Esimate CoAP over BLE power consumption

[marcmar]

Hi, I want to estimate easily, without measure it, the power consumption of sending an array of 256 float32_t elements (4 bytes each) through CoAP NON protocol using the nRF52 DK's BLE to a Raspberry Pi. I have found this very useful tool. My idea is to divide the array's byte size (256x4 bytes) by the BLE maximum payload for a BLE transmission packet (27 bytes). We can know the number of packets needed 1024/27=37,92 and we can round up it to 38 (doing another little approximation). Now using the nRF52 Online Power Profiler using:

• BLE event type: Peripheral connection -Voltage: 3.6 V -BLE interval: 25 ms (1080 bytes/sec that I like to reach, obtained rounded down 1000ms / 38 packets) -TX payload: 27 bytes
• TX power: 4 dBm

I have as result: 240 uA for 25ms.

Can I assert that my project during the BLE transmission of my array, for a time period close to 1 sec will use 240 uA more than what I have in the IDLE?

Thanks in advance for the help and explanations.

Marco

[Stian Røed Hafskjold]

Hi, your estimation seems reasonable, you just have to add the header data.

First you have the L2CAP header. Then the 6LoWPAN and UDP header. I tried to illustrate this here:

L2CAP Header:
  Length          2 B
  CID             2 B
  SDU length      2 B
  L2CAP payload:
    6LowPAN IPHC  2 B
    Source addr   16 B
    Dest addr     16 B
    UDP header    1 B
      Src port    2 B
      Dst port    2 B
      Checksum    2 B
      Payload:
        Coap head 6 B
        Payload   Your data
        End       2 B
    
Sum               55 B

So the headers alone will require 2 additional packets.

[marcmar]

Thanks for the information. It's like gold for me! , Has the payload a theoretical limit (in bytes)? I'm interested to send more byte trought CoAP over BLE in the range of 1-4 kylobytes. Thank you in advance

[Olivia]

Hi Stian,
What is this "End" of 2 Bytes? I thought of the BLE CRCbut I unerstood that would take 3 bytes...