Increasing throughput for nrf52832

[Haresh05]

We are using nrf52832 with S132.

In our application, we have 1 device as central and it will connect to 4 peripheral devices.

Central device needs to send 128 bytes of data to each of 4 device at interval of 30 msec.

We have extended the data length to send 128 bytes in 1 packet.

Currently we are sending these packets at interval of 100 msec and still packets are getting dropped.

Connection interval is set to 7.5msec.

So how can we increase throughput to achieve our requirements?

[Haresh05]

-> How consistent and how often (X fails every Y establishment attempt) does this happen in your test?
I am seeing this issue almost every time when I try to establish a connection(fails 3-4 times with reason 0x3E before making the proper connection)

->Does this happen when the Peripheral is advertising and scanning at the same time?
I have not checked with scanning disabled. Advertising and scanning are both enabled in Peripheral and Central devices as well.

->You noted that the Advertising Interval is 100ms. What are the scan parameters on these devices?
The scan Interval is 100msec and the Scan window is 80msec.
I have also tried with a scanning interval set to 160msec but the result is the same.

->Is the physical environment you are testing electromagnetically noisy?
Yes. We have nearly 50 devices in a closed environment

-> Are the devices far apart? What is the RSSI that they see from the other?
When devices are set 40ft apart, I see RSSI around 75-80.

[Hieu]

Haresh05 said:

Yes. We have nearly 50 devices in a closed environment

Are all of these devices running the same firmware?

Even if not, are many of these devices are performing Active Scanning?

Something we did not account notice with the sniffer traces is that there are a lot of SCAN_REQ from Active Scanners. 

SCAN_REQ will be sent, received, and acted upon right after the Peripheral send an ADV_SCAN_IND.
It's at the same time as when CONNECT_IND should be sent.
On top of that, both SCAN_REQ and CONNECT_IND will be sent on the three BLE advertising channels.

Therefore, if there are multiple Active Scanner in range, the CONNECT_IND packet will have a lot of competitions, and the Peripheral would easily miss it.

Besides, if the other devices are advertising, they will also bring potential collisions to the advertising channels.

Haresh05 said:

When devices are set 40ft apart, I see RSSI around 75-80.

This is also a little hard on the connection. At this range and RSSI, you will start to see some losses. This adds to the interference issue we have above.

To help improve RSSI, usually you can try to increase the TX Power of the devices. However, doing so will worsen the interference issue, so I wouldn't recommend it this time.

---

Given this situation, the difficulty to establish the connection is likely normal to be expected.

In this particular environment, I recommend you:

• Give lowering scan window to 30ms a try. 
  Our initial motivation was to help the Central's scheduler to schedule activities better.
  But in this situation, the target will be to lower on air conflicts. The scheduling benefit is still there.
  
  
• Consider using Advertising without Scan Response and/or Passive Scanning whenever possible.