Central (9) and Peripheral connections(1- mobile) - baseband scheduling

[Abishek]

HI,

  we are working with 

nRF5 SDK v15.2.0
------------------------
Release Date: Week 37, 2018

  Our use-case has a central device, which has 7 to 10 peripheral connected sensor nodes and sends back the calculated values to Mobile using the peripheral interface.

  slave latency - 0

  The connection interval used is 30ms(min 24 units) to 150ms and it always takes 30 ms as a connection interval between nodes and mobile.

  we have a requirement for 100 to 200ms latency to mobile phones. That means all the sensors should send data within 60 to 80ms to the central and the central sends the calculated value in 50ms.

   The connection and data transfer are working and it's stable. But however, we see the packets are sent 1/5 or 1/4 times of connection interval. so the packets are delayed by 200ms to 600ms normally, even in a clean environment. we do not have an OTA sniffer to see what's happening in the air.

   But our assumption is on how the connection interval is allocated on baseband time slots. Since it's always taking 30ms, I think is not possible to give connection intervals with good event length and not overlapping. So we assume the scheduler is allocating and overlapping the intervals and each slot gets changed based on priority and is not preallocated properly. 

  we tried with 40ms,50ms intervals, but in 30ms we got a good reliable connection and data transfer.

   And also we have the time_sync enabled, with is scheduled for 5 seconds and the radio access is low priority based.

   Do you have any configuration to tune this or are we missing something in our understanding of the problem? Please let us know if you need any more info on the same.

[Abishek]

Hi Hung Bui, thanks for the valuable reply. 

The connection interval used is 30ms between Nodes and Central. (all connections)

Could you please clarify one doubt:

Assume we got 1+7 connections in the central node. (Mobile and 7 Sensor nodes).

All connection interval is forced to 40ms, will it arrange the connection in free slots, like 0,5,10,....40,45,50...,80,85,90.. with 5ms event length.

or Do we have special priority for central and peripheral connections in the baseband?  Is there any best connection interval, which works better for the 1+7 device and scheduler? 

Yes, we will try the there things mentioned 

.1) Sniffer

 2)Connection Event

 3)Disable time_sync

thank you,

Abishek

[Hung Bui]

Hi Abishek,

I would suggest to have a look at section 15.2 Initiator timing in the S140 specification. In the section, we discussed about the algorithm of scheduling. 
Basically the central will try to put the new connection in a "best-fit" free time space that doesn't collide with other connection events. And it works best if they all have the same connection interval. 

There is prioritization when you establish a new connection. However, when a connection has its connection event skipped for several time, it will have higher priority than other connections. 

It's very important that the connection event length is configured small enough so that all 7+1 connection event can be scheduled inside the 40ms interval. If they are above 5ms there will be collision. I would suggest to test with 2.5ms. 

[Abishek]

Thank you Hung Bui, Its working for us. 

The problem is with the GAP interval , data packet size and connection interval usage.

Now we have full control over the data flow and the latency and the tunning values.

[Hung Bui]

Great, thanks for letting us know :) I'm happy that it works now.