Channel Access Mechanism
Last updated
Last updated
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.
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.
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.
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.
