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#1 2017-Jul-09 22:24:10

Registered: 2017-Apr-05
Posts: 23

Beacon in ap

I research Handshakes in Reference design. I understand it in MPDU RX . But i don't know how to beacon send, when and where ap send beacon in reference design.

Thanks !



#2 2017-Jul-10 07:06:29

From: Mango Communications
Registered: 2006-Aug-24
Posts: 1212

Re: Beacon in ap

Good question, beacons are more complicated than other frames because the 802.11 standard dictates that a beacon must be the next transmission following a Target Beacon Transmission Time (TBTT). This means that a beacon actually gets to jump in front of an ongoing retransmission of a different frame if the TBTT boundary is crossed between retransmissions. This important low-level channel access rule for beacons is implemented in the CPU_LOW DCF and not the CPU_HIGH AP project. The basic structure is as follows:

1) The CPU_HIGH AP project constructs a "beacon template" in a dedicated Tx packet buffer. This contains everything the AP wants to send, but leaves the actual transmission time of the packet buffer up to the CPU_LOW DCF code.
2) The CPU_LOW DCF project periodically calls the function poll_tbtt_and_send_beacon(). As the name suggests, this function polls for whether or not a TBTT boundary has been crossed and, if so, sends the beacon present in the beacon template packet buffer that the AP set up.

Early versions of the design missed this subtlety on the way that beacons are supposed to behave and had the AP directly create and pass down a beacon packet just like any other transmitted packet. This was incorrect. While more complex, the current behavior that splits beaconing between creation in CPU_HIGH and timing the transmission in CPU_LOW fully complies with the standard.



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