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.\" Copyright (c) 2009 Sam Leffler, Errno Consulting
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.Dd August 4, 2009
.Dt IEEE8021_AMRR 9
.Os
.Sh NAME
.Nm ieee80211_amrr
.Nd 802.11 network driver transmit rate control support
.Sh SYNOPSIS
.In net80211/ieee80211_amrr.h
.Ft void
.Fo ieee80211_amrr_init
.Fa "struct ieee80211_amrr *"
.Fa "struct ieee80211vap *"
.Fa "int amin"
.Fa "int amax"
.Fa "int interval"
.Fc
.\"
.Ft void
.Fn ieee80211_amrr_cleanup "struct ieee80211_amrr *"
.\"
.Ft void
.Fn ieee80211_amrr_setinterval "struct ieee80211_amrr *" "int interval"
.\"
.Ft void
.Fo ieee80211_amrr_node_init
.Fa "struct ieee80211_amrr *"
.Fa "struct ieee80211_amrr_node *"
.Fa "struct ieee80211_node *"
.Fc
.\"
.Ft int
.Fo ieee80211_amrr_choose
.Fa "struct ieee80211_node *"
.Fa "struct ieee80211_amrr_node *"
.Fc
.\"
.Ft void
.Fo ieee80211_amrr_tx_complete
.Fa "struct ieee80211_amrr_node *"
.Fa "int ok"
.Fa "int retries"
.Fc
.\"
.Ft void
.Fo ieee80211_amrr_tx_update
.Fa "struct ieee80211_amrr_node *"
.Fa "int txnct"
.Fa "int success"
.Fa "int retrycnt"
.Fc
.Sh DESCRIPTION
.Nm
is an implementation of the AMRR transmit rate control algorithm
for drivers that use the
.Nm net80211
software layer.
A rate control algorithm is responsible for choosing the transmit
rate for each frame.
To maximize throughput algorithms try to use the highest rate that
is appropriate for the operating conditions.
The rate will vary as conditions change; the distance between two stations
may change, transient noise may be present that affects signal quality,
etc.
.Nm
uses very simple information from a driver to do it's job:
whether a frame was successfully delivered and how many transmit
attempts were made.
While this enables its use with virtually any wireless device it
limits it's effectiveness--do not expect it to function well in
difficult environments and/or respond quickly to changing conditions.
.Pp
.Nm
requires per-vap state and per-node state for each station it is to
select rates for.
The API's are designed for drivers to pre-allocate state in the
driver-private extension areas of each vap and node.
For example the
.Xr ral 4
driver defines a vap as:
.Bd -literal -offset indent
struct rt2560_vap {
        struct ieee80211vap     ral_vap;
        struct ieee80211_beacon_offsets ral_bo;
        struct ieee80211_amrr   amrr;

        int      (*ral_newstate)(struct ieee80211vap *,
                      enum ieee80211_state, int);
};
.Ed
.Pp
The
.Vt amrr
structure member holds the per-vap state for
.Nm
and
.Xr ral 4
initializes it in the vap create method with:
.Bd -literal -offset indent
ieee80211_amrr_init(&rvp->amrr, vap,
    IEEE80211_AMRR_MIN_SUCCESS_THRESHOLD,
    IEEE80211_AMRR_MAX_SUCCESS_THRESHOLD,
    500 /* ms */);
.Ed
.Pp
The node is defined as:
.Bd -literal -offset indent
struct rt2560_node {
        struct ieee80211_node   ni;
        struct ieee80211_amrr_node amrr;
};
.Ed
.Pp
with initialization done in the driver's
.Vt iv_newassoc
method:
.Bd -literal -offset indent
static void
rt2560_newassoc(struct ieee80211_node *ni, int isnew)
{
        struct ieee80211vap *vap = ni->ni_vap;

        ieee80211_amrr_node_init(&RT2560_VAP(vap)->amrr,
            &RT2560_NODE(ni)->amrr, ni);
}
.Ed
.Pp
Once
.Nm
state is setup, transmit rates are requested by calling
.Fn ieee80211_amrr_choose
in the transmit path; e.g.:
.Bd -literal -offset indent
tp = &vap->iv_txparms[ieee80211_chan2mode(ni->ni_chan)];
if (IEEE80211_IS_MULTICAST(wh->i_addr1)) {
	rate = tp->mcastrate;
} else if (m0->m_flags & M_EAPOL) {
	rate = tp->mgmtrate;
} else if (tp->ucastrate != IEEE80211_FIXED_RATE_NONE) {
	rate = tp->ucastrate;
} else {
	(void) ieee80211_amrr_choose(ni, &RT2560_NODE(ni)->amrr);
	rate = ni->ni_txrate;
}
.Ed
.Pp
Note a rate is chosen only for unicast data frames when a fixed
transmit rate is not configured; the other cases are handled with
the
.Nm net80211
transmit parameters.
Note also that
.Fn ieee80211_amrr_choose
writes the chosen rate in
.Vt ni_txrate ;
this eliminates copying the value as it is exported to user applications so
they can display the current transmit rate in status.
.Pp
The remaining work a driver must do is feed status back to
.Nm
when a frame transmit completes using
.Fn ieee80211_amrr_tx_complete .
Drivers that poll a device to retrieve statistics can use
.Fn ieee80211_amrr_tx_update
(instead or in addition).
.Sh SEE ALSO
.Xr ieee80211 9 ,
.Xr ieee80211_output 9