Before a transmission or a burst of transmissions on an Operating Channel, the Initiating Device should operate at least one Channel Access Engine that executes the procedure described in the following steps 1 to step 8. This Channel Access Engine makes use of the parameters defined in Table 6-17 or Table 6-18 in Section 6.2.8.2.3.
A single Observation Slot should have a duration of not less than 9 μs.
An Initiating Device should operate at least one and no more than four different Channel Access Engines each with a different Priority Class as defined in Section 6.2.8.2.3:
- The Channel Access Engine should set CW to CWmin.
- The Channel Access Engine should select a random number q from a uniform distribution over the range 0 to CW. (1) in Table 6-17 defines an alternative range for q when the previous or next Channel Occupancy Time is greater than the maximum Channel Occupancy Time specified in Table 6-17.
- The Channel Access Engine should initiate a Prioritization Period as described in the following steps a) to c):
- The Channel Access Engine should set p according to the Priority Class associated with this Channel Access Engine. See Section 6.2.8.2.3.
- The Channel Access Engine should wait for a period of 16 μs.
- The Channel Access Engine should perform a Clear Channel Assessment (CCA) on the Operating Channel during a single Observation Slot:
- The Operating Channel should be considered occupied if other transmissions within this channel are detected with a level above the ED threshold defined in Section 6.2.8.2.4. In this case, the Channel Access Engine should initiate a new Prioritization Period starting with step 3a) after the energy within the channel has dropped below the ED threshold defined in Section 6.2.8.2.4.
- In case no energy within the Operating Channel is detected with a level above the ED threshold defined in Section 6.2.8.2.4, p may be decremented by not more than 1. If p is equal to 0, the Channel Access Engine should proceed with step 4, otherwise the Channel Access Engine should proceed with step 3c).
- The Channel Access Engine should perform a Backoff Procedure as described in step 4a) to step 4d):
- This step verifies if the Channel Access Engine satisfies the Post Backoff condition. If q < 0 and the Channel Access Engine is ready for a transmission, the Channel Access Engine should set CW equal to CWmin and should select a random number q from a uniform distribution over the range 0 to CW before proceeding with step 4b). (1) in Table 6-17 defines an alternative range for when the previous or next Channel Occupancy Time is greater than the maximum Channel Occupancy Time specified in Table 6-17.
- If q < 1 the Channel Access Engine should proceed with step 4d). Otherwise, the Channel Access Engine may decrement the value q by not more than 1 and the Channel Access Engine should proceed with step 4c).
- The Channel Access Engine should perform a Clear Channel Assessment (CCA) on the Operating Channel during a single Observation Slot:
- The Operating Channel should be considered occupied if energy was detected with a level above the ED threshold defined in Section 6.2.8.2.4. In this case, the Channel Access Engine should continue with step 3.
- If no energy was detected with a level above the ED threshold defined in Section 6.2.8.2.4, the Channel Access Engine should continue with step 4 b).
- If the Channel Access Engine is ready for a transmission the Channel Access Engine should continue with step 5. Otherwise, the Channel Access Engine should decrement the value q by 1 and the Channel Access Engine should proceed with step 4c). It should be understood that q can become negative and keep decrementing as long as the Channel Access Engine is not ready for a transmission.
- If only one Channel Access Engine of the Initiating Device is in this stage, the Channel Access Engine should proceed with step 6. If the Initiating Device has a multitude of Channel Access Engines in this stage, the Channel Access Engine with highest Priority Class in this multitude should proceed with step 6 and all other Channel Access Engines in the current stage should proceed with step 8.
- One Channel Access Engine of the Initiating Device is in this stage: This is equivalent to the equipment having no internal collision
- Initiating Device has a multitude of Channel Access Engines in this stage: This is equivalent to the equipment having one or more internal collisions
- The Channel Access Engine may start transmissions belonging to the corresponding or higher Priority Classes, on one or more Operating Channels. If the initiating device transmits in more than one Operating Channels, it should comply with the requirements contained in Section 6.2.8.2.2:
- The Channel Access Engine can have multiple transmissions without performing an additional CCA on this Operating Channel providing the gap in between such transmissions does not exceed 16 μs. Otherwise, if this gap exceeds 16 μs and does not exceed 25 μs, the Initiating Device may continue transmissions provided that no energy was detected with a level above the ED threshold defined in Section 6.2.8.2.4 for a duration of one Observation Slot.
- The Channel Access Engine may grant an authorization to transmit on the current Operating Channel to one or more Responding Devices. If the Initiating Device issues such a transmission grant to a Responding Device, the Responding Device should operate according to the procedure described in Section 6.2.8.2.6.
- The Initiating Device may have simultaneous transmissions of Priority Classes lower than the Priority Class of the Channel Access Engine, provided that the corresponding transmission duration (Channel Occupancy Time) is not extended beyond the time that is needed for the transmission(s) corresponding to the Priority Class of the Channel Access Engine.
- When the Channel Occupancy has completed, and it has been confirmed that at least one transmission that started at the beginning of the Channel Occupancy was successful, the Initiating Device proceeds with step 1 otherwise the Initiating Device proceeds with step 8.
- The Initiating Device may retransmit. If the Initiating Device does not retransmit the Channel Access Engine should discard all data packets associated with the unsuccessful Channel Occupancy and the Channel Access Engine should proceed with step 1. Otherwise, the Channel Access Engine should adjust CW to ((CW + 1) × m) - 1 with m ≥ 2. If the adjusted value of CW is greater than CWmax the Channel Access Engine may set CW equal to CWmax. The Channel Access Engine should proceed with step 2.
According to Section 6.2.8.2.3 where four different Priority Classes are defined, an Initiating Device should operate only one Channel Access Engine for each Priority Class implemented.
CW may take values that are greater than the values of CW in step 1 to step 8.