Scan Beacons loss too many packets

[ReedLi]

Hi all, I'm testing ble adv packets loss rate.I am using a simple scanner example in nRF52832 (Softdevice v2.0.0 & SDK v11), the scanner works in passive scanning, no whitelist, scan interval and scan window are both 60ms, scan timeout is 0x0000, disable timeout.The beacons advertise every 500ms with the adv packet data length 31 bytes.

Now in order to get adv packet loss rate accurately, i count scanner's BLE_GAP_EVT_ADV_REPORT Event numbers as the received adv packets number. I put scanner and 100 beacons in the shielded room, in one minute i get 9980 adv packets, but the actual adv packets should be 12000(100*60*2), then the adv packet loss rate is about 17%。

I am not sure if the packet loss rate is reasonable, or if there are mistakes in my test.Please suggest me for this.

Thanks in advance

regards

ReedLi

[natersoz]

That sounds like a pretty cool setup!

If you reduce your beacon count to just 1 advertiser then do you get nearly zero packet loss?

[ReedLi]

Hi, i test 1 advertiser just now, and i get nearly zero packet loss.

[natersoz]

Sorry - more questions:

Are you limiting your advertising and scanning to only one frequency?

Let me assume that you are.

At 31 bytes of advertising data, this is a fully loaded advertising packet.

I'm not an expert on the PHY/Link layer aspects of BLE, but that should be 376 usec of duration. Citing Robin Hayden, Bluetooth LE: The Developer's Handbook, 3.1.1 Physical Layer.

This seems right: For a full Link Layer packet there are 37 bytes, 376 bits. At a 1 Mbps symbol rate, 1 bit / symbol that is a Link Layer duration of 376 usec.

Here is how I would model this:

• Assuming you're on a single PHY layer frequency (otherwise, I don't know how this could be lossless. I would expect a hopping scanner and a single hopping beacon to experience a 2/3 transmission loss (67%)).
• There is an expected arrival time of the advertising interval / the beacon count.

I think this can be modeled as a Poisson distribution:

https://en.wikipedia.org/wiki/Poisson_distribution#Probability_of_events_for_a_Poisson_distribution

The interval is 367 usec + software latency (let's assume ero for now).

Poisson's lambda λ = 367 usec / 5 msec = 0.0734

For zero time of software latency, I get an expected packet loss of 7%:

Beacon Count             = 100
Advertising Interval     = 500 msec
Link Layer Packet Length = 47 bytes, 376 bits
Link Layer Packet Period = 376 usec
P(  0, 0.075) = 9.276e-01
P(  1, 0.075) = 6.975e-02
P(  2, 0.075) = 2.623e-03
P(  3, 0.075) = 6.574e-05
P(  4, 0.075) = 1.236e-06
P(  5, 0.075) = 1.859e-08
P(  6, 0.075) = 2.330e-10
P(  7, 0.075) = 2.503e-12
P(  8, 0.075) = 2.353e-14
P(  9, 0.075) = 1.966e-16

P packet none = 9.2756e-01
P packet none = 6.9752e-02
P packet lost = 2.6897e-03

Using this python 3.6 script:

''' Calculate advertising packet loss based on the Poission distribution '''
import math

# Link Layer packet lengths.
PREAMBLE_LENGTH         =  1
ACCESS_ADDRESS_LENGTH   =  4
HEADER_LENGTH           =  1
LENGTH_LENGTH           =  1
PAYLOAD_LENGTH          = 37    # 6 + Advertising data bytes
CRC_LENGTH              =  3

# Link Layer
LINK_LAYER_PACKET_LENGTH = (PREAMBLE_LENGTH +
                            ACCESS_ADDRESS_LENGTH +
                            HEADER_LENGTH +
                            LENGTH_LENGTH +
                            PAYLOAD_LENGTH +
                            CRC_LENGTH)

# For a 1 Mbps data rate the symbol period:
BLE_SYMBOL_PERIOD = 1.0E-6

# The Data Packet duration, this is the period of one symbol multiplied
# by the number of bits (bytes * 8) in a packet.
LINK_LAYER_PACKET_PERIOD = BLE_SYMBOL_PERIOD * LINK_LAYER_PACKET_LENGTH * 8

BEACON_COUNT                = 100
BEACON_ADVERTISING_INTERVAL = 500E-3
SOFTWARE_LATENCY            = 0.0  #3.0E-3

def poisson(k, lamb):
    return math.exp(-lamb) * math.pow(lamb, k) / math.factorial(k)

def main():
    print("Beacon Count             = {}".format(BEACON_COUNT))
    print("Advertising Interval     = {:.0f} msec".format(BEACON_ADVERTISING_INTERVAL * 1000))
    print("Link Layer Packet Length = {} bytes, {} bits".format(LINK_LAYER_PACKET_LENGTH, LINK_LAYER_PACKET_LENGTH*8))
    print("Link Layer Packet Period = {:.0f} usec".format(LINK_LAYER_PACKET_PERIOD * 1.0E6))

    p_packet_none = 0.0     # The probability that no packet is received or lost.
    p_packet_recv = 0.0     # The probability that a packet was received.
    p_packet_loss = 0.0     # The probability that a packet was lost

    lamb = (LINK_LAYER_PACKET_PERIOD + SOFTWARE_LATENCY) / (BEACON_ADVERTISING_INTERVAL / BEACON_COUNT)
    for k in range(0, 10):
        p = poisson(k, lamb)
        if k == 0:
            p_packet_none = p
        elif k == 1:
            p_packet_recv = p
        elif k > 1:
            p_packet_loss += p
        print("P({:3d}, {:.3f}) = {:.3e}".format(k, lamb, p))

    print("")
    print("P packet none = {:.4e}".format(p_packet_none))
    print("P packet none = {:.4e}".format(p_packet_recv))
    print("P packet lost = {:.4e}".format(p_packet_loss))

if __name__ == '__main__':
    main()

Here are some thoughts:

• Are you using just 1 frequency for BLE advertisements? This is a fundamental assumption I have made and if it is incorrect then none of this makes any sense.
• If you are then the most likely point of loss is that the BLE central which is receiving the advertisements is blocking and getting in the way of reception? Increasing the SOFTWARE_LATENCY value to 3.0E-3 (a very large value) will get you in the ballpark. Is your accounting on the Central side using a blocking printf()?

This is a very, very interesting setup that you have and I would like to know how your results turn out.

Thanks!

[natersoz]

Update: I realized that the method I am counting packets and you are counting packets are different.

I changed the python program to match what I think you are doing to count packets received and lost and updated the python code. It is checked in here:

https://github.com/natersoz/nrf/blob/master/scripts/packet_loss.py

It is checked in with 1.5 msec of software latency, which is still very high. 17% packet loss.

[ReedLi]

Thanks for your thoughts.Here are replies to thoughts:

• Advertisement and scan in 3 channels
• There is no printf(), i turn off it.

I can only get adv packets when receive BLE_GAP_EVT_ADV_REPORT Event  in ble_evt_dispatch. And after receiving adv packets i only count the number,  no flash and printf() operations.