RSSADAPT(9)            NetBSD Kernel Developer's Manual            RSSADAPT(9)

NAME
     rssadapt, ieee80211_rssadapt_choose, ieee80211_rssadapt_input,
     ieee80211_rssadapt_lower_rate, ieee80211_rssadapt_raise_rate,
     ieee80211_rssadapt_updatestats -- rate adaptation based on received sig-
     nal strength

SYNOPSIS
     #include <net80211/ieee80211_var.h>
     #include <net80211/ieee80211_rssadapt.h>

     void
     ieee80211_rssadapt_input(struct ieee80211com *ic,
         struct ieee80211_node *ni, struct ieee80211_rssadapt *ra, int rssi);

     void
     ieee80211_rssadapt_lower_rate(struct ieee80211com *ic,
         struct ieee80211_node *ni, struct ieee80211_rssadapt *ra,
         struct ieee80211_rssdesc *id);

     void
     ieee80211_rssadapt_raise_rate(struct ieee80211com *ic,
         struct ieee80211_rssadapt *ra, struct ieee80211_rssdesc *id);

     void
     ieee80211_rssadapt_updatestats(struct ieee80211_rssadapt *ra);

     int
     ieee80211_rssadapt_choose(struct ieee80211_rssadapt *ra,
         struct ieee80211_rateset *rs, struct ieee80211_frame *wh, u_int len,
         int fixed_rate, const char *dvname, int do_not_adapt);

DESCRIPTION
     The rssadapt module provides rapid adaptation of transmission data rate
     to 802.11 device drivers based on received-signal strength (RSS).  A
     driver needs only to provide rssadapt with indications of RSS and fail-
     ure/success of transmissions for each 802.11 client or peer.  For each
     transmit packet, rssadapt chooses the transmission data rate that offers
     the best expected throughput, given the packet's length and destination.

     rssadapt models an 802.11 channel very simply (see also the BUGS
     section).  It assumes that the packet-error rate (PER) is determined by
     the signal-to-noise ratio (S/N) at the receiver, the transmission data
     rate, and the packet length.  The S/N determines the choice of data rate
     that yields the lowest PER for all packets of a certain length.

FUNCTIONS
     ieee80211_rssadapt_choose(ra, rs, wh, len, fixed_rate, dvname,
                         do_not_adapt)
                         Choose the transmission data rate for a packet.
                         ra            Ordinarily, the rssadapt state object
                                       belonging to the node which is the
                                       packet destination.  However, if the
                                       destination is a broadcast/multicast
                                       address, then ra belongs to the BSS
                                       node, ic->ic_bss.
                         rs            A list of eligible data rates for the
                                       node; for example, the rates negotiated
                                       when the node associated with the net-
                                       work.
                         len           The packet length in bytes, including
                                       the 802.11 header and frame check
                                       sequence (FCS).
                         fixed_rate    If the operator has set the data rate
                                       using, for example, ifconfig wi0 media
                                       ds1, then fixed_rate tells the index of
                                       that rate in rs.  rssadapt obeys a
                                       fixed data rate whenever the 802.11
                                       standard allows it: sometimes the stan-
                                       dard requires multicast/broadcast pack-
                                       ets to be transmitted at a so-called
                                       ``basic rate''.
                         dvname        The device driver uses dvname to indi-
                                       cate the name of the interface for the
                                       purpose of diagnostic and debug mes-
                                       sages.  The driver sets dvname to NULL
                                       when no messages are desired.
                         do_not_adapt  If do_not_adapt is non-zero, then
                                       ieee80211_rssadapt_choose() will choose
                                       the highest rate in rs that suits the
                                       destination, regardless of the RSS.
                         The return value of ieee80211_rssadapt_choose() is an
                         index into rs, indicating its choice of transmit data
                         rate.
     ieee80211_rssadapt_input(ic, ni, ra, rssi)
                         The RSS serves as a rough estimate of the S/N at each
                         node.  A driver provides RSS updates using
                         ieee80211_rssadapt_input(), whose arguments are:
                         ic    The wireless interface's 802.11 state object.
                         ni    The 802.11 node whose RSS the driver is updat-
                               ing.
                         ra    The node's rssadapt state object.
                         rssi  The node's received signal strength indication.
                               The range of rssi is from 0 to 255.
     ieee80211_rssadapt_lower_rate(ic, ni, ra, id)
     ieee80211_rssadapt_raise_rate(ic, ra, id)
                         Drivers call ieee80211_rssadapt_raise_rate() and
                         ieee80211_rssadapt_lower_rate() to indicate transmit
                         successes and failures, respectively.
                         ic  The 802.11 state object.
                         ni  The neighbor to whom the driver transmitted.
                         ra  The neighbor's rssadapt state object.
                         id  DIsplays statistics on the transmission attempt.
     ieee80211_rssadapt_updatestats(ra)
                         An 802.11 node is eligible for its RSS thresholds to
                         decay every 1/10 to 10 seconds.  It is eligible more
                         often (every 1/10 second) at high packet rates, and
                         less often (every 10 seconds) at low packet rates.  A
                         driver assists rssadapt in tracking the exponential-
                         average packet rate by calling
                         ieee80211_rssadapt_updatestats() every 1/10th second
                         for each node's ieee80211_rssadapt object.
                         ra  The neighbor's rssadapt state object.

ALGORITHM
     rssadapt monitors the RSS from neighboring 802.11 nodes, recording the
     exponential average RSS in each neighbor's ieee80211_rssadapt structure.
     rssadapt uses transmit success/failure feedback from the device driver to
     fill a table of RSS thresholds.  The table is indexed by packet size, L,
     and a data rate, R, to find out the minimum exponential-average RSS that
     a node must show before rssadapt will indicate that a packet L bytes long
     can be transmitted R bits per second with optimal expected throughput.
     When the driver indicates a unicast packet is transmitted unsuccessfully
     (that is, the NIC received no ACK for the packet), rssadapt will move the
     corresponding RSS threshold toward the exponential average RSSI at the
     time of transmission.  Thus several consecutive transmit failures for the
     same <L, R> tuple will ensure that the RSS threshold rises high enough
     that rate R is abandoned for packets L bytes long.  When the driver indi-
     cates a successful transmission, the RSS threshold corresponding to the
     same packet length, but the next higher data rate, is lowered slightly.
     The RSS threshold is said to ``decay''.  This ensures that occasionally
     rssadapt indicates the driver should try the next higher data rate, just
     in case conditions at the receiver have changed (for example, noise
     levels have fallen) and a higher data rate can be supported at the same
     RSS level.

     The rate of decay is controlled.  In an interval of 1/10th second to 10
     seconds, only one RSS threshold per neighbor may decay.  The interval is
     connected to the exponential-average rate that packets are being trans-
     mitted.  At high packet rates, the interval is shortest.  It is longest
     at low packet rates.  The rationale for this is that RSS thresholds
     should not decay rapidly if there is no information from packet transmis-
     sions to counteract their decay.

DATA STRUCTURES
     An ieee80211_rssdesc describes a transmission attempt.

           struct ieee80211_rssdesc {
                   u_int                    id_len;
                   u_int                    id_rateidx;
                   struct ieee80211_node   *id_node;
                   u_int8_t                 id_rssi;
           };

     id_len is the length, in bytes, of the transmitted packet.  id_node
     points to the neighbor's ieee8021_node, and id_rssi is the exponential-
     average RSS at the time the packet was transmitted.  id_rateidx is an
     index into the destination-neighbor's rate-set, id_node->ni_rates, indi-
     cating the transmit data rate for the packet.

     An ieee80211_rssadapt contains the rate-adaptation state for a neighbor-
     ing 802.11 node.  Ordinarily a driver will ``subclass'' ieee80211_node.
     The ieee80211_rssadapt structure will be a subclass member.  In this way,
     every node's rssadapt condition is independently tracked and stored in
     its node object.

           struct ieee80211_rssadapt {
                   u_int16_t               ra_avg_rssi;
                   u_int32_t               ra_nfail;
                   u_int32_t               ra_nok;
                   u_int32_t               ra_pktrate;
                   u_int16_t               ra_rate_thresh[IEEE80211_RSSADAPT_BKTS]
                                                         [IEEE80211_RATE_SIZE];
                   struct timeval          ra_last_raise;
                   struct timeval          ra_raise_interval;
           };

     ra_avg_rssi is the exponential-average RSS, shifted left 8 bits.
     ra_nfail tells the number of transmit failures in the current update
     interval.  ra_nok tells the number of transmit successes in the current
     update interval.  ra_pktrate tells the exponential average number of
     transmit failure/success indications over past update intervals.  This
     approximates the rate of packet-transmission.  ra_rate_thresh contains
     RSS thresholds that are indexed by <packet length, data rate> tuples.
     When this node's exponential-average RSS exceeds ra_rate_thresh[i][j],
     then packets at most 128 x 8^i bytes long are eligible to be transmitted
     at the rate indexed by j.  ra_last_raise and ra_raise_interval are used
     to control the rate that RSS thresholds ``decay''.  ra_last_raise indi-
     cates when ieee80211_rssadapt_raise_rate() was last called.
     ra_raise_interval tells the minimum period between consecutive calls to
     ieee80211_rssadapt_raise_rate().  If ieee80211_rssadapt_raise_rate() is
     called more than once in any period, the second and subsequent calls are
     ignored.

CODE REFERENCES
     This section describes places within the NetBSD source tree where actual
     code implementing or using rssadapt can be found.  All pathnames are rel-
     ative to /usr/src.

     The code for rssadapt is in the file sys/net80211/ieee80211_rssadapt.c.

     wi(4) contains a reference implementation.  See sys/dev/ic/wi.c.

SEE ALSO
     wi(4)

     Javier del Prado Pavon and Sunghyun Choi, "Link Adaptation Strategy for
     IEEE 802.11 WLAN via Received Signal Strength Measurement", ICC'03, pp.
     1108-1113, May 2003.

HISTORY
     rssadapt first appeared in NetBSD 3.0.

AUTHORS
     David Young <dyoung@NetBSD.org>

BUGS
     To cope with interference from microwave ovens, frequency-hopping radios,
     and other sources of RF pulse-trains and bursts, rssadapt should adapt
     the fragmentation threshold as well as the data rate.

     For improved throughput, rssadapt should indicate to drivers when they
     should use the 802.11b short-preamble.

     The constants in ieee80211_rssadapt_updatestats() should be configurable.

NetBSD 5.0_STABLE               March 23, 2004               NetBSD 5.0_STABLE
