Distributed Coordination Function (DCF)

The 802.11 DCF is responsible for many behaviors. These actions include (but are not limited to):

• Acknowledging good receptions
• Retransmitting failed transmissions
• Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
• Binary Exponential Backoff (BEB)

The purpose of this document is not to explain or defend aspects of the DCF. Instead, this document is to aid in the understanding of how the standard DCF behavior is realized within the Mango 802.11 Reference Design. This document is intended to be read alongside the actual wlan_mac_dcf.c​ and wlan_mac_low.c​ code themselves.

Rather than structure this documentation as a function-by-function pseudocode equivalent to the above-linked codebases, we will instead approach the documentation from the perspective of two different roles: MPDU transmission and MPDU reception. There is overlap between these roles in what functions they execute (e.g., MPDU transmission still has an element of frame reception to deal with acknowledgments).

MPDU Transmission

From the DCF's perspective, a new MPDU transmission begins when an IPC message arrives from CPU_HIGH with a payload that must be sent. The origin of this payload is irrelevant - it might be an encapsulated Ethernet frame, or it might be a beacon created by the Access Point (AP) project, or it might even be a LTG frame. Those are all concerns of CPU_HIGH. In CPU_LOW, all that matters is that there are bytes that must be sent and certain actions may possibly be required of us to modify those bytes (e.g. timestamp insertion in a beacon transmission).

IPC Message from CPU_HIGH: MPDU Ready

When an IPC message arrives, the MAC Low Framework needs to perform a few different actions before passing the frame off to the DCF for transmission.

Pseudocode:

convert high-level MCS/rate into PHY-understandable rate parameter
	
if( insert duration )
	calculate duration of SIFS and ACK
	insert duration into MAC header

if ( insert timestamp )
	tell PHY Tx which bytes to overwrite with usec timestamp

call the frame_tx_callback()

For the DCF application, the frame_tx_callback() executes the frame_transmit() function in the DCF code.

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frame_transmit()

The 802.11 DCF Tx state machine is complex. The implementation of frame_transmit() reflects much of this complexity. In no particular order, the function needs to:

• draw a random backoff according to the binary exponential backoff
• use the MAC Support Core to transmit a frame that defers to the medium
• if needed, send an RTS instead of the data itself to reserve the medium
• if needed, look out for a response frame (i.e. a CTS from a transmitted RTS or an ACK from a transmitted data frame)
• if needed, handle a timeout and implement the complex retransmission behavior

The frame_transmit() context will only exit when an MPDU is "done." Here, "done" can mean multiple things:

• a unicast frame that is acknowledged
• a unicast frame whose every retransmission attempt has failed
• any multicast frame has been transmitted (since multicast frames are, by definition, successful since they cannot be acknowledged)

Pseudocode:

continue_retries = True
while( continue_retries )
	if( mpdu is long )
		load MAC support core with RTS
	else
		load MAC support core with MPDU

	draw a random backoff and set MAC support core (will only be used if medium is busy)

	start MAC support core Tx state machine

	do
		if( MAC support core is done )
			switch( MAC support core result )
				case immediate success (e.g., multicast):
					reset contention windows and counters
					start backoff
					continue_retries = False

				case Rx has started:
					call frame_receive() and check return value
					if( Rx was ACK and we were waiting for ACK )
						reset contention windows and counters
						start backoff
						continue_retries = True
					
					if ( Rx was CTS and we were waiting on CTS )
						frame_receive handled the MPDU Tx, so we should set
							the MAC support core status variable to pending explicitly
							so we do not fall out of the do-while

					if ( other Rx )
						increment counters and contention window
						continue_retries = True

				case Timeout has occurred:
					increment counters and contention window
					continue_retries = False
					
	while( MAC support core is pending )
	
	

frame_receive()

The frame_receive() function, when called from the context of the above frame_transmit() function block), should perform two tasks:

• If the received frame is a good FCS ACK that is addressed to this node, it should raise a flag in its return value stating this
• If the received frame is a good FCS CTS that is addressed to this node, it should raise a flag in its return value stating this. It should also start the transmission of the MPDU a SIFS interval after the reception is complete.

The MAC Support Core is capable of transmitting a frame an arbitrary interval after the completion of a reception, so we can use this for sending the MPDU after receiving a CTS.

Pseudocode:

if( RA address is to me )
	switch( packet type )
		case ACK:
			wait for FCS
			if( FCS good )
				raise corresponding return flag
		case CTS:
			wait for FCS status
			if( FCS good )
				configure MAC support core to send MPDU after SIFS interval
				raise corresponding return flag

Upon exiting the frame_transmit() context, execution returns to the MAC Low Framework, where an IPC message is sent to CPU_HIGH indicating that the MPDU transmission is done.

MPDU Reception

MPDU reception is considerably simpler than transmission. The DCF software is responsible for polling the PHY Rx by calling the wlan_mac_low_poll_frame_rx() function in the MAC Low Framework.

PHY Rx Poll

The wlan_mac_low_poll_frame_rx() function reads the status registers from the PHY and converts that information into a phy_rx_details structure that contains the following:

• the PHY mode that was used (i.e. OFDM or DSSS)
• the PHY rate that was used
• the frame length decoded from the SIGNAL field

If the reception is valid and supported, the framework calls the frame_rx_callback function.

Pseudocode:

read the MAC status register

if( the Rx PHY is currently active OR the PHY is blocked after a reception )
	if( the PHY is using the DSSS subsystem )
		update the phy_rx_details struct to reflect the DSSS mode and rate
		call frame_rx_callback()
	else
		wait for valid OFDM Rx params from the PHY
		if( params indicate a supported mode )
			update the phy_rx_details with Rx params
			call frame_rx_callback()

return status_variable the return of the call to frame_rx_callback()
	

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In the DCF, the frame_rx_callback() is attached to the frame_receive() function. In the above MPDU transmission state, we have already seen some executions of this function to deal with the receptions of ACK and CTS frames. Independently of these actions, the frame_receive() function needs to pass receptions up to CPU_HIGH and, if needed, generated control frame responses (i.e. ACK and CTS frames).

Pseudocode:

if( RA address is to me )
	switch( packet type )
		case DATA:
			wait for FCS
			if( FCS good )
				configure MAC support core to send ACK after SIFS interval
		case ACK:
			wait for FCS
			if( FCS good )
				configure MAC support core to send CTS after SIFS interval

if( frame passes CPU_LOW filter )
	call MAC Low Framework function to send IPC message to CPU_HIGH
		stating that the Rx MPDU is ready to be processed