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)

Debian                                           August 4, 2009                                 IEEE8021_AMRR(9)