In ns-3, is backoff procedure suspended when channel is busy?

[Le Tian]

Hi all,

For dcf backoff, according to wifi standard,  

"If the medium is determined to be busy at any time during a backoff slot, then the backoff procedure is

suspended; that is, the backoff timer shall not decrement for that slot. The medium shall be determined to be

idle for the duration of a DIFS period or EIFS, as appropriate (see 9.3.2.3), before the backoff procedure is

allowed to resume". 

but in ns-3, it seems no suspense in backoff when the channel is busy, slots are always decreased. do I misunderstand that ? 

void

DcfManager::RequestAccess (DcfState *state)

{

  NS_LOG_FUNCTION (this << state);

  //Deny access if in sleep mode

  if (m_sleeping)

    {

      return;

    }

  UpdateBackoff ();

  NS_ASSERT (!state->IsAccessRequested ());

  state->NotifyAccessRequested ();

  /**

   * If there is a collision, generate a backoff

   * by notifying the collision to the user.

   */

  if (state->GetBackoffSlots () == 0

      && IsBusy ())

    {

      MY_DEBUG ("medium is busy: collision");

      /* someone else has accessed the medium.

       * generate a backoff.

       */

      state->NotifyCollision ();

    }

  DoGrantAccess ();

  DoRestartAccessTimeoutIfNeeded ();

}

Kind regards,

Le

[Tommaso Pecorella]

Hi,

it could be a bug (a rather subtle one).

As usual, the best thing is to poke Sébastien (he's already reading in cc) to check if it's a known bug.

Cheers,

T.

[Sebastien Deronne]

Sorry, I had not time to look into this, but freezing should normally be working ok.

If you still have issue, I could investigate next month.

2015-08-21 9:12 GMT+02:00 Le Tian <Le....@uantwerpen.be>:

Hi,

I realised i may misunderstand the code, 

DcfManager::GetAccessGrantStart (void) const which is called by UpdateBackoff () seems consider the situation when channel is busy with " Time busyAccessStart = m_lastBusyStart + m_lastBusyDuration + m_sifs;"

Time

DcfManager::GetAccessGrantStart (void) const

{

  NS_LOG_FUNCTION (this);

  Time rxAccessStart;

  if (!m_rxing)

    {

      rxAccessStart = m_lastRxEnd + m_sifs;

      if (!m_lastRxReceivedOk)

        {

          rxAccessStart += m_eifsNoDifs;

        }

    }

  else

    {

      rxAccessStart = m_lastRxStart + m_lastRxDuration + m_sifs;

    }

  Time busyAccessStart = m_lastBusyStart + m_lastBusyDuration + m_sifs;

  Time txAccessStart = m_lastTxStart + m_lastTxDuration + m_sifs;

  Time navAccessStart = m_lastNavStart + m_lastNavDuration + m_sifs;

  Time ackTimeoutAccessStart = m_lastAckTimeoutEnd + m_sifs;

  Time ctsTimeoutAccessStart = m_lastCtsTimeoutEnd + m_sifs;

  Time switchingAccessStart = m_lastSwitchingStart + m_lastSwitchingDuration + m_sifs;

  Time accessGrantedStart = MostRecent (rxAccessStart,

                                        busyAccessStart,

                                        txAccessStart,

                                        navAccessStart,

                                        ackTimeoutAccessStart,

                                        ctsTimeoutAccessStart,

                                        switchingAccessStart

                                        );

  NS_LOG_INFO ("access grant start=" << accessGrantedStart <<

               ", rx access start=" << rxAccessStart <<

               ", busy access start=" << busyAccessStart <<

               ", tx access start=" << txAccessStart <<

               ", nav access start=" << navAccessStart);

  return accessGrantedStart;

}

hope i understand correctly this time.

Kind regards,

Le Tian
Modeling of Systems and Internet Communication (MOSAIC)
University of Antwerp - iMinds
Dept. Mathematics and Computer Science
Middelheimlaan 1
2020 Antwerpen - Belgium

Email: Le....@uantwerpen.be

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