ieee80211_beacon (9)

Leading comments

Copyright (c) 2009 Sam Leffler, Errno Consulting
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
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   notice, this list of conditions and the following disclaimer in the
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(The comments found at the beginning of the groff file "man9/ieee80211_beacon.9freebsd".)

NAME

ieee80211_beacon - 802.11 beacon support

SYNOPSIS

In net80211/ieee80211_var.h

Ft struct mbuf * Fo ieee80211_beacon_alloc Fa struct ieee80211_node * Fa struct ieee80211_beacon_offsets * Fc Ft int Fo ieee80211_beacon_update Fa struct ieee80211_node * Fa struct ieee80211_beacon_offsets * Fa struct mbuf * Fa int mcast Fc Ft void Fn 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 Fn ieee80211_beacon_alloc to create an initial beacon frame. The Vt ieee80211_beacon_offsets structure holds information about the beacon contents that is used to optimize updates done with Fn 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 Vt 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 Fn 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)