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

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 usu-
     ally 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)

BSD
								  August 4, 2009							       BSD