A configuration structure for the CSMA transmit algorithm.

One of RAIL's schemes for polite spectrum access is an implementation of a Carrier-Sense Multiple Access (CSMA) algorithm based on IEEE 802.15.4 (unslotted). In pseudo-code it works like this, showing relevant event notifications:

// Return true to transmit packet, false to not transmit packet.
bool performCsma(const RAIL_CsmaConfig_t *csmaConfig)
{
  bool isFixedBackoff = ((csmaConfig->csmaMinBoExp == 0)
                         && (csmaConfig->csmaMaxBoExp == 0));
  int backoffExp = csmaConfig->csmaMinBoExp; // Initial backoff exponent
  int backoffMultiplier = 1; // Assume fixed backoff
  int try;

  // Special-case tries == 0 to transmit immediately without backoff+CCA
  if (csmaConfig->csmaTries == 0) {
    return true;
  }

  // Start overall timeout if specified:
  if (csmaConfig->csmaTimeout > 0) {
    StartAbortTimer(csmaConfig->csmaTimeout, RAIL_EVENT_TX_CHANNEL_BUSY);
    // If timeout occurs, abort and signal the indicated event.
  }

  for (try = 0; try < csmaConfig->csmaTries; try++) {
    if (try > 0) {
      signalEvent(RAIL_EVENT_TX_CCA_RETRY);
    }
    // Determine the backoff multipler for this try:
    if (isFixedBackoff) {
      // backoffMultiplier already set to 1 for fixed backoff
    } else {
      // Start with the backoff exponent for this try:
      if (try > 0) {
        backoffExp++;
        if (backoffExp > csmaConfig->csmaMaxBoExp) {
          backoffExp = csmaConfig->csmaMaxBoExp;
        }
      }
      // Pick random multiplier between 0 and 2^backoffExp - 1 inclusive:
      backoffMultiplier = pickRandomInteger(0, (1 << backoffExp) - 1);
    }
    // Perform the backoff:
    delayMicroseconds(backoffMultiplier * csmaConfig->ccaBackoff);
    // Perform the Clear-Channel Assessment (CCA):
    // Channel is considered busy if radio is actively receiving or
    // transmitting, or the average energy detected across duration
    // is above the threshold.
    signalEvent(RAIL_EVENT_TX_START_CCA);
    if (performCca(csmaConfig->ccaDuration, csmaConfig->ccaThreshold)) {
      // CCA (and CSMA) success: Transmit after RX-to-TX turnaround
      StopAbortTimer();
      signalEvent(RAIL_EVENT_TX_CHANNEL_CLEAR);
      return true;
    } else {
      // CCA failed: loop to try again, or exit if out of tries
    }
  }
  // Overall CSMA failure: Don't transmit
  StopAbortTimer();
  signalEvent(RAIL_EVENT_TX_CHANNEL_BUSY);
  return false;
}

Public Attributes#

uint8_t

The minimum (starting) exponent for CSMA random backoff (2^exp - 1).

uint8_t

The maximum exponent for CSMA random backoff (2^exp - 1).

uint8_t

The number of backoff-then-CCA iterations that can fail before reporting RAIL_EVENT_TX_CHANNEL_BUSY.

int8_t

The CCA RSSI threshold, in dBm, above which the channel is considered 'busy'.

uint16_t

The backoff unit period in RAIL's microsecond time base.

uint16_t

The minimum desired CCA check duration in microseconds.

An overall timeout, in RAIL's microsecond time base, for the operation.

Public Attribute Documentation#

csmaMinBoExp#

uint8_t RAIL_CsmaConfig_t::csmaMinBoExp

The minimum (starting) exponent for CSMA random backoff (2^exp - 1).

It can range from 0 to RAIL_MAX_CSMA_EXPONENT.

Warnings


Definition at line 3370 of file common/rail_types.h

csmaMaxBoExp#

uint8_t RAIL_CsmaConfig_t::csmaMaxBoExp

The maximum exponent for CSMA random backoff (2^exp - 1).

It can range from 0 to RAIL_MAX_CSMA_EXPONENT and must be greater than or equal to csmaMinBoExp. If both exponents are 0, a non-random fixed backoff of ccaBackoff duration results.


Definition at line 3378 of file common/rail_types.h

csmaTries#

uint8_t RAIL_CsmaConfig_t::csmaTries

The number of backoff-then-CCA iterations that can fail before reporting RAIL_EVENT_TX_CHANNEL_BUSY.

Typically ranges from 1 to RAIL_MAX_LBT_TRIES; higher values are disallowed. A value 0 always transmits immediately without performing CSMA, similar to calling RAIL_StartTx().


Definition at line 3386 of file common/rail_types.h

ccaThreshold#

int8_t RAIL_CsmaConfig_t::ccaThreshold

The CCA RSSI threshold, in dBm, above which the channel is considered 'busy'.


Definition at line 3391 of file common/rail_types.h

ccaBackoff#

uint16_t RAIL_CsmaConfig_t::ccaBackoff

The backoff unit period in RAIL's microsecond time base.

It is multiplied by the random backoff exponential controlled by csmaMinBoExp and csmaMaxBoExp to determine the overall backoff period. For random backoffs, any value above 511 microseconds will be truncated. For fixed backoffs it can go up to 65535 microseconds.


Definition at line 3399 of file common/rail_types.h

ccaDuration#

uint16_t RAIL_CsmaConfig_t::ccaDuration

The minimum desired CCA check duration in microseconds.

The RSSI is averaged over this duration by default but can be set to use the peak RSSI, on supported platforms, using the RAIL_TX_OPTION_CCA_PEAK_RSSI option.

Note

  • Depending on the radio configuration, due to hardware constraints, the actual duration may be longer. Also, if the requested duration is too large for the radio to accommodate, RAIL_StartCcaCsmaTx() will fail returning RAIL_STATUS_INVALID_PARAMETER.


Definition at line 3410 of file common/rail_types.h

csmaTimeout#

RAIL_Time_t RAIL_CsmaConfig_t::csmaTimeout

An overall timeout, in RAIL's microsecond time base, for the operation.

If the transmission doesn't start before this timeout expires, the transmission will fail with RAIL_EVENT_TX_CHANNEL_BUSY. A value 0 means no timeout is imposed.


Definition at line 3417 of file common/rail_types.h