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/share/man/man9/ieee80211_beacon.9

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  2.\" Copyright (c) 2009 Sam Leffler, Errno Consulting
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 26.\" $FreeBSD$
 27.\"
 28.Dd August 4, 2009
 29.Dt IEEE80211_BEACON 9
 30.Os
 31.Sh NAME
 32.Nm ieee80211_beacon
 33.Nd 802.11 beacon support
 34.Sh SYNOPSIS
 35.In net80211/ieee80211_var.h
 36.Pp
 37.Ft "struct mbuf *"
 38.Fo ieee80211_beacon_alloc
 39.Fa "struct ieee80211_node *"
 40.Fa "struct ieee80211_beacon_offsets *"
 41.Fc
 42.\"
 43.Ft int
 44.Fo ieee80211_beacon_update
 45.Fa "struct ieee80211_node *"
 46.Fa "struct ieee80211_beacon_offsets *"
 47.Fa "struct mbuf *"
 48.Fa "int mcast"
 49.Fc
 50.\"
 51.Ft void
 52.Fn ieee80211_beacon_notify "struct ieee80211vap *" "int what"
 53.Sh DESCRIPTION
 54The
 55.Nm net80211
 56software layer provides a support framework for drivers that includes
 57a template-based mechanism for dynamic update of beacon frames transmit
 58in hostap, adhoc, and mesh operating modes.
 59Drivers should use
 60.Fn ieee80211_beacon_alloc
 61to create an initial beacon frame.
 62The
 63.Vt ieee80211_beacon_offsets
 64structure holds information about the beacon contents that is used
 65to optimize updates done with
 66.Fn ieee80211_beacon_update .
 67.Pp
 68Update calls should only be done when something changes that
 69affects the contents of the beacon frame.
 70When this happens the
 71.Dv iv_update_beacon
 72method is invoked and a driver-supplied routine must do the right thing.
 73For devices that involve the host to transmit each
 74beacon frame this work may be as simple as marking a bit in the
 75.Vt ieee80211_beacon_offsets
 76structure:
 77.Bd -literal
 78static void
 79ath_beacon_update(struct ieee80211vap *vap, int item)
 80{
 81        struct ieee80211_beacon_offsets *bo = &ATH_VAP(vap)->av_boff;
 82	setbit(bo->bo_flags, item);
 83}
 84.Ed
 85.Pp
 86with the
 87.Fn ieee80211_beacon_update
 88call done before the next beacon is to be sent.
 89.Pp
 90Devices that off-load beacon generation may instead choose to use
 91this callback to push updates immediately to the device.
 92Exactly how that is accomplished is unspecified.
 93One possibility is to update the beacon frame contents and extract
 94the appropriate information element, but other scenarios are possible.
 95.Sh MULTI-VAP BEACON SCHEDULING
 96Drivers that support multiple vaps that can each beacon need to consider
 97how to schedule beacon frames.
 98There are two possibilities at the moment:
 99.Em burst
100all beacons at TBTT or
101.Em stagger beacons
102over the beacon interval.
103Bursting beacon frames may result in aperiodic delivery that can affect
104power save operation of associated stations.
105Applying some jitter (e.g. by randomly ordering burst frames) may be
106sufficient to combat this and typically this is not an issue unless
107stations are using aggressive power save techniques
108such as U-APSD (sometimes employed by VoIP phones).
109Staggering frames requires more interrupts and device support that
110may not be available.
111Staggering beacon frames is usually superior to bursting frames, up to
112about eight vaps, at which point the overhead becomes significant and
113the channel becomes noticeably busy anyway.
114.Sh SEE ALSO
115.Xr ieee80211 9