comes from. 

3. Do you have any guard time parameter that can provide a fixed scan time for each connection?

I don't understand this question. Scanning is when the device is searching for advertising packets and doesn't make sense in the context of a connection.

When the device is searching for advertising packets, the scan window is how long you scan for each time and the scan interval is how often you use a scan window.

4. If yes, what is the value of the guard time, and is it guard time editable or fixed?

The scan interval and scan window are both configurable in the ble_gap_scan_params_t structure in nrf_ble_scan_init_t, and passing it to nrf_ble_scan_init().

5. Does using LE security or legacy security affect data transmission over the air? I have observed that LE security uses 98.9 Hz and Legacy uses 94.5.

The security has nothing to do with the transmission rate. How exactly have you observed 98.9 Hz? BLE uses the 2.4 GHz ISM band, so this doesn't sound correct.

Best regards,

Heidi

[VKT]

Hi,

Thanks for reply,

1. Actually with PDU size I mean range

Like for Legacy security PDU size is from 27 to 251 bytes.

and with LE security minimum PDU size increase due to LTK size 

generally it is 69, what is min PDU size available with nordic ble stack?

2. Yes, I am reading sensor data based on connection handle only but, I want to maintain sequence as well but facing difficulty in achieving it ?

Do you have any idea how this can be achieved ?

I have already tried adjusting connection parameters and intervals.

But still getting data in Burst, it is not coming in sequence.

5. my fault actually I mean to say is I am able to receive almost 98.9% data with LE security and 94% data when using Legacy security.

Best Regards,

VKT

[Heidi]

1. When using LE Secure Connections (LESC), both devices generate an ECDH public-private key pair. The two devices will then exchange their public keys and then start computing the Diffie-Hellman key. One of the pairing methods is then used to authenticate the connection. Once the connection is authenticated, the LTK is generated and the connection is encrypted. So in contrast to LE Legacy pairing,  the LTK is never exchanged and is therefore not relevant to the minimum PDU size.

Just to be sure, I have asked the SoftDevice team if there is some other reason (that I may have overlooked) that the minimum PDU size would increase when using LESC.

2. Like I said, maintaining a sequence cannot be guaranteed so you shouldn't base anything on the packages coming in sequence. Can you elaborate on what you mean when you say the data comes in bursts?

What are the connection parameters of the devices?

Why do you need the packages to come in sequence?

3. This doesn't sound right. With BLE, you will always get 100% of the data. If a package doesn't get acknowledged, retransmission occurs, and if that doesn't work you will get a time-out and disconnect. You don't lose data without getting a disconnect.