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)

Debian                                           August 4, 2009                              IEEE80211_BEACON(9)