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)