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CC3100: Medium access policies in transceiver mode

Patrick Voelker
Intellectual 590 points
Part Number: CC3100

On a custom board using the CC3100 with the latest service pack, I'm sending and receiving messages with a socket configured for transceiver mode (SL_SOCK_DGRAM / L2.)

With 11 boards running, all sending messages at various times, I see varying levels of successful message transmit and receive.  I've seen success rates as low as 30-35% for some boards messages received by other boards.

In the documentation, it mentions that a socket configured as an L2 raw socket respects 802.11 medium access policies. I would like to know which 802.11 access policies are in use in this mode and if there are any tunable parameters that I can work with to help ensure that the boards talk over each other less frequently.

I'm using these transceiver mode socket options:

#define RAW_CHANNEL                    (3)
#define RAW_RATE                    (RATE_11M)
#define RAW_POWER                    (1)
#define RAW_PREAMBLE                (SL_SHORT_PREAMBLE)

  • 0
    TI__Mastermind 20180 points

    Hi Patrick,

    With raw packets there is no guarantee that the packets are successfully transmitted, so seeing the low success rates is expected. 

    For more reliant communication, you should use a star topology communication via an Access Point that can handle all of these devices. Or you could use one of TI's mesh capable devices like the CC13xx or CC26xx series. 

  • 0 in reply to Sabeeh Khan1
    Intellectual 590 points

    Hi Sabeeh, thanks for the information on the other parts that we could use in the future.  Right now though I'm looking for specific technical information on which medium access policies the CC3100 respects with a raw L2 socket.

    In the document swru368c.pdf section 15.2, it says that a raw L1 socket has "no respect for 802.11 medium access policy [CCA]".

    Is CCA (clear channel assessment) the only medium access policy that is respected with a raw L2 socket?

  • 0 in reply to Patrick Voelker
    TI__Mastermind 20180 points

    I'm not sure of what other access policies are there for L2, I will have to dig around, but they would be in line with 802.11 standard. 

    However, the policies may not be the only reason for packet loss. I believe the maximum rate for SPI is about 5-6Mbps, so depending how often you are sending messages, you might be missing packets because the device can not transmit to your host quick enough. You can test this by slowing down how often each devices transmits, or maybe even the modulation rate. 

  • 0 in reply to Sabeeh Khan1
    Intellectual 590 points

    Thanks for taking the time to dig around, that will be helpful to know.

    Generally my boards are transmitting less than 1 kB/s in normal operation and with 11 boards running, are receiving < 4 kB/s so the SPI transfer rate shouldn't factor in.

    I'll try some other data rates including the MCS options to see what changes.

  • 0 in reply to Patrick Voelker
    TI__Mastermind 20180 points

    Hey Patrick, the medium access policy is not exposed to the driver, so yes the CCA is the only policy.