Write command and Notification in the same channel

[chinargor]

Dear Nordic Developers,

I'm trying to write some data to a slave and when the slave gets the write command sends back a notification. My problem is that I send the write command in one radio channel (for example channle 35) but I get the notification on the next radio channel (channel 3). Is it possible to get the notification on the same channel (channel 35)?

Bellow you can see the results from the wireshark.

3990 113.087487000 Master Slave ATT 34 Rcvd Write Command, Handle: 0x000e

3991 113.089248000 Slave Master LE LL 26 Empty PDU

3992 113.141960000 Master Slave LE LL 26 Empty PDU

3993 113.143982000 Slave Master ATT 34 Rcvd Handle Value Notification, Handle: 0x000b wireshark_trac_file.pcapng Lines 3990 and 3991 are using channel 35 and the other 2 lines (3992 and 3993) are using channel 3. Is it possible increase the time between the lines 3991 and 3990? Please have a look to a attached wireshark trace file.

Any advice would be highly appreciated.

Thanks, Gor

[Nathan]

Gor,

I'm actually a bit confused by this question. When you say channel did you mean characteristic? If that's the case then yes, you should be able to issue write commands from the gatt client and issue notifications from the gatts server over the same BLE characteristic, which is parsed out in code easily enough.

If you meant literal 2.4GHz radio channel, then no, that's not possible. BLE by design uses an adaptive frequency hopping algorithm to help prevent radio collisions with other devices. This behavior cannot be shut down nor suppressed.

If the question was targeted to the second case, is there any reason why you would want to use the same 2.4GHz channel continuously? There's pretty much no benefit to that as far as I can see.

Finally, you asked if there's a way to increase the time between lines 3990 and 3991. In this case the answer is no, BLE is a communication scheme where both sides talk to each other at every "connection interval". Those two events listed happened during the same connection event, which means they occurred basically simultaneously (it's used to detect link loss and make sure both sides get to communicate instead of just one side constantly blocking the connection). Think of it like a full duplex connection, if that helps. You can however modify the link's connection interval so that each of these communication "pairs" happen at shorter or longer time periods.

Nathan

[chinargor]

Hi Nathan

Thanks a lot for your answer and help. I'm sorry for not beeing clear. Yes I meant 2.4GHz radio channel. The idea to use the same radio channel (or connection event) for getting a write command and sending back notification was to make similar to our proprietary protocol, but it's not a must. Now I really don't understand how can I send more than one packets in one connection event? (As all packets in a connection event are transmitted on the same frequency). Do you have any idea?

Thanks again.

[endnode]

I believe Nathan gave the answer pretty nicely:

• BLE spec allows you to do Write and Notify over the same GATT handle (Characteristic) within the same Connection event (one or several pairs of packets exchanged over the same frequency channel).
• Nordic's stacks allows you to use this BLE feature as well...
• ...but your app must keep timing of the BLE connection which is very strict (there are only 150us between Tx and Rx windows) so I doubt you would be able to receive something from central side, process it and prepare Event Notification Tx data to the SoftDevice within that short window to send it out within same Connection Event.
• However if you have two independent data streams it should be possible to run them in parallel from both sides and effectively "share" Connection Evenets.

Cheers Jan

[Hung Bui]

Hi Gor,

This is strange. Could you upload the sniffer trace file ? In addition a screenshot would be nice. You can edit your question to add the files in. The slave shouldn't response to the master on different channel than the channel the last packet from master comes to slave.

[RK]

I don't believe Nordic's stack lets you do this at all, however fast you may be. As far as I have ever understood it, only data queued up and ready to go before the connection event begins is prepared and transmitted during that connection event. If you add anything new during the connection event, it waits for the next one.