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NAME

     ieee80211_amrr - 802.11 network driver transmit rate control support

SYNOPSIS

     #include <net80211/ieee80211_amrr.h>

     void
     ieee80211_amrr_init(struct ieee80211_amrr *, struct ieee80211vap *,
             int amin, int amax, int interval);

     void
     ieee80211_amrr_cleanup(struct ieee80211_amrr *);

     void
     ieee80211_amrr_setinterval(struct ieee80211_amrr *, int interval);

     void
     ieee80211_amrr_node_init(struct ieee80211_amrr *,
             struct ieee80211_amrr_node *, struct ieee80211_node *);

     int
     ieee80211_amrr_choose(struct ieee80211_node *,
             struct ieee80211_amrr_node *);

     void
     ieee80211_amrr_tx_complete(struct ieee80211_amrr_node *, int ok,
             int retries);

     void
     ieee80211_amrr_tx_update(struct ieee80211_amrr_node *, int txnct,
             int success, int retrycnt);

DESCRIPTION

     ieee80211_amrr is an implementation of the AMRR transmit rate control
     algorithm for drivers that use the 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.
     ieee80211_amrr 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.

     ieee80211_amrr 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 ral(4) driver defines a vap as:

           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);
           };

     The amrr structure member holds the per-vap state for ieee80211_amrr and
     ral(4) initializes it in the vap create method with:

           ieee80211_amrr_init(&rvp->amrr, vap,
               IEEE80211_AMRR_MIN_SUCCESS_THRESHOLD,
               IEEE80211_AMRR_MAX_SUCCESS_THRESHOLD,
               500 /* ms */);

     The node is defined as:

           struct rt2560_node {
                   struct ieee80211_node   ni;
                   struct ieee80211_amrr_node amrr;
           };

     with initialization done in the driver’s iv_newassoc method:

           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);
           }

     Once ieee80211_amrr state is setup, transmit rates are requested by
     calling ieee80211_amrr_choose() in the transmit path; e.g.:

           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;
           }

     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 net80211
     transmit parameters.  Note also that ieee80211_amrr_choose() writes the
     chosen rate in ni_txrate; this eliminates copying the value as it is
     exported to user applications so they can display the current transmit
     rate in status.

     The remaining work a driver must do is feed status back to ieee80211_amrr
     when a frame transmit completes using ieee80211_amrr_tx_complete().
     Drivers that poll a device to retrieve statistics can use
     ieee80211_amrr_tx_update() (instead or in addition).

SEE ALSO

     ieee80211(9), ieee80211_output(9)