source: ReferenceDesigns/w3_802.11/c/wlan_mac_low_dcf/wlan_mac_dcf.c @ 5562

/** @file wlan_mac_dcf.c
 *  @brief Distributed Coordination Function
 *
 *  This contains code to implement the 802.11 DCF.
 *
 *  @copyright Copyright 2013-2016, Mango Communications. All rights reserved.
 *          Distributed under the Mango Communications Reference Design License
 *              See LICENSE.txt included in the design archive or
 *              at http://mangocomm.com/802.11/license
 *
 *  This file is part of the Mango 802.11 Reference Design (https://mangocomm.com/802.11)
 */
/***************************** Include Files *********************************/
// Xilinx SDK includes
#include "xparameters.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "xio.h"
#include "math.h"

// WARP includes
#include "w3_userio.h"
#include "radio_controller.h"

// WLAN includes
#include "wlan_mac_low.h"
#include "wlan_mac_802_11_defs.h"
#include "wlan_mac_time_util.h"
#include "wlan_phy_util.h"
#include "wlan_mac_dcf.h"

// WLAN Exp includes
#include "wlan_exp.h"


/*************************** Constant Definitions ****************************/
#define DBG_PRINT                                          0

#define WLAN_EXP_TYPE_DESIGN_80211_CPU_LOW                 WLAN_EXP_TYPE_DESIGN_80211_CPU_LOW_DCF

#define DEFAULT_TX_ANTENNA_MODE                            TX_ANTMODE_SISO_ANTA

#define NUM_LEDS                                           4

#define RX_LEN_THRESH                                      200


/*********************** Global Variable Definitions *************************/
volatile static mac_timing             gl_mac_timing_values;
volatile static u32                    gl_stationShortRetryCount;
volatile static u32                    gl_stationLongRetryCount;
volatile static u32                    gl_cw_exp;
volatile static u8                     gl_cw_exp_min;
volatile static u8                     gl_cw_exp_max;

volatile static u32                    gl_dot11RTSThreshold;

volatile static u8                     gl_eeprom_addr[MAC_ADDR_LEN];

volatile static u8                     gl_mpdu_pkt_buf;

volatile static u32                    gl_dot11ShortRetryLimit;
volatile static u32                    gl_dot11LongRetryLimit;

volatile u8                            gl_red_led_index;
volatile u8                            gl_green_led_index;

volatile beacon_txrx_configure_t       gl_beacon_txrx_configure;

volatile static u8                     gl_waiting_for_response;

/*************************** Functions Prototypes ****************************/

int process_low_param(u8 mode, u32* payload);


/******************************** Functions **********************************/

int main(){

    wlan_mac_hw_info_t* hw_info;

    xil_printf("\f");
    xil_printf("----- Mango 802.11 Reference Design -----\n");
    xil_printf("----- v1.5.2 ----------------------------\n");
    xil_printf("----- wlan_mac_dcf ----------------------\n");
    xil_printf("Compiled %s %s\n\n", __DATE__, __TIME__);

    xil_printf("Note: this UART is currently printing from CPU_LOW. To view prints from\n");
    xil_printf("and interact with CPU_HIGH, raise the right-most User I/O DIP switch bit.\n");
    xil_printf("This switch can be toggled any time while the design is running.\n\n");
    xil_printf("------------------------\n");

    gl_mpdu_pkt_buf = PKT_BUF_INVALID;
    gl_waiting_for_response = 0;


    gl_beacon_txrx_configure.beacon_tx_mode = NO_BEACON_TX;
    gl_beacon_txrx_configure.ts_update_mode = NEVER_UPDATE;
    bzero((void*)gl_beacon_txrx_configure.bssid_match, MAC_ADDR_LEN);

    gl_dot11ShortRetryLimit      = 7;
    gl_dot11LongRetryLimit       = 4;

    gl_cw_exp_min                = 4;
    gl_cw_exp_max                = 10;

    gl_dot11RTSThreshold         = 2000;

    gl_stationShortRetryCount    = 0;
    gl_stationLongRetryCount     = 0;

    gl_red_led_index             = 0;
    gl_green_led_index           = 0;
    userio_write_leds_green(USERIO_BASEADDR, (1 << gl_green_led_index));
    userio_write_leds_red(USERIO_BASEADDR, (1 << gl_red_led_index));

    wlan_mac_low_init(WLAN_EXP_TYPE_DESIGN_80211_CPU_LOW);

    gl_cw_exp = gl_cw_exp_min;

    hw_info = get_mac_hw_info();
    memcpy((void*)gl_eeprom_addr, hw_info->hw_addr_wlan, MAC_ADDR_LEN);

    wlan_mac_low_set_frame_rx_callback((void*)frame_receive);
    wlan_mac_low_set_frame_tx_callback((void*)frame_transmit);
    wlan_mac_low_set_beacon_txrx_config_callback((void*)configure_beacon_txrx);
    wlan_mac_low_set_mactime_change_callback((void*)handle_mactime_change);
    wlan_mac_low_set_ipc_low_param_callback((void*)process_low_param);
    wlan_mac_low_set_sample_rate_change_callback((void*)handle_sample_rate_change);


    // wlan_mac_low_init() has placed a mutex lock on TX_PKT_BUF_ACK_CTS and
    // TX_PKT_BUF_RTS already. We should set their packet buffer states to LOW_CTRL
    ((tx_frame_info_t*)TX_PKT_BUF_TO_ADDR(TX_PKT_BUF_ACK_CTS))->tx_pkt_buf_state = TX_PKT_BUF_LOW_CTRL;
    ((tx_frame_info_t*)TX_PKT_BUF_TO_ADDR(TX_PKT_BUF_RTS))->tx_pkt_buf_state = TX_PKT_BUF_LOW_CTRL;

    wlan_mac_low_init_finish();

    // Print DCF information to the terminal
    xil_printf("------------------------\n");
    xil_printf("WLAN MAC DCF boot complete: \n");
    xil_printf("  Serial Number     : W3-a-%05d\n", hw_info->serial_number);
    xil_printf("  Wireless MAC Addr : %02x:%02x:%02x:%02x:%02x:%02x\n\n", gl_eeprom_addr[0], gl_eeprom_addr[1], gl_eeprom_addr[2], gl_eeprom_addr[3], gl_eeprom_addr[4], gl_eeprom_addr[5]);

    while(1){
        // Poll PHY RX start
        gl_waiting_for_response = 0;
        wlan_mac_low_poll_frame_rx();

        // Poll IPC rx
        wlan_mac_low_poll_ipc_rx();

        // Poll the timestamp (for periodic transmissions like beacons)
        poll_tbtt();
    }

    return 0;
}

void handle_sample_rate_change(phy_samp_rate_t phy_samp_rate){
    // TODO: Add an argument to specify the phy_mode in case that changes MAC timings

    switch(phy_samp_rate){
        default:
        case PHY_40M:
        case PHY_20M:
            gl_mac_timing_values.t_slot = 9;
            gl_mac_timing_values.t_sifs = 10;
            gl_mac_timing_values.t_difs = gl_mac_timing_values.t_sifs + (2*gl_mac_timing_values.t_slot);
            gl_mac_timing_values.t_eifs = 88;
            gl_mac_timing_values.t_phy_rx_start_dly = 25; //TODO: This is BW dependent. 20/25 is waveform time.
            gl_mac_timing_values.t_timeout = gl_mac_timing_values.t_sifs + gl_mac_timing_values.t_slot + gl_mac_timing_values.t_phy_rx_start_dly;
        break;
        case PHY_10M:
            gl_mac_timing_values.t_slot = 13;
            gl_mac_timing_values.t_sifs = 10;
            gl_mac_timing_values.t_difs = gl_mac_timing_values.t_sifs + (2*gl_mac_timing_values.t_slot);
            gl_mac_timing_values.t_eifs = 88;
            gl_mac_timing_values.t_phy_rx_start_dly = 45;
            gl_mac_timing_values.t_timeout = gl_mac_timing_values.t_sifs + gl_mac_timing_values.t_slot + gl_mac_timing_values.t_phy_rx_start_dly;
        break;
    }

    // MAC timing parameters are in terms of units of 100 nanoseconds
    wlan_mac_set_slot(gl_mac_timing_values.t_slot*10);
    wlan_mac_set_DIFS((gl_mac_timing_values.t_difs)*10);
    wlan_mac_set_TxDIFS(((gl_mac_timing_values.t_difs)*10) - (TX_PHY_DLY_100NSEC));

    // Use postTx timer 2 for ACK timeout
    wlan_mac_set_postTx_timer2(gl_mac_timing_values.t_timeout * 10);
    wlan_mac_postTx_timer2_en(1);

    // Use postRx timer 1 for SIFS
    wlan_mac_set_postRx_timer1((gl_mac_timing_values.t_sifs*10)-(TX_PHY_DLY_100NSEC));
    wlan_mac_postRx_timer1_en(1);

    // TODO: NAV adjust needs verification
    //     NAV adjust time - signed char (Fix8_0) value
    wlan_mac_set_NAV_adj(0*10);
    wlan_mac_set_EIFS(gl_mac_timing_values.t_eifs*10);

    // xil_printf("PHY Sampling Rate set to %d\n", wlan_mac_low_get_phy_samp_rate());
}

void handle_mactime_change(s64 time_delta_usec){
    u32 current_tu;
    if(( gl_beacon_txrx_configure.beacon_tx_mode == AP_BEACON_TX ) ||
       ( gl_beacon_txrx_configure.beacon_tx_mode == IBSS_BEACON_TX )){
        //The MAC Time has changed. We should explicitly update the next TU target
        //for beacon transmission.
        current_tu = (u32)(get_mac_time_usec()>>10);

        //The current_tu can be anywhere within a beacon interval, so we need
        //to round up to the next TBTT.
        wlan_mac_set_tu_target(gl_beacon_txrx_configure.beacon_interval_tu*((current_tu/gl_beacon_txrx_configure.beacon_interval_tu)+1));
    }
    return;
}

void configure_beacon_txrx(beacon_txrx_configure_t* beacon_txrx_configure){

    memcpy((void*)&gl_beacon_txrx_configure, beacon_txrx_configure, sizeof(beacon_txrx_configure_t));

    u32 current_tu;

    if(( gl_beacon_txrx_configure.beacon_tx_mode == AP_BEACON_TX ) ||
       ( gl_beacon_txrx_configure.beacon_tx_mode == IBSS_BEACON_TX )){

        current_tu = (u32)(get_mac_time_usec()>>10);

        //The current_tu can be anywhere within a beacon interval, so we need
        //to round up to the next TBTT.
        wlan_mac_set_tu_target(gl_beacon_txrx_configure.beacon_interval_tu*((current_tu/gl_beacon_txrx_configure.beacon_interval_tu)+1));
        wlan_mac_reset_tu_target_latch(1);
        wlan_mac_reset_tu_target_latch(0);
    }  else {
        wlan_mac_set_tu_target(0xFFFFFFFF);
        wlan_mac_reset_tu_target_latch(1);
    }
}

inline poll_tbtt_return_t poll_tbtt(){
    //u32 tu_target;
    u32 mac_hw_status;
    poll_tbtt_return_t return_status = TBTT_NOT_ACHIEVED;
    u32 current_tu;

    if(( gl_beacon_txrx_configure.beacon_tx_mode == AP_BEACON_TX ) ||
       ( gl_beacon_txrx_configure.beacon_tx_mode == IBSS_BEACON_TX )){

        mac_hw_status = wlan_mac_get_status();

        if(mac_hw_status & WLAN_MAC_STATUS_MASK_TU_LATCH) {
            // Current TU >= Target TU

            if(send_beacon(gl_beacon_txrx_configure.beacon_template_pkt_buf) != 0){
                // We were unable to begin the transmission (most likely because the MAC Support Core A was
                // already actively transmitting something). So we will just return and catch it on the next poll
                return_status = BEACON_DEFERRED;
                return return_status;
            }

            return_status = BEACON_SENT;

            // Update TU target
            //  Changing TU target automatically resets TU_LATCH
            //  Latch will assert immediately if Current TU >= new Target TU
            //tu_target = wlan_mac_get_tu_target();
            //wlan_mac_set_tu_target(tu_target + gl_beacon_txrx_configure.beacon_interval_tu);
            current_tu = (u32)(get_mac_time_usec()>>10);
            wlan_mac_set_tu_target(gl_beacon_txrx_configure.beacon_interval_tu*((current_tu/gl_beacon_txrx_configure.beacon_interval_tu)+1));

            //TODO
            //If MAC time is adjusted by more than a TU (e.g a wlan_exp reset), then
            //we can potentially be waiting a while to have the next TBTT fire. We should
            //update the target when MAC time changes significantly.
        }
    }
    return return_status;
}

inline int send_beacon(u8 tx_pkt_buf){
    int return_status = -1;
    volatile int i = 0;

    wlan_ipc_msg_t                ipc_msg_to_high;
    wlan_mac_low_tx_details_t     low_tx_details;
    u32 mac_hw_status;
    u32 mac_tx_ctrl_status;
    u16 n_slots;
    u16 n_slots_readback;
    int tx_gain;
    u8 mpdu_tx_ant_mask;
    //Note: This needs to be a volatile to allow the tx_pkt_buf_state to be re-read in the initial while loop below
    volatile tx_frame_info_t* tx_frame_info         = (tx_frame_info_t*) (TX_PKT_BUF_TO_ADDR(tx_pkt_buf));
    mac_header_80211* header                        = (mac_header_80211*)(TX_PKT_BUF_TO_ADDR(tx_pkt_buf) + PHY_TX_PKT_BUF_MPDU_OFFSET);
    u64 unique_seq;
    tx_mode_t tx_mode;
    u32 rx_status;

    // Attempt to pause the backoff counter in Tx controller A
    wlan_mac_pause_backoff_tx_ctrl_A(1);

    switch(tx_frame_info->tx_pkt_buf_state){
        case TX_PKT_BUF_READY:
            mac_tx_ctrl_status = wlan_mac_get_tx_ctrl_status();

            // Check if Tx controller A is deferring (now with a paused backoff) or idle (no Tx pending)
            if(((mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_TX_A_STATE) == WLAN_MAC_TXCTRL_STATUS_TX_A_STATE_DEFER) ||
               ((mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_TX_A_STATE) == WLAN_MAC_TXCTRL_STATUS_TX_A_STATE_IDLE)) {

                i = 0;
                while( (lock_tx_pkt_buf(tx_pkt_buf) != PKT_BUF_MUTEX_SUCCESS) ){
                    // We will only continue with the send_beacon state when we are both assured that the
                    //  tx_pkt_buf_state is READY (i.e. CPU_HIGH is not currently trying to log a beacon transmission)
                    //  and we are able to lock the tx_pkt_buf. The only reason a lock should fail is that CPU_HIGH
                    //  is actively modifying the contents of the beacon packet buffer. This is a short duration
                    //  operation so we should just wait.
                    if(i > 1000000) {xil_printf("ERROR (send_beacon): stuck waiting for CPU High to unlock Tx pkt buf\n");}
                    else {i++;}
                }

                // We've locked the beacon template packet buffer. We should set its state to LOW_CTRL
                // so CPU_HIGH can know that we are just about to transmit it.
                tx_frame_info->tx_pkt_buf_state = TX_PKT_BUF_LOW_CTRL;

                // Compare the length of this frame to the RTS Threshold
                if(tx_frame_info->length <= gl_dot11RTSThreshold) {
                    tx_mode = TX_MODE_SHORT;
                } else {
                    tx_mode = TX_MODE_LONG;
                }

                // Update the beacon's seq num (in the MAC header) and uniq_seq (in the tx_frame_info)
                unique_seq = wlan_mac_low_get_unique_seq();
                wlan_mac_low_set_unique_seq(unique_seq+1);
                tx_frame_info->unique_seq = unique_seq;
                header->sequence_control = ((header->sequence_control) & 0xF) | ( (unique_seq&0xFFF)<<4 );

                // Configure the Tx antenna selection
                mpdu_tx_ant_mask = 0;

                switch(tx_frame_info->params.phy.antenna_mode) {
                    case TX_ANTMODE_SISO_ANTA:  mpdu_tx_ant_mask |= 0x1;  break;
                    case TX_ANTMODE_SISO_ANTB:  mpdu_tx_ant_mask |= 0x2;  break;
                    case TX_ANTMODE_SISO_ANTC:  mpdu_tx_ant_mask |= 0x4;  break;
                    case TX_ANTMODE_SISO_ANTD:  mpdu_tx_ant_mask |= 0x8;  break;
                    default:                    mpdu_tx_ant_mask  = 0x1;  break; // Default to RF_A
                }

                //wlan_mac_tx_ctrl_C_params(pktBuf, antMask, req_backoff, phy_mode, num_slots)
                switch(gl_beacon_txrx_configure.beacon_tx_mode){
                    case AP_BEACON_TX:
                        n_slots = rand_num_slots(RAND_SLOT_REASON_STANDARD_ACCESS);
                        wlan_mac_tx_ctrl_C_params(tx_pkt_buf, mpdu_tx_ant_mask, 0, tx_frame_info->params.phy.phy_mode, n_slots);
                    break;
                    case IBSS_BEACON_TX:
                        n_slots = rand_num_slots(RAND_SLOT_REASON_IBSS_BEACON);
                        wlan_mac_tx_ctrl_C_params(tx_pkt_buf, mpdu_tx_ant_mask, 1, tx_frame_info->params.phy.phy_mode, n_slots);
                    break;
                    case NO_BEACON_TX:
                        return -1;
                    break;
                }

                tx_gain = wlan_mac_low_dbm_to_gain_target(tx_frame_info->params.phy.power);
                wlan_mac_tx_ctrl_C_gains(tx_gain, tx_gain, tx_gain, tx_gain);

                write_phy_preamble(tx_pkt_buf,
                                   tx_frame_info->params.phy.phy_mode,
                                   tx_frame_info->params.phy.mcs,
                                   tx_frame_info->length);


                wlan_mac_tx_ctrl_C_start(1);
                wlan_mac_tx_ctrl_C_start(0);

                // Immediately re-read the current slot count.
                n_slots_readback = wlan_mac_get_backoff_count_C();

                if((n_slots != n_slots_readback)){
                    // For the first transmission (non-retry) of an MPDU, the number of
                    // slots used by the backoff process is ambiguous. The n_slots we provided
                    // to wlan_mac_tx_ctrl_A_params is only a suggestion. If the medium has been
                    // idle for a DIFS, then there will not be a backoff. Or, if another backoff is
                    // currently running, the MAC Config Core A will inherit that backoff. By
                    // immediately reading back the slot count after starting the core, we can
                    // overwrite the number of slots that we will fill into low_tx_details with
                    // the correct value
                    n_slots = n_slots_readback;
                }

                tx_frame_info->num_tx_attempts   = 1;
                tx_frame_info->phy_samp_rate     = wlan_mac_low_get_phy_samp_rate();

                // Here, we are overloading the "create" timestamp to mean something subtly different
                //  than when it is used for data MPDUs since beacons are not created and enqueued in
                //  CPU_HIGH. By explicitly filling the current MAC time into the create timestamp,
                //  we allow CPU_HIGH to determine whether or not a backoff occurred before the beacon transmission
                //  when it is creating the TX_LOW log entry for the beacon.
                tx_frame_info->timestamp_create  = get_mac_time_usec();
                tx_frame_info->delay_accept      = 0;

                low_tx_details.tx_details_type  = TX_DETAILS_MPDU;
                low_tx_details.phy_params_mpdu.mcs          = tx_frame_info->params.phy.mcs;
                low_tx_details.phy_params_mpdu.phy_mode     = tx_frame_info->params.phy.phy_mode;
                low_tx_details.phy_params_mpdu.power        = tx_frame_info->params.phy.power;
                low_tx_details.phy_params_mpdu.antenna_mode = tx_frame_info->params.phy.antenna_mode;

                low_tx_details.chan_num    = wlan_mac_low_get_active_channel();
                low_tx_details.cw          = (1 << gl_cw_exp)-1; //(2^(gl_cw_exp) - 1)
                low_tx_details.ssrc        = gl_stationShortRetryCount;
                low_tx_details.slrc        = gl_stationLongRetryCount;
                low_tx_details.src         = 0;
                low_tx_details.lrc         = 0;
                low_tx_details.flags       = 0;

                // The pre-Tx backoff may not occur for the initial transmission attempt. If the medium has been idle for >DIFS when
                //  the first Tx occurs the DCF state machine will not start a backoff. The upper-level MAC should compare the num_slots value
                //  to the time delta between the accept and start times of the first transmission to determine whether the pre-Tx backoff
                //  actually occurred.
                low_tx_details.num_slots   = n_slots;

                 // Wait for the MPDU Tx to finish
                do { // while(tx_status & WLAN_MAC_STATUS_MASK_TX_C_PENDING)

                    // Poll the DCF core status register
                    mac_hw_status = wlan_mac_get_status();

                    if((tx_frame_info->flags) & TX_FRAME_INFO_FLAGS_FILL_TIMESTAMP){
                        if( mac_hw_status & WLAN_MAC_STATUS_MASK_TX_PHY_ACTIVE ){
                            // Insert the TX START timestamp
                            *((u32*)((u8*)header + 24)) =  Xil_In32(WLAN_MAC_REG_TX_TIMESTAMP_LSB);
                            *((u32*)((u8*)header + 28)) =  Xil_In32(WLAN_MAC_REG_TX_TIMESTAMP_MSB);

                            // The below u64 approach also works, but takes 100ns longer than just dealing with the LSB and MSB separately.
                            //*((u64*)((TX_PKT_BUF_TO_ADDR(mpdu_pkt_buf) + PHY_TX_PKT_BUF_MPDU_OFFSET + 24))) = (u64)wlan_mac_low_get_tx_start_timestamp();
                        }
                    }

                    if( mac_hw_status & WLAN_MAC_STATUS_MASK_TX_C_DONE ) {
                        // Transmission is complete

                        switch(tx_mode) {
                        //TODO: Resetting the SSRC and/or SLRC needs to be checked back against the standard
                            case TX_MODE_SHORT:
                                reset_ssrc();
                                reset_cw();
                            break;
                            case TX_MODE_LONG:
                                reset_slrc();
                                reset_cw();
                            break;
                        }

                        low_tx_details.tx_start_timestamp_mpdu = wlan_mac_low_get_tx_start_timestamp();
                        low_tx_details.tx_start_timestamp_frac_mpdu = wlan_mac_low_get_tx_start_timestamp_frac();

                        // Start a post-Tx backoff using the updated contention window
                        //  If MAC Tx controller A backoff has been paused this backoff request will
                        //   successfully be ignored. If Tx A is idle then this backoff
                        //   will execute and future submission to Tx A may inherit the
                        //   this backoff.
                        // TODO: We should double check whether post-Tx backoffs are appropriate
                        n_slots = rand_num_slots(RAND_SLOT_REASON_STANDARD_ACCESS);
                        wlan_mac_dcf_hw_start_backoff(n_slots);
                    } else {
                        // Poll the MAC Rx state to check if a packet was received while our Tx was deferring
                        if (mac_hw_status & WLAN_MAC_STATUS_MASK_RX_PHY_STARTED) {
                            gl_waiting_for_response = 0;
                            rx_status = wlan_mac_low_poll_frame_rx();
                            // Check if the new reception met the conditions to cancel the already-submitted transmission
                            if (((rx_status & POLL_MAC_CANCEL_TX) != 0)) {
                                // The Rx handler killed this transmission already by resetting the MAC core
                                // Our return_status should still be considered a success -- we successfully did not
                                // transmit the beacon. This will tell the TBTT logic to move on to the next beacon interval
                                // before attempting another beacon transmission.
                                return_status = 0;
                                // We will not sent a BEACON_DONE IPC message to CPU_HIGH, so
                                // tx_frame_info->tx_pkt_buf_state should remain READY
                                tx_frame_info->tx_pkt_buf_state = TX_PKT_BUF_READY;
                                if(unlock_tx_pkt_buf(tx_pkt_buf) != PKT_BUF_MUTEX_SUCCESS){
                                    xil_printf("Error: Unable to unlock Beacon packet buffer (beacon cancel)\n");
                                }
                                wlan_mac_pause_backoff_tx_ctrl_A(0);
                                return return_status;
                            }

                        }
                    } // END if(Tx A state machine done)
                } while( mac_hw_status & WLAN_MAC_STATUS_MASK_TX_C_PENDING );

                return_status = 0;
                tx_frame_info->tx_pkt_buf_state = TX_PKT_BUF_DONE;
                if(unlock_tx_pkt_buf(tx_pkt_buf) != PKT_BUF_MUTEX_SUCCESS) {
                    xil_printf("Error: Unable to unlock Beacon packet buffer (beacon sent) %d\n", unlock_tx_pkt_buf(tx_pkt_buf));
                }

                ipc_msg_to_high.msg_id            = IPC_MBOX_MSG_ID(IPC_MBOX_TX_BEACON_DONE);
                ipc_msg_to_high.num_payload_words = sizeof(wlan_mac_low_tx_details_t)/4;
                ipc_msg_to_high.arg0              = tx_pkt_buf;
                ipc_msg_to_high.payload_ptr       = (u32*)&low_tx_details;

                write_mailbox_msg(&ipc_msg_to_high);
            }
        break;
        case TX_PKT_BUF_UNINITIALIZED:
        case TX_PKT_BUF_HIGH_CTRL:
            //  The status was set to HIGH_CTRL because CPU_HIGH is in the process of stopping beacon
            //  transmissions. If so, we should expect configure_beacon_txrx() that will prevent
            //  future calls to this function on TBTT intervals. In this case, we will return a
            //  "success" to the calling function. We successfully did not send this beacon because
            //  we were informed by CPU_HIGH that we should stop.
            return_status = 0;
        break;
        case TX_PKT_BUF_LOW_CTRL:
            xil_printf("ERROR (send_beacon): unexpected packet buffer status of TX_PKT_BUF_LOW_CTRL\n");
        case TX_PKT_BUF_DONE:
            //  CPU_HIGH is lagging behind. The previous beacon we sent is still being processed
            //  and hasn't been returned to us. We will exit this context rather than block and try
            //  again later.
            return_status = -1;
        break;
    }

    wlan_mac_pause_backoff_tx_ctrl_A(0);

    return return_status;
}

/*****************************************************************************/
/**
 * @brief Handles reception of a wireless packet
 *
 * This function is called after a good SIGNAL field is detected by either PHY (OFDM or DSSS)
 *
 * It is the responsibility of this function to wait until a sufficient number of bytes have been received
 * before it can start to process those bytes. When this function is called the eventual checksum status is
 * unknown. The packet contents can be provisionally processed (e.g. prepare an ACK for fast transmission),
 * but post-reception actions must be conditioned on the eventual FCS status (good or bad).
 *
 * NOTE: The timing of this function is critical for correct operation of the 802.11 DCF. It is not
 *     safe to add large delays to this function (e.g. xil_printf or usleep)
 *
 * Two primary job responsibilities of this function:
 *  (1): Prepare outgoing ACK packets and instruct the MAC_DCF_HW core whether or not to send ACKs
 *  (2): Pass up MPDUs (FCS valid or invalid) to CPU_HIGH
 *
 * @param   rx_pkt_buf       - Index of the Rx packet buffer containing the newly received packet
 * @param   phy_details      - Pointer to phy_rx_details struct containing PHY mode, MCS, and Length
 * @return  u32              - Bit mask of flags indicating various results of the reception
 */
u32 frame_receive(u8 rx_pkt_buf, phy_rx_details_t* phy_details) {

    // RX_LEN_THRESH is used to manage a potential pipeline bubble that can be used during a reception
    // for processing:
    //   - If the ongoing reception is >RX_LEN_THRESH, we will start
    //     processing the frame and filling in metadata into the packet
    //     buffer prior to calling wlan_mac_hw_rx_finish().
    //   - If the ongoing reception is <RX_LEN_THRESH, we'll immediately
    //     start polling the PHY with wlan_mac_hw_rx_finish() and,
    //     if need be, configure a MAC Tx core to send a response.
    //
    // This structure handles any risk of response packets (e.g. an ACK) not being configured in time
    // for the hard SIFS boundary.
    //

    int                 i;
    u32                 return_value             = 0;
    u32                 tx_length;
    u8                  tx_mcs;
    u16                 cts_duration;
    u8                  unicast_to_me, to_multicast;
    u16                 rssi;
    u8                  lna_gain;
    u8                  active_rx_ant;
    u32                 rx_filter;
    u8                  report_to_mac_high;
    int                 curr_tx_pow;
    u8                  ctrl_tx_gain;
    u32                 mac_tx_ctrl_status;
    s64                 time_delta;
    u32                 current_tu;

    u8                  mpdu_tx_ant_mask         = 0;
    u8                  ack_tx_ant               = 0;
    u8                  tx_ant_mask              = 0;
    u8                  num_resp_failures        = 0;

    rx_finish_state_t   rx_finish_state     = RX_FINISH_SEND_NONE;
    tx_pending_state_t  tx_pending_state    = TX_PENDING_NONE;

    rx_frame_info_t   * rx_frame_info;
    tx_frame_info_t   * tx_frame_info;
    mac_header_80211  * rx_header;
    u8                * mac_payload_ptr_u8;

    // Translate Rx pkt buf index into actual memory address
    void* pkt_buf_addr = (void *) RX_PKT_BUF_TO_ADDR(rx_pkt_buf);

    // Get pointer to MPDU info struct (stored at 0 offset in the pkt buffer)
    rx_frame_info = (rx_frame_info_t*) pkt_buf_addr;

    // Clear the MPDU info flags
    rx_frame_info->flags = 0;

    // Apply the mac_header_80211 template to the first bytes of the received MPDU
    rx_header = (mac_header_80211*)((void*)(pkt_buf_addr + PHY_RX_PKT_BUF_MPDU_OFFSET));
    mac_payload_ptr_u8 = (u8*)rx_header;

    // Sanity check length value - anything shorter than an ACK must be bogus
    if((phy_details->length) < (sizeof(mac_header_80211_ACK) + WLAN_PHY_FCS_NBYTES)) {
        return return_value;
    }

    // Translate the rate index into the rate code used by the Tx PHY
    //     This translation is required in case this reception needs to send an ACK, as the ACK
    //     rate is a function of the rate of the received packet
    //     The mapping of Rx rate to ACK rate is given in 9.7.6.5.2 of 802.11-2012
    //
    tx_mcs      = wlan_mac_low_mcs_to_ctrl_resp_mcs(phy_details->mcs, phy_details->phy_mode);

    // Determine which antenna the ACK will be sent from
    //     The current implementation transmits ACKs from the same antenna over which the previous packet was received
    //
    active_rx_ant = (wlan_phy_rx_get_active_rx_ant());
    tx_ant_mask   = 0;

    switch(active_rx_ant){
        case RX_ACTIVE_ANTA:  tx_ant_mask |= 0x1;  break;
        case RX_ACTIVE_ANTB:  tx_ant_mask |= 0x2;  break;
        case RX_ACTIVE_ANTC:  tx_ant_mask |= 0x4;  break;
        case RX_ACTIVE_ANTD:  tx_ant_mask |= 0x8;  break;
        default:              tx_ant_mask  = 0x1;  break;            // Default to RF_A
    }

    // Wait until the PHY has written enough bytes so that the first address field can be processed
    i = 0;
    while(wlan_mac_get_last_byte_index() < MAC_HW_LASTBYTE_ADDR1) {
        if(i++ > 1000000) {xil_printf("Stuck waiting for MAC_HW_LASTBYTE_ADDR1: wlan_mac_get_last_byte_index() = %d\n", wlan_mac_get_last_byte_index());}
    };

    // Check the destination address
    unicast_to_me = wlan_addr_eq(rx_header->address_1, gl_eeprom_addr);
    to_multicast  = wlan_addr_mcast(rx_header->address_1);

    // Prep outgoing ACK just in case it needs to be sent
    //     ACKs are only sent for non-control frames addressed to this node
    if(unicast_to_me && !WLAN_IS_CTRL_FRAME(rx_header)) {
        // Auto TX Delay is in units of 100ns. This delay runs from RXEND of the preceding reception.
        //     wlan_mac_tx_ctrl_B_params(pktBuf, antMask, req_zeroNAV, preWait_postRxTimer1, preWait_postRxTimer2, preWait_postTxTimer1, phy_mode)
        wlan_mac_tx_ctrl_B_params(TX_PKT_BUF_ACK_CTS, tx_ant_mask, 0, 1, 0, 0, PHY_MODE_NONHT);

        // ACKs are transmitted with a nominal Tx power used for all control packets
        ctrl_tx_gain = wlan_mac_low_dbm_to_gain_target(wlan_mac_low_get_current_ctrl_tx_pow());
        wlan_mac_tx_ctrl_B_gains(ctrl_tx_gain, ctrl_tx_gain, ctrl_tx_gain, ctrl_tx_gain);


        if((phy_details->length) >= MAC_HW_LASTBYTE_ADDR2){
            // Wait until the PHY has written enough bytes so that the second address field can be processed
            // If this is a short reception that does not have a second address, it is still possible to get
            // to this line of code if there is an FCS error and the WLAN_IS_CTRL_FRAME check above fails.
            // As such, we sanity check the length of the reception before getting into a potentially infinite
            // loop.
            i = 0;
            while(wlan_mac_get_last_byte_index() < MAC_HW_LASTBYTE_ADDR2) {
                if(i++ > 1000000) {xil_printf("Stuck waiting for MAC_HW_LASTBYTE_ADDR2: wlan_mac_get_last_byte_index() = %d\n", wlan_mac_get_last_byte_index());}
            };
        }

        // Construct the ACK frame in the dedicated Tx pkt buf
        tx_length = wlan_create_ack_frame((void*)(TX_PKT_BUF_TO_ADDR(TX_PKT_BUF_ACK_CTS) + PHY_TX_PKT_BUF_MPDU_OFFSET), rx_header->address_2);

        // Write the SIGNAL field for the ACK
        write_phy_preamble(TX_PKT_BUF_ACK_CTS, PHY_MODE_NONHT, tx_mcs, tx_length);

        rx_finish_state = RX_FINISH_SEND_B;

        rx_frame_info->resp_low_tx_details.tx_details_type      = TX_DETAILS_ACK;
        rx_frame_info->resp_low_tx_details.phy_params_ctrl.mcs = tx_mcs;

        // We let "duration" be equal to the duration field of an ACK. This value is provided explicitly to CPU_HIGH
        // in the low_tx_details struct such that CPU_HIGH has can reconstruct the RTS in its log. This isn't critical
        // to the operation of the DCF, but is critical for the logging framework.
        //
        rx_frame_info->resp_low_tx_details.duration = 0;

        // This element remains unused during MPDU-only transmissions
        rx_frame_info->resp_low_tx_details.phy_params_ctrl.phy_mode     = phy_details->phy_mode;
        rx_frame_info->resp_low_tx_details.phy_params_ctrl.power        = wlan_mac_low_get_current_ctrl_tx_pow();

        switch(tx_ant_mask) {
            case 0x1:  ack_tx_ant = TX_ANTMODE_SISO_ANTA; break;
            case 0x2:  ack_tx_ant = TX_ANTMODE_SISO_ANTB; break;
            case 0x4:  ack_tx_ant = TX_ANTMODE_SISO_ANTC; break;
            case 0x8:  ack_tx_ant = TX_ANTMODE_SISO_ANTD; break;
            default:   ack_tx_ant = TX_ANTMODE_SISO_ANTA; break;   // Default to RF_A
        }

        rx_frame_info->resp_low_tx_details.phy_params_ctrl.antenna_mode = ack_tx_ant;

    } else if(unicast_to_me && (rx_header->frame_control_1 == MAC_FRAME_CTRL1_SUBTYPE_CTS)){
        if(gl_mpdu_pkt_buf != PKT_BUF_INVALID) {
            // We have an outgoing data frame we should send
            //     - Configure the Tx antenna selection
            //     - The frame_transmit() context already configured the SIGNAL field,
            //       so we do not have to worry about it in this context
            //
            tx_frame_info = (tx_frame_info_t*) (TX_PKT_BUF_TO_ADDR(gl_mpdu_pkt_buf));

            switch(tx_frame_info->params.phy.antenna_mode) {
                case TX_ANTMODE_SISO_ANTA:  mpdu_tx_ant_mask |= 0x1;  break;
                case TX_ANTMODE_SISO_ANTB:  mpdu_tx_ant_mask |= 0x2;  break;
                case TX_ANTMODE_SISO_ANTC:  mpdu_tx_ant_mask |= 0x4;  break;
                case TX_ANTMODE_SISO_ANTD:  mpdu_tx_ant_mask |= 0x8;  break;
                default:                    mpdu_tx_ant_mask  = 0x1;  break;   // Default to RF_A
            }

            // Configure the Tx power - update all antennas, even though only one will be used
            curr_tx_pow = wlan_mac_low_dbm_to_gain_target(tx_frame_info->params.phy.power);
            wlan_mac_tx_ctrl_A_gains(curr_tx_pow, curr_tx_pow, curr_tx_pow, curr_tx_pow);
            wlan_mac_tx_ctrl_A_params(gl_mpdu_pkt_buf, mpdu_tx_ant_mask, 0, 1, 0, 1, tx_frame_info->params.phy.phy_mode); //Use postRx timer 1 and postTx_timer2

            rx_finish_state = RX_FINISH_SEND_A;

            return_value |= POLL_MAC_TYPE_CTS;
        } else {
            //Unexpected CTS to me.
            //This clause can execute on a bad FCS (e.g. it's actually a bad FCS ACK)
        }
    } else if(unicast_to_me && (rx_header->frame_control_1 == MAC_FRAME_CTRL1_SUBTYPE_RTS)){
        // We need to send a CTS
        //     Auto TX Delay is in units of 100ns. This delay runs from RXEND of the preceding reception.
        //     wlan_mac_tx_ctrl_B_params(pktBuf, antMask, req_zeroNAV, preWait_postRxTimer1, preWait_postRxTimer2, preWait_postTxTimer1, phy_mode)
        //
        wlan_mac_tx_ctrl_B_params(TX_PKT_BUF_ACK_CTS, tx_ant_mask, 1, 1, 0, 0, PHY_MODE_NONHT);

        // CTSs are transmitted with a nominal Tx power used for all control packets
        ctrl_tx_gain = wlan_mac_low_dbm_to_gain_target(wlan_mac_low_get_current_ctrl_tx_pow());
        wlan_mac_tx_ctrl_B_gains(ctrl_tx_gain, ctrl_tx_gain, ctrl_tx_gain, ctrl_tx_gain);

        cts_duration = sat_sub(rx_header->duration_id, (gl_mac_timing_values.t_sifs) +
                    wlan_ofdm_calc_txtime(sizeof(mac_header_80211_CTS) + WLAN_PHY_FCS_NBYTES, tx_mcs, PHY_MODE_NONHT, wlan_mac_low_get_phy_samp_rate()));

        // Construct the ACK frame in the dedicated Tx pkt buf
        tx_length = wlan_create_cts_frame((void*)(TX_PKT_BUF_TO_ADDR(TX_PKT_BUF_ACK_CTS) + PHY_TX_PKT_BUF_MPDU_OFFSET),
                                          rx_header->address_2,
                                          cts_duration);

        // Write the SIGNAL field for the CTS
        write_phy_preamble(TX_PKT_BUF_ACK_CTS, PHY_MODE_NONHT, tx_mcs, tx_length);

        rx_finish_state = RX_FINISH_SEND_B;

        rx_frame_info->resp_low_tx_details.tx_details_type     = TX_DETAILS_CTS;
        rx_frame_info->resp_low_tx_details.phy_params_ctrl.mcs = tx_mcs;

        // We let "duration" be equal to the duration field of an CTS. This value is provided explicitly to CPU_HIGH
        // in the low_tx_details struct such that CPU_HIGH has can reconstruct the RTS in its log. This isn't critical
        // to the operation of the DCF, but is critical for the logging framework.
        rx_frame_info->resp_low_tx_details.duration = cts_duration;

        // This element remains unused during MPDU-only transmissions
        rx_frame_info->resp_low_tx_details.phy_params_ctrl.phy_mode     = phy_details->phy_mode;
        rx_frame_info->resp_low_tx_details.phy_params_ctrl.power        = wlan_mac_low_get_current_ctrl_tx_pow();

        switch(tx_ant_mask) {
            case 0x1:  ack_tx_ant = TX_ANTMODE_SISO_ANTA; break;
            case 0x2:  ack_tx_ant = TX_ANTMODE_SISO_ANTB; break;
            case 0x4:  ack_tx_ant = TX_ANTMODE_SISO_ANTC; break;
            case 0x8:  ack_tx_ant = TX_ANTMODE_SISO_ANTD; break;
            default:   ack_tx_ant = TX_ANTMODE_SISO_ANTA; break;   // Default to RF_A
        }

        rx_frame_info->resp_low_tx_details.phy_params_ctrl.antenna_mode = ack_tx_ant;
    }

    // Based on the RX length threshold, determine processing order
    if((phy_details->length) <= RX_LEN_THRESH) {
        if(wlan_mac_hw_rx_finish() == 1){
            //FCS was good
            rx_frame_info->flags |= RX_FRAME_INFO_FLAGS_FCS_GOOD;
        } else {
            //FCS was bad
            rx_frame_info->flags &= ~RX_FRAME_INFO_FLAGS_FCS_GOOD;
        }

        if(rx_frame_info->flags & RX_FRAME_INFO_FLAGS_FCS_GOOD){
            switch(rx_finish_state) {
                case RX_FINISH_SEND_A:
                    wlan_mac_tx_ctrl_A_start(1);
                    wlan_mac_tx_ctrl_A_start(0);
                    tx_pending_state = TX_PENDING_A;
                break;

                case RX_FINISH_SEND_B:
                    wlan_mac_tx_ctrl_B_start(1);
                    wlan_mac_tx_ctrl_B_start(0);
                    tx_pending_state = TX_PENDING_B;
                break;

                default:
                case RX_FINISH_SEND_NONE:
                    // Do nothing
                break;
            }
        }
        rx_finish_state = RX_FINISH_SEND_NONE;
    }

    // Check if this reception is an ACK
    //TODO: we could add a unicast to me check here. It should be redundant. Then again, the POLL_MAC_TYPE_CTS does have the unicast requirement
    if((rx_header->frame_control_1) == MAC_FRAME_CTRL1_SUBTYPE_ACK){
        return_value |= POLL_MAC_TYPE_ACK;
    }

    // Update metadata about this reception
    rx_frame_info->phy_details = *phy_details;

    // This reception was a re-transmission by the other node
    if ((rx_header->frame_control_2) & MAC_FRAME_CTRL2_FLAG_RETRY) {
        rx_frame_info->flags |= RX_FRAME_INFO_FLAGS_RETRY;
    }

    // Record information about the reception in the RX packet metadata
    rx_frame_info->channel        = wlan_mac_low_get_active_channel();
    rx_frame_info->phy_samp_rate  = (u8)wlan_mac_low_get_phy_samp_rate();
    rx_frame_info->timestamp      = wlan_mac_low_get_rx_start_timestamp();
    rx_frame_info->timestamp_frac = wlan_mac_low_get_rx_start_timestamp_frac();
    rx_frame_info->ant_mode       = active_rx_ant;
    rx_frame_info->cfo_est        = wlan_phy_rx_get_cfo_est();
    rx_frame_info->rf_gain        = wlan_phy_rx_get_agc_RFG(active_rx_ant);
    rx_frame_info->bb_gain        = wlan_phy_rx_get_agc_BBG(active_rx_ant);

    lna_gain                  = wlan_phy_rx_get_agc_RFG(active_rx_ant);
    rssi                      = wlan_phy_rx_get_pkt_rssi(active_rx_ant);
    rx_frame_info->rx_power   = wlan_mac_low_calculate_rx_power(rssi, lna_gain);

    // Block until the reception is complete, storing the checksum status in the frame_info struct
    if ((phy_details->length) > RX_LEN_THRESH) {
        if(wlan_mac_hw_rx_finish() == 1){
            //FCS was good
            rx_frame_info->flags |= RX_FRAME_INFO_FLAGS_FCS_GOOD;
        } else {
            //FCS was bad
            rx_frame_info->flags &= ~RX_FRAME_INFO_FLAGS_FCS_GOOD;
        }
    }

    // Received packet had good checksum
    if(rx_frame_info->flags & RX_FRAME_INFO_FLAGS_FCS_GOOD) {
        if(unicast_to_me &&
                (gl_waiting_for_response == 0) &&
                ( (return_value & POLL_MAC_TYPE_CTS) || (return_value & POLL_MAC_TYPE_ACK) )){
            rx_frame_info->flags |= RX_FRAME_INFO_UNEXPECTED_RESPONSE;
        } else {
            rx_frame_info->flags &= ~RX_FRAME_INFO_UNEXPECTED_RESPONSE;
        }


        // Increment green LEDs
        gl_green_led_index = (gl_green_led_index + 1) % NUM_LEDS;
        userio_write_leds_green(USERIO_BASEADDR, (1<<gl_green_led_index));

        return_value |= POLL_MAC_STATUS_GOOD;

        // Check if this packet should be passed up to CPU High for further processing
        rx_filter = wlan_mac_low_get_current_rx_filter();

        switch (rx_filter & RX_FILTER_HDR_MASK) {
            default:
            case RX_FILTER_HDR_ADDR_MATCH_MPDU:
                // Non-control packet either addressed to me or addressed to multicast address
                report_to_mac_high = (unicast_to_me || to_multicast) && !WLAN_IS_CTRL_FRAME(rx_header);
            break;
            case RX_FILTER_HDR_ALL_MPDU:
                // Any non-control packet
                report_to_mac_high = !WLAN_IS_CTRL_FRAME(rx_header);
            break;
            case RX_FILTER_HDR_ALL:
                // All packets (data, management and control; no type or address filtering)
                report_to_mac_high = 1;
            break;
        }

        // Sanity check packet length - if the header says non-control but the length is shorter than a full MAC header
        // it must be invalid; this should never happen, but better to catch rare events here than corrupt state in CPU High
        if (!WLAN_IS_CTRL_FRAME(rx_header) && (phy_details->length < sizeof(mac_header_80211))) {
            report_to_mac_high = 0;
        }

        if(unicast_to_me) {
            return_value |= POLL_MAC_ADDR_MATCH;
        }

        if ((phy_details->length) > RX_LEN_THRESH) {
            switch (rx_finish_state) {
                case RX_FINISH_SEND_A:
                    wlan_mac_tx_ctrl_A_start(1);
                    wlan_mac_tx_ctrl_A_start(0);
                    tx_pending_state = TX_PENDING_A;
                break;

                case RX_FINISH_SEND_B:
                    wlan_mac_tx_ctrl_B_start(1);
                    wlan_mac_tx_ctrl_B_start(0);
                    tx_pending_state = TX_PENDING_B;
                break;

                default:
                case RX_FINISH_SEND_NONE:
                break;
            }
        }


        // Check to see if this was a beacon or probe response frame and update the MAC time if appropriate
        switch(rx_header->frame_control_1) {
            //---------------------------------------------------------------------
            case (MAC_FRAME_CTRL1_SUBTYPE_BEACON):
            case (MAC_FRAME_CTRL1_SUBTYPE_PROBE_RESP):
                // Beacon Packet / Probe Response Packet
                //   -
                //

                // If this packet was from our BSS
                if(wlan_addr_eq(gl_beacon_txrx_configure.bssid_match, rx_header->address_3)){

                    if(gl_beacon_txrx_configure.beacon_tx_mode == IBSS_BEACON_TX){
                        // Reset all state in the DCF core - this cancels deferrals and pending transmissions
                        wlan_mac_reset_tx_ctrl_C(1);
                        wlan_mac_reset_tx_ctrl_C(0);
                        return_value |= POLL_MAC_CANCEL_TX;
                    }

                    // Move the packet pointer to after the header
                    mac_payload_ptr_u8 += sizeof(mac_header_80211);

                    // Calculate the difference between the beacon timestamp and the packet timestamp
                    time_delta = (s64)(((beacon_probe_frame*)mac_payload_ptr_u8)->timestamp) - (s64)(rx_frame_info->timestamp) + gl_mac_timing_values.t_phy_rx_start_dly;

                    // Update the MAC time
                    switch(gl_beacon_txrx_configure.ts_update_mode){
                        // TODO: notify the MAC Low Framework of this change so that TBTT can be updated (if necessary)
                        case NEVER_UPDATE:
                        break;
                        case ALWAYS_UPDATE:
                            apply_mac_time_delta_usec(time_delta);
                            //handle_mactime_change(time_delta); //This call is actually not necessary here since, whether or not we adopt the
                                                                 //new MAC time, we will update next TBTT target anyway
                        break;
                        case FUTURE_ONLY_UPDATE:
                            if(time_delta > 0){
                                apply_mac_time_delta_usec(time_delta);
                                //handle_mactime_change(time_delta); //This call is actually not necessary here since, whether or not we adopt the
                                                                     //new MAC time, we will update next TBTT target anyway
                            }
                        break;
                    }

                    if(( gl_beacon_txrx_configure.beacon_tx_mode == AP_BEACON_TX ) ||
                       ( gl_beacon_txrx_configure.beacon_tx_mode == IBSS_BEACON_TX )){
                        current_tu = (u32)(get_mac_time_usec()>>10);

                        //The current_tu can be anywhere within a beacon interval, so we need
                        //to round up to the next TBTT.
                        wlan_mac_set_tu_target(gl_beacon_txrx_configure.beacon_interval_tu*((current_tu/gl_beacon_txrx_configure.beacon_interval_tu)+1));
                    }
                }

            break;
        }


    // Received checksum was bad
    } else {
        // Increment red LEDs
        gl_red_led_index = (gl_red_led_index + 1) % NUM_LEDS;
        userio_write_leds_red(USERIO_BASEADDR, (1<<gl_red_led_index));

        // Check if this packet should be passed up to CPU High for further processing
        rx_filter = wlan_mac_low_get_current_rx_filter();

        switch (rx_filter & RX_FILTER_FCS_MASK) {
            default:
            case RX_FILTER_FCS_GOOD:
                report_to_mac_high = 0;
            break;
            case RX_FILTER_FCS_ALL:
                report_to_mac_high = 1;
            break;
        }
    }

    // Wait for MAC CFG A or B to finish starting a response transmission
    switch(tx_pending_state){
        case TX_PENDING_NONE:
            // With the new CPU_LOW beacon structure, it is possible to reach this point in the code
            // while MAC Support Core A is currently pending on an unrelated MPDU. We should not wait for this
            // pending state to clear if tx_pending_state is TX_PENDING_NONE because it never will. A previous
            // version of the code relied on the fact that it was impossible for MAC Support Core A to be pending
            // At this point
        break;

        case TX_PENDING_A:

            do{
                mac_tx_ctrl_status = wlan_mac_get_tx_ctrl_status();

                if(((mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_TX_A_STATE) == WLAN_MAC_TXCTRL_STATUS_TX_A_STATE_PRE_TX_WAIT) &&
                   ((mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_POSTRX_TIMER1_RUNNING) == 0)) {
                    // This is potentially a bad state. It likely means we were late in processing this reception
                    //
                    // There is a slight race condition in detecting this state. There is a small 1 or 2 cycle window where this
                    // check can inaccurately deem a failed response transmission. As such, we'll require the condition to be met
                    // multiple times.
                    //
                    num_resp_failures++;

                    if(num_resp_failures > 2){
                        wlan_mac_reset_tx_ctrl_A(1);
                        wlan_mac_reset_tx_ctrl_A(0);

                        break;
                    }
                } else if( (mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_TX_A_STATE) == WLAN_MAC_TXCTRL_STATUS_TX_A_STATE_DO_TX ){
                    // If the PHY is actively running, we can safely quit this context and get back to frame_transmit to get
                    // ready for an ACK reception.
                    break;
                }
            } while(mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_TX_A_PENDING);

        break;

        case TX_PENDING_B:
            do{
                mac_tx_ctrl_status = wlan_mac_get_tx_ctrl_status();

                if( mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_TX_B_DONE ) {
                    if ((mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_TX_B_RESULT) == WLAN_MAC_TXCTRL_STATUS_TX_B_RESULT_NO_TX) {
                        // The MAC Support Core B has the capability of successfully not transmitting. This is not relevant
                        // for ACK transmissions, but it is relevant for CTS transmissions. A CTS will only be sent if the
                        // NAV is clear at the time of transmission. This code block handles the case the the support core
                        // elected not to transmit the frame.
                        //
                        rx_frame_info->flags = rx_frame_info->flags & ~RX_FRAME_INFO_FLAGS_CTRL_RESP_TX;
                        break;
                    }
                    if ((mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_TX_B_RESULT) == WLAN_MAC_TXCTRL_STATUS_TX_B_RESULT_DID_TX) {
                        rx_frame_info->flags |= RX_FRAME_INFO_FLAGS_CTRL_RESP_TX;
                        break;
                    }
                } else if(((mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_TX_B_STATE) == WLAN_MAC_TXCTRL_STATUS_TX_B_STATE_PRE_TX_WAIT) &&
                          ((mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_POSTRX_TIMER1_RUNNING) == 0)){

                    // This is potentially a bad state. It likely means we were late in processing this reception
                    //
                    // There is a slight race condition in detecting this state. There is a small 1 or 2 cycle window where this
                    // check can inaccurately deem a failed response transmission. As such, we'll require the condition to be met
                    // multiple times.
                    //
                    num_resp_failures++;

                    if(num_resp_failures > 2){
                        rx_frame_info->flags = rx_frame_info->flags & ~RX_FRAME_INFO_FLAGS_CTRL_RESP_TX;

                        wlan_mac_reset_tx_ctrl_B(1);
                        wlan_mac_reset_tx_ctrl_B(0);
                        break;
                    }
                }
            } while(mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_TX_B_PENDING);
        break;
    }

    if(rx_frame_info->flags & RX_FRAME_INFO_FLAGS_CTRL_RESP_TX) {
        rx_frame_info->resp_low_tx_details.tx_start_timestamp_ctrl      = wlan_mac_low_get_tx_start_timestamp();
        rx_frame_info->resp_low_tx_details.tx_start_timestamp_frac_ctrl = wlan_mac_low_get_tx_start_timestamp_frac();
    }

    // This packet should be passed up to CPU_high for further processing
    if (report_to_mac_high) {
        // Unlock the pkt buf mutex before passing the packet up
        //     If this fails, something has gone horribly wrong

        rx_frame_info->rx_pkt_buf_state = RX_PKT_BUF_READY;
        if (unlock_rx_pkt_buf(rx_pkt_buf) != PKT_BUF_MUTEX_SUCCESS) {
            xil_printf("Error: unable to unlock RX pkt_buf %d\n", rx_pkt_buf);
            wlan_mac_low_send_exception(WLAN_ERROR_CODE_CPU_LOW_RX_MUTEX);
        } else {
            wlan_mac_low_frame_ipc_send();

            // Find a free packet buffer and begin receiving packets there (blocks until free buf is found)
            wlan_mac_low_lock_empty_rx_pkt_buf();
        }
    }

    return return_value;
}



/*****************************************************************************/
/**
 * @brief Handles transmission of a wireless packet
 *
 * This function is called to transmit a new packet via the DCF+PHY. This code interacts with the wlan_mac_dcf_hw core
 * to manage MAC and PHY state. This function should be called once per packet and will return after the full transmission
 * state machine has executed for that packet. This state machine includes channel access (including carrier sensing,
 * deferrals and backoffs), ACK reception, timeouts and re-transmissions.
 *
 * This function is called once per IPC_MBOX_TX_MPDU_READY message from CPU High. The IPC_MBOX_TX_MPDU_DONE message will be
 * sent back to CPU High when this function returns.
 *
 * @param   mpdu_pkt_buf     - Index of the Tx packet buffer containing the packet to transmit
 * @param   mpdu_rate        - Index of PHY rate at which packet will be transmitted
 * @param   mpdu_length      - Number of bytes in packet, including MAC header and FCS
 * @param   low_tx_details   - Pointer to array of metadata entries to be created for each PHY transmission of this packet
 *                             (eventually leading to TX_LOW log entries)
 * @return  int              - Transmission result
 */
//int frame_transmit(u8 mpdu_pkt_buf, u8 mpdu_rate, u16 mpdu_length, wlan_mac_low_tx_details* low_tx_details) {
int frame_transmit(u8 pkt_buf, wlan_mac_low_tx_details_t* low_tx_details) {
    // The pkt_buf, rate, and length arguments provided to this function specifically relate to
    // the MPDU that the WLAN MAC LOW framework wants us to send. We may opt to first send an RTS
    // to reserve the medium prior to doing this. The tx_rate, tx_length, and tx_pkt_buf relate
    // to whatever the next waveform will be. That waveform could be an RTS, or it could be the
    // MPDU itself.

    u8  mac_cfg_mcs;
    u16 mac_cfg_length;
    u8  mac_cfg_pkt_buf;
    u8  ack_phy_mode;
    u8  ack_mcs;

    u16 rts_header_duration;
    u16 cts_header_duration;

    u8 req_timeout;

    u32 rx_status;
    u32 mac_hw_status;
    u32 mac_tx_ctrl_status;

    int curr_tx_pow;

    u32 low_tx_details_num;
    u8  tx_has_started;

    tx_wait_state_t     tx_wait_state;
    tx_mode_t           tx_mode;

    u16                 short_retry_count   = 0;
    u16                 long_retry_count    = 0;
    u16                 n_slots             = 0;
    u16                 n_slots_readback    = 0;
    u8                  mpdu_tx_ant_mask    = 0;
    tx_frame_info_t   * tx_frame_info       = (tx_frame_info_t*) (TX_PKT_BUF_TO_ADDR(pkt_buf));
    mac_header_80211  * header              = (mac_header_80211*)(TX_PKT_BUF_TO_ADDR(pkt_buf) + PHY_TX_PKT_BUF_MPDU_OFFSET);

    poll_tbtt_return_t  poll_tbtt_return = TBTT_NOT_ACHIEVED;


    // Extract waveform params from the tx_frame_info
    u8  mcs      = tx_frame_info->params.phy.mcs;
    u8  phy_mode = (tx_frame_info->params.phy.phy_mode & (PHY_MODE_HTMF | PHY_MODE_NONHT));
    u16 length   = tx_frame_info->length;

    // This state variable will inform the rest of the frame_transmit function
    // on whether the code is actively waiting for an ACK, for an RTS, or not
    // waiting for anything.
    tx_wait_state = TX_WAIT_NONE;

    tx_frame_info->num_tx_attempts   = 0;
    tx_frame_info->phy_samp_rate      = (u8)wlan_mac_low_get_phy_samp_rate();

    // Compare the length of this frame to the RTS Threshold
    if(length <= gl_dot11RTSThreshold) {
        tx_mode = TX_MODE_SHORT;
    } else {
        tx_mode = TX_MODE_LONG;
    }


    if((tx_frame_info->flags) & TX_FRAME_INFO_FLAGS_FILL_DURATION){
        // ACK_N_DBPS is used to calculate duration of the ACK waveform which might be received in response to this transmission
        //  The ACK duration is used to calculate the DURATION field in the MAC header
        //  The selection of ACK rate for a given DATA rate is specified in IEEE 802.11-2012 9.7.6.5.2
        ack_mcs = wlan_mac_low_mcs_to_ctrl_resp_mcs(tx_frame_info->params.phy.mcs, tx_frame_info->params.phy.phy_mode);
        ack_phy_mode = PHY_MODE_HTMF;

        // Compute and fill in the duration of any time-on-air following this packet's transmission
        //     For DATA Tx, DURATION = T_SIFS + T_ACK, where T_ACK is function of the ACK Tx rate
        header->duration_id = wlan_ofdm_calc_txtime(sizeof(mac_header_80211_ACK) + WLAN_PHY_FCS_NBYTES, ack_mcs, ack_phy_mode, wlan_mac_low_get_phy_samp_rate()) + gl_mac_timing_values.t_sifs;
    }


    // Retry loop
    while(1) {
        tx_has_started = 0;

        (tx_frame_info->num_tx_attempts)++;

        // Check if the higher-layer MAC requires this transmission have a post-Tx timeout
        req_timeout = ((tx_frame_info->flags) & TX_FRAME_INFO_FLAGS_REQ_TO) != 0;

        // Write the SIGNAL field (interpreted by the PHY during Tx waveform generation)
        // This is the SIGNAL field for the MPDU we will eventually transmit. It's possible
        // the next waveform we send will be an RTS with its own independent SIGNAL

        //wlan_phy_set_tx_signal(mpdu_pkt_buf, mpdu_rate, mpdu_length);
        write_phy_preamble(pkt_buf, phy_mode, mcs, length);


        if ((tx_mode == TX_MODE_LONG) && (req_timeout == 1)) {
            // This is a long MPDU that requires an RTS/CTS handshake prior to the MPDU transmission.
            tx_wait_state   = TX_WAIT_CTS;

            // This is a global pkt_buf index that can be seen by the frame_receive() context.
            // frame_receive() needs this to figure out what to send in the event that it receives
            // a valid CTS.
            gl_mpdu_pkt_buf = pkt_buf;

            mac_cfg_pkt_buf = TX_PKT_BUF_RTS;

            // The rate given to us in the argument of frame_transmit applies to the MPDU. Several
            // elements depend on this rate:
            //
            // 1) The rate of the RTS we will send (fixed NONHT phy mode for CTRL response)
            // 2) The rate of the CTS we expect to receive (fixed NONHT phy mode for CTRL response)
            // 3) The duration of the RTS/CTS/DATA frames a long with the IFS periods between them
            //
            // The below switch() sets these elements accordingly.
            //
            switch (mcs) {
                default:
                case 0:
                    mac_cfg_mcs         = 0;
                    cts_header_duration = TX_TIME_CTS_R6;
                    low_tx_details[(tx_frame_info->num_tx_attempts) - 1].phy_params_ctrl.mcs = 0;
                break;
                case 1:
                    mac_cfg_mcs         = 0;
                    cts_header_duration = TX_TIME_CTS_R6;
                    low_tx_details[(tx_frame_info->num_tx_attempts) - 1].phy_params_ctrl.mcs = 0;
                break;
                case 2:
                    mac_cfg_mcs         = 2;
                    cts_header_duration = TX_TIME_CTS_R12;
                    low_tx_details[(tx_frame_info->num_tx_attempts) - 1].phy_params_ctrl.mcs = 2;
                break;
                case 3:
                    mac_cfg_mcs         = 2;
                    cts_header_duration = TX_TIME_CTS_R12;
                    low_tx_details[(tx_frame_info->num_tx_attempts) - 1].phy_params_ctrl.mcs = 2;
                break;
                case 4:
                    mac_cfg_mcs         = 4;
                    cts_header_duration = TX_TIME_CTS_R24;
                    low_tx_details[(tx_frame_info->num_tx_attempts) - 1].phy_params_ctrl.mcs = 4;
                break;
                case 5:
                    mac_cfg_mcs         = 4;
                    cts_header_duration = TX_TIME_CTS_R24;
                    low_tx_details[(tx_frame_info->num_tx_attempts) - 1].phy_params_ctrl.mcs = 4;
                break;
                case 6:
                    mac_cfg_mcs         = 4;
                    cts_header_duration = TX_TIME_CTS_R24;
                    low_tx_details[(tx_frame_info->num_tx_attempts) - 1].phy_params_ctrl.mcs = 4;
                break;
                case 7:
                    mac_cfg_mcs         = 4;
                    cts_header_duration = TX_TIME_CTS_R24;
                    low_tx_details[(tx_frame_info->num_tx_attempts) - 1].phy_params_ctrl.mcs = 4;
                break;
            }

            rts_header_duration = (gl_mac_timing_values.t_sifs) + cts_header_duration +
                                  (gl_mac_timing_values.t_sifs) + wlan_ofdm_calc_txtime(length, tx_frame_info->params.phy.mcs, tx_frame_info->params.phy.phy_mode, wlan_mac_low_get_phy_samp_rate()) +
                                  header->duration_id;

            // We let "duration" be equal to the duration field of an RTS. This value is provided explicitly to CPU_HIGH
            // in the low_tx_details struct such that CPU_HIGH has can reconstruct the RTS in its log. This isn't critical
            // to the operation of the DCF, but is critical for the logging framework.
            low_tx_details[(tx_frame_info->num_tx_attempts) - 1].duration = rts_header_duration;

            // Construct the RTS frame in the dedicated Tx pkt buf for control frames
            mac_cfg_length = wlan_create_rts_frame((void*)(TX_PKT_BUF_TO_ADDR(TX_PKT_BUF_RTS) + PHY_TX_PKT_BUF_MPDU_OFFSET),
                                                   header->address_1,
                                                   header->address_2,
                                                   rts_header_duration);

            // Write SIGNAL for RTS
            //wlan_phy_set_tx_signal(mac_cfg_pkt_buf, mac_cfg_rate, mac_cfg_length);
            write_phy_preamble(mac_cfg_pkt_buf, PHY_MODE_NONHT, mac_cfg_mcs, mac_cfg_length);

        } else if((tx_mode == TX_MODE_SHORT) && (req_timeout == 1)) {
            // Unicast, no RTS
            tx_wait_state   = TX_WAIT_ACK;
            mac_cfg_mcs     = mcs;
            mac_cfg_length  = length;
            mac_cfg_pkt_buf = pkt_buf;
        } else {
            // Multicast, short or long
            tx_wait_state   = TX_WAIT_NONE;
            mac_cfg_mcs     = mcs;
            mac_cfg_length  = length;
            mac_cfg_pkt_buf = pkt_buf;
        }

        // Configure the Tx antenna selection
        mpdu_tx_ant_mask    = 0;

        switch(tx_frame_info->params.phy.antenna_mode) {
            case TX_ANTMODE_SISO_ANTA:  mpdu_tx_ant_mask |= 0x1;  break;
            case TX_ANTMODE_SISO_ANTB:  mpdu_tx_ant_mask |= 0x2;  break;
            case TX_ANTMODE_SISO_ANTC:  mpdu_tx_ant_mask |= 0x4;  break;
            case TX_ANTMODE_SISO_ANTD:  mpdu_tx_ant_mask |= 0x8;  break;
            default:                    mpdu_tx_ant_mask  = 0x1;  break;       // Default to RF_A
        }

        // Configure the Tx power - update all antennas, even though only one will be used
        curr_tx_pow = wlan_mac_low_dbm_to_gain_target(tx_frame_info->params.phy.power);
        wlan_mac_tx_ctrl_A_gains(curr_tx_pow, curr_tx_pow, curr_tx_pow, curr_tx_pow);

        if ((tx_frame_info->num_tx_attempts) == 1) {
            // This is the first transmission, so we speculatively draw a backoff in case
            // the backoff counter is currently 0 but the medium is busy. Prior to all other
            // (re)transmissions, an explicit backoff will have been started at the end of
            // the previous iteration of this loop.
            //
            n_slots = rand_num_slots(RAND_SLOT_REASON_STANDARD_ACCESS);



            // Configure the DCF core Tx state machine for this transmission
            // wlan_mac_tx_ctrl_A_params(pktBuf, antMask, preTx_backoff_slots, preWait_postRxTimer1, preWait_postTxTimer1, postWait_postTxTimer2, phy_mode)
            wlan_mac_tx_ctrl_A_params(mac_cfg_pkt_buf, mpdu_tx_ant_mask, n_slots, 0, 0, req_timeout, phy_mode);

        } else {
            // This is a retry. We will inherit whatever backoff that is currently running.
            // Configure the DCF core Tx state machine for this transmission
            // preTx_backoff_slots is 0 here, since the core will have started a post-timeout backoff automatically
            wlan_mac_tx_ctrl_A_params(mac_cfg_pkt_buf, mpdu_tx_ant_mask, 0, 0, 0, req_timeout, phy_mode);
        }

        // Wait for the Tx PHY to be idle
        // Actually waiting here is rare, but handles corner cases like a background ACK transmission at a low rate
        // overlapping the attempt to start a new packet transmission
        do{
            mac_hw_status = wlan_mac_get_status();
        } while(mac_hw_status & WLAN_MAC_STATUS_MASK_TX_PHY_ACTIVE);

        // Submit the MPDU for transmission - this starts the MAC hardware's MPDU Tx state machine
        wlan_mac_tx_ctrl_A_start(1);
        wlan_mac_tx_ctrl_A_start(0);

        // Immediately re-read the current slot count.
        n_slots_readback = wlan_mac_get_backoff_count_A();

        // While waiting, fill in the metadata about this transmission attempt, to be used by CPU High in creating TX_LOW log entries
        // The phy_params (as opposed to phy_params2) element is used for the MPDU itself. If we are waiting for a CTS and we do not
        // receive one, CPU_HIGH will know to ignore this element of low_tx_details (since the MPDU will not be transmitted).
        low_tx_details_num = (tx_frame_info->num_tx_attempts) - 1;

        if((low_tx_details_num == 0) && (n_slots != n_slots_readback)){
            // For the first transmission (non-retry) of an MPDU, the number of
            // slots used by the backoff process is ambiguous. The n_slots we provided
            // to wlan_mac_tx_ctrl_A_params is only a suggestion. If the medium has been
            // idle for a DIFS, then there will not be a backoff. Or, if another backoff is
            // currently running, the MAC Config Core A will inherit that backoff. By
            // immediately reading back the slot count after starting the core, we can
            // overwrite the number of slots that we will fill into low_tx_details with
            // the correct value
            n_slots = n_slots_readback;
        }


        low_tx_details[low_tx_details_num].flags                        = 0;
        low_tx_details[low_tx_details_num].phy_params_mpdu.mcs          = tx_frame_info->params.phy.mcs;
        low_tx_details[low_tx_details_num].phy_params_mpdu.phy_mode     = tx_frame_info->params.phy.phy_mode;
        low_tx_details[low_tx_details_num].phy_params_mpdu.power        = tx_frame_info->params.phy.power;
        low_tx_details[low_tx_details_num].phy_params_mpdu.antenna_mode = tx_frame_info->params.phy.antenna_mode;

        // If RTS/CTS isn't used, these fields should just be ignored
        low_tx_details[low_tx_details_num].phy_params_ctrl.power        = tx_frame_info->params.phy.power;
        low_tx_details[low_tx_details_num].phy_params_ctrl.antenna_mode = tx_frame_info->params.phy.antenna_mode;

        low_tx_details[low_tx_details_num].chan_num    = wlan_mac_low_get_active_channel();
        low_tx_details[low_tx_details_num].cw          = (1 << gl_cw_exp)-1; //(2^(gl_cw_exp) - 1)
        low_tx_details[low_tx_details_num].ssrc        = gl_stationShortRetryCount;
        low_tx_details[low_tx_details_num].slrc        = gl_stationLongRetryCount;
        low_tx_details[low_tx_details_num].src         = short_retry_count;
        low_tx_details[low_tx_details_num].lrc         = long_retry_count;

        // NOTE: the pre-Tx backoff may not occur for the initial transmission attempt. If the medium has been idle for >DIFS when
        // the first Tx occurs the DCF state machine will not start a backoff. The upper-level MAC should compare the num_slots value
        // to the time delta between the accept and start times of the first transmission to determine whether the pre-Tx backoff
        // actually occurred.
        low_tx_details[low_tx_details_num].num_slots   = n_slots;

        // Wait for the MPDU Tx to finish
        do { // while(tx_status & WLAN_MAC_STATUS_MASK_TX_A_PENDING)

            // Poll the DCF core status register
            mac_hw_status = wlan_mac_get_status();

            // Fill in the timestamp if indicated by the flags, only possible after Tx PHY has started
            if (mac_hw_status & WLAN_MAC_STATUS_MASK_TX_PHY_ACTIVE) {

                tx_has_started = 1;

                if(req_timeout){
                    gl_waiting_for_response = 1;
                }

                if ((tx_frame_info->flags) & TX_FRAME_INFO_FLAGS_FILL_TIMESTAMP) {
                    // Insert the TX START timestamp
                    *((u32*)((u8*)header + 24)) =  Xil_In32(WLAN_MAC_REG_TX_TIMESTAMP_LSB);
                    *((u32*)((u8*)header + 28)) =  Xil_In32(WLAN_MAC_REG_TX_TIMESTAMP_MSB);
                }
            }



            // Transmission is complete
            if( mac_hw_status & WLAN_MAC_STATUS_MASK_TX_A_DONE ) {
                if(tx_wait_state == TX_WAIT_CTS) {
                    // This will potentially be overwritten with TX_DETAILS_RTS_MPDU should we make it that far.
                    low_tx_details[low_tx_details_num].tx_details_type  = TX_DETAILS_RTS_ONLY;
                    low_tx_details[low_tx_details_num].tx_start_timestamp_ctrl = wlan_mac_low_get_tx_start_timestamp();
                    low_tx_details[low_tx_details_num].tx_start_timestamp_frac_ctrl = wlan_mac_low_get_tx_start_timestamp_frac();

                } else if ((tx_mode == TX_MODE_LONG) && (tx_wait_state == TX_WAIT_ACK)) {
                    // NOTE: this clause will overwrite the previous TX_DETAILS_RTS_ONLY state in the event a CTS is received.
                    low_tx_details[low_tx_details_num].tx_details_type  = TX_DETAILS_RTS_MPDU;
                    low_tx_details[low_tx_details_num].tx_start_timestamp_mpdu = wlan_mac_low_get_tx_start_timestamp();
                    low_tx_details[low_tx_details_num].tx_start_timestamp_frac_mpdu = wlan_mac_low_get_tx_start_timestamp_frac();

                } else {
                    // This is a non-RTS/CTS-protected MPDU transmission
                    low_tx_details[low_tx_details_num].tx_details_type  = TX_DETAILS_MPDU;
                    low_tx_details[low_tx_details_num].tx_start_timestamp_mpdu = wlan_mac_low_get_tx_start_timestamp();
                    low_tx_details[low_tx_details_num].tx_start_timestamp_frac_mpdu = wlan_mac_low_get_tx_start_timestamp_frac();
                }


                // Switch on the result of the transmission attempt
                //  Safe to read tx_ctrl_status here - TX_A_RESULT is only valid after TX_A_DONE asserts, which just happened
                mac_tx_ctrl_status = wlan_mac_get_tx_ctrl_status();
                switch (mac_tx_ctrl_status & WLAN_MAC_TXCTRL_STATUS_MASK_TX_A_RESULT) {

                    //---------------------------------------------------------------------
                    case WLAN_MAC_TXCTRL_STATUS_TX_A_RESULT_NONE:
                        // Transmission was immediately successful - this implies no post-Tx timeout was required,
                        // so the core didn't wait for any post-Tx receptions (i.e. multicast/broadcast transmission)
                        //
                        switch(tx_mode) {
                            case TX_MODE_SHORT:
                                reset_ssrc();
                                reset_cw();
                            break;
                            case TX_MODE_LONG:
                                reset_slrc();
                                reset_cw();
                            break;
                        }

                        // Start a post-Tx backoff using the updated contention window
                        n_slots = rand_num_slots(RAND_SLOT_REASON_STANDARD_ACCESS);
                        wlan_mac_dcf_hw_start_backoff(n_slots);
                        gl_waiting_for_response = 0;
                        return 0;
                    break;

                    //---------------------------------------------------------------------
                    case WLAN_MAC_TXCTRL_STATUS_TX_A_RESULT_RX_STARTED:
                        // Transmission ended, followed by a new reception (hopefully a CTS or ACK)

                        // Handle the new reception
                        rx_status       = wlan_mac_low_poll_frame_rx();
                        gl_waiting_for_response = 0;

                        gl_mpdu_pkt_buf = PKT_BUF_INVALID;

                        // Check if the reception is an ACK addressed to this node, received with a valid checksum
                        if ((tx_wait_state == TX_WAIT_CTS) &&
                            (rx_status & POLL_MAC_STATUS_RECEIVED_PKT) &&
                            (rx_status & POLL_MAC_TYPE_CTS) &&
                            (rx_status & POLL_MAC_STATUS_GOOD) &&
                            (rx_status & POLL_MAC_ADDR_MATCH)) {

                            low_tx_details[low_tx_details_num].flags |= TX_DETAILS_FLAGS_RECEIVED_RESPONSE;

                            tx_wait_state = TX_WAIT_ACK;

                            // We received the CTS, so we can reset our SSRC
                            //     NOTE: as per 9.3.3 of 802.11-2012, we do not reset our CW
                            //
                            reset_ssrc();

                            // At this point, the MAC tx state machine has started anew to send a the MPDU itself.
                            // This was triggered by the frame_receive() context.  We know that the frame_receive context
                            // has started the transmission of the MPDU.  This ensures we are not kicked out of the
                            // do-while loop.
                            //
                            // NOTE: This assignment is better than re-reading wlan_mac_get_status() in the case of a short
                            // MPDU, where we may skip the PENDING state directly to DONE without this code context seeing it.
                            //
                            mac_hw_status |= WLAN_MAC_STATUS_MASK_TX_A_PENDING;

                            continue;

                        } else if ((tx_wait_state == TX_WAIT_ACK) &&
                                   (rx_status & POLL_MAC_STATUS_RECEIVED_PKT) &&
                                   (rx_status & POLL_MAC_TYPE_ACK) &&
                                   (rx_status & POLL_MAC_STATUS_GOOD) &&
                                   (rx_status & POLL_MAC_ADDR_MATCH)) {

                                low_tx_details[low_tx_details_num].flags |= TX_DETAILS_FLAGS_RECEIVED_RESPONSE;

                                // Update contention window
                                switch(tx_mode) {
                                    case TX_MODE_SHORT:
                                        reset_ssrc();
                                        reset_cw();
                                    break;
                                    case TX_MODE_LONG:
                                        reset_slrc();
                                        reset_cw();
                                    break;
                                }

                                // Start a post-Tx backoff using the updated contention window
                                n_slots = rand_num_slots(RAND_SLOT_REASON_STANDARD_ACCESS);
                                wlan_mac_dcf_hw_start_backoff(n_slots);
                                return TX_FRAME_INFO_RESULT_SUCCESS;

                        } else {
                            // Received a packet immediately after transmitting, but it wasn't the ACK we wanted
                            // It could have been our ACK with a bad checksum or a different packet altogether
                            switch(tx_wait_state) {
                                case TX_WAIT_ACK:
                                    // We were waiting for an ACK
                                    //   - Depending on the size of the MPDU, we will increment either the SRC or the LRC
                                    //
                                    header->frame_control_2 = (header->frame_control_2) | MAC_FRAME_CTRL2_FLAG_RETRY;

                                    switch(tx_mode) {
                                        case TX_MODE_SHORT:
                                            increment_src(&short_retry_count);
                                        break;
                                        case TX_MODE_LONG:
                                            increment_lrc(&long_retry_count);
                                        break;
                                    }
                                break;

                                case TX_WAIT_CTS:
                                    // We were waiting for a CTS but did not get it.
                                    //     - Increment the SRC
                                    //
                                    increment_src(&short_retry_count);
                                break;

                                case TX_WAIT_NONE:
                                    xil_printf("Error: unexpected state");
                                break;
                            }

                            // Start the post-Tx backoff
                            n_slots = rand_num_slots(RAND_SLOT_REASON_STANDARD_ACCESS);
                            wlan_mac_dcf_hw_start_backoff(n_slots);
                            // Now we evaluate the SRC and LRC to see if either has reached its maximum
                            //     NOTE:  Use >= here to handle unlikely case of retryLimit values changing mid-Tx
                            if ((short_retry_count >= gl_dot11ShortRetryLimit) ||
                                (long_retry_count >= gl_dot11LongRetryLimit )) {
                                gl_waiting_for_response = 0;
                                return TX_FRAME_INFO_RESULT_FAILURE;
                            }
                            if(poll_tbtt_return == BEACON_DEFERRED) {
                                poll_tbtt_return = poll_tbtt();
                            }
                            // Jump to next loop iteration
                            continue;
                        }
                    break;

                    //---------------------------------------------------------------------
                    case WLAN_MAC_TXCTRL_STATUS_TX_A_RESULT_TIMEOUT:
                        // Tx required timeout, timeout expired with no receptions
                        gl_waiting_for_response = 0;

                        gl_mpdu_pkt_buf = PKT_BUF_INVALID;

                        switch (tx_wait_state) {
                            case TX_WAIT_ACK:
                                // We were waiting for an ACK
                                //     - Depending on the size of the MPDU, we will increment either the SRC or the LRC
                                //
                                header->frame_control_2 = (header->frame_control_2) | MAC_FRAME_CTRL2_FLAG_RETRY;

                                switch(tx_mode){
                                    case TX_MODE_SHORT:
                                        increment_src(&short_retry_count);
                                    break;
                                    case TX_MODE_LONG:
                                        increment_lrc(&long_retry_count);
                                    break;
                                }
                            break;

                            case TX_WAIT_CTS:
                                // We were waiting for a CTS but did not get it.
                                //     - Increment the SRC
                                increment_src(&short_retry_count);
                            break;

                            case TX_WAIT_NONE:
                                xil_printf("Error: unexpected state");
                            break;
                        }

                        // Start the post-Tx backoff
                        n_slots = rand_num_slots(RAND_SLOT_REASON_STANDARD_ACCESS);
                        wlan_mac_dcf_hw_start_backoff(n_slots);

                        // Now we evaluate the SRC and LRC to see if either has reached its maximum
                        if ((short_retry_count == gl_dot11ShortRetryLimit) ||
                            (long_retry_count  == gl_dot11LongRetryLimit )) {
                            return TX_FRAME_INFO_RESULT_FAILURE;
                        }
                        if(poll_tbtt_return == BEACON_DEFERRED) {
                            poll_tbtt_return = poll_tbtt();
                        }
                        // Jump to next loop iteration
                        continue;
                    break;
                }

            // else for if(mac_hw_status & WLAN_MAC_STATUS_MASK_TX_A_DONE)
            } else if (tx_has_started == 0) {
                //  This is the same MAC status check performed in the framework wlan_mac_low_poll_frame_rx()
                //  It is critical to check the Rx status here using the same status register read that was
                //  used in the Tx state checking above. Skipping this and calling wlan_mac_low_poll_frame_rx()
                //  directly leads to a race between the Tx status checking above and Rx status checking
                if (mac_hw_status & WLAN_MAC_STATUS_MASK_RX_PHY_STARTED) {
                    gl_waiting_for_response = 0;
                    rx_status = wlan_mac_low_poll_frame_rx();
                } else if(poll_tbtt_return != BEACON_DEFERRED) {
                    poll_tbtt_return = poll_tbtt();
                }
            } // END if(Tx A state machine done)
        } while( mac_hw_status & WLAN_MAC_STATUS_MASK_TX_A_PENDING );
    } // end retransmission loop
    gl_waiting_for_response = 0;
    return 0;
}



/*****************************************************************************/
/**
 * @brief Increment Short Retry Count
 *
 * This function increments the short retry count. According to Section 9.3.3
 * of 802.11-2012, incrementing the short retry count also causes the
 * the following:
 *      1) An increment of the station short retry count
 *      2) An increase of the contention window (which is technically dependent
 *         on the station count incremented in the first step)
 *
 * @param   src_ptr          - Pointer to short retry count
 * @return  None
 */
inline void increment_src(u16* src_ptr){
    // Increment the Short Retry Count
    (*src_ptr)++;

    gl_stationShortRetryCount = sat_add32(gl_stationShortRetryCount, 1);

    if (gl_stationShortRetryCount == gl_dot11ShortRetryLimit) {
        reset_cw();
    } else {
        gl_cw_exp = min(gl_cw_exp + 1, gl_cw_exp_max);
    }
}



/*****************************************************************************/
/**
 * @brief Increment Long Retry Count
 *
 * This function increments the long retry count. According to Section 9.3.3
 * of 802.11-2012, incrementing the long retry count also causes the
 * the following:
 *      1) An increment of the station long retry count
 *      2) An increase of the contention window (which is technically dependent
 *         on the station count incremented in the first step)
 *
 * @param   src_ptr          - Pointer to long retry count
 * @return  None
 */
inline void increment_lrc(u16* lrc_ptr){
    // Increment the Long Retry Count
    (*lrc_ptr)++;

    gl_stationLongRetryCount = sat_add32(gl_stationLongRetryCount, 1);

    if(gl_stationLongRetryCount == gl_dot11LongRetryLimit){
        reset_cw();
    } else {
        gl_cw_exp = min(gl_cw_exp + 1, gl_cw_exp_max);
    }
}



/*****************************************************************************/
/**
 * @brief Reset Station Short Retry Count
 *
 * @param   None
 * @return  None
 *
 * @note    Resetting the SSRC does not necessarily indicate that the contention window should be reset.
 *     e.g., the reception of a valid CTS.
 */
inline void reset_ssrc(){
    gl_stationShortRetryCount = 0;
}



/*****************************************************************************/
/**
 * @brief Reset Station Long Retry Count
 *
 * @param   None
 * @return  None
 */
inline void reset_slrc(){
    gl_stationLongRetryCount = 0;
}



/*****************************************************************************/
/**
 * @brief Reset Contention Window
 *
 * @param   None
 * @return  None
 */
inline void reset_cw(){
    gl_cw_exp = gl_cw_exp_min;
}



/*****************************************************************************/
/**
 * @brief Generate a random number in the range set by the current contention window
 *
 * When reason is RAND_SLOT_REASON_IBSS_BEACON the random draw is taken from the range
 * [0, 2*CWmin], used for pre-beacon backoffs in IBSS (per 802.11-2012 10.1.3.3)
 *
 * @param   reason           - Code for the random draw; must be RAND_SLOT_REASON_STANDARD_ACCESS or RAND_SLOT_REASON_IBSS_BEACON
 * @return  u32              - Random integer based on reason
 */
inline u32 rand_num_slots(u8 reason){
    // Generates a uniform random value between [0, (2^(gl_cw_exp) - 1)], where gl_cw_exp is a positive integer
    // This function assumed RAND_MAX = 2^31.
    // | gl_cw_exp |    CW       |
    // |     4     |  [0,   15]  |
    // |     5     |  [0,   31]  |
    // |     6     |  [0,   63]  |
    // |     7     |  [0,  123]  |
    // |     8     |  [0,  255]  |
    // |     9     |  [0,  511]  |
    // |    10     |  [0, 1023]  |
    //
    volatile u32 n_slots;

    switch(reason) {
        case RAND_SLOT_REASON_STANDARD_ACCESS:
            n_slots = ((unsigned int)rand() >> (32 - (gl_cw_exp + 1)));
        break;

        case RAND_SLOT_REASON_IBSS_BEACON:
            // Section 10.1.3.3 of 802.11-2012: Backoffs prior to IBSS beacons are drawn from [0, 2*CWmin]
            n_slots = ((unsigned int)rand() >> (32 - (gl_cw_exp_min + 1 + 1)));
        break;
    }

    return n_slots;
}



/*****************************************************************************/
/**
 * @brief Start a backoff
 *
 * This function will start a backoff.  If a backoff is already running, the backoff-start attempt
 * will be safely ignored and the function will do nothing.
 *
 * @param   num_slots        - Duration of backoff interval, in units of slots
 * @return  None
 */
void wlan_mac_dcf_hw_start_backoff(u16 num_slots) {
    // WLAN_MAC_REG_SW_BACKOFF_CTRL:
    //     b[15:0] : Num slots
    //     b[31]   : Start backoff

    // Write num_slots and toggle start
    Xil_Out32(WLAN_MAC_REG_SW_BACKOFF_CTRL, (num_slots & 0xFFFF) | 0x80000000);
    Xil_Out32(WLAN_MAC_REG_SW_BACKOFF_CTRL, (num_slots & 0xFFFF));
}



/*****************************************************************************/
/**
 * @brief Construct an ACK frame
 *
 * @param   pkt_buf_addr     - Address of Tx packet buffer where to construct new ACK packet
 * @param   address_ra       - Pointer to 6-byte MAC address of receiving node
 * @return  int              - Number of bytes in the frame
 */
int wlan_create_ack_frame(void* pkt_buf_addr, u8* address_ra) {

    mac_header_80211_ACK* ack_header;

    ack_header = (mac_header_80211_ACK*)(pkt_buf_addr);

    ack_header->frame_control_1 = MAC_FRAME_CTRL1_SUBTYPE_ACK;
    ack_header->frame_control_2 = 0;
    ack_header->duration_id     = 0;

    memcpy(ack_header->address_ra, address_ra, 6);

    // Include FCS in packet size (MAC accounts for FCS, even though the PHY calculates it)
    return (sizeof(mac_header_80211_ACK) + WLAN_PHY_FCS_NBYTES);
}



/*****************************************************************************/
/**
 * @brief Construct a CTS frame
 *
 * @param   pkt_buf_addr     - Address of Tx packet buffer where to construct new ACK packet
 * @param   address_ra       - Pointer to 6-byte MAC address of receiving node
 * @param   duration         - Duration of the CTS
 * @return  int              - Number of bytes in the frame
 */
int wlan_create_cts_frame(void* pkt_buf_addr, u8* address_ra, u16 duration) {

    mac_header_80211_CTS* cts_header;

    cts_header = (mac_header_80211_CTS*)(pkt_buf_addr);

    cts_header->frame_control_1 = MAC_FRAME_CTRL1_SUBTYPE_CTS;
    cts_header->frame_control_2 = 0;
    cts_header->duration_id     = duration;

    memcpy(cts_header->address_ra, address_ra, 6);

    // Include FCS in packet size (MAC accounts for FCS, even though the PHY calculates it)
    return (sizeof(mac_header_80211_CTS) + WLAN_PHY_FCS_NBYTES);
}



/*****************************************************************************/
/**
 * @brief Construct an RTS frame
 *
 * @param   pkt_buf_addr     - Address of Tx packet buffer where to construct new ACK packet
 * @param   address_ra       - Pointer to 6-byte MAC address of receiving node
 * @param   address_ta       - Pointer to 6-byte MAC address of transmitting node
 * @param   duration         - Duration of the RTS
 * @return  int              - Number of bytes in the frame
 */
int wlan_create_rts_frame(void* pkt_buf_addr, u8* address_ra, u8* address_ta, u16 duration) {

    mac_header_80211_RTS* rts_header;

    rts_header = (mac_header_80211_RTS*)(pkt_buf_addr);

    rts_header->frame_control_1 = MAC_FRAME_CTRL1_SUBTYPE_RTS;
    rts_header->frame_control_2 = 0;
    rts_header->duration_id     = duration;

    memcpy(rts_header->address_ra, address_ra, MAC_ADDR_LEN);
    memcpy(rts_header->address_ta, address_ta, MAC_ADDR_LEN);

    // Include FCS in packet size (MAC accounts for FCS, even though the PHY calculates it)
    return (sizeof(mac_header_80211_RTS) + WLAN_PHY_FCS_NBYTES);
}



/*****************************************************************************/
/**
 * @brief Process DCF Low Parameters
 *
 * This method is part of the IPC_MBOX_LOW_PARAM parameter processing in the low framework.  It
 * will process DCF specific low parameters.
 *
 * @param   mode             - Mode to process parameter:  IPC_REG_WRITE_MODE or IPC_REG_READ_MODE
 * @param   payload          - Pointer to parameter and arguments
 * @return  int              - Status
 */
int process_low_param(u8 mode, u32* payload){

    switch(mode){
        case IPC_REG_WRITE_MODE: {
            switch(payload[0]){

                //---------------------------------------------------------------------
                case LOW_PARAM_DCF_PHYSICAL_CS_THRESH: {
                    if(payload[1] < 1023){
                        wlan_phy_rx_set_cca_thresh(payload[1] * PHY_RX_RSSI_SUM_LEN);
                    } else {
                        wlan_phy_rx_set_cca_thresh(0xFFFF);
                    }
                }
                break;

                //---------------------------------------------------------------------
                case LOW_PARAM_DCF_RTS_THRESH: {
                    gl_dot11RTSThreshold = payload[1];
                }
                break;

                //---------------------------------------------------------------------
                case LOW_PARAM_DCF_DOT11SHORTRETRY: {
                    gl_dot11ShortRetryLimit = payload[1];
                }
                break;

                //---------------------------------------------------------------------
                case LOW_PARAM_DCF_DOT11LONGRETRY: {
                    gl_dot11LongRetryLimit = payload[1];
                }
                break;

                //---------------------------------------------------------------------
                case LOW_PARAM_DCF_CW_EXP_MIN: {
                    gl_cw_exp_min = payload[1];
                }
                break;

                //---------------------------------------------------------------------
                case LOW_PARAM_DCF_CW_EXP_MAX: {
                    gl_cw_exp_max = payload[1];
                }
                break;

                //---------------------------------------------------------------------
                default: {
                    xil_printf("Unknown param 0x%08x\n", payload[0]);
                }
                break;
            }
        }
        break;

        case IPC_REG_READ_MODE: {
            // Not supported.  See comment in wlan_mac_low.c for IPC_REG_READ_MODE mode.
        }
        break;

        default: {
            xil_printf("Unknown mode 0x%08x\n", mode);
        }
        break;
    }

    return 0;
}