Channel Access Mechanism

Two type channel access mechanism:

• In non-beacon-enabled networks -> unslotted CSMA/CA channel access mechanism.

• In beacon-enabled networks -> slotted CSMA/CA channel access mechanism.

Unslotted CSMA/CA

NB is the number of times the CSMA-CA algorithm was required to backoff while attempting the current transmission.

BE is the backoff exponent, which defines the number of backoff periods a node should wait before attempting Clear Channel Assessment (CCA).

MacMinBE constant defined in the standard.

CSMA/CA algorithm

In slotted CSMA/CA.

• The backoff period boundaries of every device in the PAN shall be aligned with the superframe slot boundaries of the PAN coordinator.

  • i.e. the start of first backoff period of each device is aligned with the start of the beacon transmission.

• The MAC sublayer shall ensure that the PHY layer commences all of its transmissions on the boundary of a backoff period.

Each device shall maintain three variables for each transmission attempt.

• NB: number of time the CSMA/CA algorithm was required to backoff while attempting the current transmission.

• CW: contention window length, the number of backoff periods that needs to be clear of channel activity before transmission can commence (initial to 2 and reset to 2 if sensed channel to be busy).

• BE: the backoff exponent which is related to how many backoff periods a device shall wait before attempting to assess a channel.

Slotted CSMA/CA

This ensures performing two CCA operations to prevent potential collisions of acknowledgment frames.

If the channel is again sensed as idle (CW = 0), the node attempts to transmit.