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NAME

     ieee80211_beacon - 802.11 beacon support

SYNOPSIS

     #include <net80211/ieee80211_var.h>

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

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

     void
     ieee80211_beacon_notify(struct ieee80211vap *, int what);

DESCRIPTION

     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.

MULTI-VAP BEACON SCHEDULING

     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.

SEE ALSO

     ieee80211(9)