ieee80211_amrr_choose (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
   notice, this list of conditions and the following disclaimer.
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   notice, this list of conditions and the following disclaimer in the
   documenta...

(The comments found at the beginning of the groff file "man9/ieee80211_amrr_choose.9freebsd".)

NAME

ieee80211_amrr - 802.11 network driver transmit rate control support

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

DESCRIPTION

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. 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.

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 Vt amrr structure member holds the per-vap state for 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 Vt 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 state is setup, transmit rates are requested by calling Fn 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 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.

The remaining work a driver must do is feed status back to 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).

SEE ALSO

ieee80211(9), ieee80211_output9