MRF24J40_08 Datasheet, PDF (101/152 Pages) Microchip Technology – IEEE 802.15.4™ 2.4 GHz RF Transceiver

MRF24J40
3.9 Carrier Sense Multiple
Access-Collision Avoidance
(CSMA-CA) Algorithm
MRF24J40 supports both unslotted and slotted
CSMA-CA mechanisms, as defined in the
IEEE 802.15.4 Standard. In both modes, the
CSMA-CA algorithm is implemented using units of time
called backoff periods. In slotted CSMA-CA, the back-
off period boundaries of every device on the PAN shall
be aligned with the superframe slot boundaries of the
PAN coordinator. In unslotted CSMA-CA, the backoff
periods of one device are not related in time to the
backoff periods of any other device in the PAN. Refer to
IEEE 802.15.4-2003, Section 7.5.1.3 “The CSMA-CA
Algorithm” for more information. This section covers
the two modes and their settings.
Note:
Acknowledgment and beacon frames are
sent without using a CSMA-CA
mechanism.
3.9.1 UNSLOTTED CSMA-CA MODE
Figure 3-6 shows the unslotted CSMA-CA algorithm.
This mode is used in a nonbeacon-enabled network
where the backoff periods of one device are not related
in time to the backoff periods of any other device in the
network. Refer to IEEE 802.15.4-2003, Section 7.5.1.3
“The CSMA-CA Algorithm” for more information.
Configuring the MRF24J40 for nonbeacon-enabled
network operation is covered in Section 3.8.2
“Nonbeacon-Enabled Network”.
FIGURE 3-6:
UNSLOTTED CSMA-CA ALGORITHM
Start
NB = 0, BE = macMinBE
Delay for Random (2^BE – 1) Backoff Periods
macMinBE
MACMINBE (TXMCR 0x11<4:3>)
Perform CCA
Y
Channel Idle?
N
NB = NB + 1, BE = min(BE + 1, aMaxBE)
N
NB >
macMaxCSMABackoffs
macMaxCSMABackoffs
CSMABF (TXMCR 0x11<2:0>)
Y
FAILURE
(Report Channel Access
Failure to Host
Microcontroller)
Transmit Pending Packet
(SUCCESS)
© 2008 Microchip Technology Inc.
Preliminary
DS39776B-page 99