IEEE80211_BEACON(9) FreeBSD Kernel Developer's Manual IEEE80211_BEACON(9)


ieee80211_beacon802.11 beacon support


#include < net80211/ieee80211_var.h>

struct mbuf *
ieee80211_beacon_alloc( struct ieee80211_node *, struct ieee80211_beacon_offsets *);

ieee80211_beacon_update( struct ieee80211_node *, struct ieee80211_beacon_offsets *, struct mbuf *, int mcast);

ieee80211_beacon_notify( struct ieee80211vap *, int what);


The net80211 software layer provides a support framework for drivers that includes a template-based mechanism for dynamic update of beacon frames transmit in hostap, adhoc, and mesh operating modes. Drivers should use ieee80211_beacon_alloc() to create an initial beacon frame. The ieee80211_beacon_offsets structure holds information about the beacon contents that is used to optimize updates done with ieee80211_beacon_update().

Update calls should only be done when something changes that affects the contents of the beacon frame. When this happens the iv_update_beacon method is invoked and a driver-supplied routine must do the right thing. For devices that involve the host to transmit each beacon frame this work may be as simple as marking a bit in the ieee80211_beacon_offsets structure:

static void 
ath_beacon_update(struct ieee80211vap *vap, int item) 
        struct ieee80211_beacon_offsets *bo = &ATH_VAP(vap)->av_boff; 
 setbit(bo->bo_flags, item); 

with the ieee80211_beacon_update() call done before the next beacon is to be sent.

Devices that off-load beacon generation may instead choose to use this callback to push updates immediately to the device. Exactly how that is accomplished is unspecified. One possibility is to update the beacon frame contents and extract the appropriate information element, but other scenarios are possible.


Drivers that support multiple vaps that can each beacon need to consider how to schedule beacon frames. There are two possibilities at the moment: burst all beacons at TBTT or stagger beacons over the beacon interval. Bursting beacon frames may result in aperiodic delivery that can affect power save operation of associated stations. Applying some jitter (e.g. by randomly ordering burst frames) may be sufficient to combat this and typically this is not an issue unless stations are using aggressive power save techniques such as U-APSD (sometimes employed by VoIP phones). Staggering frames requires more interrupts and device support that may not be available. Staggering beacon frames is usually superior to bursting frames, up to about eight vaps, at which point the overhead becomes significant and the channel becomes noticeably busy anyway.


August 4, 2009 FreeBSD